This is the noise of the cavity if we just lock the laser to the cavity using the fast actuator (no slow actuation) and look at the control signal to the laser.  I used a potential divider to reduce the voltage going to the spectrum analyser (I used 1.023kOhm and 9.32kOhm to give approx 10 times reduction).

The plot here is taking the V/rt(Hz) and multiplying it back up again to the full voltage, then converting to frequency using the piezo spec from the laser spec sheet of 4.11471MHz/V.

We maybe want to plot this on the main noise plot to see how far away we are from measuring a useful mirror displacement noise, so I have included the .mat file that I used to make the plot