DPSS Violet?

This may be a stupid question but I see that almost all green lasers start with an 808nm pump diode and then passes the light through a Nd:YVO4 crystal to convert it to 1064nm and then passes through a frequency doubler to reach 532nm. And all of this done up to a maximum of around 30% efficiency. So my question is what if you just passed 808nm through the KTP doubler to reach 404nm?

Under Ideal conditions, the KTP crystal has a conversion efficiency of 80% With a 2.5W pump diode, that could mean breaking 2 Watts on violet lasers.
There must be something I am overlooking, and if this is a stupid and ignorant question then you have my apologies.

There is 808 nm DPSS GaAlAs diode lasers frequency doubler technology for 404 -405 nm , but the efficiency is verry low just as yellow laser DPSS is even lower.

Unfortunately, with KTP, it isn't simply dumping power into one end and watching it shit out a stream of photons through the other.

Several issues come into play here; power density is one of the most obvious. You might think that 2.5W seems like a lot of power, but frankly, it isn't. Second harmonic generation (frequency doubling) has an extremely non-linear efficiency curve- the higher the power, the higher the converstion efficiency becomes. If the power density is too low, you won't get anything out at all.

To put it into perspective, your dinky 5mW green pen laser has WATTS of intracavity power at any given point. Said power is concentrated as a spot that's micrometers wide.

Shooting a diode through a KTP doubler won't get the power density you need, and not only because it makes one pass through the cavity.

Remember that power density is a combination of area and power- in the pen laser, the 1064 beam is only micrometers wide. With a diode, the spot will be significantly bigger.

Now, assuming you got the diode down to a small enough point, you'll also have issues with phase-matching the diode to the KTP. Considering that most diodes at that power are horrible multi-mode, multi-emitter devices, it certainly won't be easy. And assuming you could phase-match it perfectly (which is damn near impossible), it'd be just as hard to get the power density needed.

Anyone remember the directly-doubled Novalux Proteras? They used a special VESCEL diode (which produced a clean, Gaussian beam with very little astigmatism), which also had an external cavity. The cavity didn't just cover the laser diode itself; the doubling optic, the LBO crystal, was also part of the cavity. Consequently, the power within the cavity was higher by magnitudes than not only what was being pumped in, but also what was being emitted. Those diodes are extremely sensitive, but it was the only way to achieve direct doubling.

So, assuming you had an external-cavity, single-emitter 2.5W 808nm diode, you have a slightly bigger problem.

KTP is a glitch. By some quirk, it's brutally efficient at doubling 1064nm. By the same quirk, it can't be phase-matched to double below approximately 500nm, and it won't double at all below 490nm (if I remember correctly).

Consequently, you're left with other options including LBO (which is annoying to work with for more reasons than one) and BBO. Neither of them have the efficiency of KTP.

goninanbl00d said:

Unfortunately, with KTP, it isn't simply dumping power into one end and watching it shit out a stream of photons through the other.

Click to expand...

^^This is pretty much how life works.

No free photons.

Excellent explanation!

I didn't know that the 1064nm beam was on the order of micrometers wide..

Meatball said:

^^This is pretty much how life works.

No free photons.

Excellent explanation!

I didn't know that the 1064nm beam was on the order of micrometers wide..

Click to expand...

Yep, the intracavity beam is tiny (often the size of the pump diode die, or less (if a focusing optic is used).

Once this beam exits the cavity it's expanded and then recollimated. If the beam that exited was the same beam in the cavity, you'd need tens of watts to sustain SHG within the KTP.

Using frequency tripling it's possible to get violet, but not really visible violet. I've got a little DPSS 355nm pulsed UV head here that works quite similarly to a DPSS green but with an extra nonlinear crystal. The light is ever so faintly violet, but it's very dim to the eye even though each pulse packs almost 4kW of peak power.

Elektro, where did you manage to get something that low in the UV spectrum? I've been looking for something like that....

Even on the surplus market, heads like that sell for over a thousand dollars easily.

@ EF and what is this laser normaly used for ?

BTW I bought Quick Show today
Thanks for the help !

Hmm. Well I guess I am not getting a 355 any time soon. Maybe they will eventually make UVB diodes for better optical data storage... I mean, they started out at 980, then 808, then 780, then 670, then 405... what's next? XD

Hmm!

Turns out a 343nm LD was just prototyped... it's not practical yet, but it's been built!

http://www.opfocus.org/index.php?topic=story&v=3&s=2

AND... Thorlabs even offers a 375nm LD... but for $4k.

http://www.thorlabs.us/newgrouppage9.cfm?objectgroup_id=5400

Flaminpyro said:

@ EF and what is this laser normaly used for ?

BTW I bought Quick Show today
Thanks for the help !

Click to expand...

TBH I'm not sure what my specific head was used for.. I would guess for fluorescence experimentation or something along those lines.

Here's the webpage for the system I have: Ultraviolet (UV) pulsed (Q-switched) diode-pumped solid-state (DPSS) laser for laser microdissection, wavelength - 355 nm. "Laser-export Co."

And I would sell it, but the last time I shipped a costly laser head it was destroyed in transit.. I'd rather not go through that again. I guess I could ship insured, which I did last time, but I can only imagine what a pain it would be to collect on a claim if something happens again.

And Jeff you should really enjoy QS.. It's top notch. Just wait until you get a full-color projector together.. then you'll really see what it's capable of.

goninanbl00d said:

Shooting a diode through a KTP doubler won't get the power density you need, and not only because it makes one pass through the cavity.

Click to expand...

Well, shooting a 1064 Nd:Yag beam through a KTP crystal can actually produce green light, although its not as efficient as intracavity doubling, and requires a pulsed laser to achieve enough power density.

I've seen it with my own eyes though quite a while ago. The laser was the size of a shoebox, no idea on its specifications.

goninanbl00d said:

Now, assuming you got the diode down to a small enough point, you'll also have issues with phase-matching the diode to the KTP. Considering that most diodes at that power are horrible multi-mode, multi-emitter devices, it certainly won't be easy. And assuming you could phase-match it perfectly (which is damn near impossible), it'd be just as hard to get the power density needed.

Click to expand...

Additionally, the light from a laser diode isn't all that monochromatic, and you need 2 photons with nearly identical wavelength and phase to make the doubling process work.


The yellow lines are generated in a more complex system, where 2 lasser lines with different wavelength are created (1064 and 1319/1352 nm) in the Nd:YAG, which are then combined with non-linear optics in the cavity to generate 589/592. The efficiency of this process is perhaps 1% overall.

The 1352 nm line is also used in red DPSS lasers that output 671 nm (not a very desireable wavelength, but better optical specs than that of diode lasers).