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Entry   30Hz noise issue, posted by Ronic Chiche at Optical room about lasers and optics | detectors and electronics 
    Reply   30Hz noise issue, posted by Ronic Chiche at Optical room about lasers and optics | detectors and electronics 
Message ID: 84     Entry time: Tue Nov 26 16:35:39 2019     In reply to: 81
 Author: Ronic Chiche 
 Status: Fixed 
 Type: issue 
 Category: lasers and optics | detectors and electronics 
 Location: Optical room 
 Title: 30Hz noise issue 

The 30 Hz noise issue has been solved !

It came from translation stage of P1 and/or P4.

Fabian remembered that close to the mirror's mount translation stage end coarse, there is a mechanical instability. The mount kind of "lift up" because of the spring strength and could induce resonance.

Initial positions:

  • P1: - 1 500 000 
  • S2: + 1 400 000
  • S3: + 1 400 000
  • P4: - 1 500 000

The 30 Hz noise was removed while moving only P1 closer (then if we put back P1 to the initial position, the noise clearly appear again):

  • P1: - 1 300 000 --> no 30Hz noise
  • S2: + 1 400 000
  • S3: + 1 400 000
  • P4: - 1 500 000

The 30 Hz noise appeared again while moving P4 further:

  • P1: - 1 300 000 
  • S2: + 1 400 000
  • S3: + 1 400 000
  • P4: - 1 600 000 --> 30Hz noise

Final positions:

  • P1: - 1 300 000 
  • S2: + 1 400 000
  • S3: + 1 400 000
  • P4: - 1 500 000

 

Ronic Chiche wrote:

since some days, we observe the cavity is difficult to lock and a strange 30Hz noise has appeared on the PZT signal which normally compensate the phase noise difference between the cavity and the laser.
to test if the problem could come from the laser, we changed the OneFive laser for the Koheras but we have exactly the same problem, thus we concluded that the problem come from the cavity or from the feedback.

today, we borrowed an accelerometer measurement setup to Julien Bonis to test if we can see a clear noise at 30Hz from the seismic noise.
we placed to accelerometer directly on the top of the cavity but the spectrum we obtained do not show a clear noise line at 30Hz, only a small excess of noise in this region...
nothing which clearly indicate the cause of our problem.

yesterday, we changed the feedback setup in changing the PDH box from n°2 to N°3 without any change in the 30Hz noise line.
today, we also changed the feedback setup by introducing and amplifier of 100 just after the PDH box.
if noise is coupled after the amp we should be more immunized from it... but nothing changed again.

still looking for the origin of this problem...

 

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