Editing: Daniel pointed out that I got the wrong pdf of the results, and reported the contrast defect from the wrong measurement. Corrected homodyne angle and the correct PDF are attached now.
I used Craig's scripts from labutils (also in gitcommon/labutils) to change the pcal lines and to sweep the darm offset for a contrast defect measurement. I added an additional DARM offset step of 11 (40mA) to the list of steps that was in the code.
to run this:
- cd /ligo/gitcommon/labutils/darm_offset_step/
- python set_up_pcal_for_darm_offset_test.py
- python DARMOffsetStep.py
- open the plotting script (plot_darm_optical_gain_vs_dcpd_sum.py) and add the filename of the log file the above script created, along with a description to the list of filenames and descriptions.
- conda activate labutils
- python plot_darm_optical_gain_vs_dcpd_sum.py
Attached is a screenshot of the OMC trans camera when the DARM offset was at it's lowest setting, with 8.3mA on teh DCPDs. It does not look like a 00 mode. This camera was realigned a few weeks ago 70875, which may explain why we didn't notice this before, it could be a camera artifact.
The contrast defect seems to be 1.6 mW. Following Vicky and Craig's alogs 68870:
contrast_defect = 1.6 mW
total_dcpd_light = 46.6 mW
darm_offset_power = total_dcpd_light - contrast_defect = 45 mW
homodyne_angle = arctan2(sqrt(contrast_defect), sqrt(darm_offset_power))
homodyne_angle = 10.7 deg
deleted
This OMC (Serial 003) does not have a low-order HOM "near miss" until 9th-order, according to the past tabletop test at CIT.
- If it's one of the 2nd order modes as the CCD image indicates, it requires a huge amount of 2nd order to be incident on the OMC (see Attachment). The 2nd order modes are almost at the anti-resonance regardless of the carrier or a sideband. The transmission of the 2nd-order modes are expected to be 30~100ppm. If this had the transmitted power of 1.6mA (i.e. ~2mW), it'd have the incident power of tens of W. So it's impossible.
- If it's 9th order and the mode seen in the image is an artifact somehow, I'd say it's possible. Indeed, Jenne already saw a 9th-order mode transmission on the OMC trans cam back in 2016. [LHO ALOG 29395]
- How about the 0th-order 45MHz sidebands, which are the strongest at the OMC incident? They are +/-0.17 FSR away from the carrier TEM00 and expected to have the transmission of ~200ppm. Assuming the incident power is the order of 100~200mW, we expect to see them as 20~40uW. So it's too small to explain 2mW transmission.
