I measured the beat signal from 3 different power input levels, the signal goes down with the power mostly at higher frequency.

When HEPA filters above the table are off, the noise also goes down, this might suggest that we still have scattering light problem.

 We are hunting for the noise source for our experiment. We want to see if our setup is limited by intensity noise or not

so, we try to change the power level and see the beat signal.

 I used 3 levels of power (measured after PMC) to be 10 mW (original value), 20 mW , 2mW, (the settings are listed below.)

The reason that the level at 10 mW decreases from previous beat msmt is that the gain for FSS and AOM loops are optimized

PDH box for AOM loop is also modified (R12, 1k -> 10k), the power into both cavities are adjusted to be about the same.

 The noise level at 10 and 20 mW are almost the same except a hill at 60 kHz on 20mW, and

  the noise from 2mW setup is lower than other settings at high frequency, starting ~ 20kHz.

 At 1 mW, I can't lock the PMC (the transmitted beam on the PD is so faint), so I increase to 2 mW to make locking easier.

At 2mW power, the range for feedback signal for PLL loop becomes smaller.

If it exceeds approximately +/- 50 mV [3.5kHz], the lock loses, but I still keep the frequency range at 100kHz.

I tried to increase the gain on SR560 which is a servo for PLL loop, but

the beat signal was worse at high frequency, starting at 2kHz.  Fortunately, the temperature drift at night (~10mV/min)is much smaller than

the drift during the day(~5mV/sec), so I can wait long enough to measure the beat at low frequency, with the gain level of 1, as usual.

At 2mW setting, I have the HEPA filter on and off in comparison. The noise level decreases a bit when the fan off, this might suggest that

scattering light is still a problem.

Even though the noise from beat measurement change with the power, the amplitude does not go with the power change.

i.e, the noise in the beat note does not go up with the same factor of the power change.

So the setup is not limited by intensity noise.  

setting for the measurement ["-" means the same as previous value]

                                      10 mW              20 mW             2mW

PMC: common gain         16 dB                 -                     30dB                       

PMC:RF                            5.7V                 -                       -

FSS:FAST gain               18.5 dB              -                      -

FSS:common                  22 dB               18 dB             20dB   

FSS:RF                           10 V                  -                      -

PDH for AOM:GAIN        4.45 (knob)       3.2                 7.0

Pin RCAV/ACAV            2.1/2.1 mW        4.8/3.9 mW    0.4/0.4 mW