To improve the utilization efficiency of interleaver bandwidth in 10Gbit/s plus 40Gbit/s mixed system, an all-fiber MZI different bandwidth optical interleaver based on linear chirped fiber grating is proposed and discussed in this paper. The same two linear chirped fiber gratings are written at the output of conventional Mach-Zehnder interferometer (MZI), which constitute a reflective cascaded MZI filter. Based on its structure, the output spectrum expression is established and described by using the principle of fiber transmission, reflex theory of fiber grating and the matrix transfer function. Influences on its transmission characteristics by such factors as the coupling coefficients of the couplers, structure parameters of fiber grating and the length differences of the interferometer arms are numerically analyzed and optimized. The results of numerical simulation indicate that when the above parameters are some certain values, 50GHz input single of device is separated into channel spacing of 100GHz odd-even two output signals. Their spectrum bandwidth are 64.67GHz and 36.98GHz at 3dB, they satisfy the requirement of different transmission rates on bandwidth in 10Gbit/s plus 40Gbit/s mixed system and implements different bandwidth transmission. The device has a certain ability to resist the deviation. Compared with the ordinary cascaded MZI interleaver, the advantage of the new interleaver is that reduced numbers of coupler. It has higher bandwidth utilization that compared with the equal bandwidth interleaver, and achieves different bandwidth transmission within 100GHz.

In order to improve the output spectrum flatness and bandwidth characteristics of conventional Mach-Zehnder interferometer(MZI) filter, a novel two-stage cascaded MZI interleaver is constructed by adding one 8-shaped fiber ring resonator to the interference arms of a conventional single stage MZI filter. Based on a comprehensive analysis, the output spectrum expression is established and simulated numerically. The results of numerical simulation indicates that when the length difference of interference arms and the coupling coefficient of the couplers are some certain values, it will obtain a uniform flat-top passband and similarity to rectangular output spectrum. The improved interleaver has a wider 0.5dB passband bandwidth and 25dB stopband bandwidth, and the rejection in stopband and the roll-off in transition band are improved remarkably. The device has a certain ability to resist the deviation, which reduces the difficulty degree of making it. The design and analysis should be useful in the realization of flattop all-fiber interleaver for development in dense wavelength division multiplexing (DWDM) networks of high spectral efficiency.

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