Beam finite spot size effect on angle-tolerant optical filters

Abstract

Theoretical performance analysis of optical filters is usually done assuming an incident plane wave. This may be a good indicator of the device performance when the spot size of the incident beam is much greater than the operating wavelength. However, in practice the incident beam is usually focused on a very small spot to fit the device footprint. Such focused beams contain a range of incident angles, i.e., a set of different plane waves. Thus, a degradation is expected in the filter spectral transmission behavior when compared to that theoretically determined by using a plane wave model. Angle tolerant filters are filters that preserve their spectral properties under a wide range of angles. This work aims at studying the effect of an inclined Gaussian beam finite spot size on the transmission performance of a five-layer angle-tolerant hybrid plasmonic multilayer filter structure by applying beam expansion method analysis. For spot sizes of 10 micrometers, the filter transmission preserves its shape for different angles of incidence. As the spot size narrows to 0.1 micrometers, transmission preserves its shape but loses around 8% compared to plane wave case. Further deterioration is observed for the same spot size at an angle of incidence of 50 degrees. The behavior is found to be limited by the filter’s field of view (FOV). The FOV of the filter we used in the study is around 50 degrees. Hence, with narrower beams, a larger angular divergence is present which cannot be compensated by the angle tolerance of the filter. To the best of our knowledge, similar work has not been studied in existing literature.