40m QIL Cryo_Lab CTN SUS_Lab TCS_Lab OMC_Lab CRIME_Lab FEA ENG_Labs OptContFac Mariner WBEEShop
  40m Log  Not logged in ELOG logo
Entry  Wed Jun 15 02:20:00 2011, Jenne, Update, Locking, MICH noise budget? MICH_noise_budget_measurements_15June2011.jpg
    Reply  Wed Jun 15 12:16:37 2011, Jamie, Update, Locking, MICH noise budget? 
Message ID: 4822     Entry time: Wed Jun 15 02:20:00 2011     Reply to this: 4823
Author: Jenne 
Type: Update 
Category: Locking 
Subject: MICH noise budget? 

I would like to announce my confusion with regard to the MICH noise budget, in hopes that someone else has some inspiration

If you tilt your head sideways, you will notice that in this plot (totally uncalibrated, as yet), the BLACK trace, which is my white-light measurement of the AS55 shot noise is above the AS55Q noise when the Michelson is locked (true only at low frequency).  You will also notice that the same appears to be true for the Whitening Filter + Antialiasing Filter + ADC noise (GRAY trace).  Since Black, Gray, Pink and Green should all have the same calibration factor (a constant), calibrating the plot will not change this.  Brown and Blue are the MICH_OUT (aka MICH_CTRL) for dark and bright fringes, respectively.

I measure 58mV at the DC out of the AS55 PD when the Michelson is locked on the bright fringe.  This (assuming DC transimpedance of 50ohms) gives 1.16mA of DC photo current.

So.  What is going on here?  Am I totally confused??

In other news, assuming (which I'm not 100% confident about right now) that these traces are vaguely correct, the Michelson is limited by shot noise above ~20Hz.  This is...good?  We want to be shot noise limited.  Do we want to be limited at such a low frequency?

(Also, yes I can calibrate the plot to m/rtHz, but no, I won't tonight because something is funny with my calibration for the free running noise and I'll fix it tomorrow.)


ELOG V3.1.3-