Yuhang, Michael

From 2024-01-08

OPO kept locking to higher order mode until I flipped INV switch on the servo. I should really figure out how the INV, +/- and threshold work together because it seems it isn't written down properly anywhere, but it's important for setting the proper behavior of the lock threshold to not catch HOMs, and depends on the sign of the error signal and use of transmission or reflection specturm.

To we keep BAB injected into OPO. The transmission is not so good, about 52 uW (should be about 400).

The green beam was aligned to the filter cavity using the targets inside the vacuum tube and the coils were offloaded to keep the best position around 0V actuation. The filter cavity was then internally aligned via reflection through the second target and to the squeezer bench. The END SR560s were turned to 1000 Hz LPF, Gain 100, low noise mode, 6dB/oct, DC coupling.

Filter cavity locks for green but is not super stable. If we put 1/f^4 filter it unlocks. We changed sideband frequency to 88 MHz so this might affect PDH.

We took the transfer function of the green FC lock (Source -> Signal:Perturb, Ch1 -> EPS2, Ch2 -> EPS1). The unity gain frequency is about 12.5 kHz, should be a bit higher - the gain was adjusted to give 13.256 kHz.

There is some weird behavior on the MEDM screen where the displayed FC_GR_TRA goes up when the filter cavity is unlocked. It seems to be an electronics issue regarding the displayed value.