I mentioned in the last post that I had measured the AM in the CCW beam after the beam splitter using a PDA255. I saw that the noise spectrum was flat well above where we see the excess low-frequency noise, so I ruled it out as the cause. What I didn't do is the same thing for the CW direction. I had a hunch today that AM could be generated by jitter of the input beam into the AOM caused by the EOM (Rana said that jitter was one source of EOM->AM coupling).
I put a PDA255 in the CW path after the AOM double-pass, stuck the signal into the demod setup, and---lo and behold---I saw low-frequency noise with just about the same shape we're seeing in the gyro signal:
I decided to try putting separate EOMs in each path. This was extremely messy for a few reasons:
- sheer lack of space: not only did I need room for the EOMs, but also for several waveplates to rotate the beam to S and then back to P into the faradays.
- inability to put in focusing lenses to get the beam through the crystal without fudging the whole modematching setup.
- only having one multi-axis EOM mount---one direction had to use one of the old fixed brass mounts, and I had to use what little parameter space I had to work with before the beam goes through the faraday isolator to do it.
Anyway, I found a way to do it that sort-of-kinda-maybe fit:
I split the power from the FG to the EOMs and realigned the beams into the cavity. Needless to say, the setup was far from optimal. Anyway, I decided to lock the cavity and PLL and see if I saw any improvement. There was none; in fact, the noise was worse. I then realized that I had forgotten to put the top on the box. I replaced it, and the noise went down a bit, but still slightly higher than before. I THEN realized that I didn't have the auxiliary box around the CCW injection steering mirrors. I replaced it, and the noise level was at just about where it was before. See below:
It is obvious that the sensitivity is suffering au cause d'the shitty way I put it together. I don't think we're seeing oscillator noise in the middle band since the PLL signal looks no better than the AOM signal (see previous post). I think it may be worth trying to do a better job at setting up the twin-EOM scheme, but there are two things that have me on the fence:
- Doing this right essentially means redesigning the optical layout altogether. There are too may waveplates to count at the moment, and I'm certain that with a few minutes' thought we can come up with a better layout. We will absolutely need another multi-axis EOM mount to avoid over-constraining the beam path, and we don't have one. I think Frank and Dmass have one, so perhaps we can snag theirs and order another one, but this is time and money. The bright side to this is that we will be replacing the cavity optics either way, and it might be "therapeutic" to rebuild the IO anyway.
- This is the tricky one: why do we see an equal improvement in both signals with the replacement of the box cover? No one ordered that, so to speak; the model predicts that IO noise should be suppressed by the CW loop gain, so the PLL should always look better than the AOM in bands where we're limited by IO noise. It could be that the AOM loop has $&#@all gain with my piss-poor attempt, but I think it's unlikely that we've lost over 60 dB of gain, even in this state.
I'll see what kind of loop diagnostics I can do in this limp-mode, and otherwise we'll just have to take a leap of faith that things will look better once we've reconfigured. If not, it will be the same amount of work again just to bring it back to the single-EOM setup. In the meantime, I'll try to think more about how the IO noise could couple to the PLL unsuppressed.