[Aritomi, Yuhang]

According to Henning result, the shot noise should be -132dBV for 2mW of incident light. In our case, the machine noise is -100dBV. So we use SR560 to amplify it by a factor of 100 means 40dB. So the shot noise level should be -92dBV.

First, we measured the noise spectrum when one of the PD is blocked. The result is shown in the attached figure one. It is clear that not even part of noise spectrum is dominated by shot noise. Even at highest frequency, noise level is around -88dBV.

Then we decide to change pitch of steering mirror to change power on the second PD. Maybe this can bring beam away from optimal position. Or the change of beam direction reduce detection efficiency. In the end, we reduced common noise rejection from -30dBV to -74dBV, means we reduce the modulated signal by a factor of 100. The result is shown in the attached figure 2.

Finally, we turned off modulation. And then we look at the signal from homodyne on spectrum analyzer. The result is shown in the attached figure 3. Then we improve the locking condition of IRMC, make the locking point as close to peak as possible. Then the signal becomes figure 4. This is reasonable because around peak the amplitude change is less. Also seems the noise comes from amplitude noise.

To do list:

1.We can also change the pitch of BS to see if we can increase signal on the first PD.

2. Try to improve locking performance.

3. To see difference when noise eater is engaged and not.

4. Noise hunting