Aritomi, Eleonora, Yaochin, and Yuhang
For the adjustment of OPO/FC matching and FC/LO matching, we always do the following.
1. Send BAB into the filter cavity. Align IR into the filter cavity while filter cavity alignment is kept in the best condition. Check IR 0/1/2 order modes by changing AOM frequency.
2. Align BAB reflection into AMC while the filter cavity is locked.
However, we found that the matching from BAB_ref into AMC is quite different when the filter cavity is locked and detuned. And we know that we measure FDS while FC is detuned, so we decide to align BAB_ref into AMC while FC is detuned. So now the procedure is as follows.
1. Send BAB into the filter cavity. Align IR into the filter cavity while filter cavity alignment is kept in the best condition. Check IR 0/1/2 order modes by changing AOM frequency.
2. Align BAB reflection into AMC while the filter cavity is detuned.
However, we are not very sure why the matching can be so different when FC is locked and detuned. Because I think the BAB_ref should be quite similar in both cases of lock and detuned since we had 94% of matching from OPO/FC. I put the reason for my thought as following. Please correct me if there is something wrong.
The main effect causing degradation of the overlap between LO and reflected beam form FC when it is ON and OFF resonance is the relative phase change between TEM 00 (resonating the cavity) and HOM (not resoanting). Raffaele did a quick calculation to show this and the numbers he found are in rough agreement with those we observed. I attached a pdf with a similar calulation and a plot that shows the overlap degradation as a function of the not coupled power into FC.