Abstract: A polarization modulator device for modulating a polarization of an electromagnetic wave includes a corrugated metallic waveguide having an interior cylindrical opening defined therein and situated along a longitudinal axis of the waveguide. The corrugated metallic waveguide also has a first waveguide section and a second waveguide section. The waveguide sections are separated by a dielectric break. A central structure is situated along the longitudinal axis of the waveguide. The central structure is supported substantially in the center of the interior cylindrical opening of the waveguide. The cylinder is substantially situated within the dielectric break of the waveguide. A magnetic field source is configured to permit a controllable magnetic field in the cylinder, wherein the magnetic field modulates a polarization of the electromagnetic wave by an angle related to the strength of the magnetic field. A polarimeter using the polarization modulator is also described.