R&D (FilterCavity)
MarcEisenmann - 20:07, Monday 22 August 2022 (3049)
FC relock

Marc, Yuhang

We found that GR reflection was far from overlapping with injection.

This was due to huge offset on input and end because AA loop was still closed.

After opening the loop, we could see TEM00 flashes but we had to have about 100 offset in input and end pitch.

We could not lock the FC so we checked the GR power while changing the SHG temperature.

GR power after FI (mW): 41.7, 45, 35, 44, 45

SHG temperature: 3.085, 3.071, 3.06, 3.0665, 3.075

Finally, we tuned the SHG alignment and could recover more than 50 mW after the FI.

We could still not lock so we tried to tweak the servo parameters :

gr lock servo, attenuation = 0.2, gain = 8, both changed no diff

gr reflection power > 200 uW

Finally, after changing the lock threshold we locked on a sideband. Turning off the servo seems to change the sign of the error signal and we could lock the FC.

In the end, we changed sign on DDS2 by reloading the latest configuration.