TVo, Hang
We did more AS72 tests today. The results did not seem to be extremely promising.
We played with the PLL loop which should lock the SUM signal after the secondary demodulation to the I_I phase. The PLL open-loop transfer function is attached to this entry. It had a UGF of 1Hz and an extra 1/f boost stage below 0.3Hz. We also adjusted the phase of each WFS quadrant so that the length signal showed up in the I_I phase. However, after the adjustment we still saw that about 30% of the OM angular dithering signal leaked to the I_Q phase. The gain of each quadrant was not changed because we do not know if the signal height difference was due to miscentering or gain imbalance. Also based on AS36/AS45 WFSs' setting, the relative gain mismatch seemed to be small.
Then we measured sensing matrix under different TCS setting:
| A_I | A_Q | B_I | B_Q | ||
| BS | AS36 | 44 | 1054 | 462 | 939 |
| AS72 | 0.18 | 0.35 | 0.41 | 0.46 | |
| SRM | AS36 | 94 | 94 | 726 | 825 |
| AS72 | 1.8 | 0.98 | 0.63 | 0.48 |
| A_I | A_Q | B_I | B_Q | ||
| BS | AS36 | 78 | 1386 | 410 | 1519 |
| AS72 | 0.10 | 0.37 | 0.64 | 0.70 | |
| SRM | AS36 | 546 | 205 | 63 | 2448 |
| AS72 | 1.5 | 0.30 | 0.58 | 0.17 |
For reference, the noise level of the AS72 signal is about ~0.1. It seemed that under the (relatively) large thermal distortion, the AS72 signal seemed change as much as AS36. The I_Q phase signals for SRM almost vanished, which was due to both that the current SRC geometry was close to 118MHz 02/20 mode resonance, and due to the I_I to I_Q leakage caused by PLL residuals.
On the other hand, the 600mW CO2X seemed to affect the IFO ASC quite a lot that t the iFO could not maintain locked for more than ~1H if the CO2X level was not properly set. We also tried to test under 100mW of CO2X power, however, the IFO had trouble keeping locked under this setting and we caused a locked loss shortly after switching to this low CO2 setting.
