I modulate the laser power @ 100Hz and measure RIN from PMC,RCAV,ACAV trans and beat note at 100 Hz, t
and then find The conversion dp/df for RIN to frequency noise to be 1.01 MHz/W. I also measure the TF from the power adj to PMC TRANSPD.
High coherence between RCAV and ACAV trans seems suspicious, and it changes with FSS gain setting.
My gain setup might not be optimized. I'll check it tomorrow.
I use a function generator to send sine wave signal at 100 Hz, 0.2 Vpk-pk to modulate the laser power.
The peaks at100 Hz for RIN from PMC/RCAV/ACAV and fbeat are
Vrm/rtHz at 100 Hz [V] DC [V] RIN 100Hz (Vrms/DC) [1/rt Hz]
PMC 12.49 e-3 1.5 8.327 e-3
ACAV 3.27 e-3 0.313 10.45 e-3
RCAV 11.57 e-3 0.59 19.61 e-3
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fbeat 984 e-3 with 70.8 kHz/V covertion -> 70Hz/rtHz
The coherence between beat note and RCAV/ACAV rin is ~1. So if we assume that the mechanism
that converts RIN to noise in beat note are the same in both cavities, we have
fnoise ^2 = (RIN_AC x Pin_AC x conversion)^2 + (RIN_RC x Pin_RC x conversion)^2 .
(This might not be accurate, since the peak from f beat is not that relatively high)
Pin for Acav and Rcav are 2.3 and 3.3 mW.
This gives conversion = 1.01 MHZ / V @ 100HZ
This conversion will be applied to RIN level to see how much it would be convert to fbeat noise.
(assuming the conversion is flat at all freq, we will find this conversion at different frequencies later)
TF from power mod input to PMC trans readout is plotted below. The magnitude at 100 Hz is -17dB
The source from SR785 is split by a T, one goes to ref channel A, another is sent to laser power modulation.
Response B is
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NOTE: (from the yesterday)
DC level for
ACAV_RCTRANSPD = 285 mV
RCAV_RCTRANSPD = 612 mV
PMC_RCTRANSPD = 1.48 V
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We need to see a plot of the coherence between the PMC trans and Rcav trans and the Acav trans. Also the RIN of all 3 plotted on the same plot.
Then, inject a small line into the current adjust at 100 Hz and measure the height of the peak on all three trans and the beat signal. Use this peak to scale them all and put them on the same plot in physical units.
Then measure the TF from the current adjust actuator to the PMC trans and close an AC coupled feedback loop on it.
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