Logan, Michael

Summary

The PLL coupling was measured and we attempted to lock. We could acquire both ppol and CC lock, but the CC lock is a bit weak and only holds for ~ 5 seconds.

Details

We attached a new power supply to the battery powered CC fiber photodiode. The cable was taped to the leg of the mini-table (with the QPDs) to make sure it doesn't go into the main laser path.

We measured the following fiber couplings:

Laser to fiber port
ML-ppol: 5.3 mW
ML-cc: 4.1 mW
ppol: 2.5 mW
cc: 0.51 mW

Fiber out to fiber PD
ML-ppol: 0.97 mW
ML-cc: 0.32 mW
ppol: 0.75 mW
cc: 0.17 mW

Fiber output efficiency (remembering that each fiber is 50:50 split before sending to duplicate PDs)
ML-ppol: 36.6%
ML-cc: 15.6%
ppol: 60%
cc: 66.7%

Fiber PD output (one laser at a time)
ppol (ML on): 27 mV
cc (ML on): 9 mV
ppol (ppol on): 22 mV
cc (cc on): 9.6 mV

Fiber PD output (two lasers on):
ML-ppol average: 49.4 mV
ML-ppol Vpp: 26.8 mV
ML-cc average: 10.9 mV
ML-cc Vpp: 9.2 mV
A DC level of 2.6 mV was subtracted out of the averages

While doing this procedure I noticed that maybe last time I hadn't screwed in the CC fiber correctly. This may have caused some weird polarization effect or something else. Maybe even all of the control problems... It's a bit weird that the ML-CC and CC output optical powers are different, but the PD voltage is the same. Maybe polarization is off a bit, I'm guessing for ML since ML volts should be > CC.

We could measure -8.5 dBm from CC BEAT (the fiber PD signal going into the PLL servo) and -38 dBm on CC PLL MON.

Previously there was apparently some problem with the DDS software where the ppol PLL local oscillator frequency could not be changed. This was for whatever reason fixed with the power reset. We could see the ppol frequency change on the spectrum analyzer. The laser temperature was a bit off. For the correct mode, it should be of a relatively large amplitude (although I couldn't see image sidebands), and if you rotate the temperature knob clockwise the frequency should increase. Initially it was the other way around (same for CC). I rebooted the ppol PLL software (Analog Devices ADF4002, black SDB board), loaded the most recent settings (20240625) and wrote to register. It didn't lock initially but after changing the phase detector polarity to negative it could lock.

For the CC PLL I adjusted the temperature to the correct polarity and loaded the software (Analog Devices ADF4001, black SDB board) and settings (20240625). I could not lock initially. I changed the phase detector polarity to negative and nothing. I eventually managed to lock by i) changing charge pump gain from 0 to 1 (I guess this is the equivalent of changing the loop gain of an analogue control loop), ii) changing timeout from 3 to 35 PFD cycles (I guess this something like increasing a lock loss threshold or control bandwidth), iii) changing the CC temperature to move the sidebands within 10 MHz of the main peak. Then we could see CC PLL lock, but only for about 5 seconds at a time. It would then lose lock and maybe or maybe not reacquire. So maybe the signal is a bit too small. Also because I wrote "I guess" in the description of settings I changed, I don't really know what they do. It's probably in Marco Vardaro thesis, 2018 era elogs, the Analog Devices manuals or Yuhang's distant memory.

On the CC temperature point, during the New Years visit Yuhang told me that the CC servo has some weird issue where it would only lock if the sidebands initially were brought 20-30 MHz away from the main peak. For whatever reason this is not the case right now (although it could be a possibility if I recover the good electronic settings). I could not observe any independent impulse/glitch noise however we noticed that when searching for some lost cable, tapping some of the cables causes noise in the CC spectrum. However, we didn't make too much of an effort to isolate the suspicious cable today.

I also turned on the homodyne PSU (+19V 0.066A, -19V 0.062A) and found another +25/-25V PSU which I had absent mindedly forgotten about. We should use this one for ATC experiment instead of the 18V one.