We reran the DARM offset step test around GPS 1394062645, similar to alogs 71913, 68870, 64974.
Our current measured contrast defect is slightly higher than in alog 71913, but overall pretty similar.
Matt will comment with the calibration of X0 offset cts into picometers.
Contrast Defect: 2.1 mW
Nominal Total Amps from DCPDs: 40 mA
Responsivity = e λ / c h = 0.858 A/W
Nominal Total Power on DCPDs: 46.6 mW
Nominal Homodyne Angle: 12.2 degrees
To get the offset in picometers from this plot, we use the quadratic factor from the fit, b, along with P_AS, and the contrast defect.
Using the data at 255Hz we find b = 0.672 mW / pm^2, P_AS = 46.6 mW, P_junk = 2.058 mW, which we then use to calculate the offset DeltaL_DC which we call x.
P_as - P_junk = P = ax^2
g = dP/dDeltaL_DC = 2ax
P = b g^2 = 4a^2bx^2 = ax^2, therefore 4ab = 1
We can then solve for x = 2 sqrt(P * b) = 2*sqrt( (46.6mW-2.058mW) * .672 pm^2/mW) = 10.94 pm
We can also calculate the mW / pm^2 [a] factor from the product of interferometer parameters: PRG, SRG, P_in, Reflectiviity of the arms, and the wavenumber; only I did not know them at the time (except the wavenumber).
I want to clarify here that 40 mW is the nominal DCPC power for O4a and now. I trended the DCPD sum output for the recent lock and the many months of O4a to confirm this is true.
OMC DCPD SUM has units of milliamps. One must divide by the responsivity to get milliwatts, which we have done here.
Matt, Criag, Sheila
We want to use this DARM offset stepping to look at the mode matching of the OMC.
In alog71141, Sheila stepped the DARM offset and looked at the OMCrefl power [H1:OMC-REFL_A_LF_OUT16] in order to estimate the mode matching level, which was estimated to be around 96% for cold OM2 [GPStime = 1371902599], and 85% for hot OM2 [GPStime = 1371910040]
When we looked at the same channels during this recent DARM offset step (3/9/24) [GPStime = 13940633389] we see much less change in the OMCrefl, which may be interpreted as better mode matching than the previous cold OM2. Because I cannot resolve any power changes in the noise, it is hard to give a percentage for the mode matching figure but we expect better than 96%.
A couple of notes on the recent DARM offset OMC reflected power: the reflected power is higher than before, as well as the noise, which may be concealing some of these power changes; another thing to note is the time spent ramping on each DARM step, which was much shorter in the previous analysis, and having a slower step may lead to less prominent power fluctuations in the OMCrefl.
change in HAM6 throughput estimated by AS_C
As done in Sheila's alog where she estimates the excess HAM6 losses, I have estimated that we have an additional 10% loss in HAM6 throughput.
Adding up the known losses for HAM6 we expect the throughput to be around 97%
Calibrating DCPD_SUM into mW and dividing by its change from DARMoffset by the AS_C change, we can estimate the losses in the OMC; the DCPDsum change was around 40.4 mW, while the AS_C change was around 46.3 mW, this gives us a ratio of 87%.
Given the ideal throughput of HAM6 being around 97% given known losses [ 0.993(OM1)*0.985(OM3)*0.9926(OMC QPD) = 97% ], this means that we have an additional 10% of losses in the OMC.
DCPD_SUM is converted from mA to mW by the responsivity: 0.858 mA/mW.
