I measured the power spectrum of channel C1:GCY_SLOW_SERVO1_IN1, which is the PZT driving voltage.
I converted the output to a PSD. Next, I converted counts/sqrt(Hz) to volts/sqrt(Hz) by multiplying with 40 V / 2^16 counts.
Finally, I multiplied it with 5MHz/V for the PZT to end up with Hz/sqrt(Hz).
This corresponds to a cavity length fluctuation of
with lambda = 532nm and a YARM cavity length of 37.757m (elog # 5626).
All in one plot