camilla.compton@LIGO.ORG - posted 16:59, Monday 21 October 2024 - last comment - 11:06, Thursday 09 January 2025(80804)
Automatic DARM Offset Steps
Elenna, Camilla
Ran the automatic DARM offset sweep via Elenna's instructions (took <15 minutes):
cd /ligo/gitcommon/labutils/darm_offset_step/ conda activate labutils python auto_darm_offset_step.pyDARM offset moves recorded to /ligo/gitcommon/labutils/darm_offset_step/data/darm_offset_steps_2024_Oct_21_23_37_44_UTC.txt
Reverted the attached tramp sdf diffs afterwards, see attached. Maybe the script needs to be adjusted to automatically do this.
I just ran Craig's script to analyze these results. The script fits a contrast defect of 0.742 mW using the 255.0 Hz data and 0.771 mW using the 410.3 Hz data. This value is lower than the previously 1 mW on July 11 (alog 79045), which matches up nicely with our reduced frequency noise since the OFI repair (alog 80596).
I attached the plots that the code generates.
This result then estimates that the homodyne angle is about 7 degrees.
Last year I added some code to plot_darm_optical_gain_vs_dcpd_sum.py to calculate the losses through HAM 6.
Sheila and I have started looking at those again post-OFI replacement.
Just attaching the plot of power at the anti-symmetric port (ie. into HAM 6) vs. power after the OMC as measured by the DCPDs.
The plot is found in /ligo/gitcommon/labutils/darm_offset_step/figures/ and is also on the last page of the pdf Elenna linked above.
From this plot we can see that the relationship between the power into HAM 6 (P_AS) is related to the power at the output DCPDs as follows.
P_AS = 1.220*P_DCPD + 656.818 mW
Where the second term is light that will be rejected by the OMC + that which gets through the OMC but is insensitive to DARM length changes.
The throughput between the anti-symmetric port and the DCPDs is 1/1.22 = 0.820. So that means 18% of the TM00 light that we want at the DCPDs is lost through HAM 6.