Participants : Marc, Yuefan, Yuhang

In order to study the coherence between the input laser IR power and the reflection from the filter cavity, we installed a PBS on the IR injection path and a photodiode at its reflection.

We studied 3 cases :

1) Both IR and Green resonants inside the FC (greenir.png)

2) No beam resonant inside the FC by misaligning the EM and making sure that there was no attempt to lock on the servo (no.png)

3) Only Green resonant inside the FC by detuning the EOM frequency (green.png)

We were expecting to find similar results between the cases 2 and 3 which is not the case.

We were expecting to find coherence at high frequencies due to power fluctuations only but it only appears in the only green resonant case (maybe the coherent length of IR is too short?)

The no beam case shows clearly a 200Hz peak (ventilation of the clean booth) and a 600Hz peak (turbo pump of BS).