In performing Wavelength Division Multiplexing（WDM）of the same number of channels

it is simpler to use threechannel devices than two-channel ones.This paper introduces and analyzes the new structure of an all-fiber interleaver that can perform three-channel flat output

derives its energy transmission expression and discusses in detail the effects of the variations of the coupler splitting ratio on the filtering characteristics.The results show that when the length difference between the fiber interference arms and the splitting ratio take certain fixed values

it will become a three-channel all-fiber interleaver with equal multiplexing interval and flat passband and the WDM channel interval depends solely on the size of the interference arm length difference.