Yutaro, Kiwamu,

Today we spent some time on investigating fiber phase noise and did a cavity scan measurement. No concrete results yet, but here is a short report on them.

Noise investigation:

We switched off as usual all the fans and air conditioners (7150, 7255). Then we locked the main laser frequency to the X arm (as described in 7289) while keeping the green X PDH unlocked but the green frequency was set so that is stays on a 00 resonance. In this way the green PDH signal represented its sensing or IR's sensing noise. Keeping this configuration, we switched on each of the fans and air conditioners at a time to see if which unit dominates green PDH's sensing noise.

The result is shown in the attachment below. Surprisingly, the PSL KOACH filters and PSL precision air conditioners didn't show appreciable effect while almost all the FFUs showed a monochromatic line at around 17 Hz. In the figure, the data with the BS are SR2 FFU unit on is shown only. Other FFUs showed almost identical behavior i.e., an peak at around 17-ish Hz. Note that the data is not yet calibrated. Anyhow, this may mean that the pillar and tapings for the fibers (7180, 7224) improved the coupling from them to fiber phase noise. We will work on the calibration tomorrow.

Cavity scan:

We did a measurement in which the green laser frequency was swept with respect to the X arm cavity length by locking the main laser to the X arm directly. We will analyze the data later, but here is the data hot off the press. The scan rate was set to 100 Hz/sec for the function generator that controls the beatnote frequency.