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Entry  Mon Nov 15 18:05:34 2010, tara, DailyProgress, BEAT, coherence <fbeat|RCAV>, <fbeat|ACAV> coherence_before_realign.pngcoherence_after_realign.pngbeat_2010_11_15.png
    Reply  Mon Nov 15 19:22:58 2010, rana, DailyProgress, NoiseBudget, Preparation to suppress intensity noise. 
       Reply  Tue Nov 16 03:36:55 2010, tara, DailyProgress, NoiseBudget, Preparation to suppress intensity noise. RIN.pngcoh.png2010_11_15.matcurrent_mod_TF.pngTF_current.mat
Message ID: 394     Entry time: Tue Nov 16 03:36:55 2010     In reply to: 393
Author: tara 
Type: DailyProgress 
Category: NoiseBudget 
Subject: Preparation to suppress intensity noise. 

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 



 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



NOTE: (from the yesterday)

 DC level for





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.


Attachment 1: RIN.png  28 kB  | Hide | Hide all
Attachment 2: coh.png  17 kB  | Hide | Hide all
Attachment 3: 2010_11_15.mat  165 kB
Attachment 4: current_mod_TF.png  21 kB  Uploaded Tue Nov 16 17:19:14 2010  | Hide | Hide all
Attachment 5: TF_current.mat  7 kB  Uploaded Tue Nov 16 17:22:44 2010
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