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Entry  Wed Mar 20 15:57:55 2019, Anjali, Noise Budget, 2micronLasers, Noise budget for the frequency discriminator High_pass_transfer_function.pdfShot_noise.pdfThermoconductive_noise_of_fiber_Glenn_parameter.pdfnoise_budget_combined_log_log.png
    Reply  Wed Mar 20 19:17:50 2019, rana, Noise Budget, 2micronLasers, Noise budget for the frequency discriminator 
Message ID: 2307     Entry time: Wed Mar 20 15:57:55 2019     Reply to this: 2308
Author: Anjali 
Type: Noise Budget 
Category: 2micronLasers 
Subject: Noise budget for the frequency discriminator 

I was trying to prepare the noise budget for the frequency stabilisation setup for 2 micron laser. In this test, we use a fiber based Mach-Zehnder interferometer as a frequency discriminator to convert frequency noise to amplitude(voltage) fluctuations. The different noise sources to be considered in this analysis are the following

  1. Shot noise
  2. Dark current noise
  3. Room thermal noise
  4. Fiber thermal noise
  5. Fiber acoustic noise
  6. Laser intensity noise
  7. Photothermal noise
  • Attachment #1 shows the magnitude spectrum of the transfer fucntion for the frequency discriminator. I assumed the delay fiber length in one of the arms of the Mach-Zehnder interferometer is 10 m. This corresponds to a time delay of 50 ns. The corner frequency is inversely proportional to the time delay.
  • The transfer fucntion can be interpreted as, if the frequency variation is very slow- it can't be distiguished by the discriminator and we dont get any signal.
  • Attachment # 2 and # 3 shows the shot noise and fiber thermal noise respectively in rad/rtHz
  • The shot noise is calculated  considering thorlabs DET10D detector with a responsivity of 1.2 A/W at 2 micron. The laser output power from Eblana laser is assumed to be 2 mW and the power reaching the detector is estimated to be 1.23 mW.
  • The fiber thermal noise is calculated based on Wanser, K. H. (1992). Fundamental phase noise limit in optical fibres due to temperature fluctuations. Electronics letters28(1), 53-54.
  • The dark current noise and room thermal noise are calculted to be a constant value (3.97x10-12 rad/rtHz and 1.20x10-12 rad/rtHz respectively)- but this is not correct
  • The noise in rad/rtHz is converted to Hz/rtHz by mutiplying with the frequency (f) and it is shown in attachment #4
Attachment 1: High_pass_transfer_function.pdf  16 kB  Uploaded Fri Mar 22 20:03:51 2019  | Hide | Hide all
Attachment 2: Shot_noise.pdf  29 kB  Uploaded Mon Mar 25 11:41:33 2019  | Hide | Hide all
Attachment 3: Thermoconductive_noise_of_fiber_Glenn_parameter.pdf  25 kB  Uploaded Mon Mar 25 11:41:48 2019  | Hide | Hide all
Attachment 4: noise_budget_combined_log_log.png  50 kB  Uploaded Mon Mar 25 11:42:58 2019  | Hide | Hide all
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