Polarization converters for circular waveguide modes

Small deformations with suitable azimuthal symmetry can be used to convert between rotating and stationary polarizations of any given mode in circular waveguides. Special cases include converters from linear polarization to circular or elliptical polarization, and rotators of linear polarization through arbitrary angles. Rotating (corkscrew type) deformations may also be used to convert between unpolarized TE0m modes and rotating TEpm modes. Polarization converters may be useful in single-mode (TE11) or overmoded waveguides. We consider the design of polarization converters to minimize mode conversion in overmoded waveguides for applications such as gyrotron outputs and electron cyclotron heating (ECH), and give examples for TE11, TM11, and TE23 modes. The performance of a TM11 elliptic polarizer fabricated for an ECH experiment is reviewed. Explicit formulas for all of the needed coupling coefficients are collected in an appendix for reference.