This page is a placeholder. The Appendices will appear here when the article is published.

Last modified: Sat Oct 22 03:10:19 EDT 2011

This page is intended partly as a followon to

the “Easy TEA Laser” page,
and partly as a standalone project. It describes a relatively powerful room-pressure nitrogen laser.

Before we get any further along, we need some safety information and a disclaimer.

These lasers use high voltages, and capacitors that can store lethal amounts of energy. They put out invisible ultraviolet light that can damage your eyes and skin. It is extremely important to take adequate safety precautions and use appropriate safety equipment with any laser; and it is crucially important with lasers that involve high voltages and/or produce invisible beams!

In addition, this particular laser uses two open spark gaps, which will damage your hearing if you do not use adequate ear protection. I strongly suggest that you acquire and use at least a pair of sound-protection earmuffs of the type used by shooters at rifle and pistol ranges; they look about like this:

width=”224″ height=”300″>




Figure 1: Hearing protection

(These cost me $35, and they are definitely worth it.)

Earplugs can also help, but by themselves are probably not sufficient unless they decrease the volume by at least 33 db, which some commercial ones do but only if you insert them correctly; I suspect that only special ones that are made to fit your own ears are really good enough for long-term use.

If you are not using enough hearing protection, you will probably get a nasty headache if you run the laser for a while. Take that as a warning, and get better protection! You can make a new spark gap, and you can make a new laser; but you cannot make new eyes, ears, or fingers.

This laser is based on the Charge-Transfer circuit topology, in which there is ordinarily a single main storage capacitor (the “dumper”). The dumper capacitor is charged by the power supply and then connected, by a fast switch, to a smaller and faster secondary capacitor (the “peaker”). The peaker capacitor is charged very rapidly, and it then drives the laser channel, along with whatever current is still flowing from the dumper. (Because the dumper is larger there is usually quite a bit of current from it, which contributes a significant amount to the total. Because it is slower, however, its influence is somewhat limited.) The CT topology is typically less efficient than the more familiar “Voltage Doubler” circuit, but is often capable of delivering higher output energy.

I have chosen to modify the CT circuit for this laser to use a minimal Marx generator as its dumper cap. My hope is that this will make the discharge more energetic by providing higher charging voltage for the peaker cap than the power supply can provide.

[The Marx Bank (or Marx generator) was invented by Erwin Otto Marx, in 1924. It is a fine way to increase the voltage of a pulsed system. The fundamental principle is that capacitors are charged up in parallel, and then connected by fast switches so that they discharge in series. A well-designed bank will get fairly close to an integer multiple of the initial charge voltage, particularly if it has only a few
stages.]

The design I’m using here is a stack of 4 capacitor plates. The bottom one is the baseplane of the entire system. (That is, it serves as the basis of the Marx bank, the peaker cap, and the laser channel. It is connected to the positive output of the power supply though an inductor. (More about polarity, below.)

Part of the baseplane is covered by a sheet of styrene plastic, 10 mils thick, which serves as the dielectric of the first part of the dumper. The next plate is on top of this styrene sheet, and is connected to the negative output of the power supply through a second inductor. It is also connected, by a piece of brass shim stock, to the lower side of the main switch, which is a triggered spark gap.

Then there is a dielectric layer of double or treble thickness, with the third plate on top of it. The third plate is lined up to be directly above the second plate. The extra-thick set of dielectrics allows the second and third plates to function as a small start capacitor for the spark gap, though I have added a very small (25 pf) start capacitor to it, on a just-in-case basis. [Note: this set ended up being 35 mils thick, and could be even thicker.] Like the baseplane, the third plate is connected to the positive output of the power supply, but through a separate inductor. It is also connected to the upper side of the main switch, via a piece of brass shim stock.

After that is a final sheet of dielectric and the fourth capacitor plate, which is connected to the negative output of the power supply through yet another inductor. The fourth plate serves as the output of the Marx bank, so it is also connected, through a piece of brass shim stock, to the top electrode of a passive free-running spark gap, which switches into the peaker capacitor. (See the schematic diagram, below, for clarification.)

The inductors prevent the bank from short-circuiting when the main switch is fired. They also keep some of the EMP out of the power supply. I built them by stiffening the cardboard tubes from two paper towel rolls with cyanoacrylate adhesive, and winding wire with thick insulation around them. I made two inductors per roll, with 31 turns per inductor. Here they are, with the power supply:

width=”163″ height=”300″>




Figure 2: Power supply and quad charging inductors

I got the wire at the hardware store; it has insulation that is 45 mils thick, and it is rated to handle 150 volts. I have no idea why it has such a low rating; I’ve been using a similar inductor on various versions of the “easy” TEA nitrogen laser for months now, at 12 kV and more, with no problems.

Here is the schematic:

with=”420″ height=”315″>




Figure 3: Circuit of the laser

Polarity note: at least with EG&G (now Perkin-Elmer) commercial trigatron gaps, it is suggested that for best operation the positive-going trigger pulse should go to the trigger electrode, and the surrounding (in EG&G terminology, “adjacent”) electrode should be connected to the positive side of the power supply. Because of the way the Marx stack is built, with the lower plate of the second capacitor sitting on top of the first capacitor, I was obliged to connect the positive side of the power supply to the baseplane, and the negative side to the top of the stack. I may, at some point, try reversing the polarity, especially if the laser doesn’t seem to want to work well with the circuit configured as it is in the diagram.

[Note, added later: I tried, and it didn’t seem to help, so I reverted to the original polarity.]

The baseplane is a sheet of single-sided circuitboard, as with the previous lasers in this series, but brass shim stock would work just as well. I am using six 4″ x 10″ sheets of brass, in pairs, as the other plates of the main storage caps. Because I can use wide pieces of shim stock to connect the caps to the triggered and free-running spark gaps, I don’t have to worry about connecting the pairs of brass plates to each other for fast discharge. (This should be evident from the assembly photos, below.)

The dielectric will be 10-mil styrene, unless I can find 8-mil or 10-mil polycarbonate. Polycarbonate has good dielectric constant and excellent dielectric strength, but is hard to find with a plain glossy surface on both sides. It is available in 10-mil thickness with one side matte and the other side covered with adhesive, neither of which is desirable in this application, and that form tends to be expensive. (I may try it anyway, just to see whether it works.)

[Note, added later: I did, indeed use 10-mil styrene, but later I changed the peaker dielectric to 6.5-mil acetate, as detailed in the text below.]

It is, of course, possible to allow the main switch to free-run; but that makes it difficult to predict when the laser will fire. At this small scale, it is easy enough to build a triggered switch, and that is what I have done:

width=”105″ height=”300″>
           

width=”300″ height=”300″>




Figures 4 & 5: Preassembly of the main switch

The photos show a 5/16-18 carriage bolt, which I have drilled to accept a small piece of capillary tubing. The trigger electrode is a broken piece of jeweler’s saw blade. This parallels the design I used for the final versions of the Easy TEA Laser project, but I am using the head of the carriage bolt as the face of the electrode, rather than an acorn nut screwed onto the end of an ordinary bolt. It was more difficult to drill the hole in the steel bolt than it would have been in a brass bolt, but I only had to drill one hole. Also, I was able to start the hole more or less in the center of the electrode face, which is integral to the support column for the capillary. This also meant that I did not have to be as concerned about whether the hole runs down the axis of the bolt, as long as it doesn’t interfere with mounting. (In fact, it is only modestly off center at the back end.)

The small piece of steel that you can see in the photo on the right is a knockout from an electrical box; I found it in a parking lot, and sanded the paint off
it. I am expecting to use it as the lower electrode of this gap. It is more or less just a strike plate, intended to protect the brass plate or shim stock beneath it from the sparks in the gap. If I can find another one, I will use it in the passive gap.

Here is the assembled gap:

width=”300″ height=”159″>
           
width=”300″ height=”159″>




Figures 6 & 7: The Installed Switch

(I have added a 25-pf start cap, on a “just-in-case” basis. It is currently a bit far from the gap, and I may move it closer at some point. Note, also, the insulating structures behind the gap; I had to add these to prevent flashovers from the top capacitor plate.)

Triggering:

Initially, I am triggering the switch with a TM-11 or TM-11A trigger unit; but I am working on a design for a DIY trigger generator, and will post it when I get it up and flying. (Triggering of this type of gap may be slightly tweaky, and an automotive spark coil, on its own, may not be very good for this purpose. For this
reason, the trigger generator may have to involve a bit of finesse.)

The power supply for this laser is the same type that I used in the previous lasers, a small unit that I took from an old electronic air cleaner. It delivers about 12 kV into an open circuit. When I trigger the main switch, the Marx generator should erect fairly quickly (probably within about 20 nanoseconds), and at that point it will be providing at least 20 kV at its output. The passive gap must hold off the initial charge voltage, which it is exposed to whenever the main store is being charged; but it must not withstand the full output voltage of the Marx, or the laser will fail to fire. As I write this I have not yet constructed it, but I expect to make it fairly easy to adjust. At least initially I expect to tweak it until it holds off the charging voltage easily, but will only take a modest overvoltage to fire.

[Note, added later: It turns out that the main gap is the one that needs to be adjusted so that it barely holds off the charging voltage; the passive gap wants to be nearly as wide as possible, so that it doesn’t begin to conduct until there is a high voltage coming from the Dumper.]

I eventually put a 200-pf start cap across the passive gap; it seems to help the performance a little, but may be on the large side.

Unless I change my mind, I will use another 4″ x 10″ brass plate for this. It should, ordinarily, have about 1/3 as much capacitance as the dumper cap; for this reason and because it may have relatively high voltages across it, I am likely to use a thicker sheet of dielectric for it. (I have 15-mil styrene, which will work.)

Note, added later: The hobby shop changed my mind for me by running out of brass plates of the thickness I wanted, and I ended up using a copper plate instead. Also, as I have already mentioned, I tried a thick dielectric and was not satisfied with the resulting performance, so after some thought I shifted to 6.5-mil acetate. The acetate is too thin to handle the amount of voltage involved for any length of time, but because it is being charged for only a few dozen nanoseconds at a time it has withstood hundreds of firing cycles. It performs quite well — when the laser is fully adjusted, its unfocused output will pump a cuvette of “DTC” in 70% isopropyl alcohol that is several inches away from the end of the channel. (See Figures 29 and 31, below.)

For what it’s worth, the peaker cap measures about 3.4 nf on my DMM.

It took me about two and a half days to build this laser, and then about two weeks to get it to stabilize. It kept puncturing a dielectric or sparking where I didn’t want it to, but I finally got it more or less tamed out. It then took me about two more weeks to get fairly good performance from it.

Here is a sequence of photos, taken during the initial build. (You will notice that the peaker dielectric is a piece of plastic sign. Later on, I changed that out and substituted a piece of 6.5-mil acetate sheet, which has higher dielectric constant. The peaker now measures about 3.4 nf on my little DMM.)

width=”300″ height=”225″>
           

width=”300″ height=”225″>
           

width=”300″ height=”225″>




width=”300″ height=”225″>
           

width=”300″ height=”225″>
           

width=”300″ height=”225″>




width=”300″ height=”225″>
           

width=”300″ height=”225″>
           

width=”300″ height=”225″>




width=”300″ height=”225″>
           

width=”300″ height=”225″>

Figures 8 – 18: Initial assembly sequence

Here are overviews of the completed laser, taken at an intermediate stage:

width=”300″ height=”225″>
           

width=”300″ height=”225″>




Figures 19 & 20: Overview of the Completed Laser

(The photo on the left has the spark gap and dumper capacitor at its left, and the peaker and the laser channel at the right. The photo on the right was taken from the usual output end, on the peaker side of the chassis, almost kitty-corner across from the other photo. You can see the cylindrical lens that I use when I want to focus the output, and a cuvette of dye solution.)

[As far as I know, btw, “kitty-corner” comes from “catercorner” or “catercornered”, where “cater” appears to derive from French “quatre”; it originally seems to have meant simply “four-cornered”.]

A note about adjustment: This design has a large number of adjustable parameters, and it is seriously not recommended as a first laser. You can drive yourself bats trying to figure out what you could possibly have done that caused some outré behavior. Alternatively, you can do something you’ve done several times with uniform results, and have something entirely different happen for no obvious reason. I can suggest a few rules of thumb, but you must take them with several grains of salt.

1. The electrodes of the main switch should, typically, be set as close together as possible without too much self-flashing, so that it has low inductance and relatively low effective resistance when it fires. (I tolerate an occasional self flash, perhaps once every 5 to 10 seconds. This helps prevent the voltage on the dumper cap from getting too high between shots.)

   
   
   

2. The passive gap wants to be set relatively wide, so that charging of the peaker starts only after the output from the Marx bank is approaching its peak voltage.

   
   
   

3. It seems to me that the preionizers (I have them on both sides of the channel) work best when they are perhaps 2 mm farther apart than the electrodes, and are essentially parallel to them. (See the “Easy TEA Laser” page for preionizer shaping.)

   
   
   

4. So far, it seems to me that the electrodes work best when they are very smooth, and very cleanly rounded. This agrees with the experience of Jarrod Kinsey, but not with that of Alfonso Torres Rodríguez. (Alfon’s laser designs are quite advanced, and they operate at somewhat higher voltages, so it is not entirely surprising that he obtains different results in some areas.)

   
   
   

5. I have found that it is instructive to note the behavior as I adjust the spacing between the electrodes, and then to repeat with different preionizer spacing.

   
   
   

6. If you find that you get sparking at almost any electrode spacing, it is likely that you are not getting very high voltage on your peaker capacitor.

   
   
   

7. On the other hand, if you get a nice clean discharge but no lasing, your electrodes are probably too far apart. (There is a parameter called E/p, which is discussed in various technical articles about nitrogen lasers that have appeared in the scientific literature. It relates the voltage to the channel width and the partial pressure of nitrogen in the channel. The reported optimum for E/p is something close to 100 volts per Torr centimeter, so a 2-mm-wide channel that is operating at 760 Torr takes a little over 15 kV for best performance. As the channel width increases, the optimal voltage increases with it. If you are attempting to operate a 2-mm channel and your power source cannot provide this much voltage, the operation of your laser will suffer.)

A low-pressure nitrogen laser is typically operated with a mirror at one end of its channel. This can provide a large enhancement in output power, and a beam that is more tightly defined. TEA nitrogen lasers, in contrast, are only occasionally operated this way. With a channel that is >30 cm long, it would seem pointless to add a mirror, but I had to know whether it would help, so I got one and tried it. It is 15 mm square, and I think the coating is UV-enhanced aluminum, so it probably reflects 85 or 86% at 337.1 nm.

width=”251″ height=”300″>




Figure 21: The Mirror

I quickly discovered that little plastic strips are not very good for tweaking the alignment, but at least they kept my fingers away from the high voltage during initial testing. I ended up using only one of them, and taping it in place; eventually I will figure out a better solution.

Here are some photos, showing misalignment and roughly correct alignment. They are not all precisely to the same scale, but it is definitely the case that when I get the mirror aligned nicely the bright central region of the beam gets wider, while the overall beam appears to become slightly narrower. (Compare the central part of the main beam as it appears in the first and fourth photos.)

width=”300″ height=”256″>
           

width=”300″ height=”188″>




width=”300″ height=”150″>
           

width=”300″ height=”150″>




Figures 22-25: Aligning the Mirror

(In order:

1. The return from the mirror is far too low; if it were any lower it would bounce off the dielectric and appear above the main pattern.

   
   
   

2. The return is somewhat low [notice the shadows of the electrodes!].

   
   
   

3. The return is slightly high.

   
   
   

4. The return is positioned at least roughly correctly.)

In sum, although this laser probably does not quite reach the MegaWatt power level, it provides substantial performance and can be used for various purposes.

For example, I wanted to know how accurate the timebase on our old Tektronix 7104 oscilloscope is, and as I do not have ready access to NIST (they are some miles from here, and I’m sure they have better things to do than calibrate my old 7B15 for me), I decided to fall back upon fundamental constants: in this case, the speed of light. The protocol here is to produce two light pulses with a separation in time that is known, and to compare that known timing with the timing displayed by the scope. Fortunately, the pulse from a room-pressure nitrogen laser is typically less than 1 nanosecond long FWHM [Full Width at Half of the Maximum amplitude], which makes it easy to use for this purpose.

To create two pulses with a delay between them, I ran one of the output beams of the laser (this was before I added the mirror) into a structure rather like a Michelson interferometer, but with grossly unequal pathlengths, which I built on top of the oscilloscope because the cable on the photodetector is short:

width=”300″ height=”106″>




Figure 26: Scope calibration setup overview

(Sorry about the view angle!)

Please note the carefully chosen “close-in” reflector, a threaded connector nut:

width=”269″ height=”300″>




Figure 27: Scope calibration setup detail

A real mirror would have swamped the detector, so I picked something that was a vaguely specular reflector but not a very good one, and lucked out — it works quite nicely. Note, also, the beamsplitter, which is a 1-inch sapphire window intended for a sewer camera. I chose sapphire because it has high refractive index, and thus reflects about 14% at normal incidence.

When I took the photos above, the pathlength from the beamsplitter to the far mirror was about 16 inches. I redid the measurement for the scope trace photo below
with a pathlength of 38 cm. We have to double that, because the light goes off to the mirror at the far end and then has to return. The speed of light is about 299,792,458 meters per second (a wee bit less in air, not that it makes any difference for this measurement), so it took slightly more than 2.5 nanoseconds for the light to travel the 76cm two-way path.

Here’s what the scope gave me, after I got everything aligned and applied a bit of optical attenuation at the far-end mirror to get the peak heights to be fairly similar:

width=”300″ height=”244″>




Figure 28: Scope calibration trace

I’ve marked the positions of the peaks; the scope indicates that they are just over 2.8 nsec apart, which means that it is about 10% off calibration when it is displaying 2 nsec per major division on the screen. Such is life; at least now I know what the error is. (There is an adjustment for this, but it is already at the end of its range.)

There are, of course, other uses for a nitrogen laser. One of the most common is as a pump source for small organic dye lasers. Here is a cuvette of dye solution, sitting about 8 inches off the end of the channel, being pumped by the unfocused beam of the laser:

width=”300″ height=”300″>




Figure 29: Unfocused output, lasing DTC in 70% Isopropanol

(If you look really closely at the beam pattern on the front of the cuvette, you can tell that the return beam from the mirror is very slightly high.)

I also decided to see what would happen if I focused the beam more tightly. We happen to have a few polished metal mirrors, and I put one into the beam after it left the cylindrical lens. If I let this combination focus the beam down to a dot, I get only very diffuse lasing; but as long as I let it make a bar that is at least a millimeter or two long, it directs the output fairly nicely:

width=”300″ height=”244″>
           

width=”300″ height=”244″>




Figures 30 & 31: Tight-focus setup; tightly-focused
output, lasing DTC in 70% Isopropanol

[For more information about this dye, see

my report on inexpensive dyes for the DIYer.]

Back to the previous nitrogen laser project,

a straightforward and simple “first laser”

Back to the laser projects index

Back to the home page

Contact Information:

Email: a@b.com, where you can replace a with my first name
(jon, only 3 letters, no “h”) and b with joss.

Phone: +1 240 604 4495.

Last modified: Sat Dec 17 23:59:48 EST 2011

I thought about using the blue diode laser to pump the CR-599, but there are two apparent obstacles. First, the blue diode is a multimode device, and it may not be possible to focus the beam from it down to a suitable spot size: jet dye lasers generally utilize spot size of less than 50 microns diameter, and I think they prefer 10 or 20 microns. Second, it is difficult to find a low-threshold dye that absorbs well at 445-455 nm, which is the wavelength range in which these diodes emit.

There is, however, another possibility. I have seen a report in which the researchers succeeded in pumping Rhodamine 6G [earlier name of the dye that is probably more often called Rhodamine 590 these days] in a jet dye laser, using only about 70 milliwatts from a green DPSSL. I’m sure they optimized their system fairly thoroughly for lowest possible threshold (though I don’t remember all the details), but it still seemed like a reasonable thing to try.

I have also seen a report in which the researchers mode locked a CW R6G dye laser by including the saturable absorber dye in the same jet stream, which is highly encouraging for several reasons, one of which is that I have only one circulator pump. (Even if I had a second pump, I have no easy way to install a second dye jet in the CR-599 chassis.) Before I get into mode locking, though, I have to have a laser to mode lock.

To examine the possibility of using a green module to drive R590 I built a small spreadsheet that performs a very rudimentary calculation that I learned about quite some time ago; it may even date from the MASER era. As I learned it, this calculation is known as the Townes-Schawlow Criterion. It takes a number of parameters, and gives you the lowest possible number of excited centers per cubic centimeter that you need in order to reach threshold. If you know the pump wavelength, which in this case is 532 nm, you can then calculate the energy requirement. To get from energy to power, you have to know how long it takes to get the correct number of pump photons into the active material. I decided that it was reasonable to excite one set of dye molecules per dye fluorescence lifetime (about 4.1 nsec) and I doubled the number to account for some lossage, including pump light that is not absorbed by the dye. (A typical prescription is to bring up the dye concentration until the jet stream absorbs 80% of the pump light. This means that you have already lost 20% at the outset.) Despite the doubled number, the spreadsheet suggests that I have at least some chance of reaching threshold with 200 mW from the green DPSSL. It definitely seemed worth a try.

The CR-599 had been largely stripped before we acquired it, but I was fortunate enough to acquire the necessary optics, thanks to Steve Roberts and Piotr Kucharski. The OC was too highly transmissive, though, so I have substituted a mirror I already had on hand that transmits less than 1% at 590 nm.

The 5920 circulator has some issues, but I have fussed and tweaked, and I can now run it at a pressure of at least 30 psi, which is the bottom of the usual operating range. It does, unfortunately, tend to fill the 599 with dye solution; but when I have it adjusted as well as possible that process is relatively slow. (If things are not quite right the dye return hose begins to fill up, there is a fateful gurgling sound, and I have about 1 second to stop the jet before the dye erupts. This is at least partly because I do not yet have the correct return hose; working on it.)

[Note, added later that evening: I have changed out the dye solution, and the behavior of the jet is now different. I can only run at about 20 psi now, which is probably not sufficient. I may be able to work around this, but it is late enough that I have stopped for the night; it will have to wait until tomorrow.]

Aligning the CR-599 is, expectably, a nontrivial exercise. I have a PDF file that contains instructions, but it refers to diagrams that are not present, so I had to proceed extra-slowly, learning what I should expect to see at each stage. (As time permits, I will add photos.) I have now performed the alignment several times, and I am beginning to get used to it. One added complication is that the green DPSSL module has adjustable focus, something you would not expect from the usual pump for this dye laser, which is a large-frame argon ion laser. I am still working on alignment, as the focus of the pump has important implications vis-a-vis the size of the pump spot on the dye jet. I wish I could photograph that, but it would be rather difficult: the pump spot is brighter than the surface of the sun, but the surround is not. This makes choosing an appropriate exposure nearly impossible. OTOH, it is possible to use a welding filter or equivalent to inspect the spot, which you have to do in order to align the jet, so at least you can observe it visually. (There is a small filter of this sort, built into the 599. Unfortunately, it is too small to photograph through, and it is somewhat dirty on the its inner face, which is difficult to reach.)

I have not yet succeeded in bringing the laser to threshold, but as of somewhat earlier this evening (before I changed out the dye solution) I have seen quite a few brief yellow flashes, and I am fairly well convinced that it will be possible to get this setup to run. I should probably note that I have also seen a certain number of brief flashes of green pump light, and it is very easy to tell the difference between those and the yellow ones.

(04 September 2011, early AM, ff)

After considerably more fussing and fuming, I now have the CR-599 a little above threshold. I have worked on optimizing various parameters so I can get a bit more output; it was originally pretty wobbly. Here is a photo of the interior. The dust in the air makes the yellow beam visible…

Unfortunately, soon after I took that photo the green DPSSL module crapped out, and I need to get a new one before I can proceed further with this project.

(21 May, 2011, ff)

Some time ago we were able to acquire, on eBay, an item that is clearly the head of a very small excimer laser. It has room for two preionizers; but when we got it, only one was present, and the other edge of the cathode had a white deposit on it that I was uneasy about.

           

(The deposit is not visible in these photos.)

For quite a while I didn’t do anything with this structure, but it has been on my mind a lot, and a few days ago I dragged it out and started thinking about it in earnest. I usually avoid commercial parts, but here was this very fine piece of equipment, lacking only a few capacitors, a switch, and some gas; and I do have a project or two in mind that could use it.

I have started to build a box for the head:

I have also made a new preionizer for it:

(I got some 5-mm borosilicate tubing on eBay. The vendor specified it as having 3.4-mm ID, but a 1/8″ rod failed to fit inside it, and in fact the square-cross-section 3/32″ brass tubing that I ended up using only fits in some of the pieces. Go figure.) Last modified: Tue May 24 16:27:32 EDT 2011

(February, 2004)

Back in December, we won a dye laser head on eBay. (Our thanks to “teknogod4u”, who offered it.) It had a flashlamp and a dye cell in it, and looked interesting. The light from the flashlamp was coupled into the dye cell by a diffuse white reflector, and I wasn’t sure whether that would work at all well.

In the process of working on spark-gap triggering, which I’ve been doing during the last few weeks, I needed to test a triggering setup against our GP-14 spark gap; that provided the perfect excuse to get this head running. The GP-14 is good for 12 to 24 kV in air, a range that corresponded well with the testing I had in mind; and I had already mounted it on a capacitor (also acquired on eBay, perhaps a year ago) in anticipation of setting up a lamp-pumped dye laser.

Here is a schematic. It may not be exact, but it is close. (I worked it up in a hurry; it should show a resistor rather than an inductor. This conduction path charges the little starting capacitor, which improves the operation of the spark gap by helping it form a robust conduction channel quickly when it is triggered.)

Here are an overview of the laser and a close-up of one end of the head. Sorry about the weird angle on the first photo; I stood where I could. The white rectangle that you can see in the first photo (it looks like a slightly bent sheet of extremely thick paper) is a piece of teflon that helps insulate the cathode lead of the lamp from the ground end of the capacitor — before each pulse, that lead is at the full positive supply voltage. (The structure that is barely visible at the back of the first photo, btw, is our Avco-Everett C5000 nitrogen laser head,) another eBay acquisition.

 

Yes, that’s a fuse clip on the end of the lamp. They work moderately well, though it’s preferable to coat them with silver-loaded conductive grease, if you have any, for better contact — strange things happen when you drop 10,000 amps through a fuse clip.

Note the little red “doorknob” cap across the GP-14; the mfr specifies that you want to push at least 10 amps through the gap when you trigger it, so that a righteous conduction channel will form; I find that a few hundred pf works well. That small a capacitance is also extremely fast, which is good because you want to overvolt the flashlamp as abruptly and thoroughly as you possibly can. (Yes, I know, it’s preferable to “simmer” the lamp at a few milliamps between shots, so that you don’t need to worry about this issue. It is, however, somewhat nontrivial to build a setup to do that, and I’ve opted for the simpler approach.)

The main storage capacitor is 0.1 microfarad, rated for up to 60 kV; I’m charging it to about 20.5 kV, so the stored electrical energy is about 21 joules. I did some informal measurements and found that the laser pulse is about 600 nsec long, which suggests that the electrical pulse is probably about 650 nsec long. (That’s actually longer than I’d hoped for, but it’s still fairly reasonable.) Given that pulsewidth, the power in the flashlamp during the pulse averages more than 32 MWE. The current isn’t really 10,000 amperes; but if we presume that the pulse is approximately sinusoidal and that the peak current occurs when the voltage is about 2/3 of the maximum, that puts the peak peak power at slightly over 45 MWE and the peak current at more than 3,300 amps.

For the moment, I’ve got an overcoated-aluminum-on-glass flat as the rear mirror, and what appears to be some sort of etalon serving as an output coupler. (The output coupler doesn’t show up in these photos, but if you look carefully you’ll see the aluminum max-ref near the right edge of the first picture, just below center, on a black metal mount.)

The diffuse reflector, btw, was not viable, and I ended up close-coupling the lamp and the dye cell the way I always do, which is by wrapping aluminum foil around them. (I have never bothered with ellipsoidal reflectors, having decided very early on that they weren’t worth the trouble. Aluminum foil works quite well, and is much easier to deal with. Just remember to keep it away from the high voltage. I always keep it shiny side in, and I can’t tell you whether that’s crucial because I haven’t ever bothered to try it with the dull side in.)

I used a front-surface mirror to reflect the beam up to the wall, so the spot in the next photo is maybe 8 or 10 feet from the output coupler. The dye solution was Rhodamine 6G in 91% isopropanol; I took the first picture with the camera white-balance set to “Cloudy”, to get moderately reasonable color rendition of the laser output, at the expense of making the wall look somewhat greenish. (The room lights were on, and they’re fluorescents…) The second picture was taken without room lights, and with the white-balance set (by mistake) to “Auto”.

As far as I can tell, these are to the same scale; but in the first shot the mirrors are poorly aligned, and in the second shot I have them aligned much better. Note the differences in spot size and shape. (You can tell that the alignment is better in the shot on the right by the fact that the center spot is a lot smaller and a lot closer to round. It is also intensely bright, even to the eye.)

 

Inasmuch as there’s no real sense of scale here, I should note that these photos show an area of wall that is something like 12 or 14 inches across. I haven’t made any attempt to minimize the divergence or select a single transverse mode — haven’t done anything fancy at all yet, in fact. This is just an extremely informal testing setup. It should be clear that we don’t have anything like the simple mode structure you see with HeNe lasers. There are several reasons for this, two of which are A) that this is a long dye cell with reflective walls, and B) that I do not have a carefully designed cavity around it. Perhaps some day I will have a chance to acquire an output coupler with the right curvature, and some iris diaphragms… in the meanwhile, however, I’ll take what I can get.

The Littrow Prism

(23 February, 2004)

This evening, I tried a Littrow prism as the rear reflector. I found it slightly harder to threshold the laser, but it was interesting enough to be worth playing with. I could almost swear that I saw laser-speckle in the beam (see the photos), but I don’t have an easy way of checking the coherence length, so I can’t swear to it. (Dye lasers aren’t particularly coherent unless you have a dispersive element in the cavity to tune them and narrow the output, so they don’t generally exhibit speckle.)

Here, anyway, are four photos. The last one is taken at about 23.3 kV, I think the third one is at 20.5 and I’m not really sure about the first two, though there’s some chance that the first one was taken at about 19.8. (There is a certain amount of shot-to-shot variation, even at a given voltage.) It’s possible, btw, that only the first two were taken with the Littrow prism as the max-ref rear mirror — the other two don’t seem to show the graininess that I recall, and I know I swapped mirrors several times during the course of the evening yesterday.

     

I have to say that these look to me like a sequence showing a flare-up of the star in the middle of the Zarf’s-Ear Nebula…

Tuning:

(24 February, 2004)

I put an equilateral dispersion prism (made of SF2 glass) into the cavity this evening, reverted to the aluminum max-ref, and tuned the laser. It worked reasonably well — here’s a photographic tuning curve of the Rhodamine 6G dye solution I’m using at the moment:

                   

These were taken at about 23.3 kV; they should be at the same scale as the previous ones, but I’ve made the thumbnails very small so they’ll fit better on the page and so you can see the color differences more easily. Just as with the previous images, btw, if you want the original bits (actually a 1280×960 crop from the original JPEG), just click the small image and then change “.8c.” in the resulting filename to “.12c.”.

There appears to be a gap between the second and third photos; I’m not really sure what that’s about, and I may take another look at some point. The best way to check, I think, will be to get a spectrometer on the thing and make a “real” tuning curve, with wavelength and relative (or even absolute) pulse energy. Before I can do that, however, I’ll probably have to build a more stable version of this laser, maybe on its own little table. (I am tempted to build a “soda-can” optical table for this, but I don’t have the patience to drill and tap a zillion 1/4-20 holes in a sheet of aluminum; and I hardly ever drink soda, so I’d have to enlist the help of lots of friends to get the cans.)

I should also note that the yellow regions in the last few images are camera artifacts — the beam was intense enough to “blow out” the sensor, even at that end of the tuning range.

Here is an overview of the tuning setup, such as it is — I just sat the prism on something and moved the mirror into the new beampath, then tweaked the mirror until I got lasing again. (Remember, I am using a HeNe to align this setup, and 633 nm is well outside the tuning range of R6G, so the laser is guaranteed not to work after I line everything up.)

Next Steps

(26 February, 2004)

I have taken the laser down so that I can mount it on a piece of extruded aluminum “L” that used to be the optical rail in a pulsed YAG laser. I suspect that it will be considerably more stable when everything is firmly attached.

I’m considering taking one of the reflections off the prism as the output beam, and using two “max-ref” mirrors. That won’t optimize the output power, but it may optimize the tuning range. I’ve begun this rebuild, and should have the laser running again within a few days. (If I’m really lucky, I’ll have it running tomorrow, Saturday, the 28th.)

Riding the Rails

(28/29 February, 2004)

Got the head mounted on the rail; kluged a stand for a mirror mount that’s designed for 1.25″ height (the dye cell center is just under 2.5″ off the deck); added a mount for the prism; and set up the max ref rear mirror on a cantilever. Here’s an overview, with a close-in view of the kluged stand and its mount, and a moderate close-up of the tuning section:

   

You can disregard the mirror next to the prism — it was already attached to the mount, and I just left it in place for now. I’ll probably remove it later, so it doesn’t collect too much dust.

I’m having trouble with the alignment right now, and I’m not getting the tuning range I want; but I hope to resolve that within a few days, and I’ll be putting the output through our little Oriel monochromator to collect tuning info. The wavelengths will be approximate, but I can at least calibrate it against a HeNe, which is a start. (It actually seems fairly accurate.)

(Note, added 2004 March 08)

I put a small iris diaphragm in, near the tuner, as a field stop. This seems to help a little, but I think I’ll need to put another one at the OC end. (It’s on order.) These should have two effects: first, they should keep the beam restricted more or less to the center of the dye cell, which should discourage the “Zarf’s-Ear Nebula” effect. (I know, it’s pretty, but it’s not very efficient.) Second, by keeping the beam running more or less down the enter of the dye cell, the stops should discourage walkoff caused by the tuner. (The laser, no surprise, runs best at wavelengths where the dye has high gain. As I attempt to tune toward the end of the range, lasing will occur at an angle through the cell, which detunes it, unless I forcibly prevent it from doing so.)

For other “photographic tuning curves”, please see the 4-Methyl-Umbelliferone page and my page about a simple nitrogen-laser-pumped tunable dye laser.

Email: a@b.com, where you can replace b with joss and a with my first name (just jon, only 3 letters, no “h”).

Phone: +1 240 604 4495.

Last modified: Mon May 23 10:35:39 EDT 2011

(December 19, 2009, ff)

This page details the construction of a prototype dye laser that is intended for initial checking of some parameters — for example, I want to know whether a simple design will threshold “easy” dyes with minimal input energy. In principle, the answer is already known to be “Yes”; but in practice it may not be so easy. The laser will be operating close to margins, and any sacrifice of efficiency will be difficult to work around.

This preliminary design uses a commercial capacitor and a commercial spark gap switch, both of which I hope to eliminate in later designs. The machine that I’m working toward will almost certainly use a commercial flashlamp, though, because xenon is the most efficient emitter in the wavelength regions of interest for pumping organic dyes.

It will, very likely, also use commercial laser mirrors. Quite a few dye lasers have been operated with simpler mirrors, which could be homebrewed, but I am not at all certain that these machines will have enough “oomph” to overcome the losses inherent in [e.g., aluminum] mirrors. OTOH, for those who are willing to work at long wavelengths, using red and NIR dyes, sputtered gold mirrors may be viable: a clean coating of gold has reflectance of at least 98% from the far red on out through the IR, and a thinly-sputtered coating could serve as an output coupler.)

This laser uses high voltages, and capacitors that can store lethal amounts of energy. It puts out a laser beam that can damage your eyes and skin, and it uses organic dyes, some of which are known to be quite toxic. It also uses flammable organic solvents.

It is important to take adequate safety precautions and use appropriate safety equipment with any laser; but it is crucially important with lasers that involve high voltages and present a health and/or fire hazard!

I have built a number of lamp-pumped organic dye lasers in the past, and I have read a number of articles about dye lasers in technical journals. In the course of doing so, I’ve noticed some characteristics that appear to define viability. Reasonably high optical pump power is required, and that is best obtained with a xenon-filled flashlamp. Xenon lamps can have efficiency as high as 50% or more under optimum circumstances. Granted, not all of the light can reach the dye solution, and not all of what reaches the dye solution can be absorbed by it; but those issues make xenon even more attractive as a source.

High pump power is also best obtained by driving the lamp with a short electrical pulse. This is one place where many DIYers go astray, by attempting to run their lasers with large capacitors at relatively low voltages. In order to create a short pulse you need to minimize both the inductance and the capacitance of the circuit that drives the flashlamp; operating at low voltage essentially makes that impossible.

Let’s run through a few numbers here so I can show you what I mean.

If you want to deliver 25 Joules to a flashlamp, and you are using photoflash capacitors that you charge to 400 Volts, you need 312.5 μf to store the energy. We can easily make a rough guess at how fast such a capacitor will discharge; ignoring resistive effects, the FWHM (Full Width at Half of the Maximum Amplitude) pulsewidth, in seconds, can be approximated as π times the square root of the product of the inductance and the capacitance, where the inductance (L) is in henries, and the capacitance (C) is in farads.

In an article on lamp-pumped dye lasers that was published some time ago, the authors determined that it is quite difficult to achieve system inductance of less than 125 nanohenry, so I am going to use 150 nh as the inductance value for this calculation. That’s optimistic; but it is consistent, and will do for now. (Just don’t expect an actual physical lamp driver circuit to meet the number predicted by this calculation; any real circuit probably has higher inductance unless it is extremely well designed, and in addition there are resistive effects that this calculation does not take into account.)

At 312.5 μf the expectable pulsewidth calculates to be about 21.5 microseconds, which corresponds to electrical power of about 1.16 MW. That may seem like a lot, but in fact it isn’t as much as we want. With actual photoflash or other electrolytic capacitors, which have high ESR and ESL (Effective Series Resistance; Effective Series Inductance), the discharge would probably be far slower, with correspondingly lower peak power, not even remotely close to what’s needed here. (Thanks to Jarrod Kinsey for bringing the ESR/ESL issue to my attention, and to Dr. Mark Csele and Dr. Winfield Hill for relevant information. Dr. Csele even has an oscilloscope trace on his Website, [right side, about ¼ of the way down] showing this problem directly.)

Now let’s try the same thing at 20,000 volts.

At 20 kV you need only about 125 nf to store the energy. The expectable pulsewidth is now just over 430 nsec, and the resulting electrical power is a little more than 58 MW. That is considerably more satisfactory. [Side note: if we guess that peak power occurs when the capacitor is at about 2/3 of its initial charging voltage, the peak current is a little over 4,000 Amperes (!).]

In terms of actual circuits, the situation is more extreme than I have presented it: photoflash and other low-voltage pulse capacitors are not designed to have low effective series inductance (ESL), so the system inductance will be larger than the amount I have used in the calculations. The use of wires to connect various parts of the circuit to each other also increases the inductance, slowing the discharge still further.

I trust that this explains my preference for high voltages, in spite of the difficulties and occasional annoyances involved in their use.

The circuit of this machine is quite simple: a capacitor is connected to the flashlamp by a spark gap, which serves as a switch. The other end of the flashlamp is connected to the other end of the capacitor, which is grounded. The one subtlety is that there is a small capacitor across the spark gap; it encourages the rapid formation of a good conduction channel when the gap is triggered. In order to be sure that this small cap gets recharged between pulses, the anode end of the flashlamp is connected to ground through a large-value (you want to use about 1 million ohms, or perhaps a bit more) high-voltage resistor. Here is a schematic diagram:

Wide pieces of brass shim stock are used for all of the high-current connections, to minimize inductance. (The shim stock I used for the initial build is too thin, but it seems to be working. Eventually I will probably replace it with thicker material.)

I am using a commercial spark gap here (an old EG&G GP-70), because I have one; I also have a trigger unit, which is handy. Both of these were acquired on eBay, so you could use similar items, but it is quite possible to build your own. Here is the spark gap:

The physical layout is determined by the relative shapes and sizes of the components, and by the need to avoid flashovers: we want the capacitor to discharge through the lamp, not around it.

My choice of 25 Joules in the example in the previous section was not entirely arbitrary; that’s about the maximum energy that I use in another dye laser that I put together a few years ago. This machine, however, is more “bare-bones” than that; I anticipate using only 30 nf, which stores just 6 Joules at 20 kV. I have several reasons for this, one of which is that the particular capacitor I’m using is a type that is often available on eBay. It looks like this:

Notice that the capacitor itself contributes 20 nh to the system inductance. The ESL of the spark gap is probably within a factor of 2 of this, and my guess is that the flashlamp is even worse. (There are good reasons why it is extremely difficult to build a driver circuit that has less than 125 nh total inductance.)

If someone actually wants to build a laser to this design they should be able to, even though it is not what I am actually aiming for, and despite the fact that there are some reasons why I wouldn’t actually recommend it.

The flashlamp I intend to use, at least for initial testing, is a little over 9" long. As of this writing, it is available from The Electronic Goldmine.

As supplied, the lamp has a trigger wire wrapped around it. I have removed this wire, as I do not intend to trigger the lamp; I will, instead, be firing it by overvolting it as abruptly as I can. (This is a fairly common technique. It is not as good as simmering the lamp, but it’s considerably easier, and will do for now. If this design turns out to be viable, I may attempt to build a simmer circuit later.)

Unfortunately, the lamp is not designed for fast-pulse service; it has skinny little wires out the ends, which are not good for a low-inductance design. It also has one other key shortcoming: the vendor’s listing claims that it is made of some type of borosilicate glass, rather than fused silica. That means it probably doesn’t pass much mid- to short-wavelength UV. For pumping Rhodamine 6G and other dyes that emit at relatively long wavelengths this may not be a problem; but it is likely to prevent the laser from operating blue, indigo, and violet dyes. We will do the best we can with it, and we’ll see whether that’s good enough. If not, we can change over to a different lamp (and/or, if necessary, more stored energy — I have two of the 30-nf caps).

[NOTE, added later: This lamp is significantly fragile. My first one suffered a few too many flashovers, and ceased to work after perhaps a hundred pulses. It is a good idea to provide a little extra insulation at the ends of the lamp, to keep it isolated from the ends of the dye cell; and it is a good idea to keep the aluminum foil close-reflector as narrow as convenient, even though that will lose you a little more light out the ends than is fully optimal.]

(20 December, 2009, early am)

Here is a view of the lamp driver circuit with the components positioned but not assembled:

I still need to: make the holes that will allow me to connect two of the shims to the capacitor; bolt the parts together; add the little starting capacitor across the spark gap; add charging and bleeder resistors; and clamp the ends of the relevant shims to the wires of the flashlamp. At that point the driver circuit will be done, and I can test it. Assuming that it fires the lamp and does so without exploding it or turning it purple from too much short UV, I can then attach the dye cell and attempt to threshold Rhodamine 6G in isopropanol.

The dye cell should be relatively straightforward, but it will be much longer than it needs to be for this flashlamp. This is because I do not want to cut the tubing — I may need a longer piece in future, especially if this design fails to reach threshold.

I am using compression fittings for the ends of the dye cell. They are convenient, and the tubing I have is a good fit. (I should note that I will be holding the dye cell tubing in with an o-ring, not with the usual parts, which are intended for use with either copper or polypropylene.) Here is a view of one of the fittings, with a hole drilled in it but without an end window:

(20 December, 2009, evening)

I am much closer now to having the lamp driver built and testable. At this point I need to connect charging and bleeder resistors, clamp the shims to the wires from the flashlamp, and provide power and trigger. With some luck, those things will occur later this evening.

(20 December, 2009, late evening)

I ran the lamp at various voltages. It appeared to handle all of them reasonably well, which is a good sign. Here is a view of the lamp driver, set up on the bench for testing:

Here are two closer views, first showing the lamp cold, and then showing it being fired, with 18 kV on the capacitor. (The first view is blurred by camera motion; my apologies.)

Although the flash is quite bright, I do not yet know whether it will prove to be bright enough. OTOH, at least I now know that the lamp handles as much energy as I initially intend to put into it.

(21 December, 2009, early evening)

The windows are mounted on the ends of the dye cell; I have the delrin rings and brass inserts I need for connecting the polypropylene tubing that the dye will flow through on its way to and from the cell; the cell ends are being attached to small mounting plates; I have a baseplate on which to mount the parts of the laser. I am now waiting for the epoxy to cure so I can mount the dye cell, set up and align the mirrors, and start testing.

(23 December, 2009, morning)

Two or things become clear to me after a certain amount of thought. First, I have mounted the dye cell ends incorrectly, and I need to fix that. I will be acquiring some half-inch aluminum bar. (Photo forthcoming later, after the new supports are in place.) I also want to bolt the mirror mounts to the baseplate, for stability, rather than simply attaching them with aquarium caulk. In order to do this, I will have to drill holes in the baseplate. Either way, I get to remove some of the paint.

I have also decided that the only source of cheap high-reflectance mirrors I currently know of is scrapped-out HeNe lasers. I even have several sets already on hand. These, unfortunately, are not broadband, so they limit the choice of dye to a very narrow range, but I have some Rhodamine 640, which should be a relatively good match, and will probably be the first dye I test with this laser unless I have some reason to try Rhodamine 6G or Fluorescein first. R640 may have one other advantage: it can probably be assisted by energy transfer from shorter-wavelength dyes, which would permit it to make use of more of the pump light. (This is actually true of various laser dyes, but the effect is only occasionally used. There can be complications under some circumstances, but I think I can avoid those by careful dye choice.)

(25 December, 2009, early afternoon)

After thinking about how to attach things to the baseplate, I decided to remove the small wooden plinths from the ends of the dye cell, and use something larger and metallic. Went to the hardware store to find some half-inch aluminum bar stock, and failed. Instead, I found some connectors for threaded stock. These look like very long hex nuts. I got a few, and eventually settled on the largest ones, which are intended for 7/16" thread. I sanded the irregularities off two faces of each, drilled and tapped two #8-32 holes in one of the faces, and glued the dye cell ends to the other face. Then I marked the baseplate, drilled holes, and bolted the dye cell down. This worked very nicely. I also drilled a hole for each of the mirror mounts, which worked fairly well except that for some reason I don’t quite understand, one of them is about 1/4" off to the side of where I want it. Still viable, though, at least for initial testing. (Photos of this progress are in the camera, and I will post them when I get a chance.)

Once I had everything in place I put a mirror at one end, shined a very small HeNe laser into the other end, aligned the HeNe with the dye cell, and aligned the mirror with the HeNe. Then I filled the dye cell with isopropyl alcohol, realigned the HeNe with the cell, and realigned the mirror. Then I put in the second mirror, aligned that, realigned the HeNe with the dye cell, and realigned both mirrors. (See the FAQ, below, for more information about this, with photos.)

At this point, all I need to do is wrap aluminum foil around the lamp and cell, fill the cell with dye solution, and start testing. It may take quite a few shots before I find the correct dye concentration, and I will probably need ot tweak the mirror alignment, but I am hoping to have a running laser some time this evening. (Fingers crossed; there is still no guarantee that this will actually work, and I may also have to swap out one or both mirrors — I was only able to find one HeNe mirror of a usable size; not sure where my other ones have wandered off to.)

(26 December, 2009, early AM)

I have not yet been successful in getting this thing to lase. I tried several obvious possibilities, none of which worked. My next move is to look for my other Maxwell 30-nf capacitor, and try 12 Joules instead of 6. Adding more capacitance is going to slow down the discharge, so I will get less than double the peak power, but I can still expect some improvement, and if the laser is only moderately below threshold, perhaps that will put it over the bar. OTOH, if there is some more fundamental flaw, adding another capacitor probably won’t help much.

In the meanwhile, here are some photos. First, one of the ends of the dye cell, which was filled with plain 70% isopropanol at the time, and a mirror (it’s the HR from the back end of a dead HeNe laser). Then the other end, with a mirror and an alignment laser.

It is not easy to see in these photos, but the bore of the dye cell does not receive all of the light that hits the outside. Here is a photo that makes this easier to see:

(The tubing appears to be curved, but of course it actually isn’t. I took this photo with my iPhone and a small lens, which accounts for the distortion.)

(26 December, 2009, early afternoon)

I have added the second capacitor —

— but the laser still did not reach threshold. Later today I will switch to Rhodamine 6G, and we’ll see whether that behaves any better.

(27 December, 2009, early AM)

After some thought, I decided that instead of trying R6G it would be a better idea to rebuild the dye cell...

(27 December, 2009, early AM)

Looking through my tubing stash, I found a piece that was just about the right diameter and length, and had a thin wall, so I built a new dye cell from it.

I looked for 1/4"-to-3/8" right-angle adapters, but didn’t find any, so I compromised by getting 3/8" compression to 1/4" MPT right-angle adapters and 1/4" compression to 1/4" FPT straight-through adapters. This is a bit more complex than I wanted to get, but I guess it will do. When I put the adapters together I used the thick pink teflon tape that is intended for waterpipes.

NOTE: If you build a dye cell of this sort, you will have to drill through the brass to provide a path for the dye laser beam. I drilled a small hole from the dye-cell side, because there was already an indentation left from the manufacture of the adapter that served in lieu of a centerpunch. I then turned the adapter around and enlarged the pilot hole from the other side, one or two drill sizes at a time, with the adapter firmly clamped.

I have run out of AR-coated windows for the moment, so I used small pieces of microscope slide instead; they are held on with silicone rubber aquarium caulk. Here are views of the ends:

You can see a tapped mounting hole in each of the “plinths”. It is about a quarter of an inch in from the end.

Because the glass or fused silica tubing is fragile, it is important to take the steps in the right order when you install a cell of this type. First, I will gently remove the tube. (It is present now so I can be sure that everything is in the correct position as I glue the ends to the plinths.) Then, holding onto the fittings (not the plinths), I will attach the polypropylene hoses that the dye runs through, and tighten the caps that hold them. Then I will mount one of the ends on the base. Then, with the fused silica tubing in place but held only loosely, I will bolt the other end into place. Finally, I will hand-tighten the caps that hold the tubing. This procedure worked well with the other cell, and I hope it will work with this one. [[Note, added the next afternoon: it did.]]

I have decided to swap out the baseplate, which I chose primarily because it was long enough for the initial dye cell. That is now out of the picture, and I have found a smaller plate that is much stiffer and will serve better. Here’s a photo, added the following day after I drilled the mounting holes in it:

This plate is brushed aluminum, about 1/8" thick, and it has stiffening ribs on its underside. It is far more stable than the previous base.

I also found my old HeNe output coupler and put it in the second mirror mount. There is some chance that I could use it with its concave side toward the dye cell, as I suspect that its radius of curvature is long enough, but I think that for the moment I will leave it with the AR-coated side facing the dye.

(27 December, 2009, afternoon)

Here is the laser on the bench, with the new dye cell in place, ready to go...

...and here it is, lasing Rhodamine 640 in 91% isopropanol:

The alignment is perhaps a bit sloppy, but I can deal with that later.

(A Helpful Hint, added some time later: If you use the original bottles from the drugstore, as I often do, you will find that when they are nearly empty they have a tendency to tip over. You can easily prevent this by going to the hardware store and buying two toilet flanges of appropriate size. These are relatively inexpensive, and they turn out to be quite handy for any number of purposes; I have even used them as telescope parts.)

I added more R6G, and got a considerably brighter spot:

There was also a very small amount of output from the opposite end of the laser. It was very difficult to photograph, though, and is just visible here, slightly less than ¼ of the way down from the top of the image and a bit less than 1/6 of the way in from the left edge...

(30 December, 2009, early AM)

Yesterday evening, as I mention above, the lamp ceased to flash. Today I dug through my supplies and found a very robust lamp that I am using until the new ones I’ve ordered arrive from the Electronic Goldmine.

(Clear evidence that the alignment is off.)

Just so you should be aware of the cooling issue, here’s what it looked like when I fired it again before giving the dye enough time to flow through the cell:

After a bit more tweaking, I got things lined up fairly well...

I should note a few things here.

1. That’s a HeNe output coupler (at the lower left, in the photos), turned around backwards, just as I was using with Rhodamine 640 and Rhodamine 6G. It works passably well with Fluorescein, but is not likely to be good enough for dyes in the blue-green to violet range.
   
   
2. The Fluorescein I have is intended for use as a biological stain; the label proclaims it to be at least 85% pure. (Proper laser-grade Fluorescein is almost certainly more than 99% pure.) Nonetheless, it lases quite nicely.
   
   
3. As I already mentioned, the direction in which the dye is flowing seems to make a difference in the quality of the output from the laser.
   
   
4. Fluorescein does not dissolve well in 99+% isopropanol, and it would be entirely reasonable to use 70% isopropyl rubbing alcohol as the solvent for it.
   
   
5. Fluorescein performs significantly better in a basic solution than in a neutral one, and adding a few drops of strong ammonia to the dye solution is helpful. I have refrained from doing that here, however, because ammonia attacks brass. (I would guess that almost any strong base will attack brass, but I have not yet checked.)
   
   
6. Patience is not just a virtue; it is crucial to this kind of work. (See the “too soon” photo, above, and the “not quite” photo, below..)

(05 January, 2009, evening)

There are several reasons why blue/indigo/violet dyes are more difficult to lase than dyes like Rhodamine 6G. For one thing, only some of them have fluorescence quantum efficiency as high as that of R6G. At least equally important, however, is the fact that as the wavelength gets shorter it inherently becomes more difficult to threshold any laser. (This is a good part of why it took so long to develop any X-ray lasers.)

I have not yet received another flashlamp from the Electronic Goldmine, so I’m still using the more robust lamp pictured above, which definitely has a fused-silica envelope. Thus, I don’t know whether this laser in its original state will be able to lase any of the blue dyes. I should also mention the fact that I am currently using a high-quality broadband “Max Ref” flat mirror as the rear reflector, and a helium-cadmium output coupler turned around backwards as the output coupler; these are probably about as good for ordinary blue dyes as HeNe mirrors are for R6G, and almost as good as they are for R640. They are considerably better than enhanced aluminum or protected silver.

(Later that afternoon)

It turns out to be even easier to lase 7-Diethylamino-4-Methyl-Coumarin. Here it is in 91% isopropyl alcohol, also acquired at the drugstore:

It looks to me like the mirrors are not aligned quite as nicely as I would like, and I will probably tweak them in an effort to improve the results a little.

I should perhaps point out that it is necessary to rinse the dye cuvette quite thoroughly when you switch from one dye to another. This is more important when you are going from long wavelengths to shorter; for example, my experience is that even a small amount of Rhodamine B will effectively quench a solution of Fluorescein, preventing it from lasing, while the Fluorescein output photos above are from a mixture of 7-Diethylamino-4-Methyl-Coumarin and Fluorescein, which lases quite happily. Not all such combinations, however, will be successful.

(08 December, 2010, afternoon)

The replacement lamp has arrived, and I have installed it. It does, indeed, appear to be borosilicate glass, as advertised: I have been unable to threshold 7-Diethylamino-4-Methyl-Coumarin with it, which suggests insufficient UV output. There is still some possibility, though, that the problem is partly a matter of pulsewidth rather than a lack of UV, and I will probably have to make some measurements to see which issue is more important.

I added some Fluorescein to the dye solution and was able to get that to lase. It was not as strong with this lamp as it was with the more robust lamp, but there is a different output coupler in the mirror mount now, which may help account for the difference in output.

It is good to know that even with the relatively inexpensive lamp this laser can provide green output.

(NOTE, added 08 January, 2010, evening: if you check the absorption and emission spectra of Fluorescein and the absorption spectrum of Rhodamine 6G, you will find that they are pleasantly compatible. This fact led me to try them together, and I was not at all surprised to find that adding some Fluorescein to a solution of Rhodamine 6G enhances the output.)

(09 January, 2010)

It was suggested to me by Harald Noack, of Graz University of Technology, that I simmer and prepulse the lamp. He referred me to an article, which sounded very interesting. I tried simmering, and found to my dismay that the laser barely worked at all. In order to understand this, I took some measurements of the light output from the flashlamp.

Here is an oscilloscope trace, showing the first obvious problem:

At 2 μsec per division you can see at least three peaks in the trace; my lamp driver circuit is underdamped, and is ringing. This deprives the initial pulse of some of the energy stored in the capacitor, and thus decreases the peak power. I’m considering what to do about this.

(I suspect, btw, that the sustained voltage between the peaks is an artifact; it doesn’t seem likely to me that the lamp would stay that brightly lit without much current going through it. Harald Noack suggests, however, that in fact the lamp does continue to emit a substantial amount of light for some microseconds after the end of an electrical pulse. It doesn’t really matter; the real issue here is not what goes on between the peaks, but the fact that there is more than one.)

I also photographed traces with and without a simmering current. These were taken at a faster sweep rate, and mostly show the first peak. The polarity of the simmer supply is negative, but the difference in performance is not pronounced. Simmer on the left, no simmer on the right. (In the photo on the right, the lower end of the spark gap and its little starting capacitor are connected to ground through a 400 K resistor. If I float them, the performance is not as good.)

           

If you examine these carefully, you will notice that the photo on the right has a slightly shorter risetime, and a significantly higher peak. These differences are more than enough to explain the fact that the laser barely works at all when I simmer the lamp.

(Later that evening)

For the sake of comparison, I put the more robust lamp (pictured above) back into the laser, and took two scope traces. The first shows the lamp output with the 400 K resistor to ground; for the second, I used a piece of aluminum foil to reflect some R6G output into the photodiode. It is probably instructive to note the longer delay between the triggering event and the beginning of the pulse, and the fact that the pulse itself is narrower than the lamp pulse. Both of these, of course, are entirely expectable.

           

The lamp pulse is single (I checked at slower sweep rate) and more symmetric, with pulsewidth of 400 or 450 nsec, while the dye laser pulse has a fairly abrupt risetime, begins near the peak of the lamp pulse, and has pulsewidth of about 150 nsec in this photo, though I have seen them as wide as perhaps 200 nsec. Both traces shown here seem very reasonable to me.

I then changed the simmer supply back to positive polarity and connected it to the lamp. I found that I had to use 800 K in series in order to get a sustained discharge, and it became clear that this lamp does not begin to conduct until the voltage across it is far higher than was necessary with the other lamp. This very strongly suggests that it has higher effective impedance, and that fact alone may be enough to account for the single pulse. Fortunately, the circuit does not appear to be overdamped with this lamp in place. Even so, simmering does not work well with it; here is a scope trace:

This clearly shows a slightly longer risetime and a noticeably wider pulse than the one just above, which was taken without any simmer current. I still do not understand why this should be the case, but it obviously is.

I then took a look at the laser output, with and without simmering. You will notice that even though I am just using a piece of aluminum foil as a reflector to get some of the laser light over to the detector, I have had to cut the sensitivity of the vertical amplifier in half because the lasing is significantly brighter with this lamp than with the other one. (It is easy to see the difference by eye when the beam is hitting the paper target.) These two photos were taken in fairly quick succession; the reflector and the photodiode probably didn’t move between the exposures, so the vertical scale should be close to the same in both. The one above, however, was taken at a different time and with things in different positions, so it is not directly comparable.

           

Both of these are about 200 nsec or a bit more, FWHM. The one without simmering is considerably brighter, which goes hand in hand with the pulsewidth and peak power. (Even if the difference in trace heights is partly an artifact, it matches my visual observations.)

(11 January, 2010, evening)

NOTE: This project continues on the next page, as I am making some changes to the design of the lamp driver.

(21 December, 2009)

Here are some questions that people may think to ask, with some answers. Anything that does not have to do specifically and exactly with this particular laser is my opinion, and should be treated with mild skepticism.

• Why do you include all of the mistakes, and the things that didn’t work?

  The errors and failures are your main pathways to a deeper understanding of the issues you’re facing. I present mine in the hope that they will help you reach a deeper level of understanding more quickly. To some extent they also expose my thought patterns and my approaches to issues and problems, in case those are of any interest or could in some way be helpful to you.
  
  

• Why aren’t you using an elliptical reflector?

  I built my first dye laser in 1970. At that time, I was advised by some very savvy people that close-coupling the lamp to the dye cell would probably work. They also pointed out that wrapping aluminum foil around things is very much easier than trying to make a precise shape; that the interior of an elliptical reflector has to be kept very clean; and that a large amount of light escapes out the ends unless you use reflective end caps. I thought about that, and used aluminum foil. It worked just fine, and I have close-coupled every laser I’ve built since then.

  (I did try a diffuse reflector once, because it was available; but that setup failed to threshold R6G. Then I wrapped foil around the flashlamp and dye cell, and the laser worked very well.)
  
  

• Why are you using a long lamp?

  This is best explained with a diagram. Until I have a chance to make a proper one, I will use ASCII-pictures.

  Here is a laser amplifier. (This is easier to explain with amplifiers than with oscillators, but the principle is exactly the same.) We will pump it with 28 units of energy. I am going to specify that it is operating well over threshold; let’s say it takes 4 units of pump energy to bring it to threshold, so the remaining 24 units are available to produce amplification. This particular laser is capable of amplifying by a factor of 10, so if the input is 1 unit of energy, the output is 10 units of energy...

        X10 amp 1 -> |LASER| -> 10       ^^^^^      28 pump 

  [[Please remember that these numbers have nothing to do with real life; I am just giving you an example here. In fact, I am going to call the length of this laser “five letters” to emphasize that fact.]]

  Now let’s double the length of the laser medium, to 10 letters, without changing the amount of pump energy. For clarity I am going to diagram this as two lasers, each of which is pumped with 14 units of energy. 4 of those units bring it to threshold, leaving 10 units to power the amplification...

         X4 amp          X4 amp 1 -> |LASER| -> 4 -> |LASER| -> 16       ^^^^^           ^^^^^      14 pump         14 pump 

  Notice that each amplifier now has only 10 units of available pump energy instead of 24, so it can only amplify by 10/24 as much. (That’s really almost 4.2X, but let’s call it 4X for simplicity.) Even so, we now get considerably more output. This will, though, be true only if:
  
  A) The system is running significantly over threshold,
  
  and
  
  B) You don’t saturate the gain of the laser medium.

  If the gain is saturated, more length adds to the output arithmetically rather than geometrically, so you quickly lose the advantage. With dye lasers (and with nitrogen lasers) it is unusual to saturate the gain, so we aren’t going to worry about that issue. Ordinarily, all other things being equal, a longer active region will reach threshold more easily and will give you more output for a given amount of input energy, once you are above threshold. (I use a flashlamp with 15" arclength in my larger dye laser, and it works quite well at 20-25 Joules input energy.)

  As Jarrod Kinsey points out, the tradeoff here is that if you have a shorter lamp (and thus a shorter active length of dye), you can compensate by using mirrors with higher reflectance. There are two or three aspects to that; the first is availability and price — can you find mirrors with high reflectance, and are they expensive? The second has to do with the output coupler: if your OC is 99.5% reflective, you are not likely to get much output from the laser. (If you just want to know whether it has reached threshold, that probably isn’t a problem for you.) The third has to do with lamp life: a longer lamp, all other things being equal, has a higher explosion energy. That isn’t likely to be applicable to this laser, though, as we are not putting much energy into the lamp. The bottom line, then, is primarily the issue of whether you can find and afford the mirrors you need.
  
  

• Why use such a high voltage?

  I explain this above, in the text.

  (In a sense, the specific voltage was chosen for me by the fact that I have a small 20 kV power supply, the fact that the capacitor is rated to handle up to 35 kV, and the fact that the GP-70 spark gap works well at 20 kV.)
  
  

• Why all that brass shim stock? Why not just use wires?

  Broad connection paths help keep the inductance down, which is necessary.

  There is also another effect to bear in mind: with DC, a thick wire can carry lots of current, which is what we need to do here — you can figure that the peak current in this laser is on the order of 4,000 Amperes, as I mentioned above. With fast pulses, however, the current travels in a thin layer at the surface of the conductor. (Look up “skin effect”.) The surface area of a cylindrical conductor goes up linearly with the diameter, so it would take a very large wire to carry that much current. It is much easier to get large surface area with a piece of shim stock. (It also lets me get better contact to the long thin wire sticking out the end of the flashlamp, but in general I avoid lamps of this type, so that’s a special case.)

  I should note that the shim stock I used in my initial construction is actually somewhat too thin. I would recommend 5 or 6 mils if you want to build a laser of this type.
  
  

• No Brewster-angle windows?

  It is difficult to cut things at the correct angle without a rotary table, which I do not have, and I don’t need polarized output in any case, so I punted this one. I wanted to use anti-reflection-coated windows on the second dye cell, as I did on the first one, but I didn’t have any more of that type on hand, so I cut pieces from a water-white microscope slide and used those. They are far from optimal, but I guess they’ll do for starters. (I have some AR-coated windows on order, and I may use a pair to replace the pieces of microscope slide at some point.)

  If I decide, in the future, that I want or need polarized output, I will put a Brewster plate in the cavity. That’s considerably easier than trying to find a way to mount the dye cell end windows at Brewster’s angle.
  
  

• Why do you turn the output coupler around backwards?

  If you look at how an ordinary HeNe laser works, you will notice that the rear reflector is flat, and is at the center of curvature of the output coupler. This is a stable configuration, and fairly easy to align, but the active region is roughly conical. That, in turn, means that some of the excited lasing medium is not actually part of the laser, and any energy that goes into it is wasted.

  Moreover, it requires that the distance between the mirrors has to be set fairly precisely. If they are too far apart, only a much more narrow “cone” of medium can lase. If they are too close together the mode structure probably suffers, though that’s not likely with a dye laser of even modest size unless the mirrors came from a really long HeNe.

  I usually turn the OC around backwards because I want to use a larger proportion of the active material, and I also do it when the radius of curvature is too short for the distance between the mirrors.

  The outer surface of an ordinary OC is curved so as to allow the laser to produce a clean Gaussian beam. If you turn it around backwards so that it faces the active region it should act almost like a flat mirror [at least, at the right wavelength], and it may be very slightly easier to align. True, there is a small amount of reflection, but the surface is always AR-coated, so this is not usually an issue.

  There are unstable cavity designs that are more efficient because they use a larger percentage of the excited medium, but I don’t have appropriate mirrors to construct such cavities at visible wavelengths.
  
  

• Why the hugely complicated alignment procedure?

  If you have never built one of these things, you may not have thought through the process of aligning the mirrors; it is unavoidably tweaky.

  Let me give you a runthrough. I hope that will make it easier for you to see why I had to jump through so many hoops, and I also hope it will make it easier for you to do when the time comes.

  First things first: you have to make sure that the path through the laser is unobstructed, and you need to choose an alignment tool that will work well. I use small CW lasers for this. If your alignment laser happens to be at a wavelength that is not reflected well by one or both mirrors you will probably have a hard time doing the alignment; likewise, if it is reflected too well by the mirror that is closer to it, you will have a hard time seeing the return from the mirror that is at the far end of the dye cell. I have several things I can use for performing alignments: a cheap green laser pointer, a variety of little HeNe lasers, a cheap red laser pointer or two, and a small violet diode laser. (I haven’t needed the violet diode for aligning anything yet, but it is handy anyway — I use it to check things for fluorescence.)

  It is also possible to use a borescope. A straightforward design is shown, IIRC, in the Scientific American “Amateur Scientist” column on either the homebrew HeNe laser, the homebrew argon laser, or both. I think it uses a small incandescent bulb, for which you could easily substitute an LED, as its light source; a microscope cover slip as a beamsplitter; an eyepiece from a microscope; and possibly one or two other lenses. If you decide to use a borescope, that column has good directions, so the rest of this discussion will cover the use of a laser.

  One suggestion for your alignment laser: make a paper target with a small hole in its center. (I like to print some convenient pattern of narrow lines on it, either vertical and horizontal bars, a series of rings around the hole, or both.) Tape or glue this target to the front of the alignment laser, positioned so that the beam emerges through the hole. The return is easy to see on the paper, and the lines or rings can make it slightly easier to figure out what is happening as you make adjustments. If you do not have such a target it is hard to tell where the return is and what it looks like, and that makes it much more difficult to achieve alignment.

  Once you have a viable alignment laser:

  1. Line it up with the dye cell. You want the beam to go right down the center from one end to the other. Even doing this can be very difficult, particularly if the laser is long, as this one is. I think it took me about 10 or 15 minutes to get my alignment laser aimed reasonably squarely down the middle of the dye cell, but that is partly because I was not using a good set of mounts for it.

     I find it convenient to put a paper target at the far end of the dye cell, so I can see what the beam looks like as it emerges. The patterns are extremely confusing, and it will take you a while to begin to understand them, but once you have an idea of what you are looking at it becomes somewhat easier to tell when your alignment beam is actually coming through the middle of the cell, and not bouncing off the walls on the way.

     If you darken the room, it is easier to see reflections from the walls of the cell, and you can “walk” the beam down the wall to the far end by making tiny changes in the position of the alignment laser or by moving the laser that you are aligning. My personal experience is that this is about the easiest way to get it lined up. For one thing, it is easy to tell if the alignment laser is aimed slightly up or down, because the beam will hit higher or lower on the wall of the dye cell as you walk it along. Depending on the dye and the wavelength of your alignment laser, you may be able to see the beam inside the cell, even where it isn’t hitting the wall. This can be extremely helpful.
     
     

  2. When you have the tool aligned with the laser that you want to set up, you can install the mirror at the far end. Tweak this mirror until it reflects the alignment beam straight back through the dye cell to the target on the front of the alignment laser.
     
     
  3. If you have not yet filled the dye cell with liquid, now is the time to do so. You will have to redo both parts of the alignment, but you will have had a little practice, which should make it slightly easier.
     
     
  4. Once you have the dye cell filled with liquid and the far mirror aligned, install the near mirror and align it. You will almost certainly find that the far mirror is now out of alignment, and you are also likely to find that the beam is no longer correctly aligned with the dye cell, so you will have to tweak a little; but everything should be very close at this point, so the tweaks should be relatively minor. (Should... a bad word!)
     
     
  5. If you can get the dye laser to operate, you may be able to tweak the mirrors again by viewing the output beam on a target some distance away. Assuming that your dye cell has a circular bore, the pattern should have a bright circular center. (See the output photos, above.) There is almost always some noise around this dot; the noise is caused by reflections from the walls of the dye cell. (It may be possible to eliminate or drastically reduce the amount of noise by including an aperture inside the cavity of the laser. I haven’t tried that yet, but I am setting up to do so, and if I get decent results I will post them.)

  If you have followed that description, you will begin to realize just how involved it is. In fact, I would hazard a guess that some people have been unable to get their dye lasers to work because their mirrors were misaligned, rather than from any lack of pump power/energy.

  Here is a set of photos, taken very late in the process. The first one shows what the target looks like when the mirror at the far end of the dye cell is blocked. Notice the bright line in the dye solution; Rhodamine 640 absorbs enough at 633 nm that you can see the alignment beam. (It is very hard to tell from the photo, but the dye emission is much closer to orange than the alignment beam. This is a quantum-mechanical refrigerator, and I should have patented it when I first noticed it, back in 1970 or 1971, with Rhodamine B. [[It was patented, just a few years ago. I grit my teeth.]])

  Notice the bright spot, above and to the right of the hole in the target. That’s the reflection from the front window. It will appear in all of these photos, and we are going to ignore it.

  

  (The color balance is also off in these images; I tried to figure out how to correct it, but that was not easy.)

  The next few photos show what happens as I adjust the mirror. Notice that the return spot in the first one is not round. This indicates some disturbance in the optical path through the dye. In the last photo, the mirror is approximately aligned. It may look like it’s aligned in the next-to-last photo as well, but if you examine it carefully you will see that it is slightly high and to the right.

                                 

  Here, for comparison, are three photos of returns that show serious disturbances in the dye solution. These are direct crops from the originals, so there are no larger versions. As mentioned above, the bright spot to the right and slightly above the central hole is a reflection from one of the end windows of the dye cell, and is not part of the return beam.

  

Here is a photo showing part of the dye cell, close to the mirror at the far end (away from the alignment laser). You can see two beams in the dye, evidence that the mirror is misaligned. That can also indicate a distorted optical path through the dye, though, so you need to be careful not to jump to conclusions. If you have any doubt, move some dye solution through the cell and watch to see how things change. If you can still see two beams after everything settles, you need to tweak the mirror alignment.

(The large version of this image is only 700 px across. It is a direct crop from the original file; I don’t have anything larger.)

Annals of the New York Academy of Sciences
Volume 168, Issue "Second Conference on the Laser" (February, 1969), Pages 703-714
DESIGN AND ANALYSIS OF FLASHLAMP SYSTEMS FOR PUMPING ORGANIC DYE LASERS
J. F. Holzrichter, M.S. and A. L. Schawlow, Ph.D.

(Yes, that A. L. Schawlow, the one who got the Nobel Prize.)

On to the second page of this set, in which I try out some possible improvements...

Back to the Index

Home

Contact Information:

My email address is a@b.com, where a is my first name (jon, only 3 letters, no “h”), and b is joss.

My phone number is +1 240 604 4495.

Last modified: Wed Jan 13 00:15:08 EST 2010

Interim Research Reports

of
The Joss Research Institute

1. Recent Ceramic Happenings, early Spring, 2005
   
   
2. Yohen Tenmoku and a plant note or two, late April, 2005
   
   
3. Bringing up the Molectron Nitrogen Laser
   
   
4. Results of a Late Spring Δ10 Gas Reduction Firing
   
   
5. Thoughts About High-Performance Nitrogen Lasers, with a set of follow-on pages in which I actually try to do something about it.
   
   
6. Item #6 is on hold at the moment.
   
   
7. “Platzepuss”: An electric-powered radio-control motorized Platz-Gleiter model (Work in progress, currently on hold while I deal with other issues.)
   
   
8. The Hughes “M-60” rangefinder laser (Likewise on hold for a bit.)
   
   
9. An Ultraviolet Laser Dye
   
   
10. Inexpensive Laser Dyes for Do-It-Yourselfers, with follow-ons about pumping and tuning
    
    
11. A brief report on one way to construct a dye cuvette for nitrogen- or excimer- laser pumping
    
    
12. A simple and straightforward 5″ refracting RFT built from surplus lenses, PVC pipe (or leftover wood veneer), and a few other things…
    
    
13. Some Handy Techniques, including a mirror mount that is entirely built from pieces you can buy at a hardware store and a hobby shop…
    
    
14. An RGB “White Light” dye laser using a single cuvette of dye solution…
    
    
15. Toward a Simple and Straightforward Lamp-pumped Organic Dye Laser, Part 1: initial proof of principle
    
    
16. A Hollow-Cathode Laser Design for the DIYer
    
    
17. More work with Translucent Porcelain
    
    
18. Handy Techniques for Fountain/Calligraphy Pens
    
    
19. A Voss (Töpler-Holtz) Electrostatic Generator
    
    
20. The Elusive Red Temmoku Glaze (I should note that I do not mean “tomato red” here, nor do I mean orange. When I say red, I mean red.)
    
    
21. Room-pressure nitrogen lasers, possibly with relatively high performance.
    
    
22. Adventures with a Commercial Excimer-Laser Head Last modified: Mon Apr 4 16:22:43 EDT 2011
    ]]> http://jossresearch.org/2011/05/21/the-joss-research-institute-interim-research-report-series/feed/ 0 http://jossresearch.org/2011/05/16/your-diy-nitrogen-laser-is-not-a-blumlein/ http://jossresearch.org/2011/05/16/your-diy-nitrogen-laser-is-not-a-blumlein/#comments Mon, 16 May 2011 00:36:26 +0000 Jon Singer http://localhost:8888/jossresearch/?p=31
    

    An Examination of the Amateur Scientist Circuitboard Nitrogen Laser

    Contents:
    Abstract
    Preliminaries
    Blumlein and His Circuit
    The Issue of Latency
    Travelling-Wave Excitation
    Issues Related to Scale
    Power and Energy
    Closing Remarks
    Some Interesting Papers

    
    

    Abstract

    Many Do-It-Yourselfers have built nitrogen lasers, often from a design published in the Amateur Scientist column of Scientific American magazine. This page discusses the text of that column in some detail, and shows several ways in which the explanation of the design and how it operates is faulty.

    
    

    To Begin

    In the Amateur Scientist column, on page 122 of the June, 1974 issue of Scientific American, there was a design for a tabletop nitrogen laser. It was written by someone named Jim Small, who was a student at MIT at the time. The article was later republished in the Scientific American book Light and Its Uses, and is also on the CD of Amateur Scientist columns, which you can get from The Society for Amateur Scientists. I have also found this CD available from The Surplus Shed, and from American Science and Surplus.

    The design isn’t bad at all: it’s easy to build, easy to operate, and puts out enough energy to drive a

    » Read the rest
    ]]>
    

    An Examination of the Amateur Scientist Circuitboard Nitrogen Laser

    Contents:
    Abstract
    Preliminaries
    Blumlein and His Circuit
    The Issue of Latency
    Travelling-Wave Excitation
    Issues Related to Scale
    Power and Energy
    Closing Remarks
    Some Interesting Papers

    
    

    Abstract

    Many Do-It-Yourselfers have built nitrogen lasers, often from a design published in the Amateur Scientist column of Scientific American magazine. This page discusses the text of that column in some detail, and shows several ways in which the explanation of the design and how it operates is faulty.

    
    

    To Begin

    In the Amateur Scientist column, on page 122 of the June, 1974 issue of Scientific American, there was a design for a tabletop nitrogen laser. It was written by someone named Jim Small, who was a student at MIT at the time. The article was later republished in the Scientific American book Light and Its Uses, and is also on the CD of Amateur Scientist columns, which you can get from The Society for Amateur Scientists. I have also found this CD available from The Surplus Shed, and from American Science and Surplus.

    The design isn’t bad at all: it’s easy to build, easy to operate, and puts out enough energy to drive a small dye laser. In fact, people are still building lasers from it today. Unfortunately, there are serious problems with the author’s explanation of how it works.

    I’m not about to violate copyright by reproducing the drawings from the article, and I don’t have time to redraw them, so it will help you to have a copy in front of you. If you don’t already own Light and Its Uses or the collected Amateur Scientist columns on CD-ROM, you can probably find the book or the magazine at your local public library or the nearest college or university library of Physics, Engineering, or Sciences. Alternatively, if it is still up on the Web, this page has copies of the illustrations on it. They aren’t very large, but you should be able to see enough to follow what I have to say.

    Mr. Small’s explanation of the general principles of operation of the nitrogen laser appears, for the most part, to be reasonable. For example, he identifies one cause of the short pulses as bottlenecking in the lower laser level: the lifetime of the upper laser level is on the order of 40 nsec at low pressures, and is perhaps 20 or 30 nsec at the pressures ordinarily used in low-pressure nitrogen lasers; the lifetime of the lower level, on the other hand, is some tens of μsec, literally about a thousand times as long. Broadly speaking, this limits the pulsewidth to less than the lifetime of the upper level.

    That is certainly correct as far as it goes; but in practice, the pulses from many nitrogen lasers (including the Scientific American laser) are considerably shorter, often in the 6 to 8 nanosecond range. This is because most small-scale driver circuits “run out of steam” — within a few nanoseconds after lasing starts, they cease to be able to give the electrons in the discharge enough energy to pump nitrogen molecules to the upper laser level at a sufficiently rapid rate, and the existing inversion is then depopulated by the lasing process. Lasing ceases long before there is time for a large lower-level population to build up.

    It is, of course, possible to get a very short pulse from a nitrogen laser by pumping (and presumably lasing) about half of the available nitrogen molecules. At that point you have bottlenecking in the lower level, regardless of the duration of the output pulse. At any reasonable pressure, however, doing this in just a few nsec takes far more electrical input than the Scientific American laser could possibly provide, and I have seen only one or two reports of high-power nitrogen lasers that appear to operate in this regime.

    It is also possible, though not particularly common, to create a resonant shortening of the laser pulse; this also has to do with the design of the driver circuit, but in a different way. See the Tsui, Silva, Couceiro, Tavares Jr, and Massone reference, below, for more information.

    Please note: some references claim that the short lifetime of the upper laser level limits the pulsewidth. That’s idiotic nonsense. Lots of organic laser dyes that have upper-level lifetimes of just a few nanoseconds will happily run for 1 microsecond or longer under flashlamp pumping, and can even be operated CW with laser pumping.

    Let’s take a look at some of the claims in the article, see what they mean, and find out how they stack up against observable reality.

    
    

    1. The Renowned Blumlein Circuit

    First of all, Small describes his laser as a Blumlein circuit, and speaks of “the Blumlein phenomenon”.

    The real Blumlein phenomenon is the fact that Alan Dower Blumlein essentially invented stereophonic sound. He even got a patent on it. Among audio engineers, he is rightly famous. There are Web pages about this, and someone has written a biography of him. Among electrical engineers, however, he is also known for his work on transmission lines. He came up with something called a “Blumlein line” or “Blumlein circuit”, or sometimes just “Blumlein”.

    This circuit involves two matched transmission lines, with a matched load between them that has twice the impedance of either line. There’s an explanation of it among the pages of Kentech Instruments.

    Note that the two transmission lines do not have to be of the same length; but they do have to have the same characteristic impedance, and the load must be matched to both of them. This implies that the load must also have a specific impedance, which does not change. (The Kentech page includes diagrams showing idealizations of what happens when the impedance of the load is or is not matched to the impedances of the transmission lines.)

    It is important to note that this is a transmission line circuit we’re talking about here, and that, as such, it involves relatively well-behaved and well-matched impedances. The impedance of a nitrogen laser’s discharge channel changes constantly during the discharge cycle, and is nontrivial even to define. It is not really possible to match such an impedance with a transmission line, which has fixed parameters. Various articles (see, for example the Tsui et al. and Persephonis references, below) have discussed this or related issues.

    The Blumlein circuit also requires extremely fast switching, because otherwise the energy storage devices behave as discrete capacitors, not as transmission lines. This is crucial, and is a major point of failure of Small’s explanation. (More about this shortly.)

    If you read the references I list at the end of this rant, you will find repeated statements to the effect that the measured risetimes of these lasers are much too long for any transmission-line behavior to occur, at least on the switched side. (There is, however, some chance of observing transmission-line behavior on the unswitched side in a well-designed laser of this type.) Note that I’m not talking about theory here — these are actual measured risetimes of real lasers, most of them a lot better than Small’s. Some of them, in fact, put out several megawatts of power, whereas Small’s design puts out perhaps 50 or 100 kilowatts. (I will provide a relevant diagram later.)

    
    

    1A. Switch Closure Timing, a Central Issue

    In his article, Small states that “at the instant the switch closes”, a discharge wave is initiated in the circuitboard capacitor that presumably forms one of the transmission lines of the device. Let’s think about that for a moment.

    First off, the word “instant” is not defined in physics, electronics, or engineering, except when people are discussing mathematical entities (“…the instantaneous value of the second derivative…”). It’s not appropriate here, and in plain point of fact, it’s meaningless. (That should serve as a pertinent warning about any description of a physical device that employs this word.)

    Second, even if we were to pretend that “instant” had a meaning, that it meant, say, “appreciably less than 1 nanosecond,” there wouldn’t and couldn’t be any such instant in any case. The switch in question is an untriggered spark gap, designed and constructed so that it includes a nice big one-turn inductor. Even excellent spark gaps, well designed and carefully triggered, take several nanoseconds to initiate; and the free-running spark gap in this laser is slower than even a reasonably good triggered one.

    (2011.0510, afternoon and evening)

    Let me show that to you.

    Here are two oscilloscope traces. The first shows the output of a TEA nitrogen laser that I built. It is here to demonstrate that my scope [a Tektronix 7104, with a 7A19 vertical amplifier (600 MHz bandwidth)] and photodetector [a Motorola MRD500 photodiode (risetime specified at 1 nsec or less), in a commercial mounting] are actually fast enough to support this measurement.

    The pulse from a TEA nitrogen laser is much shorter than the pulse from the SciAm laser. If you measure it at half of its maximum amplitude, it is generally a bit less than 1 nanosecond long. It is showing up in this photo at just under 1.5 nsec, which is very reasonable — the risetime of my setup is 1 nsec or a bit more, and it is difficult to show a pulse that is shorter than the risetime of your detector/scope combination. Unless the risetime of the spark gap is still shorter, however, it will show up.

    

    The next photo is a trace of the light from the spark gap of that same nitrogen laser. As you can see, the 0-100% risetime of the spark gap is about 18 nsec.

    

    Here’s a photo of the gap, so you can see the design for yourself:

    

    The top electrode is mounted on a broad piece of brass shim that comes off the top capacitor plate, so it avoids part of the inductance of the large one-turn coil that is inherent in Small’s design. In addition, the gap has a starting capacitor across it [the small brown cylinder just to the left of the gap], which speeds it up even further. As a result, this gap is at least as fast as the one in the SciAm laser, and in fact it is probably significantly faster.

    The first part of the bottom line is that I don’t want to hear any idiocy about “the instant” the spark gap switches, because there isn’t any such animal.

    Because much of the rest of Small’s explanation depends upon the switch closing in an unrealistically short time, it cannot possibly accurately reflect what is actually going on inside the laser. There are, however, other issues.
    

    ---

    
    
    

    1B. Formation of a Discharge Wave

    Light travels at a finite velocity, which is very roughly 300,000,000 meters per second in a vacuum or in air. In materials with higher density (and higher refractive index), it is correspondingly slower. As Small points out, a discharge wave in a transmission line travels at the speed of “light” too, but that speed turns out to be related to the impedance of the line — the electrical equivalent, if you will, of the refractive index.

    In a piece of circuitboard, the speed is on the order of 5 nanoseconds per meter (see the Schwab and Hollinger reference). That’s roughly 8 inches per nanosecond. If a discharge wave travels 8 inches during 1 nsec, then it takes 125 picoseconds to go 1 inch, and 12.5 picoseconds to go 1/10 of an inch. Please take a good look at the diagram of “The Blumlein switching phenomenon”, either on the Web or in a copy of the article. In this diagram, edge of the discharge wave is shown as a vertical wall, which is totally ridiculous.

    Even if we take it to represent a 10-psec risetime, there isn’t any such thing as a spark gap that switches in 10 picoseconds, except perhaps in a very carefully designed transmission line, pressurized to about 1500 psi.

    In fact, as you can see from the oscilloscope trace, above, it takes literally hundreds of times as long as that for even a good spark gap of the regular sort to turn on at these voltages and currents. If such a switch could make a discharge wave at all, the leading edge of that wave would be several meters wide, not the vertical wall that is shown in the diagram; and you obviously can’t have a wave that is several meters across in a device that is, itself, less than half a meter wide.

    As Schwab & Hollinger point out in their excellent article, for a Blumlein generator that is built from transmission lines with characteristic impedance of 0.160 Ω (a fairly reasonable value compared with the effective impedance of a laser channel that is fully conducting), it would take a spark gap with 0.2 nh series inductance to create a risetime even as short as 2 nsec. That’s about the size of Small’s entire laser.

    10 psec? In a free-running gap that has a nice large series inductor built into it? I don’t think so!

    In addition, Small never addresses the fact that the laser channel can’t be a well-matched load, because its characteristics are constantly changing during the electrical pulse. This makes it difficult to get any such device to operate fully in transmission-line mode, even if it is correctly designed and constructed. (If you read the references, though, you will find that it is possible to get some transmission-line behavior in a circuit that is sufficiently well designed, at least on the unswitched side. See the Shipman reference, in particular, for a fine example. There is also relevant information in the Fitzsimmons et al.; Schwab & Hollinger; and Iwasaki & Jitsuno references.)

    It is interesting to note that Small says, “In effect the assembly behaves as an adjacent pair of interconnected capacitors.” It’s not just “in effect”; his assembly is just a pair of interconnected capacitors; it is not a Blumlein circuit.

    Unfortunately, instead of using the term “doubler circuit”, which would be at least vaguely appropriate, he gives a distorted version of what would happen in a Blumlein device, including the claim that “As the charge rushes through the spark gap a steep difference of potential appears within the plate across a narrow boundary that separates the charged and discharged regions of the metal.” Well, no. Not with the design he’s describing.

    (Small also indicates “no voltage” in the region of the “transmission line” where the discharge wave has passed, which would not really be correct even if the device were operating as a transmission line; but the issue is not particularly important to this discussion, and we don’t need to get into it. If you want more and better information, read the Kentech page referred (and linked) to above, and a few of the articles cited at the end of this page.)

    
    

    The Issue of Latency

    Something Small never addresses (possibly because it had not yet been examined or measured when he wrote his article) is the fact that it takes time to pump enough nitrogen molecules into the upper laser level to establish a population inversion, which means that lasing does not start as soon as the spark gap begins to conduct. Here, for example, is a diagram that I have adapted from one that appears in a published paper:

    

    (Click the small image if you want a larger one.)

    First, note that this is a charge-transfer laser, so the voltage risetime is probably slower than that of a simple voltage doubler like Small’s.

    Second, note that the current in the laser channel really starts to flow about 100 nanoseconds after the voltage across the channel begins to rise. While it is true that with Small’s design this time will be shorter, it is certainly going to be measured in dozens of nanoseconds.

    Third, note that lasing does not begin until something like 8 nanoseconds after the channel starts to conduct; it takes time to create a population inversion. (The nitrogen laser is pumped by direct electron impact, so there can’t be a significant amount of pumping going on until there is a significant amount of current flowing in the channel.)

    Fourth, note that lasing ceases while there is still quite a bit of current flowing in the channel. This laser has FWHM pulsewidth of 13 nanoseconds, which puts it in the high-performance class and suggests that lower-level bottlenecking is likely to be what terminates the pulse. (The term FWHM refers to the Full Width of the pulse at Half of its Maximum value.)

    
    

    3. The Travelling Wave

    Another problem with Small’s explanation is that he claims to have produced a travelling optical wave. Let’s think about this, too, for a moment.

    By Small’s own admission, the output pulse from his laser is about as long as a broomstick, or a bit longer; let’s say 6 feet, which is about 6 nanoseconds. If we think about a time during which the entire laser channel is above threshold, and is lasing, light that starts at either end will be amplified by the discharge in the channel, and will reach the other end just over 1 nanosecond later, because the laser channel is a little over 1 foot long.

    If we assume that one end of the channel reaches threshold first, and then a hypothetical discharge wave “walks along the channel” as Small proposes, to create a travelling optical wave, what do we see from the two ends of the laser? The “back” end, where lasing starts first, should show a small amount of output, which will increase as the leading edge of the electrical discharge wave gets farther away. That is, the back end will lase first, but not very strongly, and the output from that end will increase during the first nanosecond or so until the entire channel is above threshold. (As pumping continues, the output from this end will continue to increase until either too much of the nitrogen in the channel has collected in the lower laser level, or the capacitors can no longer push enough power into the channel.)

    After that first nanosecond, the discharge wave (and the initial laser light from the back end) simultaneously reach the front end, and lasing begins there. Thus, the pulse from the front end should have a much sharper leading edge than the pulse from the back end.

    After that point, however, the entire channel is above threshold, so for the rest of the pulse, which is to say the next several nanoseconds, output from both ends will be identical, or nearly so. Needless to say, this fails to match Small’s description of the action; but Small’s description fails to match his own statements about the laser and what it does.

    There are only a few ways in which such a laser, which is only 1/6 as long as the pulse it emits, can produce dramatically higher output from one end than the other, and the primary ones involve something interfering with the output at one end. In an ordinary TEA nitrogen laser, this can be a disturbed discharge that doesn’t have much gain, or it can be an arc or spark. (It is, however, possible for an arc to form after lasing has ceased, so an arc by itself is not a reliable indicator.)

    If anybody can show me such a laser that puts out a large pulse from only one end of its channel without any arc or spark formation and with no obvious visual difference between the discharge at one end of the channel and the discharge at the other, I would very much like to see it. (I’m not sure of the title of this one)
    Opt. Quant. Electr. Lett. 8 (1976), p. 565

    This paper is cited by the Oliveira dos Santos et al. paper, and is included here for completeness. I haven’t read it yet.

    —————————————————–

    B. Oliveira dos Santos, C. E. Fellows, J. B. de Oliveira e Souza, and C. A. Massone
    “A 3% Efficiency Nitrogen Laser”
    Applied Physics B (Photophysics and Laser Chemistry) 41 (1986), pp. 241-244

    This is a strange and wonderful article that illustrates an entirely different approach. Using a coaxial capacitor of only 800 pf, driven by one of three “dumper” caps (1.5, 10, or 20 nf), they achieved up to 3 MW output power at efficiencies ranging as high as 3%. Peculiarly, their pulsewidth decreased as the amount of stored energy increased, which may suggest that they are pumping a substantial fraction of the nitrogen molecules in their laser. Well worth reading and thinking over very carefully.

    —————————————————–

    K. H. Tsui, A. V. V. Silva, I. B. Couceiro, A. D. Tavares, Jr., and C. A. Massone
    Resonant Narrowing of the Nitrogen Laser Pulse by Plasma Impedance Matching
    IEEE Journal of Quantum Electronics, Vol. 27 No. 3 (March, 1991), pages 448-453

    This article, though not necessarily easy to follow, contains a valuable discussion of a topic that is seldom discussed in the nitrogen laser literature. It may explain (at least partly) the occasional high-performance laser operating at relatively low pressure but producing extremely short pulses, for example the Armandillo and Kearsley laser (see below).

    —————————————————–

    A. D. Papadopoulos and A. A. Serafetinides
    “Characteristics of Doubling Circuits Used in Gas Laser Excitation: Application to the N₂ Laser”
    IEEE Journal of Quantum Electronics, volume 26 number 1, January 1990, pages 177 to 188

    Note that this laser closely resembles the Scientific American laser, but is much faster and produces considerably higher output. Nonetheless, the authors describe it as a doubling circuit, not as a Blumlein; and they analyze it in terms of lumped constants, not transmission lines. The oscilloscope traces of the current and voltage waveforms in their laser and of the laser output pulse support this approach.

    —————————————————–

    P. Persephonis
    “Electrical behavior of a Blumlein-line N₂ laser”
    Journal of Applied Physics, volume 62, pages 2651-2656, 1987

    This early Persephonis article is good, despite the misuse of the term “Blumlein”, but see the next reference.

    —————————————————–

    P. Persephonis, B. Giannetas, J. Parthenios, C. Georgiades, and A. Ioannou
    “Capacitance Allocation and Its Role in the Performance of Doubling-Circuit Pulsed Gas Lasers: Its Application to the N₂ Laser”
    IEEE Journal of Quantum Electronics Vol. 29, No. 8, August, 1993, pages 2371-2378

    This is a beautiful look at the optimum capacitances and capacitance ratio for the doubler circuit nitrogen laser. (Note that by 1993, Persephonis had ceased to refer to these as “Blumlein-lines”.) The findings in this article are somewhat surprising, in that they obtain best results with relatively large capacitances; but entirely expectable in that they confirm the general wisdom, which is that the capacitors in a doubling circuit should be of about equal value. To say that this article is seriously worth reading would be an understatement.

    —————————————————–

    Imre Sánta, László Kozma, Béla Német, János Hebling, and M. R. Gorbal
    “Experimental and Theoretical Investigation of a Traveling Wave Excited TEA Nitrogen Laser”
    IEEE Journal of Quantum Electronics, vol. QE-22, Number 11, (November, 1986), pages 2174-2180

    These people figured out how to angle the electrodes in order to cause the discharge to form at one end and walk down the cavity to the other. Because a TEA nitrogen laser has an output pulse that is only about 600 psec long, it is possible to make a TW laser that is only about a foot long, and they appear to have done so. DiY folks take note.

    —————————————————–

    K. R. Rickwood and A. A. Serafetinides
    “Semiconductor Preionized Nitrogen Laser”
    Rev. Sci. Instr. 57(7), July 1986, pp 1299-1302

    A rather intriguing paper for its general premise; also has some good information about optical cavity considerations, and about the effects of adding helium to the gas. Well worth a careful read.

    —————————————————-

    E. Armandillo and A. J. Kearsley
    “High-power nitrogen laser”
    Applied Physics Letters, volume 41 number 7, (1 October, 1982), pages 611 through 613

    This article covers the design considerations of a nitrogen laser that delivered 5 MW (!), the highest output power reported in a discharge-pumped nitrogen laser up to the time of the article’s publication, and probably still one of the highest power levels ever achieved in N₂. Oddly, their pulses were only 4 nsec long, which is quite unusual for high-performance nitrogen lasers. The article is good, if a bit brief.

    Crucial points here include the dimensions of their channel, which used electrodes a full 4 cm across, spaced 25 mm apart; and the fact that the addition of Helium, while it did not increase the output energy or power of their laser, did give them better pulse-to-pulse uniformity and a cleaner discharge. In addition, they were able to operate their laser with enough He to bring the total pressure up to more than 1 atmosphere. I have taken advantage of that in at least one of my own lasers: it allows you to operate without a vacuum pump, which can be very convenient.

    —————————————————–

    F. Encinas Sanz and J. M. Guerra Perez
    “A High Power High Energy Pure N₂ Laser in the First and Second Positive Systems”
    Applied Physics B, volume 52 (1991), pages 42 through 45

    This article concerns a charge-transfer (“dumper-peaker”) laser that developed 20.5 mJ in the UV (!). Because it had a relatively long output pulse, however, the peak power was only 1.5 MW. One interesting thing about this article is the fact that they found an optimum interelectrode spacing of about 38 mm, much wider than is common in circuitboard (or other) low-pressure nitrogen lasers, but similar to the spacing in the high-energy laser built by Rebhan et al., which is cited below.

    Another key point is that the article shows voltage, current, and laser output traces taken from oscilloscope photos. These clearly demonstrate the fact that their laser didn’t reach threshold until about 10 nsec after current began to flow in the laser channel, and also the fact that current didn’t begin to flow until dozens of nsec after voltage began to appear across the channel. Granted, their design was a charge-transfer circuit, not a voltage-doubling circuit, so the voltage risetime was slower than you would expect in a Small-type laser; still, there is definitely some nsec delay between the onset of the discharge and the onset of lasing.

    —————————————————–

    U. Rebhan, J. Hildebrandt, and G. Skopp
    “A High Power N₂ Laser of Long Pulse Duration”
    Appl. Phys. 23, 341-344 (1980)

    This is another of the best nitrogen lasers ever constructed. With some SF6 in the gas mix, it delivered 30 mJ over 19 nsec, and even without any SF6 it delivered 16 mJ over 14 nsec! It uses a liquid-dielectric peaker cap of very ingenious design. It describes the use of long electrodes to avoid sparking at the ends, an important technique.

    —————————————————–

    Godard, Bruno
    “A Simple High-Power Large Efficiency N₂ Ultraviolet Laser”
    IEEE J-QE vol QE-10 no 2, February 1974, pp. 147-153

    This is very likely Godard’s fairly infamous article in which he claims to have derived 9 MW (!) from a laser built out of kapton circuitboard. Inasmuch as nobody has ever been able to repeat the result, there is considerable skepticism. I’m not 100% sure about the reference, btw; my copy of the article was handed to me by Godard himself in either 1973 or 1974, and is not from J-QE. It says on it…
    
    “LABORATOIRES DE MARCOUSSIS
    CENTRE DE RECHERCHES DE LA
    COMPAGNIE GENERALE D’ELECTRICITE
    DEPARTEMENT RECHERCHES PHYSIQUES DE BASE
    Section Sources d’Ondes Cohérentes
    91460 – MARCOUSSIS – FRANCE”
    
    …and is dated “MAI 1973”. The title is also slightly different; it begins “A VERY SIMPLE HIGH POWER….”

    —————————————————–

    Ernest E. Bergmann and N. Eberhardt
    A Short High-Power TE Nitrogen Laser
    IEEE Journal of Quantum Electronics vol. 9 no 8, August, 1973, pages 853-854

    Bergmann (not to be confused with H. M. von Bergmann, a South African researcher who did pioneering work with TEA nitrogen lasers) and Eberhardt note that their laser’s unfocused beam could pump several dyes to superfluorescence, and that sparks could be produce by focusing the beam on various metal surfaces. This laser had 200 kW peak output power, so these results provide a rough diagnostic.

    —————————————————–

    A. Vasquez Martinez and V. Aboites
    “High-Efficiency Low-Pressure Blumlein Nitrogen Laser”
    IEEE J-QE vol QE-29 no 8, August, 1993, pp. 2364-2370

    This is another important paper, though the theoretical investigation is not as thorough as in some others, and also despite the fact that the authors speak of “the instant the spark gap triggers”, which is nonsense. Even so, there is some very interesting information here.

    —————————————————–

    C. H. Brito Cruz, V. Loureiro, A. D. Tavares, and A. Scalabrin
    “Characteristics of a Wire Preionized Nitrogen Laser with Helium as Buffer Gas”
    Appl. Phys. B 35 (1984) pp. 131-133

    This is a small laser, used to investigate both preionization and helium; mixing nitrogen and helium 50-50 doubled their output power. With preionization, they measured best output at E/p of 87.

    —————————————————–

    Peter Schenck and Harold Metcalf
    “Low Cost Nitrogen Laser for Dye Laser Pumping”
    Applied Optics, Vol. 12 # 2, February, 1973, starting on page 183

    Bert Pool used to have a copy of this fine article on his Web page, but I don’t find it now. It is a nice easy design that develops more than 100 kW peak power under optimum conditions. I believe that it uses a thyratron as a switch, but you could very easily build it with a spark gap instead. I will, however, advise you to use a triggered spark gap — they’re a lot faster than free-running spark gaps, and speed is the reason why you would want to use a spark gap rather than a thyratron in the first place.
    

    ---

    
    

    If you want to build a nitrogen laser that puts out considerably more power than Small’s, I have published a design that delivers approximately 250 kW and is capable of making sparks when the beam is focused onto a metal surface. I am currently (late 2006) working on a laser that will be less expensive to build and should put out at least 500 kW.
    

    ---

    
    

    Finally, I need to point everyone at a remarkable site put together by Thomas Rapp, in Germany. He really knows how to build lasers, including TEA nitrogen lasers. (You can have The Babelfish translate his pages; it does a fair job, considering, and although you’ll still have a lot of figuring out and thinking to do, it’s definitely worth doing.)
    

    ---

    
    
    This work is supported by
    The Joss Research Institute
    19 Main St.
    Laurel  MD  20707-4303   USA
    
    

    ---

    
    
    

    Contact Information:

    Email: a@b.com, where a is my first name (jon, only 3 letters, no “h”), and “joss” replaces “b”

    Phone: +1 240 604 4495.

    Last modified: Sat Dec 17 23:58:32 EST 2011

    ]]> http://jossresearch.org/2011/05/16/your-diy-nitrogen-laser-is-not-a-blumlein/feed/ 0 http://jossresearch.org/2011/05/15/joss-institute-projects-a-my-first-laser-project-for-the-diyer/ http://jossresearch.org/2011/05/15/joss-institute-projects-a-my-first-laser-project-for-the-diyer/#comments Sun, 15 May 2011 15:32:26 +0000 Jon Singer http://localhost:8888/jossresearch/?p=34

    Joss Institute Projects:

    A Straightforward TEA Nitrogen Laser for the Do-It-Yourselfer

    (A “My First Laser” Project
    
    That Evolves into a Higher-Performance Laser)

    [Started on April 8, 2011.]

    
    width=”450″ height=”300″

    This photo shows a fairly complex version of the laser in operation. (The initial version is simpler, and easier to construct.) The output is not visible to the eye; the fluorescent objects on the left and right indicate the presence of the beams: partly because there are no mirrors, this laser produces two.

    
    

    ---

    
    

    Prolog:
    

    Amateurs have been building lasers since fairly shortly after the laser was invented. Several laser projects even appeared in the late (and much lamented) Amateur Scientist column in Scientific American, which is now, fortunately, available in its entirety on CD-ROM. There are also various pages on the Web that provide information about DIY lasers of various sorts, and I provide links to some of them at the end of this page.

    Unfortunately, I see quite a few videos on YouTube in which someone has bought a little laser module and hooked it up to a battery; they then proudly claim that they have built a laser. That’s pretty sad, especially when almost any of them actually » Read the rest

    ]]>

    Joss Institute Projects:

    A Straightforward TEA Nitrogen Laser for the Do-It-Yourselfer

    (A “My First Laser” Project
    
    That Evolves into a Higher-Performance Laser)

    [Started on April 8, 2011.]

    
    width=”450″ height=”300″>

    This photo shows a fairly complex version of the laser in operation. (The initial version is simpler, and easier to construct.) The output is not visible to the eye; the fluorescent objects on the left and right indicate the presence of the beams: partly because there are no mirrors, this laser produces two.

    
    

    ---

    
    

    Prolog:
    

    Amateurs have been building lasers since fairly shortly after the laser was invented. Several laser projects even appeared in the late (and much lamented) Amateur Scientist column in Scientific American, which is now, fortunately, available in its entirety on CD-ROM. There are also various pages on the Web that provide information about DIY lasers of various sorts, and I provide links to some of them at the end of this page.

    Unfortunately, I see quite a few videos on YouTube in which someone has bought a little laser module and hooked it up to a battery; they then proudly claim that they have built a laser. That’s pretty sad, especially when almost any of them actually could have built a laser. This page is for you if you really want to build a laser, and not just buy one.

    The lasers I describe here are TEA nitrogen lasers. (TEA stands for Transversely Excited, Atmospheric [pressure].) That is, they do not involve either vacuum or compression. The basic design is sufficiently straightforward that it can be built by a high school student who is particularly interested, or possibly even a middle school student who is truly determined. A laser of this type that is constructed with some care and is properly adjusted should put out more than enough power to drive a small dye laser, as you can see in the addendum near the end of the page. There is also an upgrade path, which can become quite challenging.

    Before we get any further along, we need some safety information and a disclaimer.

    
    

    ---

    
    

    !!   WARNING   !!
    

    
    If you build this project you do so at your own discretion,
    on your own responsibility, and at your own risk.
    

    These lasers use high voltages, and capacitors that can store lethal amounts of energy. They put out invisible ultraviolet light that can damage your eyes and skin. It is extremely important to take adequate safety precautions and use appropriate safety equipment with any laser; and it is crucially important with lasers that involve high voltages and/or produce invisible beams!

    In addition, these designs use open spark gaps, which will damage your hearing if you do not use adequate ear protection. I strongly suggest that you acquire and use at least a pair of sound-protection earmuffs of the type used by shooters at rifle and pistol ranges; they look about like this:

    
    width=”224″ height=”300″>
    
    
    
    
    Figure 1: Hearing protection

    (These cost me $35, and they are definitely worth it.)

    Earplugs can also help, but by themselves are probably not sufficient unless they decrease the volume by about 33 db and you put them in correctly; I suspect that only special ones that are made to fit your own ears are really good enough.

    If you are not using enough hearing protection, you will probably get a nasty headache if you run the laser for a while. Take that as a warning, and get better protection! You can make a new spark gap, and you can make a new laser; but you cannot make new eyes, ears, or fingers.

    
    

    ---

    
    

    A Preliminary Look
    

    In the process of working up this page I built several versions of the laser, increasing the complexity each time I revised it. Here
    
    (Video 1)
    is an informal video that begins with a simple laser, similar to the first version I present on this page, though with a different spark gap design. The laser was not very sophisticated, but as you can see in the video, it worked. (In retrospect, though, I will point out that in the later part of the video it is firing too often.)

    It is important to note two things about this. The first is that building a simple machine provides you with experience that helps you build more advanced versions. (No surprise there, I trust.)

    The second is that this first simple machine can easily evolve, in stages, into a considerably more advanced laser with far better performance. You don’t have to throw it away and start again from scratch, because you can create a more advanced version by modifying it, as I do in the course of the first video.

    Of course, if you want to start over again, nothing prevents you from doing so. The version that you see in (for example) Figures 22 and 23 is a complete rebuild, as is the version in Video 2b. That’s another handy thing about these lasers: after you have built several, you will probably find that it takes you only an hour or two. Don’t expect your first few to be that quick, though; it does take some practice.

    Although there are, fortunately, lots of good ways to build TEA nitrogen lasers, there are also lots of bad ways. It is particularly important to remember that if you try something and it doesn’t work, you need to document it carefully anyway, because you will almost certainly need the information later on, in order to figure out something that does work. You will probably also need the information in order to avoid repeating the same error[s]. It is a great relief (and sometimes a large surprise) to return to your notes, possibly months or years later, and find something you did that you may have forgotten about, and to have at least some of the information you need in order to understand how it worked …or didn’t.

    
    
    

    ---

    
    

    Parts and Materials
    

    In order to build one of these, you will need the following things:

    • A power supply. You can build one, although that’s a project in and of itself. I bought an old electronic air cleaner for a few dollars at a thrift store, and took the power supply out of it, as you can see in the photos below. (I am also using this power supply to drive the more advanced versions, though you can move to higher voltages if you want to.)

    
    
    

    • A base of some sort. It is important to find one that is smooth and flat. If you are using a bipolar power supply you will want to be sure that the base does not conduct electricity, even at high voltages. (Some materials that are good insulators at low voltages turn out to behave rather differently at high voltage. Wood, for example, is slightly conductive, as are bricks.)

    
    
    

    • A sheet of insulating material to serve as a dielectric. This laser design is based on two small high-voltage pulse-discharge capacitors; such a capacitor is most easily constructed from two sheets of conductive foil (or metal plates), with an insulating material between them. I used styrene plastic from the hobby store, 0.010 inches (10 mils, about 0.4 millimeter) thick for the early versions, but several other materials are suitable if you have trouble finding styrene. You need a sheet that is large enough to provide a margin of at least 5/8″ all around the capacitor plates, else you may find that sparks form across the surface. The material also needs to be thick enough to withstand the voltage that your power supply delivers. If you use a supply like mine, 10 mils (1/100 of an inch, about 0.25 mm) is probably thick enough.

    
    
    

    • The plates of the capacitors. The lower plate serves for both capacitors; I used a thin piece of single-sided circuit board, but it may be easier to use brass shim stock. Alternatively, you can use copper flashing or even aluminum foil. I like the circuit board because it is fairly thin and nicely flat, but my primary reason for using it was because I had it on hand. Brass shim stock is available at some hardware stores, and also on eBay; depending on the sizes of your upper plates, you may be able to use material that is 6″ wide, or you may need the 12″ width. Shim that is 3 mils (0.003″) to 5 mils (0.005″) thick should work well; if you get thicker material it will also work, but it will be somewhat harder to cut. (I suspect that 1 or 2 mils is actually thick enough, but I haven’t tried it yet.)

      You want your sheet of shim stock, or whatever you end up using, to be smooth and flat; if it is buckled or wrinkled, it won’t make good contact with the dielectric. However, a small amount of buckling or wrinkling that is near the edge, as long as it is well away from the upper capacitor plates, should not be a problem.

      For the upper plates I used small sheets of brass (2″ x 12″) that I got at the hobby shop. In the first version, these also serve as the electrodes of the laser. They are available in various thicknesses; I found 32 mils or even 64 mils to be convenient, for reasons I’ll get into when I describe some improvements.

    
    
    

    • A spark gap. This is the main switch of the laser, and if it doesn’t work right the performance suffers, often very badly. Jarrod Kinsey has had good results with a spark gap that is constructed from a pair of large lugnut covers that were intended for use on a truck. You can see one of Jarrod’s spark gaps near the right edge of this photo.

      These lugnut covers are available at truck stops, and they are not very expensive, but I decided to use a different design here, partly to find out how well it would work and partly because the parts that I needed for it are available at hardware stores.

    
    
    

    • Some way to conduct electricity from one capacitor plate to the other, so that they charge together. I use an inductor. (See the text, below, for details.)

    
    
    

    • Various small items that round out the project. For example, it is very helpful to have some pieces of high-voltage wire, though you can probably work around that. (You can put one or two layers of heat-shrink tubing around ordinary wire to give it more insulation, but that will make the wire very stiff, which could be a problem. For example, you are not likely to make a charging inductor from wire that has shrink-tube on it. Alternatively, you can wrap electricians’ tape around ordinary wire. Two thicknesses may be enough, but it’s a good idea to test. I would, btw, avoid the kind of tape that shrinks into place when you stretch it — unlike regular heat-shrink tubing, it emits some very nasty chemicals.)

      I use crimp connectors, but you can avoid them if you want or need to.

      One unusual item that I have found helpful is a special glue that is electrically conductive. The version I used to buy has been replaced, and I have not tested the new version yet, but I expect it to work just as well. I get this glue from the Electron Microscopy Sciences division of emsdiasum.com.
      (The link is to their catalog page for adhesives; look for the word “Conductive” a little less than 1/3 of the way down the page if you want to see all of the relevant offerings; the material I will be using in the future, when I have used up the bottle I currently have is “Silver Conductive Adhesive 503”.)

      Strictly speaking, conductive glue is not necessary; but it does help avoid sparking where close contact is needed. You can see it, for example, under the back end of the spark gap in the overview photo of the laser where I mention replacing the resistors with an inductor, just before the first photo that shows the output.

      You will also need some weights, to hold things down. It is important to get the capacitor plates into good contact with the dielectric and the dielectric into good contact with the baseplate, because air will decrease the amount of capacitance; because of that, it also decreases the amount of energy that the capacitors can store at a given voltage, and interferes with the performance of the laser. In the more complex versions of the design you also need to insure good contact between the electrodes and the capacitor plates, and you need to hold the electrodes in place so they don’t slide around. I have used a wide variety of objects for this purpose, several of which you can see in the photos: bricks, worn-out sealed-cell batteries from a UPS, pieces of scrap metal, and so on. One of my weights is a spice jar filled with small nuts and bolts. (Jarrod Kinsey has sometimes used cans or jars that he has filled with pennies; I like nonconducting weights, because if you accidentally touch one you won’t get shocked, so I would use glass or plastic jars, rather than metal cans.)

    
    
    
    

    ---

    
    

    Construction:
    

    1. The Power Supply
    

    For ease and convenience, I have taken the power supply out of an old electronic air cleaner. Here is the air cleaner before I disassembled it:

    
    

    
    
    
    Figure 2: The air cleaner

    This is what the right side looked like with the cover off:

    
    
    
    
    
    Figure 3: Interior of the air cleaner

    The power supply consists of a transformer and a small circuit board. Here it is on its new base, with a switch so I can control it:

    
    

    
    
    
    Figure 4: The power supply

    (Please note that this is just a temporary setup. Although the high voltages are insulated it is not safe to have line voltage exposed as it is here, and I will eventually enclose this supply in a well-ventilated insulating box.)

    I made a voltage divider by putting a 100-million-ohm high-voltage resistor in series with an ordinary 10,000-ohm resistor, and I used the divider to measure the output voltage from this supply. The schematic diagram on the bottom of the case says that the supply puts out 5500 VDC at 0.3 milliamps, but that turns out to be a description of one polarity, not both: I measured roughly 5980 volts on the positive terminal and roughly 6390 volts on the negative terminal, for a total of over 12 kV. This is the open-circuit voltage; when the supply is providing current to a load, the voltage is lower.

    
    

    ---

    
    

    Construction:
    

    2. Parts
    

    I have been using two bases for this laser; both of them are glass, and I got both at thrift stores. (I used glass because the lower electrical plate of the laser is at high voltage, and I wanted to keep it isolated from the table. In addition, glass is generally flat, which is important.) The glass pieces I found are intended for use in the kitchen, and they have pebbly top surfaces; I decided to live with that, but if you find one that has an extremely flat bottom surface you may want to turn it over and use the flat surface as the top. Alternatively, a plain piece of window glass will work, but you should make sure that the edges are not sharp. If you prefer to avoid glass, various kinds of plastic sheet can be used to insulate the high voltage from the bench or table; just be certain that the upper surface is clean and flat.

    I used a piece of single-sided circuit board as the baseplane of the laser (it would ordinarily be the ground plane, but because of the way the power supply is configured, it is definitely not at ground potential), largely because I have several pieces, acquired on eBay some time ago, that are of an appropriate size. Also, although it is quite thin, this material is just stiff enough that the pebbly surface of the glass underneath it is not a problem. (The board is so thin, in fact, that it can be cut with a pair of scissors. I don’t recommend doing that if you have a tool that is better suited to the job, however, because cutting circuit board isn’t a very nice thing to do to your scissors.)

    Brass shim stock is a good alternative that you can get at some hardware stores, and also on eBay. It works at least as well as the circuit board. Shim that is 0.003″ or 0.004″ thick should be suitable; if it is any thicker it starts to become more difficult to cut, so you may want to obtain a pair of tinsnips if you don’t already have one.

    The next step is to find a piece of plastic that can serve as a dielectric. I had originally intended to use overhead projection transparencies, but then I went to an office supply store and priced them: $40 for 100 sheets. I eventually found some at a thrift store for a much more bearable price, but they are only 0.004″ thick, and that isn’t enough to handle the full output of the power supply, so I went to the hobby shop and bought some styrene sheets that are 0.010″ thick and about 18″ long. These work quite well, but it is important to remember to get the long size if you are using long electrodes: the brass sheets that I used as the upper plates of the capacitors in my initial versions of the laser are 12 inches long, which meant that I needed a dielectric sheet at least 13 inches long, and preferably longer. High voltage will jump across insulators if it can, and you need to provide a margin of more than half an inch all the way around the capacitors. Likewise, when I use brass sheets that are 4″ x 10″ (another size that I can get at the hobby shop) it is easier to deal with the length, but I need to use a piece of plastic that is well over 8½ inches wide, and because of the spark gap, which I have positioned at one edge of the laser, I really need at least 10 inches of width.

    You will need a spark gap to conduct electricity from one of the capacitors to the ground plane. I made my gap out of a pair of 1/4-20 carriage bolts, as you can see from the photos, below. (Figure 9 shows the initial version.)

    Note: It is not necessary to position the spark gap where I did. You can put it almost anywhere you want, provided it doesn’t interfere with some other aspect of the design. There are some people who claim that it has to be in or near a corner of the capacitor, particularly if you want to achieve what is referred to as travelling-wave excitation; but if you think about that claim you will notice that in order for it to be valid, the gap would have to switch in a rather small fraction of a nanosecond. That’s considerably faster than is physically possible for a design of this type. (If you actually have a fast photodiode and a fast oscilloscope, you can check this for yourself. You will find, as I did and as you can see in Figure 26, below, that there is exactly zero chance of it being accurate. OTOH, it is typically possible to wedge the electrode spacing so that you get most or all of your output from one end of the laser.)

    
    
    
    

    ---

    
    

    Construction:
    

    3. The Laser Itself
    

    (I’m going to presume that you have acquired the parts and materials you need, and that you have a power supply.)

    Start by rounding the corners of the brass sheets that will serve as the top plates of the capacitors. (In this version, they also serve as the electrodes of the laser.) Sand the edges at the corners and ends to smooth them — sharp edges can cause sparking where you don’t want it, and can sometimes even make it easier for the high voltage puncture the dielectric, which immediately causes the laser to fail until you put a new dielectric into place. You can also make sure that the edges that face each other and serve as electrodes are straight and parallel to teach other, and it’s a very good idea to smooth them and then polish them. Here are two views of one of the electrodes from the first version I built:

    
    
               
    
    
    
    
    
    
    Figures 5 & 6: Electrode edges

    As you can see, the laser can be made to operate even if you don’t smooth the edges of the electrodes; but it won’t work as well, and it won’t be as easy to adjust. Here is a detail of two new electrodes that I have smoothed and started to polish:

    
    
    
    
    
    
    Figure 7: Polished electrode edges

    Because the brass pieces are formed by stamping them out of larger sheets, one face of each is slightly convex and the other is slightly concave, and the sheets are almost never really flat. It’s a good idea to put the convex faces down, as this prevents air from being trapped under the sheets. Also, if there are sharp edges it holds them a tiny distance up above the dielectric, which helps avoid punctures.

    The fact that the sheets are not fully flat is another reason for using weights. In addition to the slight edge-to-edge curvature imposed by the manufacturing process, they can also be slightly bowed from rough handling. If the middle of the sheet is high, you will want more weight there. (That was what I found with some of mine, but “your mileage may vary”. If the ends are high, you can either put more weight there or very cautiously bend the strips so that the middles are slightly higher than the ends.)

    You need a way to conduct high voltage from one sheet to the other, so that they both charge correctly. I originally used resistors, but the first ones I tried had too much resistance, and the channel sparked every time the laser fired, so I changed over to an inductor that I made by winding a few turns of high-voltage wire around a surplus ferrite core. (You can see this inductor in several of the photos; it is the blue toroid with the red wire wrapped around it.) Later I returned to resistors, but I used a lower value. Either method can work, but when I tried 200 ohms I found that it is not enough; a value that low will steal some power from the discharge. A combination of resistance and inductance works better than resistance alone, and if the inductor has enough turns resistors are not even necessary.

    If you use resistors at all, btw, it’s a good idea to make sure that they are rated for high voltage; alternatively, as you can see in some of the photos here, you can use resistors that are encased in ceramic envelopes and are rated to dissipate several watts: they seem to withstand the voltages involved. As I say, though, if your inductor is big enough you won’t need resistors.

    For your inductor, you will need something like 25 or 30 turns around a nonconducting cylinder that is perhaps an inch and a half in diameter. (I haven’t taken the time to find the minimum viable number of turns; it may be less than 25, and it will depend on the diameter of the cylinder and the turn-to-turn spacing of your coil.) For convenience and stability you can wind the wire around anything strong enough to hold it, perhaps a piece of PVC plastic plumbing pipe if you want to be relatively fancy about it. I am not fancy; I use the cardboard cylinder from a roll of toilet paper, which I stiffen by soaking it with cyanoacrylate adhesive [“superglue”]. I glue the ends into place with cyanoacrylate adhesive. This works well, at least on the type of wire I use.

    Here is a photo of a combination that I made; the inductor is 25 turns, and the resistors are 100 ohms each. The wire has fairly thick insulation (0.045″, a little over 1 mm); it was the thickest I found at my local hardware store. It works quite well, and my later versions do not use resistors.

    
    
    
    
    
    
    Figure 8: Resistor-inductor combination

    The upper electrode of the spark gap in my initial version is mounted on a small piece of brass shim stock that is attached to the capacitor plate of my laser with conductive glue, though it is probably sufficient to hold it down with a weight. In the first version of the laser I set the lower electrode of the spark gap down so that it was partly on the ground plane and partly on the dielectric, in a configuration similar to the one that Jarrod Kinsey uses (I have a link, above, to a photo of one of his lasers), and although I used some conductive glue I also put a weight on it to help hold it in place. Here is that original gap in operation:

    
    
    
    
    
    Figure 9: Spark gap, initial version, in operation

    You don’t necessarily need the shim stock: it is probably fine to connect both sides of the gap the way I did the lower side, and hold them both down with weights. That may be trickier to adjust, though — the spark will jump along the surface of the plastic unless you put some sort of blockage in place to prevent it from doing so. (The laser could possibly work with surface sparks in the gap, but probably not very well; and the surface sparks would eventually damage the plastic, after which the high voltage would puncture it and you would have to replace it. I often use a short piece of plastic I-beam from the hobby shop as a block in this sort of circumstance; you can see a piece in Figure 25, behind the gap. It is necessary to glue the I-beam (or whatever you use — a cable tie or a piece cut from one could also work) to the dielectric with something that is a good insulator at high voltages, because otherwise the sparks will just sneak under it. I use “corona dope” (there are several versions, any of which should work).
    
    

    ---

    
    

    Construction:

    3A. Assembly

    Here are some stages in the assembly process:

    
    
           
    
    
           
    
    

    
    
    
    
    Figures 10-12: Assembly

    (Note: this sequence shows wider plates, a different dielectric, and a more advanced version of the spark gap than you see in Figure 13. I hope I don’t have to keep saying that there are lots of good ways to build these devices.)

    
    
    
    
    
    
    Figure 13: Overview of an early version

    (At a later point I replaced the inductor shown here with the combination of inductor and resistors that you can see in Figure 8 and in the photo at the top of the page.)

    Note: I connect one side of the power supply to the baseplane, and the other side to the capacitor plate that doesn’t have the spark gap on it; but the laser should work with the connection on either top plate, as long as both capacitors get charged. You can test to see whether one way works better with your laser than the other. You should also test to see which polarity works better. I usually find that I get better operation if I connect the positive output of the power supply to the top plates of the capacitors and the negative output to the baseplane, but your laser may be different.

    Here is a picture of the output (the small bright spot), causing a piece of white paper to fluoresce. (The large diffuse area is light from the spark gap.)

    
    
    
    
    
    
    Figure 14: Output from the simple version of the laser

    A verbal description of the process of putting one of these together is likely to be somewhat confusing, so here is a video to accompany and supplement the photos above. In this video I build a slightly later and more advanced version of the laser. (I had just taken it apart, which is why the voiceover begins the way it does.)

    (Video 2a)

    (Note that the spark gap design you see in this video and the next one is different from the ones I show in the photos above. Any of these designs will work.)

    Because all of the pieces were already shaped and ready, because I had just disassembled the laser, and because I have a fair amount of experience, it took me only 3 minutes and 30 seconds to get it running reasonably well.

    Here
    
    (Video 2b)
    is a video in which I assemble and operate a more recent version. [Note, I did not compress this video, and the filesize is about 40 MB.]

      –> CAUTION <–

    Although these videos can give you a fair sense of how to assemble a simple laser of this type, they are essentially guaranteed to give you an exaggerated notion of how easy it is to get one of these machines correctly adjusted. For one thing, lasers of nearly any sort almost never work when you first put them together. I had already built each of these several times, though, so I had a fair sense of some key parameters — for example, the best distance between the electrodes, which is about two millimeters or a bit less in this voltage range. In addition, I have to confess that I have, on more than one occasion (and as recently as a day before I wrote this paragraph), spent several hours at a time trying to adjust a TEA nitrogen laser so it would work correctly. You need to be patient with the laser, and you need to be especially patient with yourself.

    You also need to remember safety precautions when you are tweaking, as there is often high voltage on parts of the laser after you turn off the power supply. Always turn off the power and short out the HV before you touch any part of the machine.

    
    
    
    
    

    ---

    
    

    Signs and Symptoms:

    Working and Nonworking TEA Nitrogen Lasers

    First, here is a view of one section of the channel in normal operation. The tiny white dots on the surface of the cathode are expectable. They probably indicate that the channel is being driven strongly, and they may help you adjust the laser for good output, but they are not necessary for lasing.

    
    
    
    
    
    
    Figure 15: Normal discharge in the channel

    Notice that the discharge is not very bright. This is normal. In fact, it is common to obtain good laser output from a discharge that appears quite dim to the eye. (Remember, we are looking for strong UV emission here, and that does not necessarily correlate with strong visible emission.)

    Note, added later: As I continue to work with these lasers and to refine my designs, I get to adjust and observe them quite a bit. In the process I begin to suspect that I get best output from a discharge that is just on the edge of sparking, or is showing occasional bright sparks. My initial guess is that although more preionization produces a smoother and cleaner discharge, it also takes more of the energy that is stored in the capacitors. This decreases the amount that is available to drive the laser channel.

    Here are some conditions that typically do not result in lasing:

    1. Electrodes too close together. [You will see lots of thick bright sparks between them.] Sometimes only one end is too close, as in this photo:
       
       
       
       
       
       
       Figure 16: Electrodes too close together

    
    
    

    2. Electrodes slightly too far apart. [This is likely to produce a nice-looking discharge, but with little or no output from the laser. ( Video 3 shows what this looks like.)]

    
    
    

    3. Electrodes much too far apart. [You will see a few thin bright sparks between them. Occasionally only one end is separated far enough for this problem to show up.]
       
       
       
       
       
       
       Figure 17: Electrodes much too far apart

    
    
    

    4. Every time the spark gap fires, there are bright sparks in the channel. If these occur even when you first turn the power supply on, before the laser has a chance to fire, that’s a key to this issue. [Do you have a connection between the capacitors? If not, then the channel is the only way the high voltage can get from one capacitor to the other. If you do have a charging path in place, is it working? If you can verify that it is intact, then something very strange is going on; please email me.]

       If you do not get bright sparks when you first turn on the power, but you do get them every time the laser fires, the real question is whether they interfere with lasing. Sometimes they do, but not always; and it is possible to get significant output even with some bright sparks, as you can see in these photos:

       
       
              
       
       
       
       
       
       
       Figures 18 & 19: Lasing with sparks in the channel

       If your laser makes a lot of these sparks, you should readjust the channel so the electrode surfaces are not eroded by them. That readjustment may also get you an improvement in performance. As I’ve mentioned, though, the presence of occasional bright sparks during some firings is usually just fine.

    
    
    

    5. The spark gap won’t fire unless the distance between its electrodes is really small, less than perhaps 3 mm. [Possibly a punctured dielectric, though that usually results in a complete inability to get the spark gap to fire; see below. It is somewhat more likely that you have excessive corona loss from sharp edges, and your power supply just doesn’t have enough “oomph” to overcome the losses and charge the capacitors to a high enough voltage. You may be able to check by turning out all of the lights in the room and looking at the laser. If you see a dim violet spray of light going out across the dielectric, check to see whether there is a sharp edge or corner at that location. (Smoothing the edges of the top capacitor plates and making sure that all sharp corners have been rounded will also help your dielectric last longer.) If you don’t see an excessive amount of corona discharge, is your power supply working properly?]

    
    
    

    6. The laser stops running, even though the power supply is on. (Version 1) [If your spark gap is not stable, and the distance between its electrodes in it becomes too large, the power supply won’t be able to charge up the capacitors enough to cause the gap to fire. Turn off the power supply, and be extremely careful to discharge the laser before you mess with it!]

    
    
    

    7. The laser stops running, even though the power supply is on. (Version 2) [Punctured dielectric; the laser stops discharging, even though the spark gap spacing has not changed, and occasionally there may be a rapid ticking sound, not particularly loud. It is a good idea to turn off the power supply as soon as possible, so you don’t damage it. Again, be extremely careful to discharge the laser before you disassemble it to check the dielectric for tiny holes.]

    If you encounter problems that I don’t list here, you may want to send me an email message. I can’t guarantee to be able to help, but at least there’s a chance.

    
    
    
    

    ---

    
    

    Continuing Progress:

    Improved Performance and Advanced Versions

    There are various changes you can make, to get better performance from your laser[s]. Some of these involve fairly extensive rebuilding, some don’t.

    First: separate electrodes

    As you can see in Videos 1 and 2, I added separate electrodes to the laser. This helps in several ways. First, moving the discharge away from contact with the surface should cause it to take longer to damage the dielectric. Second, the edges of the dielectric aren’t always very flat, and sometimes the bumps or ripples obstruct or interfere with the beam. Raising the discharge up, even a little, usually eliminates this problem. Third, having separate electrodes makes it easier to adjust the channel width, because the electrodes are not held to the dielectric by electrostatic attraction, the way the capacitor plates are. Fourth, having separate electrodes makes it easy to test various thicknesses and different edge profiles. (See the addendum about this, below.)

    About electrodes: many people who build TEA nitrogen lasers use cylindrical electrodes, but my experience has been that round bars want to roll out of position at the most inconvenient times, so when I built this laser I chose a different path. I acquired some brass strips at the hobby shop instead. They turned out to work quite well. Jarrod Kinsey uses steel rods as electrodes, but he bends the ends, as you can see in the photo of one of his lasers that I link to above (where I discuss the spark gap), partly to prevent them from rolling.

    Addendum: Electrode profile

    I can’t tell you how your laser will behave, but I can tell you that when I smoothed and rounded the edges of the electrodes I was using in mine I got a significant improvement in performance, even though the discharge didn’t actually look much different to the eye.

    Addendum: Angled or wedged electrode spacing

    If you’ve built and operated one of these lasers, you will have observed that there is almost always more output at one end of the laser than at the other. You can adjust the electrodes to optimize either end, but (at least on my lasers) one end is usually easier to optimize than the other. (Which end this is can change as you tweak things, though, so you need to be alert.) It is usually possible to adjust the channel so that it is slightly wider at one end of the laser, and you can sometimes get most or all of the output to come from that end.

    I should note, however, that my best current designs have not been behaving that way. If I attempt to minimize the output from one end I soon begin to get less output from the other, and in fact I seem to obtain best output [at whichever end of the laser is better at the moment] when the output from the other end is only a little below its best level.

    Second: Preionization

    If you have already built and tweaked the initial version of this laser, you know that it is difficult to position the edges of the channel to produce a clean and even discharge. If the electrodes are too close together or too far apart (or if at least one of them is not straight), you get arcs and sparks. Finding the “sweet spot” where the spacing is optimal is not easy, and can take quite a bit of time and a lot of fussing.

    One thing you can do to make adjustment slightly easier (and improve the performance at the same time) is to create a much smaller, separate discharge that starts before the main discharge and fills the laser channel with ions and UV light. (Hence the name, “preionization”.) Preionization is absolutely crucial for high performance.

    If you have separate electrodes, the simplest way to accomplish this is by sharpening the edge of one of the capacitor plates. The best shape is not necessarily obvious, and the positioning of the preionizer is also important, so you are in for some fussing and fiddling.

    After some thought and experimentation, I added separate preionizers between the capacitor plates and the electrodes. (Figures 22 and 23 show this configuration.) Although this makes more things you need to adjust, so it takes quite a while if you really want to optimize the laser’s performance, I think it may be worth the trouble. On the other hand, since I first wrote this posting I have had very good results using the capacitor plates as preionizers; they are ideally positioned to create a surface discharge on the dielectric.

    I am using a shape that is actually similar to the edge profile that Alfonso Torres Rodríguez uses for the electrodes of his high-performance TEA lasers. I create the shape by rough-sanding the upper edge of one of the capacitor plates at about a 45° angle, but not all the way down — I leave a little bit of the original edge. It is difficult to photograph, but
    here is Alfon’s image of his electrode profile (red circle, with diagram below it),
    
    
             
    
    

    
    
    
    
    Figures 20 & 21: Edge profile of my preionizer

    [Remember that because I am using this for preionization, not as a channel electrode, it is positioned with the narrow edge down, close to the dielectric.]

    It is surprising how rough and informal the profile can be and still work reasonably well, though a smoother and more even profile will give you better performance.

    You will have to tweak the various spacings to find what works best. I must caution you (again) that this process is usually quite slow, and can sometimes be tedious. I often find that when I am attempting to tweak at one end, the performance at the other end changes even more.

    In the best-performing versions of the laser, btw, I have preionizer profiles on both sides of the channel, and I find that the spacing between them seems to need to be slightly larger than if I use one plain edge and one shaped edge. (When the capacitor plates are also serving as preionizers, the optimal spacing seems to be about twice the channel spacing, or possibly even slightly more than that.)

    Here are two photos taken during assembly. In the first, the preionizers are in place, in preliminary locations; they may end up somewhat closer together after I finish tweaking for best output. In the second I have added the electrodes, also in preliminary locations. (The next step is to add weights to hold everything firmly in place and ensure good electrical conduction. In addition to the half-brick you see in these two photos, weights of several sorts are visible in several places, including the photo at the top of the page and Figure 13.)

    
    
         
    
    
    
    
    
    Figures 22 & 23: Preliminary positioning of preionizers and electrodes

    Third: the spark gap

    The original gap design (see Figures 9 and 13) worked, but I wanted something that was physically more stable and that I hoped would switch faster, so I changed to the design that I show in Video 1. Here is a photo:

    
    
    
    
    
    
    Figure 24: A faster and more stable spark gap design

    (The spacer you can see in this photo did not work well, and is obsolete. I tried various alternatives, but eventually I changed over to the version of the spark gap that is shown in Figure 29, which is stabilized by its components. See the text for details.)

    The spark gap spacing determines the voltage at which the laser will fire. If the electrodes of the gap are too far apart, the power supply will be unable to deliver enough voltage to cause the gap to conduct. If they are too close together, the gap will fire very often, and typically the lasing (if you get any) will be weak. If nothing other than the piece of brass shim stock holds the upper electrode in place, it will bounce up and down when you run the laser. This can cause peculiar variations in the firing rate, but at least in some cases it can actually stabilize operation.

    Important: remember to use an insulated tool when you adjust the spacing of the gap, unless the power supply is off and you have shorted out the HV.

    Addendum: a “start” capacitor

    There is another thing you can do to the spark gap that will improve the operation of the laser. It turns out that an additional capacitor, connected directly across the gap and positioned close to it, causes it to form a better conduction channel and to form the channel more quickly. This has two results: the laser works better in general, and the pulse-to-pulse uniformity is much improved. You can see this capacitor in Figure 25, below; it is the small brown cylinder, just to the left of the gap. (It should be as close to the spark gap as is practicable.)

    Because the capacitors that comprise the laser are not very large themselves (perhaps 1500 to 5000 picofarads each, depending on construction details), the “starting capacitor” has to be quite small; I am currently using a 25-picofarad “doorknob” capacitor that I had in my junkbox, but there should certainly be other ways to accomplish this. The precise value is not very important, as long as the value is small compared with the capacitances of the laser. You do need to be sure, btw, that it will handle whatever voltage your power supply can deliver. Here is a photo, showing the gap with a start cap in place:

    
    
    
    Figure 25: Revised spark gap design

    Here, in case anyone is interested, is an oscilloscope trace:

    
    
    
    
    
    
    Figure 26: Scope trace, showing the laser pulse and the light from the spark gap

    (My apologies for the electrical noise that is superimposed on the trace; it is difficult to shield the detector and the oscilloscope from the nasty EMP that the laser generates when it fires. The noise, btw, looks very much the same from trace to trace. I guess that the way it is generated and the way the scope responds to it don’t change much.)

    In this trace it takes about 24 or 25 nanoseconds for the electrical pulse to reach peak power. (In some others it seems to be as fast as about 16 or 18 nsec.) The laser pulse occurred at about 18 nsec, but the light from the laser had to travel about two feet farther than the light from the spark gap (it went out to the side, to a mirror that reflected it into the detector), so it reached the detector about 2 nsec later than it would have if the pathlengths had been the same. I have put a mark on the trace where it should be. (I find it interesting that even with a simple DIY laser we are in a regime where this is an issue. Making comparative measurements involving really short laser pulses must be a nightmare.)

    It would be best for the laser to reach threshold when the electrical power is just reaching its peak, but at least it’s fairly close. I also suspect that it’s better to be on the upslope than on the downslope. Either way, the laser pulse is only about 1 nanosecond long, the electrical pulse is far longer, and it is therefore clear that most of the energy stored in the capacitors is wasted. This is not (and cannot be) an efficient laser.

    Special Note:

    If you are really interested, you can redesign the spark gap so that it is externally triggered. That will give you far more control over when and how often the laser pulses. It is also likely to improve the performance, partly because it allows the power supply to charge the laser up until you trigger the gap, rather than when the gap fires on its own, and the capacitors store more energy at the higher voltage.

    I must, however, note that a triggered spark gap without a trigger pulse is an untriggered spark gap. I am, at least initially, using a commercial trigger unit that we bought on eBay, because I don’t have time to design and build one myself right now. (If I do end up building one for this project, though, I will post the design here. I will note that a triggered gap of this type tends to require a that doesn’t have to have much energy in it, but it does need to be fairly fast — the risetime should be less than 1 microsecond. The pulse from an automotive spark coil is much too slow, though there may be ways to finesse that.)

    To construct the new upper electrode for the gap, I took a short brass 1/4-20 bolt and drilled a hole down the middle of it, or as nearly down the middle as I could. I then drilled out the hole with a slightly larger bit, so that I could easily fit a piece of glass capillary tubing down it. I am using melting-point capillaries that we acquired on eBay for a different project. The brand is not crucial, and in fact there are even several types of capillary tube that will work, not just the ones for determining melting points. The ones I’m using came in a container that looks like this:

    
    
    
    
    
    
    Figure 27: Capillary tubes

    One end of each tube is closed, but I already had some that I had shortened by cutting off the closed end. (You need to be quite careful when you do that, as these are thin-walled and very delicate. It’s a good idea to wear gloves. I moistened the outside of the tube, scored the wall lightly, and just pulled. I cut several of them this way, and most of them broke cleanly.) I also drilled a smaller hole, one that the capillary tubing would just fit into, through an acorn nut, as close to the peak as I could get it. Here is the preassembled head, but without the trigger electrode:

    
    
    
    
    
    
    Figure 28: Triggered spark gap, head end

    NOTE: unless the holes line up very nicely, it is important to attach the tubing to the bolt with a flexible glue. (Yes, I found this out the hard way, by using CA glue at first, and had to remove the remains of the tubing from the bolt after I tried to put the acorn nut on. As you can see in Figures 29 and 30, aquarium caulk is your friend here.) In case it isn’t obvious, you do not want to glue the tubing to both the bolt and the acorn nut, as that would make it difficult or impossible to adjust. You could glue it to the acorn nut instead of the bolt, but the bolt has more contact area and is not going to be exposed to hot plasma. For this reason I didn’t try plastic tubing, though that might also work. Jarrod Kinsey has suggested the thin tubes that often come with (for example) aerosol cans of lubricating oil as a possibility here.

    I am using a piece of broken jeweler’s saw blade as a trigger electrode. It was handy, and it is fairly hard steel, so I hope it will last a little while. (This is really only the second time I have built a triggered gap, and the first one was much different, so I do not yet know whether the saw blade was a good idea. I’ve put over a hundred shots on the laser with the new gap in place, though, and it seems to be fine.)

    Here is a photo:

    
    
    
    
    
    
    Figure 29: Triggered spark gap

    As seems to be usual with this style of gap (if I recall correctly, it is called a “trigatron”), I have the positive output of the trigger-pulse generator connected to the trigger electrode and the negative output connected to the acorn nut. I also have the positive terminal of the power supply connected to the upper plates of the capacitors. Initially, I was using only one “starting capacitor”, as you can see in some of the previous photos, but later I added a second one.

    (Parenthetical note: I tried to get an oscilloscope trace of the light from this gap, but the electrical noise was so bad that I was unable to do so.)

    I eventually changed the gap design again. I am now using a carriage bolt as the top side, just as I do for the free-running versions of the gap; I drill the hole starting at the head end of the bolt, so that it is centered where that’s important.

    Fourth: the dielectric and the capacitor plates

    I was able to get some acetate sheet (which you can see in Figures 10-12 and 22-23) from a vendor on eBay. Acetate has higher dielectric constant than most of the other plastics I’ve been using, which means that at any given voltage, a capacitor made with acetate will store more energy than a capacitor made with, for example, styrene, provided the sheets are of the same thickness. The capacitance is higher if the dielectric is thinner, and the acetate sheet that I acquired is only about 6.5 mils thick, which gives it another advantage over the 10-mil styrene sheet I was using earlier.

    Because this sheet is so thin, however, it can’t handle as much voltage as a thicker sheet, and I have to adjust the spark gap so it fires about 2 or 3 times a second. At one point I allowed the time between pulses to get too long, which let the voltage on the capacitors rise too high; a puncture promptly developed in the sheet I was using, and the laser stopped working until I swapped it out and tightened the spark gap spacing. (Even 2 pulses per second doesn’t really seem to be enough; the voltage has punctured two more pieces of acetate sheet since I first wrote this paragraph, one of them twice. Fortunately the first hole was very close to a corner, and I was able to move the capacitor plates slightly to avoid it. [I put a droplet of corona dope over the hole to prevent sparks from going through it.]

    In addition to finding a thinner dielectric with higher dielectric constant, for this version of the laser I changed to capacitor plates that are 4″ x 10″, so they have more area. (You can see these plates most easily in Figure 11, but they are also visible in Figures 12, 22, and 23.) This also increases the capacitance, and thus the amount of energy the capacitor stores at any given voltage. (With these plates, though, I have to use separate preionizers because the electrodes are 12″ long.) The capacitance, btw, calculates at just over 5.5 nf; but when I measured it I found that the actual value was only about 4.2 nf. I am not entirely sure what is responsible for the difference, though it is entirely possible there is still a small amount of air trapped between the plates and the dielectric.

    When I built the triggered gap I returned to 10-mil styrene and 2″ x 12″ plates, because I wanted to be able to pause between pulses. With the thin acetate sheet, that would not have been possible.

    
    

    ---

    
    

    Alternative Drive Circuits
    

    All of the lasers on this page use a circuit topology that is probably best described as a simple voltage-doubling circuit, even though it doesn’t actually double the voltage. (There doesn’t seem to be a better term for it, at least not yet. Many people call it a “Blumlein Circuit”, but that’s entirely incorrect; a Blumlein Circuit involves matched transmission lines and a matched load.) There is a different topology that you can also build, a Charge-Transfer Circuit.

    In the simple voltage doubler you charge up both capacitors and then short-circuit one of them to ground (or, in the machines I describe here, to the opposite power supply polarity). For reasons that are described elsewhere, this causes a large voltage to develop across the laser channel, which is between the two capacitors, very rapidly. The flow of current from one cap to the other, across the channel, drives the laser.

    In the CT circuit, on the other hand, you charge up one capacitor, which serves at the main energy storage point for the laser. You then discharge it into a second, smaller capacitor through a high-voltage switch (typically a spark gap) that is between them. The smaller capacitor starts the laser channel conducting, and then both capacitors drive the laser.

    Each of these circuit topologies has advantages and disadvantages. The voltage double circuit is efficient, symmetrical (it has been found, both theoretically and experimentally, that it provides best performance when the two capacitors have the same value), and easy to construct. The CT circuit is not symmetrical, is usually less efficient, and is often somewhat less easy to build; but at its best it can perform quite well. (See the link to the nitrogen lasers built by Alfonso Torres Rodríguez, below.)

    The main store of a CT circuit (often referred to as the “dumper” capacitor) can have relatively large value, and it does not have to be as fast as the capacitors in the voltage doubler circuit, both of which must drive the channel directly.

    One other advantage of the CT design is that you can eliminate the charging inductor or resistor-inductor combination. You are only charging one capacitor, so you don’t need or even want a connection to the other one. (In principle, you could put a connection between the peaker capacitor and the ground plane, to discharge the peaker between pulses; but for some reason this seems to interfere with performance, and it is best omitted.)

    Here are photos that show the assembly of a straightforward CT design that is closely related to the voltage doubler designs I’ve presented above. The dumper capacitor is a 6″ x 16″ piece that I cut from a brass kickplate. The peaker capacitor is a brass strip, 2″ x 12″. This laser uses separate electrodes, which raises the channel up from the surface of the dielectric and allows the top plate of the peaker to be used as a preionizer. The spacing between the top plate of the peaker and the other brass strip is not optimized in these photos. Note: on the side of the channel that is connected to the baseplane, there is no dielectric. The brass strip on this side, which supports the electrode, should be enough thicker than the top plate of the peaker cap to compensate. (With 10-mil styrene as the dielectric, I used a 16-mil strip as the peaker and a 25-mil strip on the low side. When I changed over to 5-mil Dura-Lar™ [a polyester film that is similar to Mylar], I used a 25-mil strip as the peaker, and a 32-mil strip on the low side. I use the same combination when I have two of the 5-mil sheets under the dumper, because I am still using only a single sheet under the
    peaker.)

    I used a thick glass plate as a base, partly because there is enough weight on the laser when it is assembled that if I didn’t provide a stiffener it could flex the top of the table, depending on where I position it. That would put a curve in the channel and prevent the laser from working properly.

    
    
    
    
    Glass plate in position
    
    
    
    
    
    
    
    Baseplane in position on the glass
    
    
    
    
    
    
    
    5-mil polyester sheet (the dielectric) on the baseplane
    (See remark, just after Figure 40.)
    
    
    
    
    
    
    
    Upper plate of the dumper cap in place
    
    
    
    
    
    
    
    Upper plate of the peaker cap in place
    
    
    
    
    
    
    
    Ground connection in place
    
    
    
    
    
    
    
    Upper side of spark gap, shown in position
    
    
    
    
    
    
    
    Channel electrodes in place
    
    
    
    
    
    
    
    Weights in place
    
    
    
    
    
    
    
    The laser, in operation
    
    
    
    
    Figures 30-39: Assembling a Charge-Transfer version of the laser

    Here is a view of the channel, with two sheets of 5-mil Dura-Lar as the dielectric for the dumper and a single sheet as the dielectric for the peaker. The dumper for this version of the laser was a piece of brass shim stock, 6″ wide and about 28″ long. (The baseplane for this version of the laser, which I built in February of 2012, is a piece of 12″-wide brass shim stock, also about 28″ long.) I measured the dumper capacitance as just about 10 nf and the peaker capacitance as 2.9 nf, but I don’t know how accurate my meter is. The channel was adjusted extremely well, and the unfocused beam easily drove a cuvette of “Optic Whitener” that was at least 8″ away from the end of the channel, with the laser pulsing a bit faster than 1 Hz. There are occasional bright sparks, but very few, and they do not appear every time the laser fires. The tiny white sparks (which Jarrod Kinsey calls “icicle sparks”) do, on the other hand, appear every time. They are on the cathode (negative) side. There is a dim violet glow in the channel when the laser fires, but it can be hard to see, and the camera doesn’t seem to pick it up very well. (Remember, we are looking to get UV out of the laser, not visible light, so a discharge that appears dim to the eye is not necessarily underpowered!)

    
    

    
    
    
    
    Figure 40: Channel of the Charge-Transfer version

    With 5-mil polyester film as the dielectric for both capacitors the channel needs to be wider for best operation, and it has more and longer “icicle sparks”. It may also require more preionization. Unfortunately, unless the laser is pulsing several times a second, a single 5-mil film used as the dielectric for the dumper typically lasts only a few hundred to a few thousand pulses, so I am currently using two sheets.

    
    Here is another Charge-Transfer design.
    It uses a small Marx bank as its “dumper” capacitor and provides even better performance, but at the expense of added complexity.

    
    
    

    ---

    
    

    Relevant Milestones
    

    Here are some reference points you can use if you want to.

    1. You have read (and at least mostly understood) this Web page, and possibly others that deal with this subject.

    
    
    

    2. You have acquired all of the necessary parts, and you have a working power supply.

    
    
    

    3. You have built a device; it may or may not be a laser yet.

    
    
    

    4. At least some of the time, you get laser light out of one or both ends of the machine. (This is, obviously, crucial. If you get this far you are owed congratulations, especially if this is your first laser.)

    
    
    

    5. You get laser light almost every time the laser pulses, and there are relatively few bright sparks in the channel. (Another way of putting this is that you are now learning how to adjust the laser for good operation.)

    
    
    

    6. If you have some fluorescent material that is suitable for use in a dye laser (for example,
       “Optic Whitener” from Dharma Trading Co.,
       Noodlers’ “Blue Ghost” invisible fountain pen ink,
       or even ink that you’ve extracted from a highlighter marker), the focused beam from the nitrogen laser is powerful enough to drive it. At this stage you may need to fill the channel with nitrogen, but as you continue to improve the laser it will eventually reach the point at which it will drive a small dye laser even if the channel is filled with air.

    
    
    

    7. Your laser has separate electrodes, and uses its capacitor plates as preionizers. [This could easily happen before the preceding item; it depends largely on you.]

    
    
    

    8. The unfocused output will drive a dye laser, but only if the channel is filled with nitrogen.

    
    
    

    9. The unfocused output will drive a dye laser even if the channel is filled with air.

    
    
    

    10. The unfocused output will drive a dye laser even if the channel is filled with air and the cuvette of dye is more than a foot away from the nitrogen laser. [As of 11 May, 2011 my laser has not yet reached this level of performance. It may be necessary to change power supplies, and rebuild the laser to use higher voltages.]

    At some point, you probably start coming up with your own ideas for improvements, or for entire designs. This isn’t something that can be placed in the sequence, so I am not giving it a number.

    
    In closing:
    

    I encourage you to try various ways to improve the performance of your laser or lasers, and I strongly suggest that you keep a comprehensive notebook that includes photos, and you should take photos not only of things that worked but also of mistakes, and of things that didn’t do what you wanted them to or thought they would. It can save you from much headpounding, and from having to learn things repeatedly.

    
    
    
    

    ---

    
    

    Addendum:
    

    Dye Lasers as Indicators of Performance
    

    One of the most common uses for the nitrogen laser is as a pump source for organic dye lasers. As I mentioned above, in the listing of milestones, this can serve as a rough indication of the output power.

    The total energy stored in the original version of the laser (10 mil styrene and 2″ x 12″ plates) was probably close to 200 millijoules. That version could drive a small dye laser if I used a cylindrical lens to focus the beam onto the front of the dye cuvette. Some of the later versions can drive a small dye laser even if I don’t focus the beam. (I’ve mentioned this as one of the progress steps, above.) The photo on the left, taken from in front of the cuvette, shows this with nitrogen in the channel. After I got that version adjusted a bit better I was able to get enough output even with air in the channel. (Photo on the right, taken from behind the cuvette.)

    
               
    
    
    
    
    
    Figures 41 & 42: Unfocused output driving a small dye laser

    (The brighter spot is the dye solution using the walls of the cuvette as mirrors. The tall diffuse stripe is lasing without feedback, often referred to as ASE [“Amplified Spontaneous Emission”].)

    Although I used a commercial dye and a fused silica cuvette for these two photos,
    Video 4 shows a dye cell I built out of microscope slides, glued together with silicone aquarium caulk. In this video I use the focused output of the nitrogen laser to drive three different commercial fountain pen inks. (I have actually lased five fountain pen inks so far.) I’ve already mentioned “Optic Whitener”, which is another excellent laser dye for the DIYer, and you can even get fluorescent dyes from some highlighter markers, though they are not always optimal for nitrogen laser pumping. (If you use isopropyl alcohol to extract the dye from a Sharpie “Accent” yellow-green marker like the one that you can see in the photo at the top of the page, you get a solution that seems to work fairly well. It is quite concentrated, and you will need to dilute it, just as I diluted the inks I used in the video.)

    If I focus the output, the nitrogen laser can also drive a piece of commercial fluorescent plastic:

               

    
    
    
    
    Figures 43 & 44: Fluorescent plastic sheet, lasing

    The edges of the sheet are not smooth or glossy, so its output is very diffuse. I decided to clean up one edge, but polishing it would have been difficult, so instead I glued a microscope slide to it, using cyanoacrylate adhesive (“CA”; “superglue”). (In retrospect, I should have used either a different type of CA, or epoxy. The thin glue is difficult to control, and it got on some places where I didn’t want it.)

               
    
    
    
    
    Figures 45 & 46: Improved fluorescent plastic sheet

    This resulted in a slightly improved output pattern:

    
    
    
    
    
    Figure 47: Improved plastic sheet, lasing

    (Taken with air in the channel of the nitrogen laser.)

    One slide provided enough improvement that I decided to add a second one. When I line up the beam of the nitrogen laser so that it is perpendicular to the slides and the dye can use them as mirrors, this works quite well:

               
    
    
    
    Figures 48 & 49: Plastic sheet with 2 slides

    
    
    

    ---

    
    

    Acknowledgements
    

    My designs started as variants of designs developed by Jarrod Kinsey; I have introduced a number of changes, partly for ease of construction, partly because of the materials I can get, partly because of the way I think about the issues involved here, and partly to point up the fact that this is not a “one right way” situation. Just because I do something a certain way does not mean you have to do it the same way if you can’t find the part or material that I use, or even if you just want to do it differently. If you decide to do something differently, though, or if circumstances oblige you to, you will want to think it through before you build it, so you’ll have some idea of how your version is likely to work, …unless you are willing to spend some time and effort on what could end up being a lengthy trial-and-error method. [I am hoping to provide some background information on a separate page, to help you understand how these lasers work and what the important parameters are. I also provide links to pages that other people have written about their nitrogen lasers, for comparison.]

    Jarrod’s designs, in turn, are based partly on designs and suggestions from other DIYers including myself (there are no one-way streets here!) and Milan Karakas; partly on his own experiments; and, though not as directly, partly on articles that have been published in the scientific literature.

    Just as I made a number of cogent suggestions to Jarrod as he developed his designs, he has made a number of cogent suggestions to me as I developed the designs on this page, and I am indebted to him for his help on this project. I am likewise indebted to Milan Karakas, who asked several key questions and provided crucially important advice relating to the performance of the lasers, particularly with regard to preionization and switching.

    The preionization method that I use in the improved versions of the laser was inspired by a feature that Alfonso Torres Rodríguez uses in his TEA lasers. My spark gap design was partly inspired by two high-speed spark gap designs that I encountered in the scientific literature, and partly by what I could get at the hardware store. The power supply is my own idea.

    I must also acknowledge my indebtedness to Ernest E. Bergmann, who developed some of the earliest room-pressure nitrogen lasers, and whose papers have been extremely helpful to me; to Professor Mark Csele, who has done superb work with a number of DIY-feasible lasers and has published fine Web pages about them; and to Sam Goldwasser, for his amazing Laser FAQ. (Links, below.) If I have left anyone out, I hope they will forgive me…

    
    

    ---

    
    

    Links

    ===============================================

    There are various DIY laser projects on the Web, including several TEA nitrogen lasers. No two of these are alike, which underscores the fact that there are lots of ways to think about the requirements and parameters of these lasers.

    • Jarrod Kinsey has pointed me to a fine page about homebrew TEA nitrogen lasers, written by Nyle Steiner. His design is perhaps even easier to construct than the ones on this page.

    
    
    

    • Thomas Rapp, in Germany, has an excellent pageset about the lasers he has built. To get to the TEA nitrogen lasers, go to the list on the left side of the page, click “Stickstofflaser”, then click “TEA Laser”. (The other lasers are just as interesting.) [If you don’t read German you can use one of the translation utilities on the Web, but be prepared to do a little work. As of 2011, translation on the Web is not well adapted to technical information about lasers.]

    
    
    

    • Hubert Pissavin has a TEA nitrogen laser that uses air as its lasant. His project is a good one: easy to build, and not expensive.

    
    
    

    • Renato Salles has a page about his nitrogen laser.

    
    
    

    • The TEA nitrogen lasers of Alfonso Torres Rodríguez are more complex and considerably more difficult to build than the ones I present here, and they operate at higher voltages. Their performance, however, is admirable — even with air in the channel, the unfocused beam from any of Alfon’s best machines can drive a dye laser at a distance of more than 50 cm. [If you don’t read Spanish you can use one of the translation utilities on the Web, but (as with Thomas Rapp’s pages in German) you need to be prepared to do a little work.]

      Alfon also has a number of YouTube videos showing some of his lasers in operation.

    
    

    • Professor Mark Csele has written
      a page about TEA nitrogen lasers with much information and with various references.

    
    
    

    • Peter Terren has many interesting pages,
      including one about the nitrogen lasers that he has built.

    [Although it is somewhat peripheral, you may want to read
    my rant about the explanation that is in the Scientific American “Amateur Scientist” nitrogen laser project.
    —There are some potentially useful references at the end.]

    ===============================================

    Sam’s Laser FAQ is an incredible resource, covering a tremendous variety of laser types. My one caveat is that in addition to a lot of important information there are many opinions, some of which are more credible than others. When you see descriptions of a laser or a project, or specifications of a commercial device, they are generally quite reliable; when you see someone stating that dye lasers are not worth the trouble (because he barely managed to threshold Rhodamine 6G with 200 joules into his flashlamp), that’s not necessarily as reliable. (It is entirely possible to threshold some dyes
    with just a few joules into a flashlamp,
    but it is extremely difficult to threshold R6G or any other dye if you drive your flashlamp with capacitors that are not well adapted to fast-pulse service. This clearly includes photoflash capacitors, though they are admirably suited to slow pulse service.)

    ===============================================

    Here is a page on which I hope to provide additional technical information about TEA nitrogen lasers, the principles on which they work, circuit topologies, and so on. As of mid-May, 2011 I am just starting to work on it, so please bear with me if you find it mostly empty. (You can always email me with questions.)

    
    

    ---

    
    
    This work is supported by
    
    the Joss Research Institute
    19 Main St.
    Laurel MD 20707-4303 USA

    
    
    

    ---

    
    

    Contact Information:
    

    Email: a@b.com, where you can replace a with my first name (jon, only 3 letters, no “h”) and b with joss.

    Phone: +1 240 604 4495.

    
    Last modified: Sat Dec 17 11:46:14 EST 2011
    

    ]]> http://jossresearch.org/2011/05/15/joss-institute-projects-a-my-first-laser-project-for-the-diyer/feed/ 0 http://jossresearch.org/2011/05/10/joss-research-institute-web-report-5-part-1/ http://jossresearch.org/2011/05/10/joss-research-institute-web-report-5-part-1/#comments Tue, 10 May 2011 03:09:13 +0000 Jon Singer http://localhost:8888/jossresearch/?p=43
    

    TJIIRRS: Number 5 of an Ongoing Series;

    Nitrogen Laser Considerations for the DIYer,
    With a View Toward the Design and Construction
    of a High-Performance DIY Laser

    Part 1 of a Multipart Report (see Links)

    (16 September, 2009: this is a 2006 rewrite and continuing revision of a page I originally wrote in July of 2005.)

    I have been rereading some papers on nitrogen and excimer lasers, and rethinking my understanding of the important characteristics of a nitrogen laser. Some of the issues are simple and some are fairly obvious, but some are not so easy to understand. This page attempts to examine and clarify issues pertinent to DIY high-performance nitrogen lasers, and to take a look at what you need to know in order to build one. Follow-on pages examine specific designs and the performance that you can expect to achieve with them.

    If you want a high-performance nitrogen laser and you can’t afford to buy one, it is certainly possible to construct one; but even if you start with a good design, there is only a modest chance that you will obtain the specified performance level on your initial attempt. The nitrogen laser is not a high-tech device, but

    » Read the rest
    ]]>
    

    TJIIRRS: Number 5 of an Ongoing Series;

    Nitrogen Laser Considerations for the DIYer,
    With a View Toward the Design and Construction
    of a High-Performance DIY Laser

    Part 1 of a Multipart Report (see Links)

    (16 September, 2009: this is a 2006 rewrite and continuing revision of a page I originally wrote in July of 2005.)

    I have been rereading some papers on nitrogen and excimer lasers, and rethinking my understanding of the important characteristics of a nitrogen laser. Some of the issues are simple and some are fairly obvious, but some are not so easy to understand. This page attempts to examine and clarify issues pertinent to DIY high-performance nitrogen lasers, and to take a look at what you need to know in order to build one. Follow-on pages examine specific designs and the performance that you can expect to achieve with them.

    If you want a high-performance nitrogen laser and you can’t afford to buy one, it is certainly possible to construct one; but even if you start with a good design, there is only a modest chance that you will obtain the specified performance level on your initial attempt. The nitrogen laser is not a high-tech device, but it is sufficiently complex and depends upon the values of enough parameters that there is no substitute for repeated hands-on experience. It is probably a good idea to plan on building your laser three times; if you manage to get it to work well in one or two, you can count yourself either lucky or very accomplished. I will note that over the course of writing this pageset I built four to six lasers (depending on how extensive a rebuild has to be, before you consider it to be a new machine), and I rebuilt almost every one of them at least twice. I needed that experience in addition to what I learned from the articles I read. There is also the plain fact that if you don’t buy one and you don’t build one and you don’t find one and nobody gives you one, you don’t have one.

    
    

    Background and Preliminaries

    The nitrogen laser was discovered (not invented) in 1963, by H. G. Heard. He published his discovery in Nature. (You can find a proper citation by following the “References” link, below.) Nitrogen was, if I recall correctly, the first convenient pulsed UV laser, and is still a DIY favorite because it is straightforward, easy to construct, and doesn’t involve poisonous or corrosive or horrendously expensive gases. I think that a lot more of us would be building excimer lasers if we could afford the xenon (a large tank of xenon costs many thousands of dollars), or if we were too stupid to be afraid of fluorine, nitrogen trifluoride, and chlorine.

    Unfortunately, although nitrogen is cheap and easy to handle, it is not well behaved as a laser. The nitrogen laser has a few undesirable characteristics, the most prominent of which is a problem of lifetimes. When you excite a nitrogen molecule (which we do in our lasers by slamming an electron into it), it can be raised to any of several states. The good news is that it’s easy to get it to go into the particular excited state that forms the upper laser level we want. That state has a lifetime of about 40 nsec at very low pressure, but the lifetime decreases as the pressure goes up, and at 1 atmosphere it is only about 2.5 nsec. (It is important to remember that this is about the partial pressure of nitrogen in the gas mixture; if you have 1 atmosphere total pressure, of which 30 or 40 Torr is nitrogen and the rest is helium, your laser is still a low-pressure nitrogen laser. This turns out to be extremely convenient, and I discuss it below.)

    A short upper-level lifetime doesn’t have to be a problem; with dye lasers, for example, it is essentially a non-issue. (The upper-level lifetime of Rhodamine 6G is a little over 4 nsec; but R6G easily lases for a microsecond or more in flashlamp-pumped lasers, and it can even lase CW with appropriate laser pumping.)

    The lifetime of the lower laser level, however, is significant; it needs to be shorter than the lifetime of the upper laser level. With organic dyes this is not a problem, but with nitrogen it is: the lifetime of the lower laser level is about 1,000 times as long as the lifetime of the upper level. A laser only operates when there is what is called a “population inversion” — when the number of excited centers (atoms, ions, whatever) in the upper laser level is higher than the number in the lower laser level by a wide enough margin to overcome the losses in the device (and remember, the output counts as one of the losses). An excited center in the lower laser level can and will absorb light at the laser wavelength. True, that puts it in the upper laser level, and it can then re-emit the light; but there is no guarantee that will do so as part of lasing, or even in a useful direction. It certainly doesn’t help, and if there are too many centers in the lower level (anywhere near as many as there are in the upper level, for example), lasing is not possible.

    In order to get your nitrogen laser to turn on, you must excite lots of nitrogen molecules into the upper laser level; they fall out of that level, many of them to the lower laser level, most of them (though not all — nothing is 100% efficient) emitting photons in the process, …and then they sit there. Within a very short time (almost always less than 30 nsec for low-pressure nitrogen lasers, after lasing begins) it becomes impossible to sustain a population inversion, and lasing ceases. It doesn’t matter how strong your discharge is, the gas can’t lase any more. You have to stop and wait for many microseconds while the nitrogen molecules in the lower laser level return to the ground state. This limits the repetition rate of the nitrogen laser, probably to a few kHz.

    (The timescales are different for atmospheric pressure nitrogen lasers, which cannot lase for longer than a nanosecond or so, but the principle is essentially the same.)

    In addition, cold nitrogen lases better than hot nitrogen, so it helps if you flow the gas through the laser and allow enough time between pulses for cool gas to get in and replace the gas you’ve just heated by running millions of watts of electricity through it.

    This issue also limits the repetition rate of a nitrogen laser. If you want full peak power, either you have to flow the gas extremely fast (which wastes gas and costs money), or you simply don’t pulse the laser more than 4 or 5 times a second. If you want maximum average power, on the other hand, you can pulse your laser more frequently, possibly as often as a few hundred times a second if your power supply can sustain that and if you can cool the parts of the system that tend to get hot. (At a few joules of input energy per pulse, a rep rate of 100 Hz takes hundreds of watts, most of which comes out as heat.)

    
    

    A Brief Aside About Latency

    Something we don’t often think about is the fact that lasing does not start “instantly” after current begins to flow in the channel. There is always some latency before enough nitrogen has been excited to create a population inversion. (There’s no such thing as an “instant” in physics or electronics in any case.) Here is a diagram, adapted from one I found in a research paper, showing the voltage, current, and laser output curves of a high-performance charge-transfer laser:

    

    (If you click the diagram, you will get a larger version.)

    The latency typically ranges between 10 and 15 nsec; in the example given here, it is about 11 nsec. (Because of the lower level bottleneck problem, it probably can’t be longer than about 30 nsec unless the discharge starts off slowly and then ramps up to maximum power extremely abruptly.) If your driver circuit cannot deliver energy to the discharge rapidly enough, the total energy it delivers doesn’t matter; your laser still won’t reach threshold.

    Moreover, because the lifetime of the upper laser level is 30-40 nsec under actual conditions, which means that the laser pulse can easily be 15 or 20 nsec in duration, the driver circuit must deliver considerable voltage (and power) for at least 30 nsec in order for bottlenecking to get a chance to occur. Small lasers, as I mention elsewhere on this page, “run out of steam” long before that point.

    
    

    A Brief Aside About Wavelengths

    Something very few DIYers are aware of: there are at least FOUR nitrogen laser wavelengths. The “usual” nitrogen laser, operating in the second positive system, emits at 337.1 nm in the UV, often at several closely-spaced wavelengths. It is also reported to emit at 357.6 nm in the UV under some conditions, though very few articles mention this.

    Then there is an ionized molecular nitrogen laser, which puts out blue light at approximately 428 nm. That laser doesn’t typically operate without mirrors, so very few DIYers have seen it, but Mark Csele mentions it on his excellent site — his students sometimes see it by filling an excimer laser with helium and adding a very small amount of nitrogen.

    In addition to these there is an infrared nitrogen laser that operates in the first positive system, and if you measure the output of your laser without absorbing IR, you may get a spurious value. I think the IR laser and the blue laser only operate in an actual resonant cavity, which requires two mirrors; but I am not entirely certain.
    

    ---

    
    
    

    Driving Circuitry

    While it should be possible to pump nitrogen with a magnetic pulse compressor, a technique that is used in some excimer lasers, pulse compressors are difficult to design and build, so DIYers have not (as far as I’m aware) used them. We typically use capacitive discharge, in one of two main circuit designs. These are the “Doubler” circuit (often inaccurately called a “Blumlein”, although it is nothing of the sort), shown in the upper half of the diagram; and the Charge-Transfer circuit, shown in the lower half. The cathode of the laser is shown as an arrowhead, and the anode as an open circle.

    

    For convenience, I have drawn the doubler circuit with the negative terminal of the power supply as the “hot” terminal and the positive terminal at Ground [Earth] potential, but this is not crucial. Two things are crucial, though. The first is that during charging the laser channel needs to be grounded [earthed], so that the high voltage does not simply run through the hose to the vacuum pump. Second, it is important to make sure that the spark gap is operated correctly. (This may vary from gap to gap; I happen to have a commercial gap that is specified to operate as shown in this diagram, with a positive-going trigger pulse on the trigger pin, which is adjacent to the positive electrode.)

    In addition, I have drawn the doubler circuit with a charging inductor; in practice you can use either an inductor or a resistor. It’s a good idea to try both, and use whichever works better in your machine. (It is also a good idea to put both a choke and a resistor in series with the power supply, because the laser generates a hefty voltage spike when you fire it, and you can damage or destroy your power supply if it is not adequately protected. If your laser is large enough, it also generates an EMP [Electro-Magnetic Pulse] when you fire it, and can damage cameras and other equipment that are too close to it.)

    
    

    The Voltage Doubler Circuit: How it Works

    Both capacitors or banks, depending on the design, are charged. (An inductor or resistor permits them to be charged by a single power supply, as is shown in the diagram.) When they are at the desired voltage, the spark gap is triggered.

    It takes some nsec for a conduction channel to develop inside the spark gap; this is assisted by the small capacitor (labelled “Start Cap”) across it, which in the case of the commercial gaps I use needs to be just large enough to produce a current of at least 10 Amperes in the gap, as rapidly as possible; a few hundred pf is typically more than enough. Note the fact that this capacitor needs to be able to handle at least the initial charging voltage. It is a good idea to use one that is rated somewhat higher, as it is likely to last longer.

    As the switch begins to conduct and current starts to flow through it, the combination of the inherent inductance of the circuitry and the capacitance of Bank 1 produces a resonant (“tank”) circuit. If the laser does not fire, the maximum voltage across the channel will approach twice the initial charging voltage as the tank circuit “rings down”. (If you have a fast enough oscilloscope and a safe way to probe the high voltage that is present on the capacitor, you can use this fact to get a sense of the inductance and resonant frequency of that side of your circuit.)

    In normal operation, however, this never happens. At some intermediate voltage the laser channel starts to conduct, and current begins to flow through it. Within some nsec the discharge is well developed, and lasing typically begins about 10 to 15 nsec after the channel starts to conduct. Lasing ceases when the electrons in the discharge no longer have enough energy to excite nitrogen molecules effectively, or when there is enough population in the lower laser level to quench the laser.

    (Most small lasers cannot pump long enough for bottlenecking to occur; they terminate by “running out of steam”, typically in less than 10 nsec. A few fairly high-power nitrogen lasers also appear to run out of steam quickly, but they deliver large amounts of energy to the discharge in that time, which results in large output.)

    For optimum performance, the two capacitors or capacitor banks should have approximately the same value. (This has been tested by several investigators, with fairly uniform results.) They should also have extremely low inductance. In fact, in DIY terms the inductance of the entire circuit should be as low as is practical. The capacitors should be physically as close to the laser channel as possible, for example, and the connections should be broad foils, rather than wires.

    A circuit of this type was published in the Amateur Scientist column in Scientific American in 1970. It was intended for DIY construction, and is a good design that remains popular even today, but the description of its operation was seriously flawed. The author used unreasonable values for parameters like the speed of his spark gap switch, and the resulting explanation did not make sense. Unfortunately, it was unthinkingly accepted and followed by many people, and echoes of it even show up in the scientific literature on nitrogen lasers.

    
    

    Charge Transfer: How it Works

    In a Charge-transfer (“CT”) circuit, only Bank 1 is charged directly by the power supply. When it reaches an appropriate voltage, the spark gap is triggered. Within a short time (some nsec), as the gap begins to conduct, Bank 1 (sometimes referred to as the “Dumper”) begins to discharge into Bank 2, charging it. When the voltage across Bank 2 gets high enough, the laser channel starts to conduct, after which both banks push current through the channel. (Several groups have tested this, and it is clear that Bank 1 contributes to lasing, at least to some extent.)

    Because Bank 2 (sometimes referred to as the “Peaker”) provides the majority of the drive for the laser, it needs to be as fast as possible, with low self-inductance, and needs to be as close to the channel as is practical, with low-inductance connections. These characteristics are less crucial for Bank 1; but because it also contributes to some extent, faster switching and lower circuit inductance, along with careful choice of capacitors (look for those with low ESL [Effective Series Inductance] ratings), will typically give you more output and higher efficiency.

    In a CT circuit the Dumper is generally significantly larger than the Peaker, often by a factor of 2 or 3 and sometimes as much as 4. In some cases it is possible to have a resonant effect here too, which can, in principle, charge Bank 2 to more than the initial charging voltage on Bank 1. However, I wouldn’t count on it.

    
    

    Comparisons

    Voltage-doubling circuits (they don’t actually double the voltage, but there doesn’t seem to be a better descriptive term for them) are often slightly more efficient than CT circuits; but because both capacitors are generally left on charge all the time when the laser is in operation, they suffer more from aging effects. In addition, as I’ve already mentioned, if the laser is being operated at less than atmospheric pressure it is necessary to keep the channel at ground potential to prevent the power supply from discharging through the hose to the vacuum pump.

    For high-power nitrogen lasers, more workers seem to use Charge-transfer circuits; but both forms can deliver output energies of over a dozen millijoules, and both can produce pulses that are more than 10 nsec long; the performance level is largely a matter of design and construction, and the choice is yours. (See, however, the discussion of transmission lines, below.)
    

    ---

    
    
    

    Other Relevant Issues

    As I already mentioned, most small lasers cannot sustain a population inversion for more than a few nsec. As it happens, there are some high-performance lasers that also put out short pulses; these use resonant effects to deliver their energy to the channel as rapidly as possible.

    One way to accomplish this is to construct the capacitors (both banks for the voltage-doubler circuit, or the Peaker in the Charge-transfer circuit) as transmission lines. This is nontrivial for several reasons, the most obvious of which is that a transmission line works properly only if it is reasonably well matched to the load that it is driving. Because the effective impedance of the laser channel changes rapidly during the firing cycle, matching it is not fully possible. My understanding, as of this writing, is that at the peak of the electrical pulse the channel of a representative nitrogen laser presents a load of 0.1 to 0.4 ohms to the driving circuit; this is a very approximate value, but will serve for now. It is possible to make a transmission line with a characteristic impedance in the 0.1 to 0.4 ohm range, and such a line should pump a nitrogen laser reasonably well. Even in the best case it will not operate entirely as a transmission line; but to whatever extent it does, the performance of the laser will be improved.

    The characteristic impedance of a transmission line is usually given by this formula:

    Z₀ = (377/sqrt(ε))×(s/W)

    ε is the dielectric constant, s is the spacing between the plates, and W is the width of the plates. (Notice that because the formula uses only the ratio between s and W, it doesn’t matter what units they are in, so long as the units are the same for both.)

    An example: Let’s say I want to pump a nitrogen laser channel with two transmission lines, one from each side, either as peaker caps or as a doubler circuit. I have a large piece of circuitboard that I hope to make the lines from.

    The dielectric is 2 mm thick, and the board is 32 x 36 inches. (Uh-oh. Already need to make one conversion.) I will be charging the main storage capacitor to about 30,000 Volts, so I need to etch away the copper from roughly the outer inch of board; this gives me either two pieces that are 16 x 30, or two pieces that are 14 x 34. I am going to choose 14 x 34, and use the longer side as the width. 34″ is about 86 cm, give or take a bit.

    The dielectric constant of ordinary circuitboard is about 5.3, which is convenient because the square root of 5.3 is very close to 2.3. So:

    Z₀ = (377/2.3)×(.2/86)

    This comes out to be about 0.38Ω, which should at least be viable, even if it is not fully optimal. (This depends on the dimensions of the laser channel, the fill pressure, and various other parameters.)

    The capacitance of the two lines, btw, totals about 14.3 nf.

    Let’s do the same calculation for the Amateur Scientist laser. That laser is constructed from a piece of circuitboard that is 30 x 45 x .04 cm, with 2 cm etched from the margin, and a 5-cm strip removed from the middle of one side to define the capacitor plates. This gives us two capacitor plates, each 18 x 26 cm, and we will consider these to be the transmission lines. It is clear from the article that the 26 cm dimension is the width.

    Z₀ = (377/2.3)×(.04/26)

    This comes out to be just over 0.25Ω, which is quite good. The capacitance of these two lines totals about 11 nf.
    

    ---

    
    
    

    Channel Length

    If you can adequately pump a longer channel it will give you considerably more output, assuming you don’t saturate the gain.

    As a thought-experiment, imagine that you have a laser channel with gain of 10X per unit length at a given pumping level. I am going to assume a few things: first, that we are far from saturation; second, that we are well above threshold. (Both of these are reasonable for nitrogen lasers.) Third, that it takes 2 units of pumping energy to bring this laser to threshold; and fourth, that we are putting a total of 24 units of pumping energy into the device.

    For convenience, let’s pretend that our laser is being run as an amplifier, with an input pulse of 1 unit of energy. (Not the pump; remember, that’s 24 units.) The laser is 1 length unit long, and it has gain of 10 per length unit, so its output is 10 units of energy. (See “A”, below.) If you put two of these lasers in series, one after the other, you do not get a gain of 20!

     A)   1 -> | x10 amplify | -> 10  B)   1 -> | x10 amplify | -> [10] -> | x10 amplify | -> 100 

    Of course, this cannot be extended indefinitely. At some point you saturate the gain; at that point the increase ceases to be exponential, and becomes merely additive.

    If you don’t double the energy input when you double the length, things are not quite as rosy; but if you are operating well above threshold you can still expect some improvement. Let’s take our 24 units of energy input, and spread it over two of these lasers. Each device takes 2 energy units to reach threshold, which leaves 10 energy units for pumping beyond that level. Our original device also took 2 units to reach threshold, but had 22 units of pump left over; if nothing else interferes, this means we should get roughly 10/22 as much gain, or a little over 4.5X per unit length…

     A)   1 -> | x10 amplify | -> 10  B)   1 -> | x4.5 amplify | -> [4.5] -> | x4.5 amplify | -> 20.25 

    Eventually, however, if you make the laser long enough, you start having trouble with bottlenecking in the lower laser level. If you build a nitrogen laser that is 4 meters long and run it at relatively high pressure, most of the “output” will be absorbed in the gas and will never escape from the laser. (It is possible to avoid this problem by using travelling-wave excitation, but that turns out to be rather difficult to accomplish.)

    At low partial pressures of nitrogen and reasonable channel lengths, however, this is not an issue: even a channel that is more than 1 meter long is entirely viable, and in fact many high-performance nitrogen lasers are that large. (Light travels at about 300,000,000 meters per second, so it takes a little over 3 nsec to go 1 meter. If your laser doesn’t bottleneck for at least 12 nsec, which should certainly be the case unless the latency is very long, a channel length of 1 meter is obviously viable. If you are using special techniques to get a very short pulse, however, and you are populating the lower laser level very quickly, 1 m may be too long to work well. Likewise, if other processes either begin to absorb the output or disturb the optical path through the gas, you may have trouble with a long cavity.)
    

    ---

    
    
    

    Electron Energy and Pumping

    As mentioned, the electrons in the discharge need to have a certain amount of energy in order to be able to pump nitrogen molecules. This energy is usually given in Volts per Torr Centimeter; the parameter is named E_n or E_p, depending on how it is expressed. The optimal value is about 80, but some researchers seem to find that they get best performance at higher values, often around 100. (It is possible that calculations of this value are not extremely accurate, or that other factors can affect the optimum; I don’t really know for sure.)

    For example, if you were pumping nitrogen at 5 Torr pressure, with 3 cm between your electrodes, you would have to push enough current through the discharge to maintain 1200 to 1500 Volts across it; if your capacitors weren’t big enough or your system weren’t fast enough to do this, your laser wouldn’t reach maximum output, and in the worst case it might not even reach threshold.

    Achieving 1200 Volts in such a discharge isn’t very hard; but most nitrogen lasers operate at 30 to 120 Torr and at channel spacings anywhere from 1 cm to 4 cm, which means that they need considerably higher voltages. At 60 Torr and 4 cm, you need enough current to sustain about 20 kV or even more if you want to reach the optimum electron energy and get the best possible performance from your laser. If the effective impedance of your channel is 0.2 ohms, you must push 120,000 Amperes through it in order to bring the voltage up as high as 24,000. It takes a very good design (and very good components) to accomplish this.

    I once spoke with a nitrogen laser designer at Avco-Everett Research Lab; he said that anyone can build a nitrogen laser, that it takes considerable effort to build one that puts out as much as 250 kW, and that only someone who is expert at it can build one that puts out much more than half a Megawatt. As far as I can tell from my own experience, he was entirely correct. (Please bear in mind, however, the fact that he was talking about low-pressure lasers. At the time, around 1971, there weren’t any commercial atmospheric-pressure nitrogen lasers. Also, preionization techniques weren’t as well developed as they are now, which works in our favor.)
    

    ---

    
    
    

    Energy Density in the Discharge; Efficiency

    This is closely related to the previous issue, but not identical. If you dump enough current through your channel to sustain the required voltage, you are dissipating energy at a certain rate. It is fairly easy to figure out how much energy you put into the channel and what volume of gas you are pumping. The one tricky bit is whether you count only the energy that goes into the gas up to the point at which lasing ceases, which is not necessarily easy to determine, or whether you count all of the energy that is stored in the capacitors, which is not realistic because much of it is put into the discharge much too late to do any good.

    In general, my feel for this is that high-performance nitrogen lasers tend to put at least 30 to 40 joules into the channel per liter of gas that they excite. For example, if the discharge in your channel is about 3 mm thick and 40 mm wide, that represents an area of 1.2 square cm; if the active region is 80 cm long, you are pumping 96 cubic cm of gas. You have to put about 3 joules (or more) into this, in a very brief period, if you expect to get well above threshold.

    Let’s look at it a different way: if this laser is operating at 30 Torr, you need to sustain a voltage of about 10 kV across it to be in the general region of the optimum. If the effective impedance is 0.2 ohms, that takes 50,000 Amperes, which is 500 MWE (MegaWatts of Electrical power). If we think of this as the peak power, and use 250 MWE as the average power, and if we allow for 10 nsec startup time followed by 10 nsec lasing time, for total time of 20 nsec, this calls for 5 joules. (At 1 GW you are dissipating 1 joule per nanosecond; at 250 MWE it takes 4 nsec to dissipate 1 joule.) While that is not identical to the figure we got in the previous paragraph, it is certainly close.

    The problem, of course, is that this is only the amount of energy that goes into the discharge up to the point at which lasing ceases. In the real world, the driving circuit is still pouring energy into the channel, and continues to do so for many nsec. The total stored energy in the capacitors is likely to be more like 25 to 50 joules. It should be clear from this that a nitrogen laser is not a particularly efficient device. If you put 30 joules in and you get 6 millijoules out (which is quite respectable; for a 12-nsec pulse it represents average power of half a million watts), your efficiency is 0.02%…
    

    ---

    
    
    

    The Gas Mixture

    If you compare the performance of air and nitrogen, you find that air is a terrible laser. There is typically as much as 10X difference in output power between them. (Note, however, that to the eye, if you use the beam to excite a fluorescent material so you can see it, the difference appears much slighter. This is because your visual response is essentially logarithmic. It has to be: full sunlight is about a million times brighter than moonlight, and if your response weren’t the way it is, you either wouldn’t be able to see in the day, or you wouldn’t be able to see at night.)

    The difference is caused by the fact that in large quantities, oxygen poisons the laser. I have seen, however, at least one study in which the authors found that in small quantities, oxygen actually improved their performance. They found the optimum to be about 0.3%; any more than 0.5% was deleterious. Because of this, if you are building a high-performance laser you have to be careful to eliminate leaks in your vacuum system. Air is 20% oxygen; if you operate your laser at 50 Torr, as much as 1 Torr of air is probably beneficial. (1/50 of 20% is 0.4%.) Much more than 1.5 Torr, however, and you are going to see degraded performance. To put that a different way: if you can’t pump your channel down to less than 2 Torr with the inlet valve shut, you need to find and fix some leaks.

    Helium, which is readily available, is extremely helpful. If you have sufficient preionization (see the section about that), you can run a low-pressure nitrogen laser at atmospheric pressure, eliminating the need for a vacuum pump, by filling the laser with helium and adding a tiny bit of nitrogen. I have done this myself. It uses up large quantities of gas, and can be expensive, but if you don’t have a vacuum pump and you want to run a low-pressure nitrogen laser, it offers you a chance to do so. It also means that you do not need to be as careful about leaks, though they are still something of an issue.

    Even at lower pressures, helium is very handy. A 50-50 mixture of helium and nitrogen tends to show better uniformity from shot to shot, and in some studies the output power is higher. This appears to depend on various other design parameters; I have never seen a study to determine what they are, so you may or may not find a power improvement if you mix helium into your nitrogen. OTOH, once you get things adjusted, you are not likely to see any reduction in output.

    It helps to have flow meters; that way, you can (just for example) flow 4 liters of helium per minute and 0.2 liters of nitrogen per minute through your laser at 1 atmosphere, if you want to run the laser with roughly 36 Torr of nitrogen in it.

    It is widely known that electron-attaching gases, in particular SF₆, improve the operation of the nitrogen laser; but these gases are not easy to handle, they are beyond the reach of most DIYers, and my guess is that arcs and sparks in mixtures containing them could possibly release small amounts of fluorine. Even small amounts of fluorine are highly undesirable in the home, and are severely deprecated. Helium, on the other hand, is inert.
    

    ---

    
    
    

    Preionization

    In TEA (“Transversely Excited Atmospheric [Pressure]”) nitrogen lasers, some form of preionization is almost invariably required, else operation is spotty at best; in many cases, it is impossible to obtain lasing in anything more than a very small fraction of pulses without preionization.

    Many low-pressure nitrogen lasers operate reasonably well without any particular effort at preionization, but most of them do seem to benefit from it. I think I have only read one or two papers in which the researchers report that preionization did not seem to improve the performance of their lasers, and I have read at least a dozen where it did.

    There are many ways to achieve preionization. You can, for example, inject a relativistic electron beam into the channel. That, however, is not really a DIY technique, so I am going to discuss other methods.

    In some cases the structure of the laser creates its own preionization. The Levatter and Lin laser (see references), which developed 3 MW and about 20 mj, used a packed array of injector razor blades as its cathode. As the electric field began to rise at the beginning of the discharge cycle, the sharp edges of the razorblades generated lots of corona, which filled the channel with UV and ions. (This is how electrical preionization works, and is the desired condition.) They also included a separate, passive preionizer, but if memory serves they found that their cathode was sufficient by itself.

    Peter Schenck and Harold Metcalf used a piece of bandsaw blade with teeth every 2 mm in a design that should be known by more DIYers. (See the references.) Their laser was about 1 meter long, and easily put out 160 kW. Again, the structure of the cathode provides its own preionization.

    Another laser used a cathode made of 0.1-mm wires, spaced 1 mm apart. These were oriented across the cathode structure. This seems like it would be difficult to build using separate individual wires, but you could space two threaded rods a small distance apart and just wind a single long wire around them. The threads would space the windings for you, with reasonable precision. The authors of at least one paper found that 0.8 mm was the optimum spacing for their laser; this is very close to 32 per inch, a commonly available thread in the US. (I have tried using #8-32 threaded rod without any wires, depending on the threads themselves; but my construction was not good enough, and I need to revamp that design before I’ll be able to say much about how well it can work.)

    I have seen one excimer laser design in which the main storage cap was switched into an array of blunt pins, which were positioned behind a cathode made of stainless-steel screening. The discharge from the pins to the cathode charged the peaker caps, and simultaneously preionized the channel very thoroughly. I am uncertain why the authors found that blunt rounded ends were better than pointed ends, but it was clear from the paper that they did. I actually have a commercial head that uses this method, though with perforated metal mesh rather than screening:

    

    (Click the small image if you want to see an enlargement.)

    As I’ve already mentioned, it is possible to add a structure to the laser that steals a small amount of energy from the discharge, and uses it to preionize the channel. One way to accomplish this is to run a thin wire alongside the channel, typically somewhat closer to one electrode than the other, and connect it to the more distant electrode through a small capacitor. When the voltage across the channel begins to rise, the small capacitor is uncharged, and so the field strength between the wire and the electrode it is close to increases more rapidly than the field strength between the two main electrodes. A corona discharge develops, which charges the small capacitor and sprays UV and ions into the gas between the main electrodes.

    It is also possible to use either one wire and one of the electrodes, or two entirely separate wires, and just run a milliampere or so of DC. (This technique was used in the Rebhan et al. laser, which developed over 1.5 MW and produced pulses 18 nsec long; see the references.) The advantage is that it does not steal energy from the main discharge. The obvious disadvantage is that it requires a second power supply, and that the wires must be either well off to the side, or must be shielded a bit from the electrodes so that the discharge avoids them. Even so, this technique seems to be robust and reliable.

    A related method, best if the wall around the channel is thin, has a broad conductive strip on the outside, connected to one of the electrodes. Corona develops on the inner surface of the wall.

    A method that is common in CO₂ lasers involves semiconductors. This turns out to be quite practical for nitrogen lasers, and is extremely easy to construct. I have built a laser in which I simply coated one inner wall surface with epoxy and poured carborundum grit on it. When the epoxy had set, I poured off the excess grit. Then I attached the wall to the channel, making sure that there was contact between the electrodes and the semiconducting material. (I did leave a blank space, parallel to the electrodes, without any silicon carbide on it, for sparks to jump across; but that may not be necessary.) This laser ran with 1 atmosphere of helium in it, and did not require a vacuum pump.
    

    ---

    
    
    

    A Side-Issue:
    Fabry-Perot Resonators and the Definition of “LASER”

    There are people who try to claim that because a nitrogen laser doesn’t have mirrors (or has only one), and does not have a defined mode structure, it isn’t a laser. It seems to me that despite the fact that Gordon Gould specified a cavity in his original notebook, in 1957, when he invented the term “LASER”, the acronym does not have the initials “FPR” anywhere in it. Moreover, any device that operates by means of Stimulated Emission produces light that has characteristics you just don’t get from other sources. In addition, it is perfectly possible to put mirrors on a nitrogen laser; it just isn’t necessary.
    

    ---

    
    
    

    References

    Various of these are at the end of my rant about the explanation of the Scientific American laser.
    

    ---

    
    

    As a bit of a postscript, if you aren’t already on the LASERS mailinglist and you would like to join, try this page.
    

    ---

    
    

    To a page about my initial effort to produce a high-performance nitrogen laser

    To a page about my continuation of that effort, which resulted in a laser that puts out about 100 kW and can operate without a vacuum pump

    To a “How-To” page about that laser

    To an interim page about my effort to scale up a published design in order to enhance its performance

    To a page about my recent (starting mid-August, 2006) redesign of the “DKDIY” laser, which resulted in significantly enhanced performance

    To a brief “How-To” page about building the revised design

    To a page about my current (late 2006) effort to build a less-expensive laser with even better performance

    Back to the Index

    Home
    

    ---

    
    
    This work is supported by
    the Joss Research Institute
    19 Main Street
    Laurel MD 20707-4303 USA
    
    

    ---

    
    
    

    Contact Information:

    My email address is a@b.com, where a is my first name (jon, only 3 letters, no “h”), and b is joss.

    My phone number is +1 240 604 4495.

    Last modified: Sun Dec 18 00:03:21 EST 2011

    ]]> http://jossresearch.org/2011/05/10/joss-research-institute-web-report-5-part-1/feed/ 0