Tonight after IFO thermalization, I remeasured the frequency noise and the contrast defect. Earlier in the day we changed the ring heaters to be set to 1.3 W on EX and 1.1 W on EY. The contrast defect is now down to 1.7 mW, comprable to a measurement made recently by Dan. Our frequency noise has also reduced at high frequency.
I have added a new frequency noise trace to the plot I put in a comment in alog 68842. See new plot below. I was sure to turn on LSC FF this time during the measurement. Comparing the brown and purple trace, our frequency noise is reduced from the ring heater change.
We could either step ETMY up again to 1.2 W and see if we gain even more, or we can call this setting good enough and stay here. The CO2s have some effect on the frequency noise, so we may be able to tune more improvement there.
LHO:65000 Craig calculates the homodyne angle, based on measured contast defect and total dcpd power:
contrast_defect = 1.7 mW
total_dcpd_light = 20 mW
darm_offset_power = total_dcpd_light - contrast_defect = 18.3 mW
homodyne_angle = arctan2(sqrt(contrast_defect), sqrt(darm_offset_power))
homodyne_angle = 17 deg
Craig also notes this 17 deg would be an upper limit on the homodyne angle.