Yuhang, Michael

We investigated the cause of the CC PLL glitches and saw that it was not so stable as it seemed on Friday. I had to leave earlier but Yuhang continued to investigate afterward (next entry).

On the spectrum analyzer we normally look at the DC peak and visualize the CC PLL sidebands at -7 and 7 MHz. Whenever the system unlocks it seems to be precluded by small bumps about 1 MHz away from the sidebands.

Another thing to check was the magnitude of the correction signal. The CC laser has a piezo tuning coefficient of ~ 1 V/MHz. If we move the sidebands about 2 MHz away from lock, we see that the fast loop monitor output only shows 200 mV, which isn't enough to properly correct, though it moves in the correct direction. From the specifications of the ADF4001 board the fast loop should be able to provide 10 V. It seems that there is definitely some problem with the electronics. We see that ppol PLL gives roughly the correct magnitude of correction signal (2 V for 3 MHz detuning from 160 MHz).

We once again thought it was good enough, and decided to move on to characterizing optical loss and phase noise by measuring the sqz/asqz ratio for different green pump powers, since the purpose of switching to the "one EOM" layout was to reduce squeezing phase noise.

However, we then ended up seeing some CC2 phase jitter despite the board saying it was locked. We checked CC1 and CC2 error signals and saw that the glitches were correlated in each of these channels. At this point we were not sure whether or not the glitches were coming from the lasers or the electronics.

We sent the PLL CC monitor channel to a mixer and demodulated at 7 MHz. It should be flat but instead we saw spikes. We tried also for ppol, reducing the modulation to 80 MHz (DDS has a bandwidth of 110 MHz), and then demodulated at 80 MHz. We once again saw glitches in the ppol. Looking at the time domain SHG error signal on lock shows none of these glitches though. If the problem is in the main laser, then it should be in the SHG, but it isn't. The OPO error signal also has no glitches despite being locked with ppol. It should also be noted that we didn't see any of these spikes in the fast or slow output monitor channels of either PLL.

Freezing one of the glitch spikes on the oscilloscope shows it has a width of 100 us, corresponding to a frequency of 10 kHz. This seems like a mechanical frequency, but at this point I had to leave for the day.