Reducing beam diameter for AOM blanking

[weartronics]

Hi PhotonLexicon,

I am looking for comments on the best way to reduce the Laserscope beam diameter for AOM blanking (using an external AOM, not the Q-switch).

The first way I considered is to use telescopes before and after the AOM. This way, the distances are not critical and there is a small diameter, collimated beam passing through the AOM, but it requires two lenses on each side.

The second way I considered is to put equal focal length lenses on either side of the AOM at equal distances from the AOM, which will make a beam waist at the AOM. This only needs one lens on each side, but I don't want to make a hot spot in the AOM crystal at the focal point.

The third way I considered is like the second way, but to position the AOM slighty offset from the focal point so that the focal point is not inside the AOM. However this means the beam is converging or diverging as it passes through the AOM. I think this might be the best way if there is enough space to use long focal length lenses.

Has anybody done this before? Which method did you choose, or what would you recommend?

Best regards,

weartronics

[Laserman532]

I'll take a stab at it. I would not do that, you will fry your crystal if not instantly may be before that, I see no good outcome from trying it unless you have a bunch of crystals just laying around you are not using. I use the q-switch and get about 96-98% depth of modulation with NO ill effects on laser what so ever, It is the only way I have done it. Most laserscope RF drivers have FPS (first pulse suppression) or some call it Pulse Kill. If your driver does not have FPS some people prefer mechanical blanking and that is fine and works fine. You are creating a boatload of grief to do what you want to do.

[weartronics]

Hi Pat,

I understand that the pulses are many times more powerful than the CW beam, but so long as the power remains below the damage threshold of the crystal I don't see why there would be any problem... I know you have a lot of experience with Laserscopes in particular. Could you explain why you think the AOM will be damaged?

Best regards,

weartronics

[dave]

What I think he is saying is you can use the scopes q-switch to do the blanking, but the q-switch needs to have first pulse suppression. If it doesn't, bye bye crystals. Also, I think indefinite hold off time is important as well.. Mentioned by Steve somewhere else..

[LesioQ]

I think that if You use collimating optic prior to AOM crystal then not whole beam will get diffracted back into the output optic path so the losses will be high. It's assuming You will be using 1st order beam as the laser controlled output.
Addidtionally the 0th order beam is at close angle to 1st order so most likely it will hit the lens after the crystal anyway causing ghost spots.

Just a feeling

BTW: has anyone tried analog "blanking" with q-sw drive in a z-fold ?
I think I will give it a try following weeks ...
Piotr.K

[Laserman532]

Having never done it, I could be mistaken. But to my knowledge most AO cells were meant to be operated intra cavity with the exception of very low powers (have you ever seen a PCAOM operate over a watt or two of CW POWER let alone Q-SW). A q-switched laser has extreamly high peak powers, if you get past the coatings, then focused at the bragg angle and make it back out..(of course getting past absorption). what would be your extinction ratio anyway? What about the scattering / deflection anomolies making it through your lens system? Would it be better than blanking with the IC AO CELL, i think not. Will you coat your AO Cell with AR coatings for 532 or just slap a 1064 in the out put path. Out of all the methods of blanking laserscope beast...this would be the last choice I would try. Timing issues arent a huge problem, because you will be sending packets but your extinction ratio and exceeding damage threshold are the biggies. Additional COST Factors???

I reccomend you try it and prove me wrong, that is how science advances!

Dave, he is talking about exta cavity blanking, I suggested intra cavity blanking.

Oh Lesioq, By the way, hows that taking the beam out the RAM going? (did you get half the spot size with twice the divergence, and did it knock your power down???)

[mixedgas]

Neos makes both 20 watt PCAOMS and 200 Watt AOs, so it is possible.
I've also seen split and recombine pcaom systems with 2 crystals, for those big honking internal mirror Innova sabres that are not polarized, because they have no brewsters.

You can try it with a low cost AO, quartz or Teo2, but NOT a glass AO.
Thats fine because only a few glass AOs are out there, and if you hold them up to a light you can see a deep greenish tint. They were made for cheap hene applications.

TeO2 has a wopping high threshold. I would not bring the beam down to a tight focus in the crystal, but get it down to a mm or so.

Pat is right, you could fry a cheap thin crystal hene AO, but if you get one with a fat crystal, no problems. I've got one around here with a 1 cm tall 5 cm long crystal, I bet it does it no problems.

I've heard mixed results on using the Q switch as a modulator. It should be no problem if the driver has giant pulse supression.

But I have a qswitched yag with a Intraaction Q switch that warns not to hold off for more then 4 minutes , or the driver goes bye-bye from the heat.

I doubt a good quality driver such as a NEOS would melt if the case, including the 19" rack cases, have external heatsinking.

That 50 watts of RF drive means 100 watts gets dissapated in the final RF amp. So heatsink accordingly. When its pulsing away the dissapation is much, much less.

You might try pseudo blanking, with yoru external AO, where you use the oth order as on, and subtract energy out into the 1st orders when blanking. Its not 100% extinction, but it works. Who cares if you have a 2 watt retrace at 60 watts. IE no RF when unblanked, RF into the crystal when blanked. This gets rid of the ghost problem for sure. You will have to invert in software or add a inverter chip.

So your gonna have to try it.

Your not using the somewhat lame Laserscope intracavity mechanical shutter?

It goes without syaing you need to watch the ghost beams out of BOTH sides of the AO at that power. Be careful!


Steve.

[LesioQ]

To my expirience AOM should operate with parallel beam of light. This means that if You use a lens prior to AOM cell the incoming beam will be convergent, thus only a part of it will be able to meet Bragg angle criteria. Te remaining part of the beam will not get diffracted to 1st order spot past the crystal.

If AOM has a large aperture and piezo acoustic wave transmitter is generating 'thick' layer of acoustic wave then high aperture beam will be able to interact in the crystal successfully.

I think for extracavity q-sw 532nm blanking You would need a proper aperture AOM crystal, not tightening the beam to small AOM You'd like to use as this also increases energy density problems.

And what's the purpose of external AOM is interesting ...
I'm still ahead but I have a feeling that q-sw use for 532nm intensity control will be my fancy During the modelling of the FPS interface to my q-sw driver I noticed the possibility of 532nm analog intensity modulation both in q-sw and CW opertion of my z-fold. Probably far from linear response, still an analog one.

Here's the basic idea:

Top (blue) is the intensity request. High slopes would be slowed down to 1ms for FPS.

Bottom (green) is RF intensity to driver (q-sw pulsing not to time scale {i.e. divided by 100} for visibilty).

Piotr.K

[mixedgas]

[quote=LesioQ;86167]To my expirience AOM should operate with parallel beam of light. This means that if You use a lens prior to AOM cell the incoming beam will be convergent, thus only a part of it will be able to meet Bragg angle criteria. Te remaining part of the beam will not get diffracted to 1st order spot past the crystal.

end quote.
It has been my assumption that the OP had a qswitched laser and wanted to do extracavity modulation, if I am wrong, please tell me now.

You are somewhat correct, but there is another more common operation region of the crystal they dont mention in the basic and intermediate theory books.

Focusing down to increase blanking speed for video applications is a common thing in AO systems. It shifts the operation range from Bragg to Raman-Nanth and the power pretty much stays up. Bragg only works for a true Rf sinewave and first order anyways.

Defocusing and upcollimating to increase scan angle by filling the whole cell is another one.

Steve

[weartronics]

Hi everybody,

Thanks for all this information so far. Actually my idea is to use a spinning mirror to scan the beam horizontally and an AOM to blank the beam to make 360 degree radial patterns. I am also considering to use the AOM for vertical deflection, which is why I am asking about the external AOM.

There is some limited discussion of AO Q-switches in Siegman's book, but no discussion of AO deflection (though, I suppose the principle is exactly the same). I would like to find a manufacturer who has application notes with usage examples for deflection or a good discussion of the theory. Alternatively, if you know a reference book that covers this area well, I would consider to buy it. I have seen examples of both collimated beams and focussed beams in the AOM crystal, but I don't know much about either case, I am interested to learn.

Best regards,

weartronics