Eleonora and Yuhang

We tried to tilt lens by more than 10 degrees to avoid back scattering (see attached figure 1). Beside, we tried to put beam dump (see attached figure 2) or iris(SM1D12 with thread and can be mounted to the lenses before homodyne) to block the scattered light.

The comparison of shot noise with homodyne open to filter cavity(but only vacuum come from FC) is shown in the attached figure 3. Although we could see there are no obvious difference between use beam dump (iris) or not, the level of back scattering around 10Hz is less than the shot_FC measurement we did one month before. This means anyway, the tilt of lens helps to reduce back scattering.

However, the coating of lens is not optimal for non-normal incidence. So we tried to measure squeezing again. And the result is attahed as figure 4. We could see we have only 5.63dB squeezing. This means we have 3% more loss caused by the tilt of lens.

Anyway, we measured also the FIS from FC, the result is attached in figure 5. Although we have only 2.17dB squeezing from FC, but the low frequency squeezing is much better now.