Polarization-independent quasi-flat-top comb filter using an in-line Sagnac interference

We propose a novel all-fiber quasi-flat-top comb filter based on an in-line Sagnac interference. The transmission function of the proposed filter is theoretically analyzed using Jones transmit-matrix. And the influence of several factors on the output spectrum is experimentally demonstrated, such as the length of the two polarization-maintaining fibers (PMFs), the birefringence of PMFs, the rotatable angle of polarization controllers (PCs) and the temperature subjected to PMFs. A well-defined flat-top interference pattern as theoretically expected is achieved. The measured extinction ratio and free spectrum range (FSR) of the proposed filter are 25 dB and 16 nm, respectively. In addition, it has the advantages of good flattop transmission passband, high channel isolation, fully compatible with optical-fiber system and low cost. This kind of filter can be widely used in dense wavelength division multiplexing (DWDM) systems.