We had an interferometer that was nicely thermalized at 20W today, including a 900Hz frequency line to monitor the REFL9I and REFL45I gain. So we decided to investigate the influence of common TCS on both AS_90 and REFL45 gain.
We thus increased the TCS CO2 central heating by 1W on each optic (to 1.2W on X, 1W on Y). This resulted in a DECREASE (at least initially) of the REFL45 gain (the opposite of what we see going to high power), and at the same time a recovery of the AS_90 buildup.
After letting it settle for about 50min in this configuration, we went to 50W. To our surprise the sign of REFL45I changed. We can only explain that by the 45 sideband going through critical coupling. This would mean a number of our loops in REFL changes sign, so it is not surprising that we lost lock soon afterwards.
The attached plot illustrates that story. Blue: REFL45I gain, Green: REFL9 gain, Orange: AS_90
At the same time, we monitored the intensity coupling as another figure of merit. See the attached. The time t in the legend box is initialized at the start of the test so that it starts with t = 0.
The coupling above 100 Hz initially improved by a factor of few, but then later it came back up and eventually became higher than the initial coupling.
If we compare today's data against what Evan measured back in this February (25476), the latest coupling is higher in most of the frequency band. The current coupling is higher by a factor of 10 at 100 Hz and too high by a few factor at 1 kHz. The second loop was open during the test.
