Related: alog 30221

In addition to what Daniel has posted, I measured the coherence and DBB QPDs again, this time in nominal low noise.

Probably due to lower IFO noise, the coherence is larger and broader, extending down to 70Hz or so.

The differences between today and yesterday are:

• Used pre-alignment servo to center the beam on QPDs, and then held the output of the PZT filters. So this is still totally open loop but the centering is much better.
• DBB PMC is kept very close to anti-resonance by adding offset to DBB PMC length PZT manually. During the measurement, transmission PD output was lower than 1.1mV. Compare this to about 8 or 9V when in resonance.
• IFO was in nominal low noise.
• The plot shows DCPD_SUM scaled arbitrarily by 3E6 instead of DELTAL_EXTERNAL.