Secure transmission over an existing public WDM lightwave network

We develop a spread-spectrum-based approach to security-enhanced transmission over an existing public wavelength-division-multiplexing (WDM) lightwave network. Secure transmission for a dedicated user is achieved by overlaying a covert channel onto a multichannel WDM network. The covert channel is spectrally encoded using the technique of frequency hopping and then temporally spread by a dispersive element, with the resulting average power below the noise floor in the fiber. The presence of the WDM channels provides an ad hoc security expansion and increases the difficulty for an eavesdropper to intercept and decode the secure signal