Flattening spectral transmittance of birefringent interleaver filter

A new method to produce flattening spectral transmittance for a birefringent interleaver filter based on the interference of polarized light is presented. In previous publications, each birefringent crystal was required to be of equal thickness and only one group of crystal azimuth angles was obtained. In addition, the shape of the spectral transmittance was not sufficiently flat and the flatness width of the passband and stopband was not sufficiently wide. The proposed method has advantages over previously published methods. It is based on the Fourier transform, and the thickness of each crystal can be different. All numerical solutions can be obtained for crystal azimuth angles provided ripples are small in relation to the relatively wide passband and the stopband. Three birefringent crystals are used and all the azimuth angles of the crystals are obtained when ripples are limited to a uniform 0.1% in both the passband and stopband, with widths greater than 2/11 of the period. The influence of the change of azimuth angle and crystal thickness on the spectral transmittance of the system is discussed