Polarization-Independent All-Fiber Quasi-Flat-Top Comb Filter Based on a Dual-Pass Mach–Zehnder Interferometer With High Birefringence Fiber in the Second Loop

A polarization-independent all-fiber comb filter with quasi-flat-top spectral response based on a dual-pass Mach-Zehnder (M-Z) interferometer is proposed and demonstrated. The proposed filter consists of a dual-pass M-Z interferometer with a section of high birefringence fiber (HBF) and a polarization controller (PC) in the second loop. The theoretical analysis shows that the apparatus allows to operate as a polarization-independent multiwavelength periodic filter and to gain the quasi-flat-top passband filter by adjusting the polarization state of the PC. Theoretical prediction was verified by experimental demonstration.