Vicky, Cao, Dan
After some experimenting with driving this 14.7 kHz acoustic mode on ETMX this evening we've decided to try and just "push past" with a decent RH change. We suspect that as we increase the ETMX RH we are reducing the X-arm mode separation frequency and some 3rd order mode is moving down into the 14.7Hz region and causing it to become unstable very quickly. With any luck, using more RH we will keep the HOMs below the 14.7kHz area after powering up. We can see at that point whether we are keeping our improved optical gain, lower constrast defect, and how frequency noise coupling is behaving.
Both ETM RH are now set at 1W on each segment. I moved both as to try and keep the reasonable arm matching that I suspect is giving us a lower contrast defect. Before we were also able to increase the ETMX RH without exciting these modes but the ETMY RH was much higher. So perhaps there is some cross-coupling between the arms now they are better matched?
Squeezing looks much better here. ~2-3 hours after this ring heater change, we reached NLN and already, immediately see >4.5dB, approaching 5dB, of squeezing without doing really anything differently!
Just adding a reminder that the ASC REFL WFS 45 still have large offsets after thermalization. These offsets have reduced relative to a longer lock from 4 days ago, suggesting that we are moving in the right direction and possibly explaining why the ASC is muhc more stable. However, 4 days ago the REFL 9 yaw signals were centered around zero, but now they have small offsets.
We do not see these offsets in the pitch REFL WFS.
The optical gain reduced slightly with these ring heater changes, but the constrast defect did not. It seems we still have about 1mW. Optical gain drop seems consistent with the drop in arm power from 380kW to 360kW.
Frequency noise coupling looks slightly better than before and much better than before with the old ring heater settings. Still not as good as when Craig and I changed the ring heaters back in Feb (67516).
DARM spring is less now, this is still with -175 cts on the SRCL offset.