[Nutsinee, Haocun]
After putting ND filters before the OPO REFL due to the saturation issue, the OPO can be locked without problems, and the I and Q phases look good. (Nutsinee can give more details on this.)
Later on we took more squeezing measurements under this good locking condition, and I plot the data with some fitting curves as attached.
Homodyne Visibility: 97% and 96.87% (We had to do this, and before any adjusting it was ~86.7% with ~2mW unbalancing.)
So the estimated optical efficiency is 65% and 50mrad phase noise.
According to our budget here, our optical efficiency should be around 85%, which means we need to find out where are these ~20% loss from.
Next step:
- Swap PD or tweak electronics to have proper set-up for the saturation problem, instead of using ND filters.
- We'd better to adjust the crystal position again with OPO locking properly.
- Measure the optical loss on SQZT6, and dig out other potential losses.
Just re-checked the quantum efficiency of both diodes, which are pretty good. Very close to 1. The back reflection are ~10uW out of 1mW input.
However, found out that we were using 1mW on each diode instead of 0.5mW for those measurements, which could give some effects on the squeezing measurements.