Yuhang, Michael

2024-01-10

We realigned green to OPO using a rather rough method of checking the bright spots on the OPO reflection FI that separates BAB and ppol IR.

We replaced the cable for the correction signal of the CC PLL slow loop to the CC laser temperature. We took a 5m cable and made sure it wasn't broken. This fixes one issue of the CC PLL but there are still others as mentioned previously.

We changed the ppol PLL frequency for green injection to OPO and once again found it had shifted a large amount , to 205 MHz at 37.5 mW green injection (it was previously 240 MHz). We went back to our usual 25 mW green injection, but the ppol frequency this time didn't change much. The CC1 error signal which characterizes squeezing level was about 302 mV (should be about 320-340). We tried to optimize the ppol frequency but then ended up with large oscillations in the CC1 error signal (seems about 760 ns). It seems like it "should" be 185 MHz, but it is more stable at 205 MHz so we left it there. It should be noted that previously the CC PLL performance was improved by switching off rampeauto for the FC green lock. Now we need to have that on again so we go back to having CC issues. The noise seems to be common to CC1 and CC2 error signals. But there was not so much time to investigate this issue and this entry marks the end of Yuhang's visit... for now.

To do list of items noted during Yuhang's visit:

• Coherent control investigation - why is there glitch noise in the CC error signals? It seems to be an electronics problem regarding either the CC PLL board or the FC green lock servo.
• Take a more accurate optical loss/phase noise measurement (sqz vs antisqz at different green pump)
• Recover FDS and CCFC
• Cable management, especially at the bottom of the rack among the Nikhef RF amplifiers and DDS boards
• Figure out how to properly set the SR785 spectrum analyzer for open loop transfer function measurements - we seemed to not get the correct reading versus the old spectrum analyzer. The SR785 has a bit uncomfortable interface in my opinion but is also much faster.
• Figure out proper settings regarding transmission/reflection PD -> INV/NON INV -> +/- -> Error signal sign -> Lock threshold sign
• Fix SHG 3rd order mode mismatch
• Replace and retest new OPO PZT
• Update optical layout and wiki items
• Get rid of the superfluous green EOM (78 MHz, previously used for green lock of GRMC but now redundant) - requires complete overhaul of green beam shaping though. Would be quite a pain, especially because mode matching optimization is very time consuming