Wavelength-division-multiplexing (WDM) optical networks provide higher data rate and stronger security than many other transport technologies. Yet optical networks are not completely immune to security treats, among which are interchannel attacks by exploiting the crosstalk effects, including eavesdropping and jamming. In this paper, we develop a novel routing algorithm to minimize lightpath overlapping thus minimize the damage caused by an interchannel attack. The algorithm has a running time that is exponential only to the number of existing lightpaths but polynomial to the network size.