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Entry  Tue Nov 25 23:11:41 2014, Dmass, Noise Hunting, Cryo ISS, Anomalous intensity noise CavHz2RIN.png
    Reply  Tue Dec 2 16:05:06 2014, Dmass, Noise Hunting, Cryo ISS, ISS loops almost work 
Message ID: 1169     Entry time: Tue Nov 25 23:11:41 2014     Reply to this: 1170
Author: Dmass 
Type: Noise Hunting 
Category: Cryo ISS 
Subject: Anomalous intensity noise 

We still have the strangely large intensity noise in both paths.

 

 The lowest transmitted RIN we have measured in lab is:

  • White up to the cavity pole
  • 1/f after the cavity pole
  • few times 1e-5
  • Decades higher than the intensity noise on the beam measured simultaneously right before the crytostat, via a pickoff mirror

A plot of the [RIN/Hz] transfer function as a function of offset from cavity resonance is below -

Maximum possible value is 2.5e-5 RIN/Hz

When we have tuned offsets to try to make this go away (in the past), we have not been crazy offset. I think a reasonable estimate based on this, and transmitted power optimization is < 0.5 RIN/Hz

If we had a monstrous offset (~40 kHz), the frequency noise required to produce this is ~1Hz/rtHz, which is well in excess of our previous best beat

=> the intensity noise measured at transmission is not frequency noise + offset

Nic and I are turning on the ISS and taking a beat tomorrow, to see if Intensity noise -> frequency noise coupling is currently limiting (photothermal)

 

Musings -

Would there be any way to get white jitter noise which couples into intracavity power but not into the input PDs?

We discovered that we can effect the intensity noise with the half-wave plate right before the cryostat input window - the L/4 plate next to it has no effect.

I would suspect laser feedback**, but measured frequency noise at transmission, and laser output intensity noise are both too low, unless it could somehow be laser feedback induced jitter which looked white up to high frequency?

 

** because it is so easy to mode hop the lasers even with Faraday's at their output

 

 

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