Sheila, Georgia, Craig, Peter

We made several injections into the OMC PZT2.  We started with a 12 Hz 0.3 V excitiation on OMC PZT2, which is the same excitation used to verify the OMC length noise in 34048.  We haven't repeated all the OMC length noise measurements that we did in O2, but the DCPD RIN during this excitation matches the DCPD RIN during the O2 excitation, so it seems that the OMC length noise is fairly similar to O2.  

In the attached plot, the pink trace shows that when we increase the OMC length RMS enough to have a large bilinear coupling (using the 12 Hz excitation which shows up at 24Hz in DARM), we see the bilinear coupling of the resonance in HAM6 around 1080Hz, as well as the sensing noise from the OMC length dither imposed on the OMC length by the dither loop.  We increased the dither amplitude from 750 DAC counts to 1500 counts, and saw that with the same 12 Hz excitation the OMC length noise from dither sensing noise is decreased, as we would expect and as was seen in 34084. Because the 12 Hz excitation is dominating the 

Since the frequency noise around the dither frequency of 4100 Hz is down converted to 100 Hz by the OMC length noise, Craig made an injection of frequency noise around the dither line to try to reproduce the down conversion reported here: 44874.  We see a similar noise in the DCPD RIN as with the OMC PZT excitation, except that the feature at 1080 Hz is much smaller.  We then increased the OMC dither amplitude (keeping the ugf the same) and see that the downconverted noise is decreased because the signal to noise ratio of the OMC length noise is reduced.  The improvement at low frequency is larger than with the 12Hz PZT excitation,  we think because the RMS OMC detuning was decreased in this test but not when the PZT excitation was dominating the RMS.  

We don't see any of the lines observed at LLO during similar tests  41374.  

We will leave the OMC dither amplitude increased.  Note that in 2016 we turned the dither amplitude down because of a nonlinear environmental coupling near HAM6.  30380 It would be good to check on this again.