Jennie W, Sheila, Ryan C, Ibrahim, Corey

Over the last 3 weeks three DARM offstep measurements (where we change the DARM offset to look at the fraction of the light from the differential mode which makes it past the OMC) have been taken.

This is so we can get data points to compare to my model of ARM to OMC mode-matching. These were done at three different CO2 X power levels.

Measurement 1: 2025/06/18 20:44:55 UTC

CO2 central heating on ITMX: 1.698W

CO2 central heating on ITMY: 1.694 W

The test is run with /ligo/gitcommon/labutils/darm_offset_step/auto_darm_offset.py . The data is processed with plot_darm_optical_gain_vs_dcpd_sum.py .

Graphs of the power after the OMC vs. optical gain are in the first plot,  optical gain vs. offset and anti-symmetric port power vs. power out of the omc are in this document.

The average contrast defect is 1.07 mW, the junk light is 679 mW, the transmission of the differential mode light at the AS port by the OMC is 1/1.217 = 82.2%.

Measurement 2: 2025/06/20 15:11:46 UTC

CO2 central heating on ITMX: 1.698 W

CO2 central heating on ITMY: 1.711 W

The test is run with /ligo/gitcommon/labutils/darm_offset_step/auto_darm_offset.py . The data is processed with plot_darm_optical_gain_vs_dcpd_sum.py .

Graphs of the power after the OMC vs. optical gain are in the first plot,  optical gain vs. offset and anti-symmetric port power vs. power out of the omc are in this document.

The avergae contrast defect is 1.03 mW, the junk light is 677 mW, the transmission of the differential mode light at the AS port by the OMC is 1/1.212= 82.5%.

Measurement 3: 2025/06/30 15:06:50 UTC

CO2 central heating on ITMX: 1.698 W

CO2 central heating on ITMY: 1.721 W

The test is run with /ligo/gitcommon/labutils/darm_offset_step/auto_darm_offset.py . The data is processed with plot_darm_optical_gain_vs_dcpd_sum.py .

Graphs of the power after the OMC vs. optical gain are in the first plot,  optical gain vs. offset and anti-symmetric port power vs. power out of the omc are in this document.

The avergae contrast defect is 1.09 mW, the junk light is 694 mW, the transmission of the differential mode light at the AS port by the OMC is 1/1.212= 82.5%.

This is not a very good test for our purposes as I think we want a larger change in mode-matching from thermal tuning to inform our simulations of Arm->OMC mode-mis-match.

Each time we have stepped the CO2Y down (all these darm offset measurements were meant to be repeated after decreasing the CO2 power) for this test (measurement 2 on the 20th June, alog 85238 shows attempt from 23rd June, alog 85335 shows attempt from the 25th June, alog 85429 is measurement 3) we have lost lock, so we might not be able to repeat this measurement with a larger CO2 step.