Found the IR beatnote between PSL and Y end laser.
Since our goal was to find the beatnote ASAP to recover ALS, I ignored the fine details in alignment. I will revisit the setup to make some improvements in the near future.
1. Coupled the PSL IR beam leaking after the doubler into the fiber. We have only 10% coupling into the fiber at the PSL table right now (6mw); but this will be improved once I get a suitable translation stage for the telescope.
2. PSL IR --> PM980 fiber --->50-50 fiber beam splitter ---> 50-50 fiber beam combiner
AUX Y ---> PM980 fiber ---> 50-50 fiber beam combiner
The output port of the fiber beam combiner is connected to the fiber coupled broadband RF PD.
3. The RF output of the PD when connected to a spectrum analyzer shows a beatnote of -50dBm. The small amplitude of the beatnote is due to the laser power being attenuated before coupling into the fiber to keep the PD safe.
Attached is photo of how the setup is put on the PSL table. We will put all the stuff in a box once the X setup is also in place.