[Yuhang, Eleonora]
Before Christmas we decide to replace the cube BS before homodyme (where SQZ and LO are recombined) with a plate BS in order to reduce back-reflection as much as possible.
The BS ratio must be as much as possible close to 50/50 to get optimal homodyne balance. We purchased 5 plated BS and characterized them with Simon's set up (entry #1931). We use s-pol.
Here the results:
| code | ratio (T/R) at 45 deg | 50/50 ratio angle | note | |
| Thorlabs1 | BSW41-1064 | 1.2 | 55 deg | bright ghost beam |
| Thorlabs2 | BSW41-1064 | 1.17 | 37.7deg | bright ghost beam |
| Newport1 | 10Q20HBS.33S | 1.006 | 45 deg | weaker ghost beam, low dependance from angle |
| Newport2 | 10Q20HBS.33S | 1.006 | 45 deg | weaker ghost beam, low dependance from angle |
| CVI | BSNP-1064-50-1025 | 0.89 | 41.7 deg | no ghost beam |
We note that the power mesured by the power meter is quite sensitive to the incidence angle. So, we adjusted power meter orientation every time to optimize it. Newport BS has the most balanced splitting ratio at 45 deg. and it is very much insensitive to the angle. Anyway CVI has no ghost beam, and the mirror quality should be better so we decided to go on with CVI BS and adjust the angle to get optimal balance.