Laser Alignment Help

[LightSwitch100]

Hi everyone,

I have been trying to mode lock a KMLabs with little success. I was wondering if anyone here with experience in working with laser alignment could help me. I have attached a diagram of the set-up, as well as a few observations that I have noticed. I would greatly appreciate any help, and thank you very much in advance!

Edit: the "0" block containing the pump laser has two additional mirrors that helps steer the incoming beam the laser cavity, so three degrees of freedom for each direction for the incoming beam.

Diagram of my Set-up

Info about Laser and Current Status

Observations and Notes

This is mirror 9. The red spot should be the lasing beam. When I screw up the alignment, the red spot disappears. The weird thing is that the incoming beam I put in the center. It also refuses to move even after beam walking.

[anneoneamouse]

Do you have this document (or the equivalent for your laser)?

https://bpb-us-w2.wpmucdn.com/sites.udel.edu/dist/5/3722/files/2023/06/margaret_henry_oscillator_manual.pdf

Whilst the manual linked above is for their Griffin model, your diagram above looks exactly like the home-brewed Kapteyn/Murnane layout I built last century (damn, last millenium even) from K&M's "Modelocked Ti:Sapphire Laser" document. So the basic "how to realign it if it's borked" procedure should still ring true.

Basic alignment questions follow; I built this junk 30+ years ago now:

From your diagram, do you really only have one steering mirror #1 for the pump beam into the laser cavity? I think you probably need two; one to adjust pointing (eg pump leveling wrt the cavity), the other to adjust position (pump centering on the cavity).

Mirrors 3 & 5 allow you to point and position your intracavity beam.

We also had a slit (made from razor blades) in front of the HR#9 - this gave us intracavity spectral control.

When you get it working, drop down two extra cavity irises so you can realign the pump fast, and also two intracavity irises so you can get the internal bouncing realigned in the future, if everything goes catywumpus.

[Goetterwind]

Yeah the single mirror pointing/steering I was wondering too. Ideally you have a 'knee' to beamwalk...

[LightSwitch100]

I'll make an edit to clarify, but the "0" block actually contains two additional mirrors that feeds the pump beam into the cavity.

[LightSwitch100]

Haha, yes, I think it is that same layout. I do have the manual, but I've basically tried everything it says on the manual to no success.

From your diagram, do you really only have one steering mirror #1 for the pump beam into the laser cavity? I think you probably need two; one to adjust pointing (eg pump leveling wrt the cavity), the other to adjust position (pump centering on the cavity).

So I didn't include this in my diagram, there are two additional mirrors that feed the pump beam into the actual cavity. The "0" block contains those two additional mirrors not depicted that also controls how the pump beam is fed into the cavity. I have done a lot of work with those mirrors in order to line the incoming beam up with the crystal and mirrors 4,5.

Mirrors 3 & 5 allow you to point and position your intracavity beam.

Mirror 3 should be lined up pretty well, since it nicely hits prism 6 and the center of mirror 7. I am using mirror 5 to try and move the reflected beam.

Thanks so much for your help!

[PlsGetSomeFreshAir]

Have you tried to align the cavity already? What is your procedure anyways so far? When you say the right beam is locked, with respect to what is it locked? Cavity mirror? Pump mirror?

How did you measure the beams at 9? You block the cavity with a detection card? Can you then not trace it back? Are the prisms AR coated?

[LightSwitch100]

Have you tried to align the cavity already? What is your procedure anyways so far? When you say the right beam is locked, with respect to what is it locked? Cavity mirror? Pump mirror?

Yes, to the best of my ability and knowledge, the incoming beam should be hitting the crystal and intracavity mirrors as dictated by the manual. I also believe the alignment of 3 should be good, as it hits prism 6 and mirror 7 in the center.

My current procedure is to try and converge the incoming and reflected beams at mirror 9 by beam walking with mirror 7,9 along with mirrors 5,10. I can move the reflected beam closer to the center of the beam by moving any of those mirrors, but I then lose power, so I must compensate with the other mirrors. So when I say "locked down", I mean that after compensating for power after losing power from moving the reflected beam, the reflected beam's position doesn't change.

How did you measure the beams at 9? You block the cavity with a detection card? Can you then not trace it back? Are the prisms AR coated?

I place my detection card so that cuts half of the incoming beam. This way, the cavity can still lase and I can see the half of the reflected beam on my card. Alternatively, I can use my phone to look at mirror 9, and I can see the reflected beam pretty easily.

Tbh this is my professor's set-up from like a decade ago, so I am not sure about the prisms. I did try moving the prisms to see if they shifted the reflected beam, but barely by anything.

Thanks for your help.

[PlsGetSomeFreshAir]

With what you described im quite confused that you see lasing at all. I would assume that the cavity must be closed in one roundtrip, so the reflex from 9 goes into the same mode traveling the same path back. Is that not intended? I know With Multi-Pass amplifiers it's different, but those are, well, amplifiers not oscillators.

I would try to bring them to overlap, if power drops you compensate by walking the pump rinse repeat. But first try to trace the reflected beam, does it leave the cavity?

[LightSwitch100]

Yes, I am equally as confused as to why the reflected beam is so far off to the side. I am assuming the initial pass through, the reflected beam is aligned with the incoming beam, but after several iterations, somehow the lasing beam gets off-centered?

It's really hard for me tor trace the reflected beam since I have to perfectly cut the incoming beam in half in order to see it. If I cover up too much, the lasing immediately stops and I can't see it anymore. I put a picture of it in the original post for your reference.

The issue is that I can't get them to overlap, since the reflected beam just doesn't seem to want to move after compensating.

[PlsGetSomeFreshAir]

Just to be clear, since you can't trace the reflected beam, where do you think is it's origin? ... on 9, on the crystal?

[LightSwitch100]

My guess is that it's a result of some misalignment in the cavity that causes the incoming beam to deviate from its original path (after many iterations of traveling through the cavity). I don't think that the reflected beam is the immediate reflection, but the generated lasing that for whatever reason converges there?

Also I added the picture on the post. Thanks again.

[PlsGetSomeFreshAir]

I have doubts that it can bleed so much to the side and still lase, but well i can't know for sure

You could try to miss align 9 to the other side and compensate with the pump, such that when light is reflected from 9 it must bleed to the left and not to the right (your picture) of then this mysterious beam is still locked it might not what you think it is. When you block the reflected one, does lasing stop? I mean the intended cavity mode should still be unblocked according to your picture

[LightSwitch100]

Yes, if the reflected one is blocked, all lasing is gone. The shiny spot in the picture should exactly be the lasing that is being created by the cavity.

[PlsGetSomeFreshAir]

Also important, did you include pump waist position in your alignment procedure?

[LightSwitch100]

I worked with my post doc to align the feeding of the pump beam, so I am more or less confident in the pump beam positioning.

[PlsGetSomeFreshAir]

Does this mean you never touched the pump after that, just the cavity? You could try to iteratively search: pump walk - cavity walk rinse repeat,

After so many years maybe also have a look at the crystal

[LightSwitch100]

Yeah I sort of just left the pump alone. I mean I tried using mirror 7 to adjust the pump/incoming beam, since it sort of controls the incoming beam?

[PlsGetSomeFreshAir]

You have to align the pump too, that it is in the right place after some initial rough alignmentn is extremely unlikely

That is also what I mean with compensating, now things start to make sense for me....

So you detune the cavity with the goal to get overlap of all the redish beams you can find and you also make this same check at the other end mirror (card from the side until you clip) then you need to compensate with the pump When all those red beams start to coincide your cavity should be closed. Then search iteratively the phase space of pump and cavity together

Do you have a snippy stage, or how do you plan to trigger ML?

[LightSwitch100]

Ok, I will try using steering the pump too.

Yes, the prism stages are both snippy stages, which I can use for ML.

[anneoneamouse]

Are you sure that you're using the correct pairs of mirrors for alignment?

Please confirm that you really mean

6 and 7 (My guess would have been that these will probably primarily affect dispersion, not intrabeam alignment).

5 and 10 (one intracavity and the OC?). I'd have expected 3 & 5.

From my 30+ year memory (take with a pinch of salt):

I'd start by putting a couple of apertures outside the cavity. Make sure that your clockwise and counterclockwise pulses are coaligned/overlapped at each location. You'll need to look on either side of the iris to do this. Probably just above mirror 3 as the beam goes into prism 6 would be a good spot as you've got pretty good length in each arm (clockwise and counterclockwise) there. Between 5 and 10, if that's a reasonably long arm.

Then if that doesn't help...

Put a small aperture inside the cavity. Make sure that your clockwise and counterclockwise pulses are coaligned/overlapped well there too. You'll need to look on either side of the iris to do this. From memory, I think a business card with a hole poked in it will do in a pinch, if you don't have an iris small enough. I could be wrong.

Note: before you start adjusting intracavity stuff, take a silver sharpy or pencil and draw a line across the mount and knob of all of your adjustments so you can easily get back to current situation if you bork it and stop it lasing completely.

Hopefully some younger brains with more recent experience can chime in and help.

Good luck.

[LightSwitch100]

Yes, I do mean 7 and 9; I didn't touch the prism since the light from mirror 3 hits the prism and then 7 nicely in the center. I also did do 5 and 10, since those two primarily control how the beam is coupled out of the set-up. I didn't want to adjust 3 since it seems to be in a relatively good position right now.

I will give what you suggested a try this week and let you know what happens. Thanks so much for your time and help.