We got some AS72 results today with the IFO at Nominal Low Noise (NLN).
First, as a sanity check we dithered the OM1 in pitch and then looked at the AS72 MHz response. If the PLL is perfect, all the spot position signal should be in the I phase, while the PLL residual will rotate the I phase signal into the Q phase. We varied the overall loop gain by factor of 10 and the largest I/Q ratio was 2.5 (~40% spot centering leaking into Q phase). Adjusting the phase of each quadrant did not noticeably improve nor degrade the PLL performance so we just put them back to zero.
Then we measured a sensing matrix at NLN
| A_I | A_Q | B_I | B_Q | ||
| SRM | AS36 | 171 | 38 | 648 | 176 |
| AS72 x1e+6 | 3.1 | 1.9 | 1.7 | 1.2 | |
| BS | AS36 | 100 | 1780 | 748 | 1301 |
| AS72 x1e+6 | 0.54 | 0.81 | 1.5 | 1.5 |
It seemed possible to form a non-degenerate 2x2 matrix for SRM/BS alignment using RF72_A_Q/RF72_B_Q according to the measurement. This means that if in the future we can sync the phases of 118.3MHz and 45.5MHz without needing a secondary demod loop, we can then decouple the wavefront distortion from DC spot centering: even the OMs locked with offset, the BS/SRM alignment signal should not be contaminated by the mis-centering. Alternatively, we can also use the AS72_A/B_I signals for RF centering (AS_A_RF72_I -> OM2, AS_B_RF72_I -> OM1) to avoid carrier junk lights.
An earthquake came in shortly after we finished the measurements so more measurements under different TCS settings will be preformed tomorrow.