Kiwamu, Lisa, Sheila, Dan, Evan

Summary

A preliminary measurement shows that we are not quite limited by intensity noise between 50 Hz and 1 kHz, but we need to take another round of measurements in order to say anything conclusive.

This was taken before switching to the ETMY ESD and before feedforward subtraction of MICH.

This result is roughly consistent with Gabriele's conclusion that the intensity noise coupling above 100 Hz is a factor of a few below the total DARM noise.

Details

Kiwamu wired up the extra LSC DAC channel (LSC-EXTRA_AO_2) to drive the excitation point of the ISS second loop (PSL-ISS_TRANSFER2_INJ).

Then we drove a swept sine using this DAC channel and looked at the transfer function IMC-IM4_TRANS_SUM → LSC-DARM_IN1. The data aren't great, since it seems hard to get good coherence around 120 Hz and below 50 Hz. I've attached a calibrated version of the transfer function. IM4 trans is nominally calibrated into microwatts, and the conversion to DARM_IN1 from OMC-DCPD_SUM for this lock is 5.4×10⁻⁷ ct/mA (the DCPD_SUM channel is nominally calibrated into milliamps). The transfer function looks very feature-rich, and we would greatly benefit from a more careful measurement.

A good time to look at IM4 during this lock configuration is 2015-03-24 05:08:20 to 05:10:20 UTC. Using the ASD of the IM4 sum power during this time, one can use the above TF to propagate it forward to DCPD sum photocurrent. This can be propagated foward to DARM noise using the coefficient 4.6×10⁻¹³ m/mA. I've undone the DARM pole, but not the DARM loop (since our measurement is more or less above the loop UGF).