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An all-fiber comb filter with flat-top spectral response based on a Mach–Zehnder interferometer (MZI) cascading a Michelson interferometer (MI) is proposed. The proposed filter consists of a rotatable polarizer, two 3-dB couplers, and two fiber Faraday rotator mirrors (FRMs). The numerical simulation results show that the birefringence of the single mode fiber (SMF) can be neglected when the polarization state of linearly polarized incident light respect to one principle axe of fiber. By selecting the path difference of the MZI and MI and adjusting the rotation angle of the two FRMs, the flat-top comb spectral response can be obtained. [ABSTRACT FROM AUTHOR]