I measured the RIN of PMC/ACAV/RCAV and noise in beat note when 250mV white noise was injected in the laser current actuator.

The result is note quite reasonable because the noise grows with f^1.5 instad of 1/f  .

The intensity servo is working now and reduce the noise from beat measurement a bit.

I did this to find out how RIN couples to frequency noise in beat note measurement, thus I can refine my noise budget.

I used SR785 as a source for white noise  @100mV and sent it to OUTPUT ADJ CH on the laser driver. It is the current acutator

which adjusts both intensity and frequency of the laser.

RIN from three cavities change almost at all frequency, but quite alot around 100 Hz -10 kHz, see fig1.

The noise from beat measurement also increases around that frequency range as well, see fig2.

and the coherence between RIN from each cav and the beat measurement are similar, so I plot only the coherence between RCAV_RCTRANSPD and beat note (fig3).

The sum of noise is
 

S_beat^2 = S_rin^2 + S_other^2     ----(A) , so

S_beat_with noise^2 = S_rin with noise^2 + s_other^2    -----(B)

 (B) - (A)  will give

 (extra frequency  noise due to RIN)^2 = (RIN)^2 * (conversion factor)

should tell us how RIN couples into S_beat

 The residue from white noise injection is plotted on fig4.

The input power to ACAV is 2.2 mW, RCAV is 2.5 mW.

I match the frequency noise residue with RIN by   quadrature sum of RIN * Pin * 2e3 * f^1.5, see fig 5.

This is very unreasonable. We expect noise due to RIN to be 1/f. 

 Meanwhile, suppressing intensity noise is still a good idea. I used SR560 to stabilize laser intensity.

The PD used for monitor the laser power is AC coupled to SR560, and the output is fed back

to the laser driver OUTPUT ADJ ch,

Gain setting 2 INV with 30dB attenuator at the output.

Low pass, with a pole at 100 kHz

 To my surprise, the noise of the beat note is reduced a bit, despite verey small coherence  between RIN and fbeat

we observed before. I compare with the data from this morning and I measure the beat frequency noise again

right after I finished with the data with intensity servo (2am), see fig 6.

I'll verify the data again by comparing RIN from each cavities, with and with out intensity servo.