Anamaria, Varun

Summary of the test: Over the weekend before the IFO went down, we had done injection studies to project OMC-ASC Noise:

• The LHO uses the OMC-QPDs for OMC-ASC loops. However, when we tried putting an offset in these loops (alog 58969) we saw noise projections greater than DARM, which is not meaningful and we still do not understand why this test didn't workout.
• We transitioned from OMC-QPD to dithers according to (alog 58960). The key thing to note is that: We didn't see any significant changes in the DARM noise spectrum apart from the optical gain, which went down slightly.
• After making the transition, we made the measurement of the OMC-ASC noise with dithers on (alog 58970). The results presented below are for this measurement. For a given OMC-ASC loop the following approach was applied in the measurement. 
  • STEP1: An offset was added to the loop, which was increased till we could see the noise visible in darm (theta). We will refer to this spectrum as DARM_OFFSET for future reference.
  • STEP2: We band excited the loop at different offset values from 0 to theta. Using the rms area in the band excited, we estimate the loop crossing (alog 58970).

Analysis: 

• After calculating the loop crossing in STEP2, one can rescale the DARM_OFFSET by crossing/offset to project the noise in DARM.
• We define a quad-subtracted noise spectrum by the quad difference of DARM_OFFSET and DARM_QUIET. The projection is meaningful only where the quad subtracted is above the quite DARM. We define a threshold accordingly for each of the loop measurements, see Plot1.
• In the measurement, we saw significant coupling between the OMC-ASC loops, which needs to be corrected for in the analysis. To quantify this coupling, we use the output drive at DC in each OMC-ASC loop corresponding to the added offset in one of the loops. Using this coupling coefficient, we can estimate the contributing noise from each of the OMC-ASC loops upon projection, see Plot2.

Results:

• We estimate the OMC-ASC noise is a factor of 2 below darm in the frequency band of 20-60 Hz. Moreover, the 89Hz peak seems to be related to OMC-ASC.
• We didn't see any excess noise in the ANGY loop without hitting the saturations of the output drive (alog 58978). Therefore, it is not included in the noise projections. 
• There is significant coupling between the OMC-ASC dither loops which needs to be addressed.

Caveats and the next things to do:

• In the band excitation, we didn't see the noise at the 2-omega band.
• We still do not understand why the QPD measurement did not work.
• When we added the offset, the optical gain changed significantly. To eliminate the thresholds in the analysis, we can use the shot noise from the noise budget and scale it in accordance with kappa_c.