Alberto and Bram.

With the strange increased power readings on the REFL_PD, which seemed to be linked to the Beckhoff PLL temperature feedback, and not being able to lock the PLL we headed to EY. After trying to find the beatnote (fiber output with the local laser), it turned out there was no light coming out of the fiber (even after Alberto relocked the RefCav). Keita re-relocked the RefCav.

With light coming out of the fiber, no beatnote was found until we tuned the crystal temperature from the original 31.27 deg C to 38 deg C (which seems to be it maximum). Even at that setting, it was at a few hundred MHz. All Beckhoff settings were turned 'off/cleared', as indicated on the screens.

To get the beatnote within range of the EY laser, I changed the RefCav to the next FSR (up in laser crystal temperature), with only a small change in beatnote frequency.

Adjusting the Lightwave (= RefCav) temperature down by ~0.5 deg C, requires the Innolight (EY) laser to adjust by 0.2 deg C (down), to follow the beatnote.

In the end I reduced the RefCav laser down by 5 deg C (to ~34.6 deg C), but tuning the laser temp of the Innolight laser did not to bring the beatnote down to ~40 MHz (still just above 100 MHz).

There is something fishy going on, but so far we didn't manage to find it.

Next we will unplug the temp feedback cable from the laser controller, just to be sure we don't have a secret offset.