Modelling the Laserscope resonator

[weartronics]

Steve alerted me to PSST,
http://www.st-andrews.ac.uk/~psst/
what a great modelling tool!

I created a model of the Laserscope 700/800 series Z-fold resonator,
http://external.weartronics.com/lase..._resonator.mcv
but I have a question...

I am not seeing a beam waist in the KTP, and I am not seeing the expected 2.5X magnification from the end face of the Nd:YAG rod to the KTP. Kuizenga says the 4.0mm diameter rod should image to 1.6mm diameter in the KTP. PSST only models TEM00, but at least I expected the ratio to be the same.

I suspect this is because the nonlinear component of the Nd:YAG refractive index is wrong. I realise this depends on laser power and temperature, but if you're interested, could you download the model and see if you have a better estimate than me?

[LesioQ]

I'm having problems feeling the thermal lensing in the YAG rod being expressed as lens duct parameter... How do it's parameters relate to focal length ?
Piotr.K

[weartronics]

Hi Piotr, relating n0, n2 of the Nd:YAG rod to the focal length is my problem too...

According to Siegman, thermal lensing can be approximated with a thin lens. He says for typical Nd:YAG rods, the typical focal length is 1000mm for a few kW of input power (this is on p843), so the effect is weak, but significant. In the patent, Kuizenga says the back focal length (which is set at 170mm in the resonator design) varies from ~150mm at max power to ~400mm at low power due to thermal lensing. I calculate that 1/170mm (physical resonator BFL) + 1/1250mm (thin lens compensation) = 1/150mm (resonator BFL at max power), therefore "Lens 3" at position 169.99mm with focal length 1250mm can approximate the thermal lensing. This makes the distribution in the resonator more like I expected (there is a beam waist at the KTP), but I am not really confident with this model without understanding n0, n2 of the Nd:YAG rod, for example I can not set the n2 to zero. If you can find any forumla relating n0, n2 of the Nd:YAG rod to focal length, please tell me.

[LesioQ]

I think 1200mm range for thermal lens will be too long. Using 3mm rod I've expirienced as short as 200-240mm of rod's f.l.
My memory remids me that Kuizenga was mentioning like 40cm of f.l. and resonator stability range around that value. My expirience is that the thermal lens has the biggest impact on z-fold resonator. And rod f.l. is dependent on many factors like rod dia, doping, pump cavity reflector type (ceramic=diffuse or gold ellipse) rod side finish (frosted or polished), pumping schema and power and others.

My basic claim was actually related to z-fold mirror reflectivity, not curvatures.
In typical situation the 532nm output beam emerges through OC optic (50cm cc). My observations are that if RAM optic was allowed to pass 532nm then the green beam would be available for further manipulation closer to i's origin (i.e. KTP) thus having better initial specs, than the beam we get from OC optic.

As a coincidence I do have a mirror HR@1.06/AR@532nm 15cm r.o.c. I would consider to put in place of RAM optic for evaluation in my hybrid KTP / Greenlight setup.

Piotr.K

[Laserman532]

I think 1200mm range for thermal lens will be too long. Using 3mm rod I've expirienced as short as 200-240mm of rod's f.l.
My memory remids me that Kuizenga was mentioning like 40cm of f.l. and resonator stability range around that value. My expirience is that the thermal lens has the biggest impact on z-fold resonator. And rod f.l. is dependent on many factors like rod dia, doping, pump cavity reflector type (ceramic=diffuse or gold ellipse) rod side finish (frosted or polished), pumping schema and power and others.

My basic claim was actually related to z-fold mirror reflectivity, not curvatures.
In typical situation the 532nm output beam emerges through OC optic (50cm cc). My observations are that if RAM optic was allowed to pass 532nm then the green beam would be available for further manipulation closer to i's origin (i.e. KTP) thus having better initial specs, than the beam we get from OC optic.

As a coincidence I do have a mirror HR@1.06/AR@532nm 15cm r.o.c. I would consider to put in place of RAM optic for evaluation in my hybrid KTP / Greenlight setup.

Piotr.K

for what it is worth the rod in a LSCP is 5 x 79mm... How much power are you getting out Piotr? Very nice build! I am trying to wrap my head around that if you make ram transmissive you would wipe some gain out of the cavity (green is generated both directions) I worked with Dirk Kuizengua for about 2 years during our development of the KTP for entertainment there is probably only one person who knows more and that is Iaasik Bass at LLNL (or was) My guess is Dirk has pretty much dialed in the parameters. Maybe so, maybe not but he is good

[LesioQ]

Thanks Pat!
For quite a time I ws working with Lasersonics pump chamber with 3x100mm rod and gold elliptic reflectors, but the results were not too good.
Now I switched to Greenlight YAG pump chamber with 7x125mm rod and huge 6kW lamp. My current single phase setup is limiting me to 25A to the lamp, giving ca 20W 532nm, q-sw. I'm considering modifying my PS to go up to 50A to the lamp to see what's waiting there for me Big Bang I bet

Outcoupling through RAM should just improve just beam quality, not power, due to smaller initial diameter at exit, thats all I guess. *What if* optic relay was set to 3.3 factor (from 2.5) is another uncertainity.

Piotr.K

[mixedgas]

Whose single phase PSU are you using? May I ask? I have a 3-5 watt green yag that I need a single phase 18-20A supply for.

Steve

[Laserman532]

when it comes to cavity design I would consult with steve R. I still think any output coupling at the 532 hr will remove doubling from cavity. I have to think...if that would have worked...they would have done it. (you know they wrote the book on it) 6kw should be fine if your lamp is rated for that You are only a tad short on power with 25 amps. But I am most concerned about your cavity not being completely enclosed. All it takes is one dust particle settling on KTP and.............well you know But it looks excelent otherwise!

[LesioQ]

Whose single phase PSU are you using? May I ask? I have a 3-5 watt green yag that I need a single phase 18-20A supply for.

Steve

Oh, it's a DIY buck switcher, SG3525 + IGBT. No line isolation. Plus a starter from a HeNe PS and a couple of microwave HV caps.

For below 25A I'd give a go to Analog Modules. But no starter, just a basic current source.

when it comes to cavity design I would consult with steve R. I still think any output coupling at the 532 hr will remove doubling from cavity. I have to think...if that would have worked...they would have done it. (you know they wrote the book on it) 6kw should be fine if your lamp is rated for that You are only a tad short on power with 25 amps. But I am most concerned about your cavity not being completely enclosed. All it takes is one dust particle settling on KTP and.............well you know But it looks excelent otherwise!

Yeah, enclosing the beam path is the nice thing in LS-KTPs. Too bad it's hard to build . But I do have a box of spare KTPs at hand .

These 80W Greenlights / Niagaras have the whole cavity sealed, not serviceable outside the factory. That must be sad not to be able to stick fingers anywhere !

Piotr.K

[weartronics]

*What if* optic relay was set to 3.3 factor (from 2.5) is another uncertainity.

Hmm, the beam waist/spot size in the KTP will be smaller (1.2mm instead of 1.6mm)... which means more green and maybe burned KTP .

I still think any output coupling at the 532 hr will remove doubling from cavity.

Could you give any more detail about this? I don't understand why it matters whether "OC" or "RAM" is HT532, as long as they are both HR1064.