Nergis, Peter, Stefan,
Currently, as we transition from 10W to 57W input power, our recycling gain drops from about 34 to 27. It seems like we need to tune the common TCS:
Attached are 3 plots:
Plot 1: DC signals:
PR_GAIN and normalized arm power (both blue)
AS_DC A and B (green and red)
POP_DC A and B (cyan and purple)
REFL_DC A and B (yellow and black)
Note the interesting behaviour of the REFL DC, all while AS_DC is linear in power.
Plot 2:
Arm and power recycling cavity powers, as well as test mass pitch control signals. The test masses have to compensate for radiation pressure, making the pitch control signal proportional to the arm cavity power.
Plot 3: RF signals:
Power and signal recycling cavity sideband buildups.
[Jenne, Stefan, Peter]
We sent a lot of CO2 power to the ITMs today for a short period of time, once at 20W PSL power and once at 40W PSL power. The idea was that if the recycling gain drop was due to central heating from the intracavity power, we should be able to mimic that by heating with the CO2 lasers and see a drop in recycling gain. However, we don't see a drop in recycling gain, so it's not a heating / mode matching problem. The drop in recycling gain really is due to misalignment effects, mostly SOFT yaw.
We set the Yarm TCS to 2.4W, and the Xarm TCS to 4.0W for the durations of these tests. See attached that we didn't see any effect in any buildups or recycling gain.
Here are some plots that show that the X arm is getting misalinged in yaw durring the lock that Stefan was invesitagting power recycling gain loss for. The first plot shows the transmitted green power, which is dropping for the X arm but not the Y arm. We don't use any ASC control for the green light while in full ock, but by leaving it injected we can tell when the alignment of an indivdual arm is changing. The second plot shows the Transmon QPDs for both pit and yaw, showing that X yaw has the largest change durring this lock.
