Analysis of the beam aberration and transmission coefficient's variation of frequency selective surfaces in quasi-optical feed system

Full-wave simulation is widely adopted for the calculation of the frequency response of frequency selective surfaces (FSSs). Generally, uniform plane wave excitation and an infinite FSS array are assumed, and the simulation results have good agreement with the measurement data. However, the simulation cannot provide accurate determination of the transmission coefficient in the optical path in a quasi-optical system because the incidence wave is Gaussian instead of the uniform plane wave. In the conventional simulation, the mode conversion loss is ignored, and the reflection loss is less than the one for Gaussian beam. It is related to the sensitivity of the FSS to the incidence angle, which results in the beam's aberration and converts the mode of the transmitted beam from the fundamental Gaussian beam mode to higher order of Gauss-Hermite mode. In this letter, the Spatial Spectrum Analysis (SSA) method is applied to calculate beam's aberration, transmission coefficient, and conversion of the transmission beam's mode. The influences of the beam waist radius on the transmission coefficient are analytically discussed. The transmission coefficient and the conversion of the transmission beam's mode in different beam waist radius are measured, which agrees well with the results by Spatial Spectrum Analysis.