Polarization-Controlled Tunable All-Fiber Comb Filter Based on a Modified Dual-Pass Mach–Zehnder Interferometer

We propose and demonstrate a novel tunable all-fiber comb filter based on a modified dual-pass Mach-Zehnder (M-Z) interferometer. The proposed filter consists of a polarization-dependent isolator, a polarization controller (PC), and a modified dual-pass M-Z interferometer composed of a standard 3-dB and a non-3-dB fiber coupler with a PC in one arm. By adjusting the polarization state of the input light, dual-function operation, the spectral spacing tunable and the wavelength switching (interleaving) operation can be easily achieved. The theoretical prediction is verified by the experimental results.