To improve the utilization efficiency of interleaver bandwidth in 10 Gb/s plus 40 Gb/s hybrid system,an all-fiber asymmetric bandwidth optical interleaver based on a cascaded Mach-Zehnder interferometer is proposed and discussed.Influences on its transmission characteristics by such factors as splitting ratio of the coupler of the directional fiber couplers and the physical length differences of the interferometer arms are numerically analyzed in detail.The preference condition of the interleaver which meets the maximal isolation and the characteristic of the output spectrum is obtained.One set of optimized data is validated in the experimental result.The results of numerical simulation and experiments show that all-fiber optical interleaver with -3 dB passband in odd channels and even channels could be obtained,which have more than 60 GHz passband and 30 GHz passband,for transmission speed of 40 Gb/s and 10 Gb/s,respectively.Experimental results are in good agreement with the simulation results.Comparing with the conventional interleaver with uniform bandwidth channel,the kind of interleaver with higher bandwidth utilization efficiency has more advantages as multiplexer/demultiplexer during upgrade from 10 Gb/s to 10 Gb/s+40 Gb/s.