an optical signal filter for providing a periodic transfer function in transmitting signals within a selected bandwidth , by which passbands are interleavered into groups of separate outputs . the filter employs the transmissivity characteristic of birefringent crystals in conjunction with splitting the input beam into orthogonal and separate components , while compensating for temperature variations by pairing crystals of different types . the transmissivity functions are independent of the polarization of the input beam , and are shaped to flatten transmissivity peaks by the use of cascaded stages of birefringent crystal pairs .