How can I overcome alignment issues with a bow-tie cavity?

[kelly0303]

Hello! I am having some issues with a bow-tie cavity I am aligning and I would really appreciate any help. The cavity is not locked, so it does have some vibrations (it is on damping legs), and I want to lock a laser to it. I am seeing some peaks, but the output power is lower than expected. When I try to adjust one of the mirrors, I get to a very bright and round spot, but soon after I take my hand off from the mount it goes away (I tried using a piezo to rotate the mirror, too, but I get the same result, the spot disappearing soon after I see it on the CCD camera). During this whole process I am scanning the laser frequency over more than 1 cavity free spectral range. I would imagine that, due to vibrations, the spot to not be very stable, but given that I see it, so I am close in parameter space to that well-aligned point, how come it vanishes completely, given that I still scan the laser frequency? Shouldn't I see it regularly? Can someone help me figure out what is happening? Thank you!

 

[Twigg]

Here are some suggestions you could troubleshoot:

1) Your CCD's framerate may be aliasing the laser's frequency scan. This happens if your CCD's framerate is too low relative to the frequency at which you scan the laser. If this is the case, then you would notice that the brightness of the spot on the CCD varies as you adjust the laser frequency within a full scan range around resonance. To test this, I would look at your transmission photodiode trace. When the CCD spot goes away, does the transmission photodiode peak go away too? If there's still a transmission peak, then the problem is the CCD framerate and not the cavity alignment.

2) Relative alignment drifts. Are your laser and your cavity mounted on the same breadboard? If not, the cavity might drift in position relative to the laser and that will kill your alignment fast. This is especially true if the cavity's breadboard is mounted on vibration damping material (sorbothane, viton legs, air bags, etc.).

3) It is possible that you are burning an optic inside the cavity. When you realign the cavity, you are moving the beam off the burned spot, and so you get resonance. But as that new spot on the optic starts to burn, the cavity will go dark as the internal scattering skyrockets. To test this, I would turn your laser power down by a factor of 10 and see if that solves the issue.

4) Thermal lensing. It is possible that one of your cavity optics is bending in response to the heat it feels when hit with the full cavity-enhanced power. This would have the same effect as (2), and I would test it the same way (turn down the power by x10 and see if the resonance becomes stable).

 

[kelly0303]

Here are some suggestions you could troubleshoot:

1) Your CCD's framerate may be aliasing the laser's frequency scan. This happens if your CCD's framerate is too low relative to the frequency at which you scan the laser. If this is the case, then you would notice that the brightness of the spot on the CCD varies as you adjust the laser frequency within a full scan range around resonance. To test this, I would look at your transmission photodiode trace. When the CCD spot goes away, does the transmission photodiode peak go away too? If there's still a transmission peak, then the problem is the CCD framerate and not the cavity alignment.

2) Relative alignment drifts. Are your laser and your cavity mounted on the same breadboard? If not, the cavity might drift in position relative to the laser and that will kill your alignment fast. This is especially true if the cavity's breadboard is mounted on vibration damping material (sorbothane, viton legs, air bags, etc.).

3) It is possible that you are burning an optic inside the cavity. When you realign the cavity, you are moving the beam off the burned spot, and so you get resonance. But as that new spot on the optic starts to burn, the cavity will go dark as the internal scattering skyrockets. To test this, I would turn your laser power down by a factor of 10 and see if that solves the issue.

4) Thermal lensing. It is possible that one of your cavity optics is bending in response to the heat it feels when hit with the full cavity-enhanced power. This would have the same effect as (2), and I would test it the same way (turn down the power by x10 and see if the resonance becomes stable).

Thanks a lot for your reply. Just for context, I am currently using a power 1000x smaller than the final one I aim for (and that the mirrors are supposed to withstand), so I don't think (I hope so) it's an issue with the laser power. However, I am exactly in the second situation. The laser and cavity are on the same board, but the cavity has viton legs! However, given that I scan the laser frequency, shouldn't I go back and forth between different points in the cavity phase space i.e. shouldn't I regularly see the bright spot? The laser and the cavity will vibrate differently, but, assuming it's some period to that vibration, they still should bring the bright spot back after a while, no? Also, even if they were on the same board, don't they have different responses to the vibrations? So won't they behave differently regardless? Thank you!

 

[Twigg]

Also, even if they were on the same board, don't they have different responses to the vibrations? So won't they behave differently regardless?

The issue isn't vibrations. The issue is that those viton legs are not dimensionally stable. When you come into the lab on Tuesday morning, the viton legs will each be a different length than when you left on Monday. Consequently, your laser beam (which doesn't move because the laser is bolted to the board) will no longer be aligned to your cavity. This is what I mean by "drift". It's a notorious issue that comes up a lot in AMO experiments. Usually people see this on a day-to-day or hour-to-hour timescale for millimeter-sized beams. However, in your case a small movement makes a big difference, since you are trying to overlap the laser onto a mode that is only several hundred microns across.

The usual solution to this problem is to fiber couple your laser beam from breadboard #1 through a single-mode fiber onto breadboard #2. The fiber exit port is then rigidly bolted to the same board as the cavity, so the laser and cavity stay aligned since there's no relative motion between them. But you have high power requirements, so that might not work for your final setup. (If you can find an SM fiber that will support your final optical power, then by all means use that!)

My gut feeling is that you need to remove the viton legs. In principle, you could keep the laser beam aligned onto the cavity using some servo-motor mirrors and a CCD in a feedforward configuration if you really need to, but it's a lot of work and a bit of a gamble.

Also, even if they were on the same board, don't they have different responses to the vibrations?

The effect of vibrations on the alignment of the incident beam should be tiny if you're working on a good optical table or breadboard. Where vibrations really hurt is you is when they affect the relative positions of the cavity mirrors to each other (not the pointing of the input beam). In this case, the big problem is that your laser stops being resonant with the cavity. The fact that the vibrations cause you to lose some degree of mode overlap is a much smaller effect.

If you're worried that removing the viton legs will make your cavity very noisy because of vibrations, you might consider a feedforward scheme to adjust the frequency of the laser according to the output an accelerometer placed near the cavity. For an example, check out Figure 2 of this reference. In your case, you could replace the frequency modulation on the AOM with current modulation on your laser.