Mode filtering in grating spectrometers at millimeter wavelengths

Abstract

Grating spectrometers employed as electron cyclotron emission diagnostic systems use as entrance and exit slits pieces of rectangular or circular waveguide with a scale size of 5 → 10λ. Since only the extraordinary mode polarization of the radiation is wanted, a wire grid polarizer is used to reject the other radiation. A combination of polarizer, finite size of slit, finite grating size, and limited etendue of the instrument severely limits the number of modes of the radiation field transmitted through the instrument.