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We observe that the performance of wavelength- routed (WR) networks often suffers from asymmetric traffic and network traffic patterns differing from the original design plans. WR networks with a fixed number of channels in a given transmission direction are inflexible. Therefore, we propose reversible wavelength channels. Like a reversible lane in highway systems, a reversible wavelength channel has the flexibility of its transmission direction being configurable at the setup of a lightpath. So far reversible wavelength channels have not been discussed in WR networks even though we observe that most of the required technologies are already available. In this paper, we discuss all the required technologies for implementing reversible wavelength channels in WR networks. We demonstrate that reversible wavelength channel can provide significant performance improvement for WR networks when the traffic is asymmetric. Even if the traffic is symmetric, we also have nontrivial performance improvement with reversible wavelength channels, i.e., the blocking performance of WR networks with reversible wavelength channels will be similar to that of normal WR networks with doubling the number of fibers per link. Different implementation approaches for reversible wavelength channels are discussed. Among them, the performance of the reversible waveband approach is discussed in detail.