Following this alog thread about the AS_C and OMC DCPD discrepancy, Daniel advised me to reduce the DARM offset and look at AS_C and OMC DCPD SUM to see if the ratio changed. I reduced the DARM offset such that the OMC DCPD SUM went from 20 mA to 5 mA. This reduces the local oscillator power by one fourth, or the local oscillator field by half. So, we expect carrier signals which are constant, e.g. the suspension and photon calibrator lines, to also decrease by half. Results Looking at the peaks on the photon calibrator (third line in the plot), we getDC offset power 20 mA 5 mA Ratio of PD with itself (5 mA / 20 mA) ----------------------------------------------------------------------------- AS_C NUSM [W] 3.89e-6 2.89e-6 0.745 OMC DCPDs [mA] 1.92e-3 1.08e-3 0.563Peak heights are in watts of input into HAM6 for AS_C, and incident mA for OMC DCPDs. It looks like the OMC DCPDs pretty much see the expected dropoff in response, but AS_C does not. Could mean that AS_C is seeing significant power in the PCAL line getting to the AS port through a circuitous route, most likely HOMs. PD Ratios Assuming a responsivity = e λ / h c = 0.858 A/W for the DCPDs, and converting DCPD SUM to W, the OMC DCPD SUM / AS_C ratios areDC offset power DCPD SUM / AS_C [W/W] ------------------------------- 20 mA 0.578 5 mA 0.437So as the DARM offset decreases, the ratio gets worse. Could make sense if the 00 carrier beatnote is becoming less dominant in AS_C as we reduce the DC offset. Template is in:/ligo/home/craig.cahillane/Git/IFO/ASC/data/darm_offset_vs_asc_as_c_omc_dcpds_low_high.xml
The alignment of the arm light into the OMC changes with this test.
Top plots in the first attachment show HAM6 PD/QPD signals demodulated at the PCAL frequency.
Bottom left plot in the first attachment shows OMC QPD pitch/yaw signals (normalized to NSUM by the front-end, as usual) demodulated against PCAL at 17 Hz, showing that the yaw pointing changes as the DARM offset is changed. The bottom right plot is similar, but I have undone the usual dc NSUM normalization of pitch/yaw and instead normalized by the amount of PCAL light in the NSUM channel at 17 Hz. With this normalization, we see both pitch and yaw pointing changes as the DARM offset is changed.
For completeness I've uploaded a second attachment with the dc trends of the various PD/QPD channels. Nothing too surprising here: QPD pitch/yaw signals continue to be servoed to their setpoints during the offset reduction.
See also Keita's proposal for QPD beacon: 64626
