After the SQZ MM tuning action of Wednesday, there was some indication of extra noise from scattered light on external SQZ bench ESQB. So yesterday afternoon I tweaked the alignment upstreams the triple Faraday Isolator.
In order to identify noise from ESQB I took two reference spectra with the slow shutter (on ESQB) closed, see timing on Fig1 and a comparison of Hrec spectra in Fig2 with shutter open (magenta) and closed (blue).
To check how the noise from scattered light can be reduced with proper alignment on ESQB, I stopped the SQZ AA loop, then I sligtly mistuned the SDB1 Faraday Isolator waveplate to send more light on ESQB, and I increased the frequency and amplitude of a dither line on one SQZ AA mirror. Some noise is visible in Hrec around the dither frequency (85 Hz) and between 45 and 50 Hz, see Fig3. By slightly tilting the SQZ AA mirror closer to the ITF (Mirror1) the noise can be largely reduced, see Fig4.
Then tuned back the SDB1 FI waveplate, then I moved all SQZ AA dither lines to higher frquencies and amplitude, until noise on Hrec is clearly visible at the dither frequencies; then I modified the AA Matrix in SQZ_PLL_AA server to increase the loop bandwidth. I entered the DET lab and moved the tilt of output mirror of the MM telescope on ESQB to reduce such noise, see Fig6. Unfortunately I had to stop before reaching the optimal alignment, due to lack of internet connection.
After restoring the usual conditions for the squeezer AA, I chiecked that the noise in Hrec around 50 Hz is similar to the case with slow shutter closed, see Fig 7 where blue spectrum is with shutter open before alignment, yellow shutter closed, magenta shutter open after alignment.