Spurred by exploding demand for transmission capacity, several techniques have been developed in dense wavelength division multiplexing(DWDM) systems for packing as many channel as possible into fibre-amplifier active regions. An interleave filter, which separates DWDM signal channels with equal spacing into two groups with twice the spacing, has received much attention. To be effective, this filter must have a wide flat-top passband, low insertion loss, minimal crosstalk and low dispersion. A polarization-based interleave filter can be fabricated by using cascaded birefringent crystals. Filter of this type can provide superior peak flatness and isolation for narrow channel spacing over what can be obtained in traditional interferometric devices. In such filter, the crystals are cut with their optic axes perpendicular to their length. By using a method based on an analogy to the synthesizing of finite impulse response(FIR) digital filters, the angles of the optic axes and the length of the crystals can be determined. An example of the design of a three cascade birefringent crystals, 50GHz polarization-based interleave filter is described. The allowable parameter error for this filter is also discussed. Furthermore, a polarization-based filter, which can separate the upper and lower channels of an input DWDM signal, can be designed by using this synthesizing method.