??? I still don't understand. What principle are you rely on?
I could not understand why you rotated the HWP to the "minimum" transmission
and then minimized the transmission by rotating the output PBS. What is optimized by this action?
Probably there is some hidden assumption which I still don't understand.
Something like: Better transmission gives best isolation, PBS has some leakage transmission
of the S-pol light, and so on.
Tell me what is the principle otherwise I don't accept that this adjustment is "to get a good isolation with the Faraday".
P.S. you could flip the faraday without removing it from the V-shaped mount. This does not roll the Faraday.
The procedure you wrote down as a standard is right. I explain reasons why we didn't do such way.
For our situation, we can rotate the polarization angle of the incident beam by using a HWP in front of the Faraday.
This means we don't have to pay attention about the PBS_in because the rotation of either PBS_in or the HWP causes the same effect (i.e. variable transmission ). This is why we didn't carefully check the PBS_in, but did carefully with the HWP.
Normally we should take a maximum transmission according to a instruction paper from OFR, but we figured out it was difficult to find a maximum point. In fact looking at the change of the power with such big incident (~1W) was too hard to track, it only can change 4th significant digit ( corresponds to 1mW accuracy for high power incident ) in the monitor of the Ophir power meter. So we decided to go to a minimum point instead a maximum point, and around a minmum point we could resolve the power with accuracy of less than 1mW.
After obtaining the minimum by rotating the HWP, we adjusted the angle of PBS_out to have a minimum transmission.
And then we was going to flip the Faraday 180 deg for fine tuning, but we didn't. We found that once we remove the Faraday from the mount, the role angle of the Faraday is going to be screwed up because the mount can not control the role angle of the Faraday. This is why we didn't flip it.
I could not understand this operation. Can you explain this a bit more?
It sounds different from the standard procedure to adjust the Faraday:
1) Get Max transmittion by rotating PBS_in and PBS_out.
2) Flip the Faraday 180 deg i.e. put the beam from the output port.
3) Rotate PBS_in to have the best isolation.