Rishabh, Michael

We intended to go and tweak the IRMC control loop gain for better control bandwidth and stability, as well as measuring the GRMC/MZ transfer functions. But there were a few problems.

The IRMC seems a bit misaligned. We only have about 0.33 mW/1.66 mW = 19.9% transmission. We tried unlocking and relocking several times. Trying to view the free spectral range on the IRMC reflection PD showed some weird jagged spectrum with not very high amplitude. Power meter showed nothing, although Marc later said this is because the bandwidth of the power meter needs to be set to High. Perhaps the PLL alignment process affected the IRM alignment again. The injection and reflection power are basically the same as when it was working properly though.

On the green path after the SHG, the 90/10 green beamsplitter sends reflection to the filter cavity green AOM, which was optimised as per recent elog entries. Accordingly, due to optimisation of the 90/10 BS reflection alignment, there will be a small amount of change in the transmitted path length to the GRMC and MZ. Indeed, some change in the mode spectrum could be seen in the GRMC transmission (figure 1). We used the steering mirror after MZ and now the mode matching is (1.25/(1.25+0.40)) = 96.9% (figure 2, 3). We also tweaked a bit the PDH error signal - the phase was adjusted from 164.993 to 185 degrees (DDS2 Channel 2 GRMC Demod), so the error signal looks a bit more symmetrical (figure 4).

We couldn't lock the GRMC though. According to a previous entry from Yuhang regarding this issue, we looked at several troubleshooting points:

0. grmc has a good alignment.
97% mode matching (figure 2, 3)

1. PDH signal has 316mV pk-pk checked from EPS1.
120 mV pk-pk (figure 4)

2. grmc has loop sign of INV, which is as design.
Yes

3. The RF source phase is reloaded. The phase of RF source is 125deg. When it is changed to 35deg, the signal around resonance becomes flat. This indicates the RF signal phase is still a good one.
Due to the change of EOM the DDS phase is different, but we flipped by 90 degrees and saw that the signal around resonance became zero so indeed the error signal and DDS configuration are fine.

4. There is a switch which has +/- sign. This doesn't decide the sign of control loop. But when we use this type of servo for CC1/2 controls, we need to flip this switch. I tried to flip this switch, but it doesn't help to close loop.
I tried this but no change

5. grmc transmission is checked to have 1.13V peak. This is two times smaller than the value written by Pierre.
The main peak reads 1.25V from the PD signal (figure 3)

6. Loop gain is 3 as usually used.
Yes. Likely this will need to be changed later though due to the EOM change.

7. Threshold for peak identification is -0.55V. This is as required.
The threshold knob was set to 3.3 but I tried changing it to a few values and didn't get any lock.

8. The GR power reaching AOM is measured to be 44mW, whose nominal value is 50mW.
We have 47 mW reaching the AOM

We have good mode matching and a good error signal shape. So the alignment, transmission PD, DDS configuration and PZT are all fine. It seems like the issue is coming from the servo module, or perhaps the amplitude of the error signal. Hopefully it's just to do with the settings and not something for which we have to take the board outside of the cleanroom.