A W-band polarization converter and isolator

A 95-GHz printed low-loss linear-to-circular polarizer is designed as a component of an active direct-detection millimeter-wave imaging system. The periodic printed grid structure presents different reactances to the TE and TM polarizations, resulting in equal amplitude and phase quadrature upon transmission through four parallel grids. The polarizer is measured in both a Gaussian beam system and a plane wave system, and demonstrates an axial ratio of 0.23 dB, polarization isolation of 38 dB, and transmission loss of 0.3 dB for normal incidence. The quarter-wave plate is characterized up to +/- 35 degrees off the optical axis, and exhibits an axial ratio better than 1 dB up to +/- 17 degrees off the optical axis.