I've started working on a general routine to measure noise couplings in our interferometers. Often this is done with swept sine measurements, but this misses the nonlinear part of the coupling, especially if the linear part is alreay reduced through some compensation or feedforward scheme. Rana suggested using a series of narrow band-limited noise injections.
The structure I'm working on is a python script that uses the AWG interface written by Chris W. to create the excitations. Afterwards, I calculate a series of PSD estimates from the data (i.e. a spectrogram), and apply a two-sample, unequal variance, t-test to test for statisically significant increases in the noise spectra to try and evaluate the nonlinear contriubutions to the noise. I've started a git repository at github.com/e-q/ifoCoupling with the code.
So far, I've tested one such injection of noise coupling from the ETMX oplev error point to the single arm length error signal. It's completely missing the user interface and structure to do a general series of measurements, but this is just organizational; I'm trying to get the math/science down first.
Here's a result from today:
Median, instead of the usual mean, PSDs are used throughout, to reject outliers/glitches.
The linear part of the coupling can be estimated using the coherence / spectrum height in the excitation band, but I'm not sure what the best what to present/paramerize the nonlinear parts of each individaul excitation band's result is.
Also, I anticipate being able to write an excitation auto-leveling routine, gradually increasing the exctiation level until the excited spectrum is some amount noisier than the baseline spectrum, up to some maximum amount configurable by the user.
The excitation shaping could probably be improved, too. It's currently and elliptic + butterworth bandpass for a sharp edge and rolloff.
I'm open to any thoughts and/or suggestions anyone may have!