An RGB "white light" dye laser

The nitrogen laser has mostly been behaving itself, though it did coat the inside surface of one of its windows with powdery glop, obliging me to remove and replace that window. I can’t really complain, though. For the most part it is a happy little camper, and it appears to be delivering a little over 200,000 watts. (Should be more, but I may have screwed up some parameters in the design. I am expecting to rebuild another of my nitrogen lasers fairly soon, and we’ll see whether I can coax better performance out of that one.)

So, what do you do with a nitrogen laser? I suppose that depends on lots of things, but one common use for them is to pump organic dye lasers. (See, for example, the photo in my previous posting.)

I have a cold, and I have very little energy, but I can only sit around blowing my nose for so long, after which I get kinda stir-crazy. Hmmm, there’s this nitrogen laser on the bench; I wonder if I can make RGB “white” laser light from one single cuvette of dye...

It is not too horribly difficult to get two colors at once if you choose your dyes carefully. Two-color dye lasers have been reported in the literature, probably several times. I’m also sure that people have pumped multiple cuvettes of different dyes, and combined the outputs to get an RGB mix; but that isn’t what I wanted to do. I wanted 3 dyes in the same container, all lasing at the same time.

That was clearly not going to be easy, for a number of reasons. First off, shorter-wavelength dyes can often transfer energy to longer-wavelength dyes. Sometimes you can make use of that, to help the longer-wavelength dye lase better, or to assist a dye that doesn’t absorb nitrogen laser output very well by transferring energy to it from a dye that does; but if enough energy gets transferred away, it can stop the shorter-wavelength dye from lasing. For example, adding even a very small amount of Rhodamine B [which lases red] to a solution of Fluorescein [which lases green] in ethanol will quench the Fluorescein and stop it from lasing. (I know this because I tried it.) It should come as no surprise that finding a compatible set of three dyes is considerably more difficult than it is with only two.

Once you have three dyes that appear to play well together, and a solvent to put them in, you then have to adjust the concentrations of all of them. This (he said, through slightly gritted teeth) turns out to be absolute flaming madness. I got 3-color lasing, just barely, several times, ...and lost it several times while trying to balance the colors. At one point I diluted the solution slightly, and the red dye just stopped lasing, pouf. Gone. Argh. It is also regrettably easy to add too much dye, which tends to stop all of the dyes from lasing.

One ends up doing a dance in which one dilutes the solution just a little, adds a barely-visible amount of dye, tests it again and notes what is off balance this time; ...lather, rinse, repeat. I won’t even get into the issue of aligning the optics every time the cuvette goes back into place, which are also fairly tweaky. (Yes, I know, I should build a strong and upright cuvette holder, and I should tie down all of the mounts that hold the lenses and mirrors, not just some of them. I will get there, eventually.)

Some of the intermediate results, though not quite what I was looking for, were fairly pretty:



(All three colors can be seen in that pattern, but they are not correctly balanced, and there isn’t any part of the beam that is more or less neutral in color. Sigh. At least it was nice to look at.)

This effort ended up taking me about two and a half days (...and I still have the lousy cold; feh!). Here are two views of the final result, which I arrived at earlier this evening. The first shows the output of the dye laser on a paper target; the colors, at least in the central spot, are [finally!] moderately well balanced, though I’m sure the CRI is appreciably less than 100. There are, after all, limits.







The second was taken with a diffraction grating in front of the camera; the dye output is on the right, and the dots on the left are the spectrum of the output. The bright red object at the bottom of the image is the top of the cuvette that contains the dye solution.



“A pleasant afternoon’s entertainment.”

[Ahem.]

Note: I have added a report with more technical detail to the Joss Research Website.

Further note, added later: the Institute’s Website has been redesigned and reimplemented, and they are still working out some bugs. If you go to www.jossresearch.org and click the “TJIIRRS” button, you should be able to get to a list of reports. With some luck, you’ll be able to navigate to report 14 from that list.

Cheers —
jon