Weird beam behavior in a cavity

[kelly0303]

Hello! I am trying to align a bow-tie cavity and I noticed a behavior of the beam spot that I can't explain with my (limited) knowledge of cavities. For reference, I am scanning the current of the laser in order for the laser to follow the cavity. I attached below 3 frames from a video I took of the beam spot size (they are not consecutive frame, but the video is too long to upload). In the first frame, it is the way the beam spot looks like when (I think) I am well aligned. However I tried to rotate one of the mirrors quite a lot and the result is the following 2 frames. I expected that rotating the mirror would basically make everything disappear, as the beam doesn't replicate itself in the cavity anymore (I checked that one path is not enough to give any signal on the CCD camera I am using). However it looks like I have 2 independent beams which still (kinda) amplify in the cavity even when their path is quite far from the original (well-aligned) one. Why is this happening? Just as a side note, the 2 beams that appear seem to split smoothly from the original one. It is not like I have the nice 00 mode and then suddenly I change to a 10 mode. It literally behaves like in the first picture I have the 2 spots overlapped and then I slowly take them apart. Any insight would be appreciated. Thank you!

 

[Twigg]

Some follow-up questions:

For reference, I am scanning the current of the laser in order for the laser to follow the cavity.

Are you adjusting the laser current by hand to stay on cavity resonance, or are you scanning the laser by applying a modulating waveform?

I attached below 3 frames from a video I took of the beam spot size

Based on past threads, the camera is looking at the transmitted beam, not the reflected beam, right?

However I tried to rotate one of the mirrors quite a lot and the result is the following 2 frames.

Rotate how? Did you rotate the mirror in the xy plane or tilt it vertically (like by using adjustment screws on a kinematic mirror mount)? Or did you rotate/roll/spin the mirror about its cylindrical symmetry axis? Based on the pictures, I'm guessing you meant tilt in the xy plane, but just making sure.

I've seen something similar on a low finesse cavity. My interpretation of the effect back then was that I was getting an effect similar to Lloyd's mirror where the fringes were caused by the interference between the multiple passes of the light through the cavity. Try rotating the mirror both horizontally and vertically. Do you get an oval of fringes on the camera? That's how it worked on mine.

 

[kelly0303]

Some follow-up questions:Are you adjusting the laser current by hand to stay on cavity resonance, or are you scanning the laser by applying a modulating waveform?Based on past threads, the camera is looking at the transmitted beam, not the reflected beam, right?Rotate how? Did you rotate the mirror in the xy plane or tilt it vertically (like by using adjustment screws on a kinematic mirror mount)? Or did you rotate/roll/spin the mirror about its cylindrical symmetry axis? Based on the pictures, I'm guessing you meant tilt in the xy plane, but just making sure.

I've seen something similar on a low finesse cavity. My interpretation of the effect back then was that I was getting an effect similar to Lloyd's mirror where the fringes were caused by the interference between the multiple passes of the light through the cavity. Try rotating the mirror both horizontally and vertically. Do you get an oval of fringes on the camera? That's how it worked on mine.

Thank you for your reply. I will try what you suggested tomorrow when I get back to lab, but here are the answers to the other questions.

I am using a servo, and currently I am in the scan mode. I am using a triangle wave signal at 100 Hz and amplitude such that I cover a bit more than one cavity linewidth in terms of frequency.

I have a kinematic mount (this one) and yes, the mount is fixed, I am adjusting one of the screws to tilt it in the xy-plane.

Actually my cavity is relatively high finesse, about 10000 (at about 1m total length - bow tie cavity).

 

[kelly0303]

@Twigg sorry for the delay. This is what I get when I go in the other direction, too.

 

[Twigg]

Sorry for the slow reply.

This is similar to the pattern I saw on that Fabry-Perot cavity I described (finesse somewhere around 1,000 - 10,000), except that on the FP cavity the pattern was elliptical not rectangular. I believe it's the same mechanism just in a different geometry. What I meant earlier when I said that this effect shows up on "low finesse" cavities is that I don't see this effect on another Fabry-Perot cavity I work on with a finesse of 250,000.

I believe the explanation is that the light is still interfering with its multiple passes through the cavity, even though those multiples passes don't perfectly overlap. This interference produces near-constructive interference. That contrast drops as you further misalign the cavity.

 

[kelly0303]

Sorry for the slow reply.

This is similar to the pattern I saw on that Fabry-Perot cavity I described (finesse somewhere around 1,000 - 10,000), except that on the FP cavity the pattern was elliptical not rectangular. I believe it's the same mechanism just in a different geometry. What I meant earlier when I said that this effect shows up on "low finesse" cavities is that I don't see this effect on another Fabry-Perot cavity I work on with a finesse of 250,000.

I believe the explanation is that the light is still interfering with its multiple passes through the cavity, even though those multiples passes don't perfectly overlap. This interference produces near-constructive interference. That contrast drops as you further misalign the cavity.

Thanks a lot for this! Do you know if this means that when I see the central bright spot I am well aligned, or am I still in a weird place in the parameter space, where it happens to see a bright spot, without actually being in the global maximum in terms of alignment? I spent some time trying to find a better alignment but haven't managed to so far and I am actually not sure when to stop and conclude that I am done aligning the cavity.

 

[Twigg]

I believe (very strongly) that when you see a single spot you are best aligned.

If you want to be absolutely sure, try measuring the peak brightness in each of your images by loading them in matlab/python/whatever. You'll have to do this by eye because there are always a few random "hot pixels" that are saturated in any image.