Today I spent some time on the LSC, because SRCL has been one of the dominant noises in DARM from 20Hz-60Hz for a while.
1) Retuning POP45 demod phase. I changed the POP45 demod phase from 86.5 to 67.1, to minimize the SRM signal in POP 45Q. I also checked that the current phasing of POP9 does a good job of minimizing the PRM signal in POP9Q. I did this following the procedure Evan Hall describes in 24933.
- As I rotated the phase, the MICH loop shape changed, as well as the couplings from MICH to PRCL and SRCL. (1st screenshot). SRCL and PRCL didn't change, but the scross couplings from MICH to those loops did change, which is also shown in the screenshot.
- The second screenshot shows that all 3 DRMI control signals had small reductions in the control signals after changing the demod phase.
2) Retuning PRCL-SRCL subtration in input matrix. I found that the input matrix element subtracting POP9I from POP45I for SRCL wasn't doing its job of making SRCL insensitive to PRCL very well. By changing the element from -0.024 to -0.03195 I saw a 15dB reduction in the coupling from a PRM drive to the SRCL error signal, but this didn't reduce our control signals much. The third attached screenshot shows the impact on the coherence between DARM and DRMI signals of the demod phase change and the input matrix tuning. MICH and SRCL are both improved a bit.
Other things we did today:
- took measurements for reutning SRCL FF which I will use to calculate filters tomorrow.
- Measurements for LSC sensing matrix
- Tried old settings for MICH->PR2, which made the interferometer unstable. We might want to retune this to reduce the PRCL control signal. (25656)
- Tried not reducing the modulation depth for RF9, which also caused something to go unstable. Next time we try this we might want to go through noise tunings step by step to see what the problem is.
- We have observed that since the vent the wind seems to have a negative impact on our range. We have mostly been in commissioning mode during the windiest times, but if we are in observing and it is windy we might want detchar to take a look.
Here is the sensing matrix measurement:
| PRM | BS | SRM | |
| POP9I (W/um) | -0.66 | 1.1 | -0.012 |
| POP45Q (W/um) | 2.1e-4 | -0.029 | 0.013 |
| POP45I (W/um) | 4.6e-4 | 0.064 | 0.035 |
I used the suspension calibration listed in alog 24933, and the sensor calibrations listed in 24959 with 12dB of whitening gain for POP9 and 30dB for POP45.
Since the time of Evan's measurement in alog 24933, the 9MHz modulation depth has been reduced by 3dB while the 45MHz modulation depth has been reduced by 6dB, which is not enough of a change to explain the discrepancy between this measurement and that one.