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Entry  Sat Jun 30 05:01:04 2012, tara, DailyProgress, BEAT, beat measurement with 2mW input power beat_2012_06_29_3.png
    Reply  Mon Jul 2 00:46:05 2012, tara, Notes, NoiseBudget, mechanical loss in spacer spacer_loss.png
Message ID: 1009     Entry time: Sat Jun 30 05:01:04 2012     Reply to this: 1010
Author: tara 
Type: DailyProgress 
Category: BEAT 
Subject: beat measurement with 2mW input power 

I changed the power input from 1mW to 2mW for both cavities and measure the beat signal. From the measured results, there is no difference between the beat signal at different power input levels. This means that the current signal is unlikely to be sitting on shot noise or electronic noise that depend on power level.


     With the current laser (500mW), we can increase the power to both cavities up to 2mW. With higher input power, shot noise and electronics noise can be brought down. Even though the noise budget suggests that we are not limited by shot noise and electronic noise, I think it is a good idea to verify it . If we are limited by the shot noise due to low input power or electronic noise due to low frequency discriminator gain, we can see the change if we increase the power.


  1. The power into both cavities are increase to 2mW.
  2. For RCAV, common/fast gain are 630/750.
  3. For ACAV I used an ND filter to reduce the power incident on RFPD. I chose the one that reduce the power roughly in half.

==results and comments==


I plot the beat measurement with  1mW (black) and 2mW(pink) power together to show that there is no apparent improvement. Note that the noise budget is corrected for 2mW input power for both cavities. (I have not include the noise due to power fluctuation yet, I'll add that soon). 

 The slope around 50 Hz to 200 Hz is strongly go with 1/f^0.5 which is the slope of Brownian noise. However, it is ~ a factor of 1.2 above the upper limit of our estimation. I will check with Peter if he still has some information about the coating or not, and also the parameters in the noise budget calculation (for example, the loss in SiO2 for spacer, substrate to see if the values I use are sensible or not.

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