Rhodamine 6g & 532nm

[jonsinger]

@johnsinger, That's got to be one of the best replies in history, very in depth and informative. I touched Rhodamine 6G about 15 years ago for this job but ended up giving it up as a bad effort oh and Welcome to PhotonLexicon.

Andy

Hi, Andy. Thanks for the kudos! I came upon the thread while searching for info about CW dye lasers, and even though it didn't seem active, it was very interesting; definitely cool enough to merit registering and responding. (Thanks also for the welcome!)

One thing I've consistently found to be extremely useful over the entire course of my involvement with what I will loosely call research is the nearest University Library. We (DIYers) ask many questions of each other, and sometimes you get conflicting opinions about something, or incomplete info. A quick look at Sam Goldwasser's Laser FAQ, for example, will show you places where people contradict each other, or appear to because of incompleteness or differences of approach. An occasional trip to the library can clear away a whole lot of undergrowth and let some light in (as it were). Personal example: until 2 or 3 days ago I thought 69 mW from a green DPSSL was _the_ record low for pumping a CW dye laser. Then I was Websearching and found mention of the HeNe work from 1986, so when I went to the library to check papers, I read that one. Totally amazed me.

The reason why I was looking for info, btw, is that I've been thinking about Optical Frequency Comb Generators. A DIY OFCG is probably about at the limit of what's possible, or maybe even beyond the limit, but I think it would be interesting to make an attempt or a run-up toward one. The baby-steps involve some sort of mode-locked laser, and it seems to me that a dye laser is probably about as cheap and easy as it's likely to get -- Ti:Sapph is really well suited to this, but it's expensive. We have a somewhat dead CR-599 here, and I'm thinking about trying to set it up and get it going with either Fluorescein or Fluorol 555, pumped by a ~1-Watt blue laser diode that I recently got from cajunlasers.com. Finding optics, however, is not going to be trivial. The two mirrors that surround the jet, for example, need to be about 7.5 cm RoC. I may end up doing something completely different if I can't locate anything suitable in a while; we'll have to see how it goes.

As to R6G, how did you end up deciding that it was a bad effort? (I suspect that the usual way is to try to build something like the SciAm dye laser, which is on the tweaky/marginal side. It can be done, but it ain't easy...)

Cheers --
jon
(no "h", btw, just 3 letters)

[dsli_jon]

...never mind! (again...

[mixedgas]

OK, one thing came up on a trip to the sekret barn some place in Mo. I was reading a Lexel factory setup manual for a ring dye system. Their favorite trick is to start with a known lasing mixture and align the dye laser. They then flush the laser, load a pure master solvent in the circulator, and make a saturated solution of R6 or other dye in Methanol. Like one hundred milligrams of dye in 50 mL of MeOH. They then place a power meter AFTER the pump beam. They then add small amounts of the dye concentrate to the circulator to start lasing. Once lasing, they adjust for 80 to 90% pump adsorption.
This corresponds to maximum lasing in their setup.

I knew about the hene pumped dye, but .4 mW of IR output (REPEAL STOKES LAW NOW!) just did not seem that useful to mention on PL. The really interesting ones are the metal halide and xenon CW arc pumped dyes, but 5 mW of dye red for 7 kilowatts of metal halide just did not seem more then a curiosity.

Something to watch would be diode pumped metal vapor, then frequency double it. Metal vapor is back for military apps, pumped by direct diode pumping in the IR. Low melting point metals, not copper or gold. Potassium, Lithium, that sort of thing.

Stev e

[planters]

I like where this thread is going. I have a couple of flash lamp pumped dye laser heads extracted from medical lasers a few years ago. I'm thinking about running one of them with a R6 50:50 methanol: water mix. The original set up was with very long, as in tens of milliseconds, pulse duration with several kilojoules discharge energy and a few joules output; efficiency be dammed. COT was used and the output was tunable from 590nm to 605nm with an inter-cavity dichroic. The manufacturers added large amounts of series inductance to produce these long pulse lengths. I was hoping to set up one of the heads with a fast discharge as in a few microseconds with low input energies, a high pressure circulation pump and as rapid a PRF as possible to create a quasi-continuous (visually) output. I will stick with the R6, should I avoid the COT? Should I up the output coupler reflectance to lower the threshold? I don't plan to bother with the tuning and will live with whatever wavelength optimizes the PRF/output efficiency. Any thoughts would be greatly appreciated.

[mixedgas]

[QUOTE=planters;200071]I like where this thread is going. I have a couple of flash lamp pumped dye laser heads extracted from medical lasers a few years ago. I'm thinking about running one of them with a R6 50:50 methanol: water mix. The original set up was with very long, as in tens of milliseconds, pulse duration with several kilojoules discharge energy and a few joules output; efficiency be dammed. COT was used and

Take a long, hard, look at the ends of the lamps, I just scrapped one of those for a friend and the quartz to to metal seal at the ends of the lamp was very flimsy. I doubt it can do quasi CW, the metal was a small fraction of the diameter of a CW lamp, so I'd worry about its heat/shockwave capacity.

Very high rep rate, very high energy dye lasers used to have the dye running down the center of the flashlamp bore.

Google patents is your friend, search : Coaxial Flashlamp or assignee name: Candela

Steve

[planters]

I read about these coaxial designs years ago and I thought their principal attraction was there close coupling geometry and hence their high efficiency (up around 1.5%). One significant limitation was the transmitted shock and heat and the effect this had on the refractive index-homogeneity on subsequent pulses. My thought is that if a Cynosure laser can operate for pulses lasting ten's of thousands of usec. then the peak powers available during such a long pulse are not actually that high and the effect of accumulating triplet state molecules is not sufficient to shut down lasing during a period within which there can not be any significant dye exchange within the cavity. The actual thermal loading of the flash lamp in the normal function of this laser was 3KJ/sec. at 1Hz. and I'm thinking 30J at say 100 HZ. I have several replacement lamps for this laser and they are 5mmID x7mmOD by 60cm and have the typical pinched seal around a short stem supporting a small cylinder held free from the walls.The gas fill is 500 torr Xenon. I'll keep looking at the patents. I am surprised that Candela held some of these.

[planters]

OK did some more research and there is definitely a PRF advantage to coaxial flash lamps. The Google patents don't go into detail re. the physics of high repetition frequency, but it seems pretty likely that it is due to quenching(cooling) of the gas by the envelope walls that allows PRF up to 1000Hz.. The greater surface area to cross sectional area in an annular discharge explains this. Nevertheless, because of cost and availability I plan to move forward with a conventional linear flash lamp that based on the manufacturer's data as well as these same patents may still be usable up to 100-200 Hz. I think limiting the individual pulse energy will minimize gas heating and allow me to reach up toward the higher limit. Balanced against this is the need for sufficient energy for each pulse to bring the plasma to an efficient radiating temp and this may be on the order of 100J for the present lamp no matter how quickly the discharge occurs. I like your previous reference to titrating dye concentrate into a running system to optimize absorption. This would also have the benefit of showing the best dye concentration re. mode volume, wavelength and power. To keep it simple, I will just use a rectified HV charging circuit discharging (by over voltage) through the lamp and an adjustable non-triggered spark gap. I'll worry about stability later. I'll add the COT to the optimized system to see if there are any benefits ( or it screws it up). Any one with some experience with these dye mixtures might be able to save me some time /money. This should be fun.

[jonsinger]

Hi!

Nice photo, rays into the night.

12-15W of blue? ZOWIE! I thought I was doing pretty okay with my cute little diode putting out about 660 mW... though unfortunately it is very obviously multimode. Here it is on the wall, defocused so you can see the pattern easily -- http://www.jossresearch.org/pictures...thresh.14c.jpg

I am strongly into the DIY end of things, but it ain't easy to do a DIY CW dye laser of any sort, so I'm thinking about whether I can use the blue diode to pump a surplus CR-599-04 that is missing its tuner (I can live without that -- irrelevant to at least one project I have in mind), and also (dammit!) missing the 3 mirrors that turn it into a dye laser instead of just a dye jet with a nice bright fluorescent spot on it. Not going to be easy to find those, especially the 2 green/yellow MaxRef ones with 7.5 cm RoC. Sigh. (At least the wretched thing has a jet orifice and a pump mirror in it.)

Anyway, I'm in the middle of 77 things as usual, including making dinner because I got back late from what I was doing this evening, so I don't think I can chat now. Also, I gotta come up to speed on some of the terminology, as there was a certain amount of stuff on the chat page that I had difficulty following. (I can definitely understand photos of heavy-duty lasers, though!)

Cheers --
jon

[jonsinger]

I think I may possibly have a point about flashlamps -- I once saw an article in which it was claimed that if you push xenon hard (lots of Amps per square cm) the plasma becomes very hot ...and quite opaque. If this is accurate, it means that coaxial lamps have an extra advantage: you lose only about half of the light that comes out of the lamp, because the gas layer is typically a thin annulus around the dye cell. It also suggests that a regular linear lamp, if you hit it fairly hard, basically turns into a thin layer of "lit" gas near the wall, and a core that might as well be dark.

For this reason, I tend to use lamps with relatively small bore; but that's a luse in a different way: the narrower the bore, the higher the ESL and ESR, so the pulses get stretched out and the peak power is decreased. The one way you can turn this to your advantage is if your driver circuit actually expects a relatively high ESR. If so, with a large-bore lamp (at least, one with relatively short arc length) the circuit can be underdamped, in which case it will ring, which is not good for your capacitors and also decreases the peak power.

...At least, that's my understanding.

I have the tiniest smidgen of supporting data for one of my driver circuits -- I got slightly long pulses from a driver circuit with 100 nf capacitor and a narrow-bore lamp with 15" arc length. (I think the electrical pulse was ~650 nsec FWHM when I expected ~400, but don't quote me because that's memory from several years ago.)

Unfortunately, I have no personal data at all about xenon plasma opacity; have to take what I can find in the literature. I read that article quite some time back, though, so you can take the claim with a few grains of salt.

Just by the bye, 100J/pulse at 100 Hz is 10 kW, so you will have some "interesting" cooling challenges for both the lamp and the dye cell.

I have thresholded one or another Rhodamine (I think it may have been R640 rather than R590) with as little as 6J/pulse into a lamp with (IIRC) 6" arc length; but something tells me that just reaching threshold is not precisely your ambition. ;o)

Cheers --
jon

[mixedgas]

I can attest that the plasma is very opaque from other experiments. Any adsorbed pump light goes right into heat. I've had to fold light back through CW metal and xenon arc and flash lamps, as part of high speed photography projects. You end up seeing the "hole" in the plasma in the far field, if you do.

Before you assume every thing you've heard about triplets is wrong, Keep in mind something. If you look at the dye solution for a CW dye, its nearly opaque with dye. The pulsed lasers I've looked at have visibly weak concentrations compared to CW. This has much to do with triplet states. The secondary adsorption is low because the concentration is low.

In the long pulse lasers, with the weak concentration, you have time for the dye to relapse, and re-emit.

The down side is, the UV from the long lamp kills the dye fast. The 580 nm system I just stripped had a pump system that scrubbed the dye past a exchange resin and injected more dye.

Before you hook up a pole pig to a cap bank, take a look at:

1. ^ F. P. Schäfer (Ed.), Dye Lasers 2nd Edition (Springer-Verlag, Berlin, 1990).
2. ^ F. J. Duarte and L. W. Hillman (Eds.), Dye Laser Principles (Academic, New York, 1990).

I think Schafer is up to the 4th edition. Interlibrary Loan is your friend, if you can bribe or sweet talk a librarian into doing it. Universities don't mind, but public libraries hate to pay the 6$ and shipping.

Your better off getting your hands on a Q-switched green pump.

As for the warning about plasticizers and detergents, its in the Coherent, and Lexel CW manuals, and in Schafer and Lankin. CW is a far different beast then pulsed.

You might also wish to get a call into Exciton and see what they say with regards to high rep rate lamp pumped lasers.

Steve