J. Kissel

As we're continuing to investigate the nonsensical low-frequency behavior of the DARM loop's sensing function (some times referred to as detuning of the SRC causing an optical spring), for the past month's worth of weekly calibration measurements, we've been gathering PCALX to DARM transfer functions as well as the "nominal" "reference" PCALY to DARM transfer functions. 

Remember, this measurement of the sensing function is the ratio of two transfer functions:
      DARM IN1         C
(1) -----------   = -------
    PCAL X or Y     (1 + G)

      DARM IN2      1 
(2)   -------- = -------
      DARM EXC   (1 + G)

       (1) 
  >>  ----- =  C
       (2)

So, to create these comparisons, I divided either the PCALX or PCALY to DARM IN1 transfer function with the same DARM IN2 / DARM EXC = 1 / (1 + G) transfer function taken at the immediately after. 

I attach a comparison of those processed measurements taken on each day compared against each other. 

The conclusion: we see the same frequency response at low frequency on both PCALs, and yet that shape is changing from measurement to measurement. So, the change is in the DARM IN2 / DARM EXC transfer function over time.

This strongly suggests that this low-frequency feature is not result of mis-centering of PCAL spot positions.

More data analysis of past data to come.

A detail-oriented person might see that the "optical gain" of the sensing function reported by PCALX is slightly different than PCALY, to the tune of somewhere between 0.5 and 1%. We'll chew on this as well.