Naoki, Vicky

We injected the band limited white noise in FC-IR loop and could measure the FC backscatter noise in DARM (attached figure). It seems that the FC backscatter noise is lower than the DARM by one order of magnitude.

This is the similar measurement as LLO63858. We injected the band limited white noise in FC-IR DOF2 excitation port. The top plot shows the FC-IR error signal without excitation (brown) and with excitation (purple). The bottom plot shows the DARM spectrum without excitation (blue), with excitation (red), and the projected backscatter noise.

We are not very sure the calibration from the FC-IR error signal to the DARM, but the FC length excitation matches well with the backscatter in DARM when we calibrate the FC IR error signal by a zero at cavity pole (37 Hz) and some calibration factor. This calibration needs to be considered more.

As you can see, the projected backscatter (brown) is ~one order of magnitude lower than the DARM (blue) below 200 Hz. Above 200 Hz, the IR error signal is limited by the dark noise, so the projected backscatter is just the upper limit.

Note: As reported in alog67586, we also need a radiation pressure gain for the projected backscatter.

GPS time of measurement: 1362963475