Is there any way other than the twisted periscope configuration that can be used to preserve the circular polarization? We are presently using a normal periscope in our setup and would like to modify around that in order to reduce reflection induced variations in the circular polarization of the light.
Reflection from a mirror at non-zero angle of incidence usually induces a phase shift between the S and P field components. This is in addition to the expected 180 degree shift which is basically a geometric effect and changes RCP to LCP and vice versa, and which is cancelled if the total number of reflections is an even number. However, this extra phase shift can be made to be zero at a given (non-zero) angle of incidence, e.g. 45 degrees for a periscope application, by proper design of the coating. This is particularly easy for a dielectric mirror; only the topmost layer thickness need be adjusted. I believe that "ultrafast" mirrors may feature such a design; check with Newport. The product page is: https://www.newport.com/f/low-gvd-ultrafast-mirrors
Hi Garima,
Did you try to turn your incident circular polarization in linear polarization in order to add some polarization filtering in your set-up?
Hi J.M. Poyet
If the incident light is linearly polarized, it wasn't getting affected by the Periscope setup. We changed our set up and introduced a Quarter-wave plate at the end of the periscope setup to avoid distortion of the circular polarization. Can you please elaborate on the polarization filtering point you mentioned, Sir?
Hi Garima,
As you said, if the incident light is linearly polarized, it shouldn't be affected by the periscope setup. Then you can use a first quarter-wave plate at the input of your periscope to go from circular to linear polarization and a second one at the output to go back to a circular polarization.
You can also add a linear polarizer between the two quarter-wave plates in order to optimize the conversion from circular to linear or to filter out potential parasitic effects (an incident elliptical polarization for instance).
Good luck.
If an arbitrary (not just horizontal or vertical) polarization is unaffected by the periscope, then circular should be OK as well. That would mean that the mirrors have zero S-P phase shift, which is possible but only if that was explicitly designed into the mirror coating; generally, instead, at 45° angle of incidence the phase shift would be many degrees. That would make linearly polarized somewhat elliptical, but maybe the ellipticity was not being noticed. In general, though, to have good circular polarization would require deliberately correcting the polarization, either at the output or the input of the periscope. Correcting at the output can always be done with two quarter-wave plates, one to make the polarization linear and the other one to then convert it to circular. Visualizing this on the Poincaré sphere is probably the easiest way to understand this. The QWPs would need to be readjusted if you change the angles of the periscope.
On the other hand, if you start with linear and then want to "pre-distort" it before the periscope so it emerges as circular, it's not obvious to me that you can do that with two QWPs; I believe a HWP followed by a QWP would always work, though.
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