daniel.sigg@LIGO.ORG - posted 11:11, Friday 07 October 2016 - last comment - 09:23, Thursday 13 October 2016(30305)
RAM measurements: Take 3
Evan, Daniel
17:12:30 UTC Oct 7 2016:
- RF modulation off for 9 and 45 MHz
- 45 dB whitening gain compensated by a filter gain of 0.0056
- 1 stage of whitening, compensated
- V/ct filter engaged, referred to TNC ouptut of 2-chn demod board
- Dark
17:16:30 UTC Oct 7 2016:
- same input configuration
- 29 mA of photocurrent
17:18:30 UTC Oct 7 2016:
- turn DC centering of REFL WFS on
17:24:30 UTC Oct 7 2016:
- turn DC centering off again
- 14.8 mA of photocurrent
17:32:00 UTC Oct 7 2016:
- 9 MHz modulation on
- 14.8 mA of photocurrent
- DC values: 9I = 3.7 mV; 9Q = 9.5mV
17:34:30 UTC Oct 7 2016:
- 9 MHz modulation on
- 29 mA of photocurrent
- DC values: 9I = +7.2 mV; 9Q = +18.1mV
18:06:30 UTC Oct 7 2016:
- 9 MHz and 45 MHz modulation on
- 29 mA of photocurrent
- DC values: 45I = –14.3 mV; 45Q = +62 mV
Spectra attached.
Coherence (modulation on)
Using 2600 V/W for the demod gain and transimpedance, and 29 mW of dc PD power, this implies the following AM depths:
Using 0.22 rad and 0.28 rad for the 9 MHz and 45 MHz modulation depths, this implies the following AM/PM ratios:
The attachment contains a budget of the expected CARM residual. The in-loop error point is taken from the CM board control signal, as was done previously. Here I used 2600 V/W for the transimpedance and demod gain.
The other measured traces are taken from the REFL9I readback (not from the CM board), so in principle there could be some extra dark noise at the error point from the summing node board or CM board. However, based on the O1 level this is of the same order as the shot noise (so we are not missing a huge amount of extra noise in this estimate).
Attaching earlier RAM plot, this time with informative labels
Here is a time series of REFL LF during the modulation depth reductions that happen during lock acquistion.
During the 9 MHz depth reduction (from 0.22 rad to 0.11 rad), the dc power changes from 4.83(3) mW to 4.27(3) mW. That means that after the modulation depth reduction, 4.08(4) mW of the dc light is from the carrier and 0.19(2) mW of the dc light is from the 9 MHz sideband (this assumes the 45 MHz contribution is negligible).
Note that the dc level is still settling to its final value of ~3.7 mW, so it's possible that these power ratios are evolving during the lock.