  In order to improve the transmission efficiency of optical-fiber
communication system with 10Gb/s+40Gb/s, an all-fiber interleaver with unequal
passband is proposed and discussed, which is based on a two-stage cascaded
Mach-Zehnder interferometer (MZI). The optimum value of structural parameters,
such as splitting ratios of the couplers and the physical length differences of
the interferometer arms, were chosen. One set of optimized data is validated in
the experimental result.The experimental results and the theoretic analysis
indicate that an all-fiber optical interleaver with -3dB passband width in odd
channels and even channels could be obtained, which having more than 60GHz
passband and 30GHz passband, for transmission speed of 40Gb/s and 10Gb/s,
respectively.By assigning different portions of spectrum to the 10Gb/s and the
40 Gb/s channels, the bandwidth efficiency requirement of the 40 Gb/s channel
is relieved, and therefore longer transmission distance can be
achieved.Comparing with the conventional asymmetric interleaver, the most
obvious benefit of the proposed interleaver is all-fiber structure.
