I added a Faraday isolator after PMC and 35.5 MHz broadband EOM, now the noise becomes less susceptible to the
input power to the cavity.
The isolator is installed between the 35.5 MHz EOM and a lens just before the beam splitter that splits the beam into
ACAV and RCAV path. I measured the beat signal at 10mW (measured in front of the BS, as before).
The beams going into both cavities have the same power level ~0.4 V, see blue plot.
Then I realized that I had not re-aligned the beams into both cavities. The isolator significantly
alters the beam paths, I aligned the input beam, then measured the beat noise again, at 4mW and 2 mW, see green and red plots.
*I'm very surprised to see no significant difference between before and after alignment.
All new three results are very comparable to 2 mW measurement before the isolator was installed, see pink plot.
However, if we compare 10mW from today and yesterday measurements, there is a significant broad band effect at high frequency.
By adding the isolator, we reduce the back reflected power to the PMC, and the laser. The power dependent noise we saw before
might come from this back reflected beam to the laser. Tomorrow, I'll try adding an EAOM.