Rhodamine 6g & 532nm

[planters]

So many issues. And all good points. Based on my reading the opacity of the plasma, just as with the sun, prevents the inner volume from radiating and only through reradiation and conduction to the outer layers is energy lost. This occurs above temperatures where the plasma radiates primarily at emision lines and more as a simple blackbody and to my understanding above 5-10K depending on the fill gas. The consequence is a lower peak power and prolonged pulse.
I've mixed RG6 before and the concentration that produced the most output (10-5 to 10-4 M) also produced the worst beam probably due to very limited pump penetration. And your right that UV kills dye. The system I used had a parallel low flow circuit that scrubbed the dye as well as the degradation products completely out of the solvent and continuously replaced the loss with fresh dye concentrate The only time I have ever used COT was when Cynosure included it in their "proprietary" kits.This was when I used to tote this laser and lease it to physicians( and before it was no longer supported and I tore it to bits). It's my understanding that COT is important for these long pulse lasers, but that it reduces florescence efficiency as a side effect while quenching the triplet state and is not needed nor used in short pulse systems. I may be wrong. The dye relapse and re emitting in long pulse lasers (sort of a quasi saturatable absorber) as well as the accumulation of triplet absorption may explain the low efficiency (0.1%) vs. short pulse systems (1.0%).
10Kw is correct, however I expect that the high PRF and low pulse energy are an ideal to shoot for. But I expect that one thing or another will limit this product ie. min. threshold, max. dye exchange, flash lamp cooling or even a suitable power supply ( 10Kw at 10Kv... I wish).
I agree a Q-switched green pump is a better solution, but I don't have one. What about it dsli_jon?

[planters]

Yes. But lets keep it public. This is interesting. You've heard the saying if all you have is a hammer all the world is a nail. Aside from the diode based systems the only hardware I currently have is some extracted dye heads and so the temptation is to work around these. However, the wavelength flexibility of dye based systems is exciting and proven pump sources make sense. Mobility is not a priority, and I am always interested in exploring novel/interesting sources such as mixedgas' reference to diode driven, up converted,metal-alkali lasers...Oh boy. I'm not kidding. Whats out there? Fiber? Amplifiers? I really like low cost because something that is affordable can actually be done and isn't just brainstorming/fantasizing. I'm going to SELEM and I'm sure I'll learn a lot. Regarding blue; huge powers with moderate beam quality are very practical. The 445 diodes are just game changers in terms of the output/cost-convenience trade off. What are your thoughts?

[jonsinger]

Hi.

@MixedGas --

(Lemme do this in pieces; hope that's okay.)

A) [re: opacity of plasma] Very cool -- thanks! Good to know that the info I remembered is accurate.

B1) "Before you assume every thing you've heard about triplets is wrong,..."

[That obviously isn't addressed to me. I've read bunches of stuff about triplets since 1970, and some of it was clearly right.]

B2) "...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."

Uhhh, could you explain how it relates to triplet states? [If we are merely examining dye solutions that aren't excited, there aren't any triplets around, so I don't see how it would relate at all. ...Or do you mean something other than just visual inspection of an unexcited liquid when you say "If you look at the dye solution"?]

B3) "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 energy of the lowest triplet is lower than the energy of the lowest singlet, so the dye molecules have to return to S0 and then be excited again before they can re-emit; aside from that, though, this certainly makes sense to me.

I don't see that a CW dye laser is so horrendously different from a long-pulse dye laser in this regard, btw. At maybe 10 meters per second (jet dye laser, number chosen because it occurs in the literature), if we figure that the pump spot is perhaps 15 microns across (again, I seem to be seeing this number in the literature), then the dye is "in the spotlight" for about 2/3 of a microsecond, right? With R6G, at least, that is a reasonable amount of time for many of the triplets to fall back to the ground state, get re-excited, and re-emit, assuming that they aren't just cycling several times between T1 and T2. (I could easily believe that _some_ of them cycle that way, but I'd be hard-pressed to think they were all doing it for the entire time they were in the pump spot.) [[Reference: http://www.sciencedirect.com/science...30401875901091 -- they measured R6G triplet lifetime in EtGly in a jet dye laser, and got 125 nsec, give or take a little. Others seem to have gotten numbers that were slightly shorter, but I think they were working with methanol solutions.]]

C) "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."

That's very cool.

I saw a Russian article about a high-power flashlamp-pumped dye laser in which they were putting about 10 kJ into the lamps for each pulse. They found that their dye solution lifetime varied a lot, depending on the solvent. IIRC, in ethanol they got about 6 pulses before they were down to 50% output, but in isopropanol it was more like 16. ...But don't quote me -- it has been a year or two since I saw the article, and I may be misremembering.

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

A nice Q-switched green pump laser would be A Very Happy Thing for almost any of us to have, I think.

E) "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."

Plasticizers, sure -- I've never heard anything good about them; but detergents? As I said, when I look at early CW dye laser papers (which I did last Sunday), it seems like lots of them had roughly 2% to 4% detergent in the dye solution, and they worked just fine. I don't know what the detergent did to the dye solution lifetime, but it certainly did good things to performance in the short run. (Triton X-100 did not show much triplet-quenching activity, but Ammonyx LO did. I can probably find a reference for that if it doesn't show up on a Websearch.)

Cheers --
jon

PS: I tried to post a reply to a specific item about triplets on page 1 of a chat set, but my reply shows up at the end of page 5, long after everyone has forgotten whatever was on page 1. Sigh. As an addendum to that item, btw: there are a few phosphorescent materials that do not absorb their own emission, and sooner or later somebody will lase one of them, ...provided they can figure out a viable pumping scheme. There seem to be quantum-mechanical reasons why the S0-S1 absorption of good phosphorescent materials is a lot weaker than it is in our regular laser dyes, and I think the S0-T1 absorption is even weaker. That makes pumping them quite difficult. I kinda doubt that we'll see any CW phosphorescence lasers any time soon, unless perhaps they are electrically pumped -- OLED materials are probably a reasonable place to look for candidates.

[jonsinger]

One more item about detergents -- although it is almost certainly the case that more flashlamp-pumped dye lasers use them than CW, a Websearch on "cw dye laser" and ammonyx provides a number of hits, including this rather interesting one: http://www.springerlink.com/content/v12511n217222441/

Cheers --
jon

[planters]

Any more memories about the 6J threshold flash lamp pumped dye? The approximate pump duration, trigger method, flash lamp dia.and fill pressure as well as the dye tube dia. would be helpful. With the 500 torr flash lamps that I have, I'm concerned that without 50-60J I simply won't have enough energy to heat the xenon to an emission temperature let alone support any radiative output. Minimal threshold is important to allow minimal shot to shot input and allow high PRF at achievable average power levels. Regarding detergents, if I remember rightly, they are included with dye jets to reduce surface tension and inhibit the jet's tendency to break up into droplets.

[jonsinger]

@planters -- Hi.

A) 6J into the lamp: I wrote up the whole thing. The first page, which has that item, is at http://www.jossresearch.org/tjiirrs/015a.html

(I'd be curious to know the ID and arc length of your lamps. Also what your driver circuit looks like. I drive my larger machine with 100 nf at 20-25 kV, switched by an EG&G GP-15B or GP-32B, but for this laser I wanted something significantly simpler. I can now make fairly reasonable little triggered spark gaps, and when I have time I will probably rebuild the machine to make it even more DIY.)

B) Laser dyes apparently tend to form peculiar aggregates in water; the aggregates don't fluoresce the same way that individual dye molecules do, so you begin to get concentration quenching even at moderate dye concentrations. Adding detergent tends to break up the aggregates, allowing you to work with higher concentrations. Some detergents work better than others, and some are more suitable for dye lasers than others.

You can demo this on an informal basis by dissolving a little too much Fluorescein or R6G in a small amount of distilled water, enough that it doesn't fluoresce as much as you'd expect. Then add a drop of some transparent uncolored detergent (dish liquid will typically do, as long as it isn't colored and doesn't have any fluorescent brighteners in it), and notice the change in the fluorescence.

Cheers --
jon

[mixedgas]

You can demo this on an informal basis by dissolving a little too much Fluorescein or R6G in a small amount of distilled water, enough that it doesn't fluoresce as much as you'd expect. Then add a drop of some transparent uncolored detergent (dish liquid will typically do, as long as it isn't colored and doesn't have any fluorescent brighteners in it), and notice the change in the fluorescence.

end quote.

Hum, maybe the DET warning is simply for a 70-90 PSI jet? When that jet recovers into a collector that is but a 25 mm plastic tube, any bubbling or trapping of air is going to be a "bad thing". Solvent choice would have a large effect on how that jet swirls down the collector pipe. I've looked at four Lexel Mixture sheets, and they all have less then 10% difference in the solutes. Most have perhaps 1% of a solute for the dye and then balance glycol or other solvents.

One thing is for sure, I'm now convinced that there are as many dye mixtures as there were dye laser operators, squared.

Planters, how many Joules is the Sci Am dye laser? Not too many if I remember right. What, 40?

I'll pull the coherent dye manual on monday and scan it in. I'll see if my local U still has Schafer on the shelf.

I'll take a pic of a collector tube in the morning.

Jon, I have lots of broadband OC/HRs for 1-3% transmission at 647-670, they are NOT narrowband, PM me with a shipping address. In fact I'll send you some blue/green ones, some of the blue/greens reach well into the yellow.

Steve

[planters]

I built that laser many years ago and was frustrated that the 65J I expected based on the instructions was not nearly enough and not till 200 J did this laser suddenly gather all that orange fluorescence into an unmistakable far field laser spot, but when it did, man was it bright. The capacitor was a monster; 6Kv/ 60uf/ 40nH. The elliptical reflector was marginal and yes lots of pump light leaked out the ends. The cavity mirrors were harvested He Ne mirrors. What a mess, but if it worked then flash lamp pumped dye lasers must be pretty forgiving and with the right setup...

What would be the optimal mirror set up for a R6 dye? Would it be similar for cw jet and flash lamp? Should it be symmetrical vs asymmetrical, plano/concave,and a reasonable range of OC%

My lamps are 7mmOD x5mmID x 60 cm long 500 torr xenon filled and the circuit plan was very crude. The rectified switching PSU from the original laser would charge a 1 uF " you roll your own" 60cm long capacitor laying parallel to and in close proximity to the laser head. This capacitor will discharge through an adjustable air spark gap by over voltage. If too unstable or if the lamp fails to self terminate then I would add a trigger electrode.

I'm still interested in the Q-switched pump. In this kind of a set up I'm assuming a jet is unnecessary and only sufficient flow to mix and cool the dye is necessary and this can occur in a completely contained loop. Am I wrong? In any case what about dividing a single Q-switch pulse into two components; one to drive a small "seed" laser which then passes through an amplifier pumped by the second component of the original pulse? This might facilitate modulation of a powerful laser by controlling/conditioning the lower energy seed laser.

[jonsinger]

"Hum, maybe the DET warning is simply for a 70-90 PSI jet?"

Could be; could also be that EtGly (the most common solvent for CW jet lasers, AFAIK) doesn't play nice with detergents. I've never tried it, partly because I've never had a chance.

"When that jet recovers into a collector that is but a 25 mm plastic tube, any bubbling or trapping of air is going to be a "bad thing"."

Hmmm! You got that right. I wonder whether maybe Ammonyx LO doesn't bubble much... that ridiculous machine I posted a link to was a jet laser, I think... hard to see how it could have been anything else.

"Solvent choice would have a large effect on how that jet swirls down the collector pipe."

True thing.

"I've looked at four Lexel Mixture sheets, and they all have less then 10% difference in the solutes. Most have perhaps 1% of a solute for the dye and then balance glycol or other solvents."

Solute? Something screwy going on in the terminology here. A solute is something you dissolve -- the dye, for example. A solvent is what you dissolve it in. (Is this a special use of the term, just for dye lasers?)

"One thing is for sure, I'm now convinced that there are as many dye mixtures as there were dye laser operators, squared."

Heh. Yep.

"Planters, how many Joules is the Sci Am dye laser? Not too many if I remember right. What, 40?"

Gargh... I just tried to find the SciAm/AmSci stuff on line, and failed. I was reading one of the columns just a few weeks back, but I don't remember the site, and I'm not finding it with a search. Sigh.

"I'll pull the coherent dye manual on monday and scan it in. I'll see if my local U still has Schafer on the shelf.
I'll take a pic of a collector tube in the morning."

"Jon, I have lots of broadband OC/HRs for 1-3% transmission at 647-670, they are NOT narrowband,"

Hmmm. I will eventually have things going on from 612.5 or so to >700 (helium-iodine laser, we hope), so that kind of wavelength is very happy.

"PM me with a shipping address."

Will do.

"In fact I'll send you some blue/green ones,"

_Many_ thanks!

"some of the blue/greens reach well into the yellow."

Excellent -- even if they only reach into the green, they'll cover enough of what I have in mind. The problem is that because they're going into a CR-599 I need HR mirrors with 7.5cm RoC. That's just going to be a bear. If I had a complete machine shop here I could probably build tiny lens mounts and use flat mirrors; but that would probably screw up the astigmatism and the dispersion. Besides, I don't have a machine shop.

"Steve"

Cheers, and thanks again --
jon

[jonsinger]

@planters --

"I built that laser many years ago and was frustrated that the 65J I expected based on the instructions was not nearly enough and not till 200 J did this laser suddenly gather all that orange fluorescence into an unmistakable far field laser spot, but when it did, man was it bright.

I remember when my first one actually worked. I just about screamed.

"The capacitor was a monster; 6Kv/ 60uf/ 40nH."

Oof. That's huge.

"The elliptical reflector was marginal and yes lots of pump light leaked out the ends."

I got lucky -- I was advised right from the git-go to forget the ellipsoidal reflector and just tightwrap aluminum foil around the lamp and the dye cell. Works just fine.

"The cavity mirrors were harvested He Ne mirrors."

Near as I recall, you tend to get beams out both ends with R6G and HeNe mirrors, but that's okay -- the gain is high enough to handle it. I 'spect that HeNe mirrors would work better with RB if you're looking for low threshold. RB is peculiar, though -- the fluorescence quantum efficiency seems to depend a lot on solvent viscosity, so it works nicely in EtGly but not so great in methanol.

"What a mess, but if it worked then flash lamp pumped dye lasers must be pretty forgiving and with the right setup..."

Best believe it. I've never tried to maximize the output energy, but I've pumped them with lots of drivers, and they are _very_ forgiving. I will admit, though, that my usual tactic involves small capacitors with very low ESL ratings, at rather high voltages. That results in fast pulses with high current (and thus high peak power), which is why I can get away with low pump energy.

"What would be the optimal mirror set up for a R6 dye?"

Geez. Kinda depends on what you're doing. (For example, whether you are looking for low threshold or high output power.)

"Would it be similar for cw jet and flash lamp?"

I don't think it can be. The CR-599 takes two HR mirrors that have about 7.5 cm RoC (the "rear" mirror and the "fold" mirror) and one [flat?] OC. I believe that most flashlamp machines, OTOH, take one flat HR and one curved OC, with the curvature depending on cavity length. Just not the same kind of animal...

"Should it be symmetrical vs asymmetrical, plano/concave,and a reasonable range of OC%"

As before, it all depends on what you're trying to do. Wanna maximize the extraction efficiency from the pumped volume? Maybe you should think about some kind of unstable resonator? This is an area I've never messed with, and I can't really even pretend to advise anybody. I have almost always just grabbed whatever mirrors I could find. My first few dye lasers were just flat-flat, which makes fairly good use of the pumped volume but doesn't create anything like a useful mode structure. I've used lots of HeNe mirrors, and when the RoC of the OC was too short for the length of the cavity, I thought about how an OC works and just turned it around backwards. Again, not a pretty mode structure, but it does get the nice orange light onto the wall. (I've done that hack a lot of times.)

"My lamps are 7mmOD x5mmID x 60 cm long 500 torr xenon filled and the circuit plan was very crude."

60 cm long? Zowie! That's outstanding. You are almost certainly going to have to deal with reflections from the walls of the dye cell, but I bet you can finesse your way around that.

http://www.jossresearch.org/pictures...littrow.8c.jpg

(That's a few feet away from the laser; the lit region is maybe 8" across. I did not make any attempt to refine or clean up the optical path inside the cavity, and as you can see I paid the price. Cute, though.)

Nothing wrong with a crude circuit plan, as long as it has low inductance.

"The rectified switching PSU from the original laser would charge a 1 uF " you roll your own" 60cm long capacitor laying parallel to and in close proximity to the laser head. This capacitor will discharge through an adjustable air spark gap by over voltage. If too unstable or if the lamp fails to self terminate then I would add a trigger electrode."

1 uf should be viable, provided you build it with very low ESL. At 10 kV that should store 50 J, and it will be significantly faster than 60 uf. If you can keep your total circuit inductance down to about 250 nh, you could easily see pulses that are only about 1.6 usec FWHM. (I'm assuming that the driver circuit matches the lamps properly; any mismatch, either underdamped or overdamped, will do bad things to the peak power. An overdamped circuit will have fairly short risetime, but the pulse will go on and on and on. An underdamped circuit will ring, so the energy will get spread out over more than one pulse...)

"I'm still interested in the Q-switched pump. In this kind of a set up I'm assuming a jet is unnecessary and only sufficient flow to mix and cool the dye is necessary and this can occur in a completely contained loop. Am I wrong?"

I haven't built one like that, but I think you are quite correct. It partly depends on the rep rate, though. You have to be able to get at least one full exchange of dye between shots, and I think you really want 2 or 3 exchanges so you can also cool the walls. Don't quote me on that, though. As I say, I haven't done any high-rep-rate work, and my dye flow is decidedly leisurely.

"In any case what about dividing a single Q-switch pulse into two components; one to drive a small "seed" laser which then passes through an amplifier pumped by the second component of the original pulse? This might facilitate modulation of a powerful laser by controlling/conditioning the lower energy seed laser."

Definitely. The small machine is much easier to control, and the amp gets you some decent energy. Something sorta vaguely analogous was done by some people (I think maybe even including Ted Hansch [how do you get an umlaut here?]) to amplify a pulse from a modelocked laser. They took a Nd:YAG machine, doubled it, split the resulting green pulse 3 ways, and pumped 3 amplifier sections with it. I think the energy break was 5%, 15%, and whatever was left (some losses in the optics, so maybe 70-75%?). The third amplifier was longitudinally pumped from its output side, and they had a saturable absorber cell between the 2nd amp and the last one to keep the pulses clean. Pretty hefty, and apparently worked nicely.

One thing you probably need to worry about is ASE -- the amplifier may add a lot of noise to your carefully tailored signal from the oscillator. (I say this because I vaguely recall having seen something about it somewhere; it has been too many years, though, and I don't remember any of the specifics. You may want to do a bit of a literature search on MOPA dye lasers...)

Cheers --
jon