[Michimura, Yokozawa, Komori]

The first measurement of the cross-spectrum between two DC PDs is shown in the attached figure (green curve).
The data was collected around 18:00 UTC on October 15th.

In this measurement, we were able to subtract out the noise contributions specific to each PD (the shot noise and the dark noise), allowing us to estimate the underlying noise common to both.
The green spectrum represents a flat structure between 500 Hz and 2 kHz.
One possible candidate for this noise source is frequency noise.
By increasing the interferometer (IFO) power, we could potentially reach the level indicated by the green line while simultaneously reducing the shot noise and dark noise contributions on each PD.