The demodulation phase of the IMC rotated by approximately 90 degrees. This is the third time to see this kind of phase rotation in the IMC. Very strange.

• A brief history of phase rotation
  • 1st  2013.July.16 (alog 7108)
  • 2nd 2013.July.17 (alog 7119)
  • 3rd  2013 July.23 (NEW !!)

Temporary fix:

Temporarily I switched the signal from the I to Q signal by changing the actual cabling at the corner field rack and flipped the control sign at H1:IMC-REFL_SERVO_IN1POL by toggling the switch in the upper left of the IMC board screen. The IMC is now locking and should serve as an interferometric monitor for the sus and seismic people. The WFS loops are currently disabled by setting H1:IMC-WFS_GAIN to be zero to avoid a meaningless control. It seems the IMC started dropping its lock after Roberts and his company entered the PSL enclosure. The temperature transient somehow caused it ? It is unclear.

By the way the attached is a picture of the current I-mon and Q-mon signals in a X-Y projection. The I-mon is x-axis and Q-mon is y-axis. You can see a line almost in parallel to the y-axis indicating that the IMC length signal shows up mostly in the Q-phase.