Today I measured beat noise again to see where we are after adjusting many parameters.
The beat measurements are plotted below for 100 kHz and 10 kHz input range.
For 100kHz input range (grey), we are limited by LO phase noise. The shape of the phase noise has changed
from previous data in pink.
For 10 kHz input range (red), something else is the limiting source. I'm checking if we are limited by RFPD noise or not, but
the result is not conclusive yet.
I also measured the noise at the error point for ACAV loop (loop closed) in green, then projected it on the nb
by multiplying by the slope of the error signal (0.252 MHz/V = cavity's FWHM / error signal pkpk value = 108 kHz / 428 mVpk-pk.)
The level of the noise match the beat around 30Hz to 1kHz, but diverges at higher frequency.
Not quite sure if it makes sense or not (PLL loop is valid upto ~ 3kHz.)
The input referred noise of RFPD and the PDH box is measured, by blocking the beam on the RFPD
and measuring the singal on PDH out. The data is divided by the TF of the PDH box ( I measured this while the loop is closed,)
and multiplied by the error signal's slope, then plotted in blue.
The result is not quite right. It's higher than the beat. I'll have to check this.
*ACAV gain is set to 1.5 for all measurements.
RCAV common gain is set to 22.0 dB
power to each cavity = 1 mW