craig.cahillane@LIGO.ORG - posted 05:43, Friday 07 December 2018 - last comment - 14:07, Wednesday 14 August 2019(45753)
DARM offset calibration
I calibrated the DARM offset at 20 watts input power. The calibration of the H1:OMC-READOUT_X0_OFFSET channel counts into picometers is 0.636 pm/cts.
I did so using the antisymmetric power on the OMC DCPDs [mA] compared to the DARM optical gain [W/m] as calculated by the ratio of the 331 Hz PCAL line [m] and OMC SUM [mA] response at the same frequency. The math looks the same as here but I have accounted for the slight drop in response from the DARM pole.
Numbers used:
-------------------------------
DCPD Responsivity = 0.858 A/W
DCPD Quantum Efficiency = 0.98
DARM pole = 421 Hz
PCAL Line Frequency = 331.8 Hz
Plot 1 is the antisymmetric power vs the DARM offset. (I also did this at 2 W.) Things here are consistent with very little contrast defect light in full lock, we'll have to lock RF DARM and the OMC to carrier to see what our actual contrast defect is.
Plot 2 is the DARM optical gain vs the DARM offset.
I put this calibration into the H1:OMC-READOUT_X0 filter module but did not engage it.
---------------------------------------------------------------------------------------
What's interesting here is that if you work out Kiwamu's theoretical calculations from page 8, eq 11 of here, things don't line up at all (~factor of 4 difference in calculated DARM offsets). I'll need to think more deeply on this, because we could learn something about the state of the IFO (signal recycling gain, arm reflectivity derivatives). In the real IFO there are some power losses here and there (OFI trans, OMC trans, OMC mode match, etc) that could confuse us as well, we can use 20 W vs 2 W to eliminate those.
Images attached to this report
Comments related to this report
This fit doesn't work for me: e.g., use x=40 in 6.7E-6 x^2 - 1.1E-7 x + 2.1E-6 and you get 10.7E-3 rather than ~27E-3.
Soz, the fits in the first plot was calibrated into W/(cts^n), where n is 0, 1, or 2, and the fits in the second were in W/(m*cts^k) where k is 0 or 1. Attached are the new plots with fixed x-axis labels and appropriately calibrated units for the fits (so if you put in x = 10 pm, you'll get mW of antisymmetric power or mW/pm of DARM optical gain)
Images attached to this comment
From the 20 W measurements of OMC Sum vs Opt Gain, I estimate the total contrast defect light to be 1.71 ? 0.24 mW, or ~85 ppm. This is a different method than the DARM offset sweep done at 2 W (alog 46142). Repeat of EH alog 30573. Also attached is the 20 W data shown in the plot. Code located in /craig.cahillane/Git/IFO/FULL_IFO/DARMoffset.ipynb
Non-image files attached to this comment
