A tunable wavelength routing scheme based on sum- and difference- frequency generation (SFG+DFG) is proposed, where the concept of double pass configuration is employed. An arbitrary channel can be dropped from a WDM signal and added to another WDM signal at arbitrary wavelength. Therefore, the tunable wavelength conversion and wavelength routing are integrated together in this scheme. The theoretical expression of the input signal wave and the converted wave are derived under the assumption of “small signal" when the phase is matched perfectly. Numerical calculations were performed which showed that the cross-talk can be suppressed provided the spacing of the WDM channels is greater than 02nm (25GHz). If the input or output of this device is changed, this scheme can be utilized as tunable wavelength add multiplexer, tunable wavelength drop multiplexer, wide tunable wavelength filter or tunable wavelength converter. A wavelength interchange cross connection (WIXC) unit can also be realized by employing two devices of this scheme.