Craig, Georgia, Danny, Sheila

We have re-tuned the SRCL feedforward for 30 W interferometer configuration. The updated feedforward reduced SRCL coupling to DARM from 60Hz to 500Hz (driven measurement shown in final attachment).

We measured the transfer function from SRCL to DARM without feedforward, and from the SRCL-FF path to DARM, using templates found here:

/opt/rtcds/userapps/release/lsc/h1/scripts/feedforward/

We then used Craig's ipython notebook to fit these transfer functions and calculate the feedforward filtering required to cancel SRCL noise in DARM. The notebook is found here:

/ligo/home/craig.cahillane/Git/Feedforward/SRCL2DARMfeedforward.ipynb

This notebook uses iirrational to fit the transfer functions. We added some human tweaking to the fit to minimise the resonant peak at ~8Hz. First attachment shows the data (blue), iirrational's fit (orange), and the human-modified fit that was implemented in the feedforward filter (green). Note that the fit to the phase of the TF is not good below 60 Hz, so perhaps there is room for iterative improvement on these filters.

The second attachment shows the old SRCL-FF filter (Nov7) compared to our new filter (Jan15).

The final attachment shows the SRCL to DARM coupling reduction: yellow is DARM (and SRCL) with no excitation, blue is with the old feedforward, purple is the new feedforward.