Cao, Ellie, Dave O and Aidan

From llo alog 21927, Aidan found that the ratio between s an p polarizations is higher than expected.  The polarization at the output port has been observed to change over time during lock stretches. In particular the s polarization reduces by 30-50% over the course of 1-2 hours and p polarization slowly increases. Due do various polarization-depenent optical elements in the interferometer, the s-polarization pick up different phase shifts. Excess of p-polarized light may couple to DC readout and introduces excess of noise at the output. Given the time dependent behaviour of the s-polarisation we would expect the DARM noise to change with time if  polarization noise was contributing in a major way to the DARM spectrum. The DARM power spectra were inspected a number of times during post power up to  investigate the dependency of the noise on the polarization  the state of the interferometer.

DARM power spectra from 19Dec15, 26Dec15 and 9Jan16 lock stretches show no apparent changes over time after power up. These lock stretches were during the O1 run and were chosen because the interferometer went to observing mode very quickly after the power was increased. Power spectra are recorded at 0.19 Hz bandwidth, 10 averages. The interferometer went to observing mode at 7, 5 and 2 minutes after power up on 19Dec15, 26Dec15 and 9Jan16 respectively.

There is no clear correlation between the polarization drift and the DARM spectra. Whereas there is a 30%-50% decrease in s-polarization over the course of 1-2 hours after power up, the DARM noise spectra remains stable in the 10 to 70 Hz range.

The DARM spectra also indicates no effect due to thermal lensing as the DARM spectra is very stable during self-heating of the interferometer. Thermal lensing relaxation time is approximately 20 minutes after power up. DARM power spectra after stabilization (occurring at minimum 2 minutes after power up) remains constantst during thermal lensing relaxation period and beyond.

We will be looking into the same problem at Livingston, which has a stronger polarization drift.