Excitation of a single hollow waveguide mode using inhomogeneous anisotropic subwavelength structures

We propose and analyze a general approach for coupling a free space uniformly polarized beam to a desired hollow waveguide mode, thus enabling a single mode operation. The required spatial polarization state manipulation is achieved by use of inhomogeneous anisotropic subwavelength structures. Demonstration is obtained by coupling a linearly polarized CO₂ laser beam at a wavelength of 10.6μm to the TE₀₁, TM₀₁, EH₁₁, EH₂₁, and EH₃₁ modes of a 300μm diameter dielectric-coated hollow metallic waveguide. Full polarization and intensity analysis of the beam at the waveguide's inlet and outlet ports indicates a high coupling efficiency to a single waveguide mode. Finally, shaping the waveguide mode to a nearly diffraction limited linearly polarized beam and to a radially polarized vectorial vortex are also demonstrated.