Laserscope collimation (or lack thereof)

[weartronics]

Hi PL,

I removed the fiber coupler from a Laserscope and measured the collimation of the beam coming off the optical table. Interestingly, the beam without the q-switch is well approximated with a gaussian distribution.

I fitted the gaussians to the beam profiles to calculate the 1/e^2 radii, then I calculated the divergence is 8.50 mrad full angle. In simple terms, this means ~100mm diameter beam with ~10m throw distance, ~200mm diameter beam with ~20m throw distance.

I assume that Laserscope already did the best job they could of collimating the horrible multimode beam coming out of the OC, but this is much higher divergence than I expected. Are your results similar, or do I have a problem?

Best regards,

weartronics

[mixedgas]

I assume that Laserscope already did the best job they could of collimating the horrible multimode beam coming out of the OC, but this is much higher divergence than I expected. Are your results similar, or do I have a problem?

[/quote]

Your actually doing better then most examples I have seen.

No problem, quite normal!

It gets worse with thermal lensing/mode hopping/beam wander when misadjusted and with the Q switch.

I've never found a good solution, as I only get to work on them for a few hours at a time, but I bet laserman532 can sell you something or point you in the right direction.

Steve

[Laserman532]

yep, thats pretty much it. A small trick is to move the provided collimating lens down the path about 6 inches if you locate big mirror scanners there you will have a better shot of not over filling them. The only other thing you can do is up collimate the beam, your divergence numbers will be better but your beam diameter will increase.

[dsli_jon]

Hey Nic -

Hi PL,

...the fiber coupler...

I assume that Laserscope already did the best job they could of collimating the horrible multimode beam coming out of the OC, but this is much higher divergence than I expected. Are your results similar, or do I have a problem?

No, no 'problem', buddy - and you answered your own question! Remember that Laserscope only had to worry about 'collimating' the beam to get down the length of the deck, into the endoscope fiber-launcher - with UNcollimated bare-fiber output as the desired-result for this surgical-purpose laser - you kinda don't want a nicely-collimated beam inside a gut!

I think (...yeah, you did get all that info I sent ya, right?) you'll find much better results with the 400mm, for on-deck galvos, or, for launch-into a topside / external beam-table, etc, a 750mm, than that 'stock' 200mm... yes, that is the best you will get out of that lens...

KIT... ciao
j

[weartronics]

Hi Steve and Pat and Jon,

Thanks for all your comments! I understand that moving the stock lens or replacing it with a longer FL could decrease the diameter, but this must be at the expense of divergence, right? If anything, I want to go the other way (increase diameter to reduce divergence). At this stage, I will keep the current ~7mm diameter and 8.50mrad divergence, though I might up-collimate later. Originally I thought I would have to buy a telescope, but maybe I can just move or replace the stock lens in the unconventional direction (good idea thanks Pat)!

The reason I measured the divergence is because I wanted to see how much worse it would get if I fiber coupled the laser source to the projector. According to my calculations, if I used an 800um core, 0.37 NA fiber, divergence would be approximately 10 times larger for the same beam diameter . However, if I could use a 100um core, 0.22 NA fiber, divergence would actually be smaller for the same beam diameter . Unfortunately this idea is not feasible because the power capacity of such a small fiber is insufficient, but is it really possible to improve the beam characteristic just by putting it through a fiber? Are there current applications for this?

Best regards,

weartronics

[dsli_jon]

Hey Nic -

Hi Steve and Pat and Jon,

Thanks for all your comments! I understand that moving the stock lens or replacing it with a longer FL could decrease the diameter, but this must be at the expense of divergence, right? ... but maybe I can just move or replace the stock lens in the unconventional direction

Umm, well, no not quite - the whole point of changing the 'stock' 200mm to a 400mm (or whatever you determine works-best for your desired 'end-result'...) FL, IS to 'wrangle' the divergence - yes, it will 'change' beam-diameter a bit, but only to it's focal-point and then back-out - the 'divergence-management' COMES-from the focal-length you choose, and like Pat said, finding the 'sweet-spot' along the beam-path, to optimize between fitting on your galvos, / gaining the 'longer-range' results you desire... really, the only reason to 'up-collimate' is to get 'better divergence' over a very, very, very long distance - or - if you just like / want a big-fat (but well-collimated) beam...

The reason I measured the divergence is because I wanted to see how much worse it would get if I fiber coupled the laser source to the projector. ...but is it really possible to improve the beam characteristic just by putting it through a fiber? Are there current applications for this?

No. It WILL 'get worse'. BUT that doesn't mean that you can't end-up with a 'reasonably well-collimated' result, out of a fiber-fed projector. ie: What we have found works best, is 200µm fiber, 20W MAX, going in, and keeping your 'run' to less-than 150', and - actually - using a OEM Laserscope INput-coupler lens (see below) AS the OUTput coupler lens.. I've experimented extensively, with all-kinds of lenses / FL's / combos / off-the-shelf telescope / colli-combos, etc - and I have never seen better 'overall' results, than the 400mm, in that on-deck 'sweet-spot' I sent you...

I'd suggest, before 'pulling your hair out' too much-more get a simple 400mm and 750mm plano-convex from Edmunds Optics (400mm, AR-coated: part# NT47-724; 750mm, AR-coated: part# NT47-388) and, well... experiment! But be careful!!! (sorry, I know that was rather 'grandmotherlike'

Yeah, sure, you can also try the 500mm, but I never found it was needed... like I said, we found 400mm was perfect for 7-8mm-aperture on-deck galvos, and the 750mm was better for a 'beam-only' unit, that, say, had a removable / umbilicalized deck to sit on a giant, external beam-table (or, for one where you built an 'upper-deck' version...)

Hope this helps, Nic... maybe re-read that PDF (??)

ciao
j

[LesioQ]

Jon,

Your recent pictures .... I thought You were on holiday .... or ... did You get Your toys to San Sal with You ??
Piotr.K

[weartronics]

Hi Jon,

Thanks for more tips and photos of your fiber coupling. I really appreciate you sharing so much knowledge about this machine, it saves me a lot of time and experimenting. I did read all the good stuff you sent me, and if I go ahead with putting a projector on the optical table I would use your advice re: collimator positioning and focal length.

However, I'm still thinking fiber coupling would be preferable...

Could you tell me a bit more about your fiber coupling? I have only used pre-assembled cables in the past, I never terminated a fiber before. Wouldn't you have to AR coat the end faces? I am not equipped for coating . Could you give me any more details about the type of fiber and connectors you're using (can I buy them from Edmund)?

Another crazy idea, I wonder if the beam from the resonator is sufficiently randomly polarised that I could use a PBS as a 50/50 splitter to get two fibers out of one machine, or is there a better way to accomplish this?

Best regards,

weartronics

[dsli_jon]

Hey Piotr -

Jon,

Your recent pictures .... I thought You were on holiday .... or ... did You get Your toys to San Sal with You ??

Hah, no these are just pix I have with me on my laptop, from the 'Bat-archives' ... but, um... "holiday"... what's that?

ciao...
j

Hey Nic -

Thanks for more tips and photos of your fiber coupling. I really appreciate you sharing so much knowledge about this machine,...I go ahead with putting a projector on the optical table I would use your advice re: collimator positioning and focal length.

Well, my only point is - experiment! There is not really an 'absolute', 'end-all / be-all' answer for collimating these things - best approach is to try a few things to see what works for your-particular systems' beam-characteristics / goals for your system is...Even though Laserscopes are very-well engineered laser-systems, you will see differences in beam chars from system to system - again, all LS had to worry about was getting each one 'in-tolerance' for THEIR specs / purposes, that is, down a fiber, with 'raw' output...pretty 'mushy' by our 'standards' / expectations... And proof-that there IS intrinsic fluctuations in beam chars, is WHY they have such an extensive beam shaping / attenuation / control-stage, post-collimator (you know, all that 'medicaly-stuff' on-deck...)

Which, btw, is a 'treasure-trove' of optics and solenoids - you can even 'R-E' the 'photocels' they use for beam-power sampling, to make your 'own' on-deck power-meter and those lucas-ledex solenoids make (in addition-to great beam-shutters) for very-stable fx-position actuators, and even fiber-launcher actuators...

However, I'm still thinking fiber coupling would be preferable...

Well, again - depends on what your 'goals' are... fiber CAN work, but it WILL give you an inherently larger, and more-divergent beam to 'work with'... if you're expecting 'crisp graphics' out of it, prolly forget it - fiber-remotes are usually best for fx / aerial / atmos projectors only...not to say, it cannot work for 'graphics', but they'll just be 'mushy', at best...

Could you tell me a bit more about your fiber coupling? I have only used pre-assembled cables in the past, I never terminated a fiber before.

Well, unless you really wanna do it for the 'mountain-climb' of-it I'd suggest just buying a 'show-ready' fiber, from someone who knows how to build fibers for KTP - ie: Precision Projection Systems, out in CA, or, PL-Members 'One Stop Laser Shop' in FL - they have actually done quite a bit with KTPs / conversions, and can def. hook you up with a good KTP fiber, with the SMA connectors, etc - even a 'polishing' kit, for when you 'burn' an end... to get into cleaving and terminating your own fibers is quite a 'juggernaut' (..not trying to discourage you, but just being honest - it's a bit of a 'learning-curve'..)

Another crazy idea, I wonder if the beam from the resonator is sufficiently randomly polarised that I could use a PBS as a 50/50 splitter to get two fibers out of one machine, or is there a better way to accomplish this?

Yeah, a simple 532-coated 50/50 dichro will be just fine - there is usually at least one in all the 'attentuators' on a 'raw-medical' deck (some are 60/40, 70/30, etc - you have-to just test to see what 'came' with yours...) and the twin-ports of the LS-deck make for PERFECT twin-launcher-ports...

Anyhoo, gotta run... can type more later..

ciao for now...
j

[weartronics]

Hi Jon,

That's an unconventional prism arrangement... what's it do?

I would really like to learn how to terminate the fibers, I have read a lot about it before and I would like to have a go. Previously I thought the reflection would be too high without AR coated ends, but I am prepared to try it.

Could you give me any clues about the 200um fiber and SMA connectors you use? I can't seem to find 200um fiber at Edmund.

Cheers,

weartronics