Chien-Ming and Yuhang

After the last very good measurement of squeezing, we start to think about improving coherent control 1 PD. Then we measured squeezing again. The result is attached in figure 1 and 2. As you can see the low frequency is covered by classical noise while high frequency seems fine. However, the squeezing level was only 4.9dB at that moment.

Then Chien-ming suggested cleaning optics. And then we checked optics together. We found optics are tremendously dirty. Especially one of the lenses is very dirty on one side of the surface. And for others, there are lots of dust on the surface of the mirror. However, after the cleaning of optics, the balance of BS is destroyed. After the alignment of this BS, we measured squeezing again. However, we didn't improve the squeezing. We observed squeezing level almost the same with last time. See attached figure 3 and 4.

There is some consideration after this measurement:

1. The fluctuation of measurement is about 4dB pk-pk. Is this coming from the measurement device?  

2. The low-frequency noise of the second measurement of squeezing is much higher than the first one. We also know that for the second measurement, the balance of the SQZ part is very bad. The unbalance signal on PD is about 50mV. Although we know this balance is not as important as the balance of local oscillator, we still need to make it stay at a decent level.

3. To keep a clean clean room is really important for squeezing to avoid some potential losses.

4. We also tried to change coherent control power, but we noticed that after we decrease coherent control power we may also increase phase noise. This measurement is better done when we have a better coherent loop. Also with some characterization of the coherent control loop.