Chris, Sheila

Our measurements of the COMM noise with the WFS locked have an rms of 20Hz, measured down to 0.1 Hz.  To make sure this was reasonable, we turned off the refl bias path so that the laser was locked only to the green beat note. The transmitted power fluctuations are noticably less than the HWHM of 40Hz, so this seems believable. (StripTool is the second screenshot attached). 

We wanted to try locking the arm half way down the fringe so we could make a measurement of the noise using the transmitted power.  Since the IR trans PD is normalized to 1 when the cavity transmission is maximized, the calibration of this signal is just a high frequency gain of 84 (FWHM of the arm cavity), and to correct for the refl bias loop gain we have a pole at zero and a zero at 3Hz.  The correct calibration has a more complicated frequency dependence, so we would have to do the real calibration to make a comparison above about 42 Hz.  Neither of these traces have the cavity pole at 42 Hz corrected for.  You can see that the side of fringe measurement agrees with the normalized refl PD signal well up to 100 Hz . (1st screen shot attached)

We repeated the measurement in the normal configuration, where we lock on resonance using the refl bias path, and the noise is the same (30Hz rms down to 0.02Hz). The RMS is dominated by the pitch mode, so we will work on increasing the WFS bandwidth tomorrow. Tonight we tried OpLev damping (this added noise around 1 Hz) and turning up the pitch OSEM damping gains (inconclusive so far). 

We redid the measurement with the WFS outputs held, and saw that the noise was high again (100Hz rms down to 0.1Hz), with the WFS on again we were back to the low noise state. So we can safely say the WFS are helping.

We also checked that the noise is the same from 3-100Hz with and without the refl bias path engaged.