Deep UV CW laser

[LSRFAQ]

Since when are invisible wavelength UV lasers used in show projetors.... :thinking:
You might wanna read the appropriate section of Sam Goldwassers repair FAQ.

AT theme parks, more often then you might think. Disney has used UV, and several touring bands have used UV, with florescent paint on sets or specialized phosphors on objects.

Think Tinkerbelle in Peter Pan, Waves in Little Mermaid, things like that.

Just do not expect the variance to be easy for this in the US.

Steve

 

[ElektroFreak]

I saw your post before you deleted it. Your input is welcome Steve.. you've got years of experience on me. Can't argue with that one bit ..

 

[anselm]

Yes, ok that makes a lot of sense with the fluorescing and all, but are they
really using 240nm-ish UVC?
Wouldn't UVA be safer and more convenient there?

 

[LSRFAQ]

Care to explain why? I think you misunderstand what I said. I was offering an alternative option so that the OP could in theory avoid buying that Spectra Physics doubling monstrosity because it is not necessary if all you want is to double the frequency of a TEM00 laser beam. Amazingly enough, none of my frequency-doubled DPSS heads contain a Spectra-Physics frequency-doubling laboratory device that I know of. .. and yet, somehow the doubling still happens.. and all it takes is the right crystal.

end quote.

Phase matching and tuning angle is easy at doubling, only three waves have to propagate in phase in the crystal. However at CW THG, it is a few orders of magnitude tighter because five or more waves have to propagate in the crystal and yet remain aligned in the crystal structure at the atomic level. You also need single longitudinal and transverse mode for CW THG, hence the expensive Sp and Coherent stabilized delta cavities, with four mirrors folding the unused pump energy back into the beam waist in the crystal.

This is why Innova Fred and Lexel SHG uses intracavity light, its already phase coherent and a about two orders of magnitude higher density then the exit beam of a normal laser.

You can do it with home made gear, you just need to know how to stabilize things thermally and control the length of the delta cavity for THG.

You can bet the 244 nm CNI has some stabilization far in advance of the simple CNI SHG greens.

High peak power pulsed laser THG is a different story. That is easy compared to CW. With 10s of kilowatts of peak power per pulse, it just needs to pass through a doubler and a doubler for THG.



Steve

 

[LSRFAQ]

Yes, ok that makes a lot of sense with the fluorescing and all, but are they
really using 240nm-ish UVC?
Wouldn't UVA be safer and more convenient there?

Usually its UV hecad or UV argon at 350-365, however in some cases, solid state at 244 is attractive because its DPSS.

Steve

 

[ElektroFreak]

Absolutely.. while my tone might have conveyed that it would be a piece of cake, that isn't what I meant necessarily. The DIY option might not be the simplest project, but I think it is certainly possible with some care.. and for a LOT less money than that doubling system that was linked to earlier. I have a 355nm THG q-switched lil beauty here and she's got some pretty serious thermal stabilization.. I'm sure the initial alignment was tough as well.

 

[goninanbl00d]

Well, don't forget that LBO and BBO needs to be critically phase-matched, as well as requiring exact temperature stabilisation (to a degree) for maximum efficiency (or for any output at all).

KTP is 'relatively' flexible when it comes to a SHG medium. Shoot photons at it, and it shoots photons with half the original wavelength back. Stick it in a Q-Switched's beam path, and you have doubling.

LBO, however, can't just do that. You can't just stick a piece of LBO or BBO into a cavity using a pair of tweezers, and expect efficient conversion (or any output at all). In a blue laser pointer, it's aligned, at the very least. Yes, it has no temperature stabilisation, but it barely gets away with it as it is (and with BiBO, efficiency has slowly climbed).

The CNI deep-UV units are all Q-Switched, even the smallest ones. The last CW ones are at 355nm, the 266nm units are all Q-Switched.

Don't forget the SHG'd ion systems need a stream of dry nitrogen to cool the crystal and stabilising assembly. The crystals are extremely hygroscopic.

If you wanted to, you could probably DIY the whole setup, but unless you know what you're doing and have experience, like Steve, it would probably be easier, and cheaper, to buy a commercial off-the-shelf solution.

 

[Bluefan]

Actually, if you look closely at mauswiesel's post he's using discrete crystals. Separate Nd:YVO4 and KTP.

It is more difficult if they are separate, you're right.. but not by that much. Look at every single Chinese DPSS 473nm unit around.. just a diode and two crystals. No crazy-awesome frequency-doubling module required. In fact, I'd wager that Spectra-Physics unit is intended to double something like a Mai-Tai or Millennium or some other of their laboratory systems.. It would be even simpler for MrAnders since he's only worried about doubling so there would be only one crystal. Can a whole bunch of cutting edge tech make doubling quite a bit simpler? Absolutely. Can frequency-doubling be accomplished successfully with a heater stage/oven and a single crystal? Again, Absolutely.

I have a 355nm q-switched head waiting to be figured out, but mine uses intracavity tripling. With q-switching the tripling (or doubling if you want) can be done outside of the cavity with a decent efficiency.

At CW however, a single pass through a nonlinear crystal will generate a higher harmonic, but just very little of it. Those lasers with discrete crystals have the doubling done inside the cavity, which is the important point. Inside the cavity circulates a lot of power, so the conversion will be much more efficient. For SHG the efficiency scales with the input power squared, and inside a cavity you can easily get 100 times more power than withotu a cavity. That gives a 10000 more efficient conversion (unless you have pump depletion or no phase matching).

 

[HIMNL9]

Care to explain why? (etc )

Sorry, no intention to start a war

I was just stating that for this type of doublers, is not so easy as for the 1064/532 operation for get a green beam, all there (but i know my English suck, sometimes )

I mean, for that type of doublers, you can use a 4th armonic doubler with a prism separator from a "standard" cavity, and get the 266nm relatively easily (as CNI do with their cavities), but using an LBO or BBO resonator for directly double a 488nm beam and obtain a 244nm output, is not so easy ..... you need the right type of crystal cut, couple it with precision in a resonant cavity with the right angle and polarization, keep it at its better working temperature, and "actively tune" the resonant cavity for stability (not speaking about the need to seal the cavity in a dry and possibly inert atmosphere, for the damn hygroscopic tendence of these crystals) ..... for this i said it may be not easy as "placing a crystal in a beam" as we usually do for get green .....

I've not said that it cannot be done at hobby level, i said just that it can end being a nightmare in fact of alignments, tunings, sealing and active-loopback stabilization, if you want to get some decent power from the assembly.

 

[ElektroFreak]

Ok.. perhaps some people just aren't reading what I was writing. I'm fairly sure I touched on pretty much all of that, but with much less detail. I covered the difficulties with alignment and temperature stability.. if you were to go back and read my posts you'd see it. I don't always have time to write down the intricate details, that's what you guys are for..

My sole point throughout all of this is that you don't necessarily need to go out and buy a $5000-10000 "frequency doubling machine" in order to double to UV. It can be done by hand for much less money. I'm just going to leave my input at that for fear of people misunderstanding that as well. Wouldn't want any posts to pop up like "EF, you cannot double to UV with ice cream.... silly guy"

 

[ElektroFreak]

EDIT: Just noticed the post about the difference between intra- and extra-cavity doubling. It's not outside the realm of possibility to create a resonator around the doubling crystal with relatively standard optical equipment.. I think it could be done on a hobbyist level, and especially in a lab environment. Based on the OPs requirements it seems like he might have scientific or industrial laboratory access.