In order to improve the output of the optical flat-top extent and expand the channel bandwidth of interleaver filters

an all-fiber cascaded Mach-Zehnder Interferometer（MZI）-typed interleaver filter consisting of one 3×3 and two 2×2 single-mode fiber couplers is developed.The expression between the output spectrum of the model and its relevant parameters is deduced using the transfer matrix method and its characteristics are analyzed.The result shows that an ideal interleaving spectrum can be produced when the difference between the coupler splitting ratio and the fiber interference arms are constant.This filter has a channel interval of 2 nm and

when assisted by double-ring fibers

its output waveform is still more flat-topped and the side-mode suppression ratio is close to the theoretical value.