Band-rejection filter with high extinction ratio using prism-waveguide cascaded coupling system.

Abstract

A band-rejection filter is proposed based on a prism-waveguide cascaded coupling system, which is composed of an equilateral trapezium prism and a deposited multilayer structure. By properly adjusting the thickness of the coupling layer and the light extinction coefficient of the guiding layer, the radiative and intrinsic dampings matching condition could be well satisfied, and then a series of reflectivity dips will appear in the reflectivity wavelength spectrum. Since the module of reflectivity is smaller than one, the extinction ratio of the rejected frequency via the cascaded coupling system is twice as high as that of the single-coupling technology. By narrowing the guiding layer to a micrometer scale, the free spectral range is broad enough to cover the Raman spectrum scattered from the frequently used sample. In addition, the numerically calculated results show that the light in the free spectral range is mostly reflected, with an insertion loss down to 0.45 dB. Compared to previously reported band-rejection filters, it is relatively simple to manufacture our device, which possesses potential applications to help distinguish the Raman signal from the elastic scattering background.