Birefringent Filter for Millimeter Waves

The theory of optical birefringent filters consisting of several equal-length birefringent plates oriented at various angles to the input polarization vector has been applied to millimeter waves. Each plate of such a filter should be ideally impedance-matched to free space, and the wave to be filtered should be a linearly polarized plane wave. The filter, in effect, rotates the plane of polarization of a band of frequencies that it is desired to pass (stop) , and permits the undesired (desired) frequencies to travel on unrotated. Polarization-selective absorbers are thus required. The input and output wave vectors are usually either orthogonal or parallel to each other.