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| 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. |
Hey, Jenne. I think there are a couple of things. First, you're missing a PD dark noise measurement, which would be useful to see.
But I think the main issue is that it sounds like all of your closed loop measurements are done with the in-loop PD. This means that everything will be suppressed by the loop gain, which will make things look like they have a noise lower than the actual noise floor. |