This project is part of a larger project I have in mind, which requires a mode locked laser. (That project, or at least its earliest stage, is described in TJIIRRS #23.) I initially tried passively mode locking a blue diode laser, but that proved to be somewhat fraught, and I need to find an appropriate detector before I proceed further with it. (I may also need to try active or hybrid locking.) For the moment, I have set that approach aside in favor of a different one. We have an old Coherent CR-599-04 jet dye laser and a Coherent 5920 dye circulator, and I decided that it might be a good idea to attempt to get these running as a possible alternative path.
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.