A novel Interleaver using a modified Michelson interferometer in which its reflecting mirrors are replaced by two Gires-Tournois resonators is presented. On basis of the theory of Michelson interference, a designing principle of the interleaver which has square-like frequency response with zero ripple, wide flat-top, unity contrast, simple structure, and stable performance is given. The device which possesses channel spacing of 50 GHz, ripple less than 0.05 dB, 1 dB bandwidth more than 0.38 nm, and close channel isolation more than 23 dB has been designed.