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Entry  Tue Mar 3 11:49:16 2020, anchal, DailyProgress, ISS, Transmitted Laser Relative Intensity Noise CTN_Trans_RIN_measurement.pdf
    Reply  Sat Mar 7 09:50:31 2020, anchal, DailyProgress, ISS, Transmitted Laser Relative Intensity Noise - Out of loop CTN_Trans_RIN_measurement.pdf
       Reply  Wed Mar 11 12:45:15 2020, rana, DailyProgress, ISS, Transmitted Laser Relative Intensity Noise - Out of loop 
          Reply  Wed Mar 11 13:16:09 2020, anchal, DailyProgress, ISS, Transmitted Laser Relative Intensity Noise - Out of loop 
             Reply  Wed Mar 11 16:43:55 2020, anchal, DailyProgress, ISS, Comparison between Out of loop vs In loop RIN CTN_Trans_RIN_OOLvsIL.pdf
Message ID: 2555     Entry time: Wed Mar 11 16:43:55 2020     In reply to: 2553
Author: anchal 
Type: DailyProgress 
Category: ISS 
Subject: Comparison between Out of loop vs In loop RIN 

I took spectrum of Out-of-loop (OOL) photodiode and In-loop (IL) photodiodes with transmitted light from the cavities when ISS is on in both paths at gain value of 2x10000.


Measurement:

  • In-loop photodiodes of ISS have been disconnected from Acromag cards as this was injecting noise into the loops.
  • So, without witnessing the DC level, we can increase the ISS loops' gain to 2x1000 with HF and LF both set at 300 Hz with 6 dB/octave roll off.
  • A major problem in this measurement is the slow drift of DC power level during the time of measurement.
  • I was measuring DC value of photodiode averaged over 10 s with an oscilloscope just before the measurement was taken.
  • In fact, one can see that between two measurements of the different spans, the DC value has changed enough to show mismatch in South In-Loop RIN.
  • The out-of-loop photodiode was measured with sequentially blocking either the north laser or the south laser.
  • The same issue with knowing the exact DC level persists here as well.

Inference:

  • As expected the out-of-loop noise is higher than the in-loop noise, but I did not expect a factor of ~5 difference.
  • However, this method of measuring RIN isn't very faithful.
  • Overall for the experiment, it is clear that increasing the gain of ISS further did not lower the beatnote spectrum much.
  • This suggests that we have damped down photothermal noise in the experiment below other fundamental noise sources.
  • My future efforts would be focused solely to get rid of the peaks in the range of 200 Hz to 1 kHz.
  • I see that the sharp peak at 480 Hz has changed into a tri-peak around the same frequency without any changes in the experiment other than ISS.
  • This atleast indicates that this peak was not undefeatable, and so must be the remaining ones.

Data

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