Robert, Dan

On some locks we've noticed very loud scattering noise, sometimes extending out to >100Hz, implying an impressive fringe velocity of tens of microns per second.  Robert pointed out to me yesterday that the OSEMs of the OMC suspension saw high amplitude motion around the time of these glitches.  The OMC SUS motion seemed to be in the vertical direction, and we guessed that the OMC ASC pitch feedback to the OMC SUS was coupling to the vertical motion of the suspension.  And, from there, some scattering path length was changing very rapidly.

Recently the OMC has been aligned using the QPD loops, which are relatively high gain (ugfs around 1Hz).  These loops drive the OMC SUS fairly hard.  Coupling to the other SUS DOFs has been a concern.

Today on our first low-noise lock we immediately noticed the scattering noise between 40 and 200Hz.  We reduced the OMC ASC gain and the noise disappeared.

The scattering noise is highly correlated with the velocity of the OMC SUS in the vertical and longitudinal directions.  In the plots attached, the top panel is a spectrogram of the whitened calibrated DARM channel.  Scattering arches are seen at 60Hz and up to 220Hz.  The bottom panel is the absolute value of the time-derivative of the OMC SUS motion, either vertical (first two plots) or longitudinal (second two plots).  The units are in velocity although I haven't gotten the calibration to make sense, and anyways we would need to calculate the motion of the bottom stage.  (These plots are reminiscient of similar work by Bas Swinkels a few years ago.)

The second plot or each pair shows 100 seconds of data, as I turned down the gain on the OMC ASC loops and the suspension was driven more and more gently.  As the velocity drops, so do the scattering glitches in DARM.

For now we'll run in a low-gain state for the OMC ASC.  After talking with Jeff, one solution might be coil balancing of the OMC SUS to decouple pitch actuation from the vertical mode.  Also, we might try some vertical and longitudinal  compensation in the OMC ASC actuation itself.  Or, we can chuck it all and use only the tip-tilts.