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Message ID: 2324     Entry time: Sat Apr 13 16:23:30 2019
Author: anchal 
Type: DailyProgress 
Category: FSS 
Subject: South Cavity Reflection Path reset 

Today I reset the reflection path from faraday isolator to the RFPD.

  • First I slightly turned the input BS on faraday isolator to make reflection beam more horizontal. Then I changed input polarization into isolator accordingly.
  • I removed one of the mirrors from the reflection path.
  • I added a 0th order half-waveplate right after the isolator. Earlier, there was no polarization control here. I tuned the waveplate to make all the light in p-polarization.
  • I also added a PLCX-25.4-46.4-UV-1064 lens in the path.
  • Beam profiler was not available, so I used my earlier a la mode calculation and reverse propagate light from the cavity to have a beam width of about 500 um at the RFPD.
  • Now, with 3.385 mW light falling on the photodiode, I see a DC output of 6.6 V. This is already more than expected value with 0.75 A/W responsivity. I think now we are definitely absorbing all of the light. I measured about 70 uW of light reflecting from the photodiode.
  • Angle of incidence is about 25 +- 5 degrees.
  • I also put back the triangular beam dump for reflected light from photodiode.

This certainly improved the amount of light reaching RFPD but the PDH error signal still looks the same. I'll directly hook up RFout of RFPD tomorrow to the oscilloscope and take some time series data to see any discrepancy with waveform shape.

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