[Yuhang, Aritomi]

We had homodyne noise spectrum bump problem reported in entry1529.

One of the reasons could be the cut issue of PD, and I checked the entry1159. I found that the 'beam analyzer' was put in a wrong position. The corrected version is attached here. In this version, the 'beam analyzer' is marking the position where the beam size(100um) is 5times smaller than the size of PD(500um). So we can decide that the homodyne PD should be 26mm~32mm after 30mm lens. This means we have only 6mm range to make sure the beam is smaller enough than the size of PD(500um).

We also measured again the shot noise after this work(attached picture 2 and 3). The bump issue sometimes is till present, but it covers region smaller than 20Hz and rarely show up. But we found small 45Hz and 54Hz peaks which are not present in previous measurement. They may due to we didn't cover bench as before(now the west side is half open and south side is open).