Sheila, Vicky, Naoki

Summary:  The sqz blrms at 80 Hz became worse October 27th and we may be able to improve the sensitivity in that band by adjusting the FC detuning.  We do not know exactly why the high frequency squeezing was restored to a better and more stable level last night.  

The first attachment is a set of SQZ BLRMs trends from the last month.  Focusing on the blue line (1.7kHz BLRMS) you can see that the crystal move on Oct 17th increased the level of squeezing available, but also caused the squeezing level to be less stable. This frequency band doesn't contribute much to the range, but you can see that the squeezing level at 1.7kHz seems to correlate with the range, presumably because it tracks the mid frequency squeezing level.  

On October 27th, Naoki centered the beam on FC2 73777, and the 80 Hz BLRMS (red trace) jumped up to +1.5dB, indicating that the squeezer is degrading the low frequency sensitivity of the IFO.  We would like to try adjusting the FC detuning to see if we can get back to the earlier performance. 

Looking at the second attachment, (again focusing on 1.7kHz blue sqz BLRMS), you can see that since the crystal move the squeezing level has drfited as the crystal changes.  The CLF_REFL_RF6 power slowly drops over time, meaning that there is less of the generated 3MHz sideband (and less nonlinear gain).  Every time the OPO temperature is adjusted, the RF6 level steps up as the nonlinear gain is restored.  The bottom left trace shows the RLF transmitted by the filter cavity, which also increases each time the temperature is readjusted, because the generated RLF sideband is increased as the nonlinear gain is restored.  In last night's lock, the FC transmission was larger than it has been in the past month, although the CLF 6MHz wasn't especially large.  We don't know why this is, but it's interesting that it corresponds with the restored (and seemingly stable) 4.5dB of squeezing.  Other things that we've looked at to explain last night's squeezing level were AM4 alignment (shifted a few days ago and doesn't seem directly related to the improved squeezing), and the OPO PZT voltage which jumped up this morning after the IFO was no longer locked to near 100V.