Eleonora and Yuhang

As we tested on Thursday, CVI mirror is ideal for our requirement. So we decide to use CVI BS. Although it takes us a lot of time to find a good position for it. We succed in balancing homodyne in the end and to find a good compromise between the position of a lot of optical components. The change of optical components is summarized in the last attached picture(the latest change is updated to naoj_wiki).

We verified that no backreflected beams could be found with the new BS. 

After balancing homodyne, we measurde back scattering and squeezing again. The comparison is between Friday and Thursday result (attached figure 1, 2 and 3).

Note that the back-scattering is measured by placing a sensor card on the squeezing path. Since the peaks observed at low frequency are at the bench resonant frequencies, we assume that the sensor card (which is vibrating with the bench), is reflecting back the scattered light towords homodyne. If we put a beam dump instead of the sensor card the peak at low frequency disapperad.

In attached figure 1, we can see the back scattering is even worse. Based on the fact that this back scattering is not stable, we think that the change of BS doesn't improve the situation.

In attached figure 2, we can see the squeezing level is improved from 5.90dB to 6.06dB. This means CVI BS really has less loss. From the loss and phase noise information measured in entry 1916, we can infer we reduce loss by ~1% by replacing BS.

In the attached figure 3, we can see there are some new peaks appear around 3kHz to 10kHz. These peaks was there several months ago. They were removed after the use of Faraday. We will investigate why they show up again.

Conclusion: the back scattering peaks at low frequency observed when we place a sensor card on the sqz path are still present even after replacing BS with a better one, with no detectable back reflection, and even after replacing homodyne lenses with superpolished ones. The source of back scattering was not the cube BS. So, what can it be? Homodyne?

Next steps: Put beam dumps around Homodyne. Check the situation with the filter cavity aligned.