The alignment into the reference cavity was tweaked. Mostly a vertical adjustment followed by a minor adjustment to the yaw. The transmission as reported on the MEDM screen went from ~2.6V to ~4.6V. Measured the power before the lens located in front of the ALS fibre to be 33.4 mW. At this time MEDM reported 4.62V. With a DMM, the DC output of the locking photodiode was measured. - locked: 0.124 - 0.134 V - unlocked: 0.772 - 0.777 V So the reference cavity visibility is 83.3%. The gain of the filter yielding the DC output of the locking photodiode was changed from -0.050 to +0.500 as this agreed better with the value measured with the DMM. Not sure why the big change other than I had noticed that since the CDS reboot of a couple of weeks ago, the reflected DC was different. Perhaps this gain setting wasn't saved? For the transmission photodiode, when the FSS was unlocked the DMM measured -1.8mV. MEDM reported ~0.121V at the same time. When the FSS was locked and MEDM reported ~4.6V in transmission the DMM measured -2.766V to -2.777V. Jason/Peter
For a comparison I just happened to measure the ref cav visibility yesterday as part of the PSL rebuild and LLO also get ~83% visibility