Preservation of polarisation orthogonality of counterpropagating waves through dichroic birefringent optical media: proof and application

It is well known that two orthogonally polarised copropagating optical waves may lose this orthogonality because of dichroism of the optical medium. It is proven that polarisation orthogonality of counterpropagating waves is exactly preserved in any linear reciprocal medium even when it is dichroic. It is shown how this property can be applied in a novel singlemode reflective star with echo reduction by using orthogonal polarisation reflectors.