[Aritomi, Yuhang]
First we made LO DC balanced and aligned LO and BAB into AMC. DC balance of BAB is 10mV. We measured shot noise of LO (Pic 1). Although there are large 50Hz peak and its harmonics , homodyne is shot noise limited above 20Hz. To remove the 50Hz peak, we floated ground of homodyne, but the shot noise was same. We also measured shot noise spectrum when LO and CC are injected into homodyne. Shot noise with LO and CC was same as shot noise with LO only.
Then we locked green and IR phase with SR560 (bandwidth ~ 100Hz) and found squeezing spectrum is very bad. So we locked green and IR phase with Pierre's servo (bandwidth ~ 1kHz) and finally could recover the 5dB squeezing (Pic 2).
We thought green power is 50mW, but actually green power was smaller than 50mW due to misalignment of GRMC at that time.
Add some points,
- We observed the unstable lock of green phase. And I think it is related to the unstable of squeezing measurement.
- From the phase control behavior, the phase noise(free-running) seems to be higher than before. Need to be confirmed.
- The green phase lock has two main resonance 500Hz and 7kHz. In the beginning, we could find a gain region between these two oscillation and lock green phase. But at the end of yesterday, we couldn't find this region when we change the gain of control servo.
- The error signal for the green phase lock usually change. As pointed out by Aritomi-san, once it is due to the misalignment of green(green power was changed). Actually, we observed a second change of error signal of green phase lock. We check PLL lock, green power(also GRMC alignment), OPO temperature. But they were fine. We should use more time to investigate what makes it change and try to avoid it.