Yuhang, Michael

From Friday last week.

We recovered frequency independent squeezing to about 7 dB (vs 5 dB in April). The CC PLL is still glitchy and occasionally bumps the noise of the squeezed spectrum but the interval is enough to take an averaged spectrum.

CC PLL 

Initially we attempted to check if the connection of the PLL actuation to the CC laser was broken. We saw that the slow signal monitor of the CC board has voltage, but the actuation cable from the slow servo did not.

After fixing this issue, we tried some other adjustments of the PLL parameters. Eventually we saw 10 MHz sideband detuning locks well. We went to lunch, came back and it was still locked.

We decided to check the sign behaviour of the PLL control loops. There are two control loops for each PLL, "fast" and "slow". The "slow" loop corrects long term drift. For each loop, we held the PLL sidebands at a location on the spectrum analyzer and plugged the output monitor to the oscilloscope. Then we saw the following:

Fast loop: When the fast loop is closed, the voltage of the correction signal goes up while the sideband frequency separation from the carrier is reduced. This implies a negative sign for the fast loop which is consistent with negative polarity set in the PLL software.
Slow loop: When the slow loop is closed, the correction signal goes down while the sideband frequency separation from the carrier is also reduced. So the slow loop has a positive sign. "Inv" should be off.

We were already using the smallest gain but the control loops still overshoot. We tried removing 12 dB attenuators, and this seemed to make the CC PLL lock good enough.

Frequency Independent Squeezing

We optimized ppol further by making the CC1 error signal large. We started from a ppol frequency of 160 MHz. But actually 160 MHz was already good enough and CC1 locked.

CC2 error signal was seen to go up and down on a timescale of 0.5s which is the correct behaviour.

Using the SR785, we saw -132.34 dBVrms/rtHz shot noise at high frequency (9 kHz) while blocking homodyne. CC2 fast loop seems to still be sending a lot of glitches to bump the squeezing spectrum every now and then, but the interval between glitches is still long enough to obtain 100 average traces.  The presence of glitches is indicated by the reading of the green phase shifter high voltage driver going to zero, which indicates that the problem is somewhere in the CC signal. To speed up the inspection we just looked at very high frequency of 80 kHz. The lowest level of noise we saw was -139.2 dBvrms/rtHz, or 6.96 dB of squeezing.