Free-space optical polarization demultiplexing and multiplexing by means of conical refraction

This paper presents a novel method to demultiplex and multiplex a monochromatic input light beam into, in principle, an arbitrary number of polarization states by means of the conical refraction (CR) phenomenon. In CR, when a focused Gaussian beam passes along the optic axis of a biaxial crystal it its transformed into a light ring at the focal plane of the system (or ring plane). Angular amplitude masks forming a star burst like pattern with n (up to 12) opened circular sectors placed at the ring plane allow passing only some sectors of the ring. These sectors form the communication channels. A second biaxial crystal identical to the first one and placed after it with opposite orientation of its plane of optic axes multiplexes the selected channels into a single beam, similar to the input Gaussian one. Finally, a third biaxial crystal is used to decode the transmitted channels at the receiver stage. With this technique, an increase in one order of magnitude in the channel capacity for free space optical communications (FSOC) of a monochromatic input Gaussian beam at 640 nm for a free space propagation distance of 4m with controlled crosstalk is observed.