Today, we found out that at high frequency, the limiting noise source is probably electronics noise might be from the Universal PDH box.

We tweaking the setup parameters, FSS gain, UPDH gain, sideband power, laser power to see what will change the beat noise.

We learned that

1) The current setup is not FSS gain limit, because we adjust the gain, but the beat noise remains the same,

and the range of the gain we can adjust is quite high, between 5 - 30 dB.

2) By adjusting side band power, the noise level changes.

However, when we added gain to compensate for the lower side band power, the noise does not go back to original level.

This is weird. We adjusted decrease 35.5 MHz sideband power, and noise level goes up. Increasing gain in FSS and ACAV loop does not change it back.

3) The peaks around acoustic frequency (mechanical peaks)can be suppressed by increasing PDH gain, but the flat level changes.

The noise level calculated from input referred noise sitll does not match with the beat noise, but they are close.

Now the beat noise is not limited by LO phase noise.From the flat shape of the noise, we think that it might be the electronic noise

from the PDH box.

To do next: increase the error signal slope for ACAV path. Now it is very small ~23 mV.