An optical communication system has a transmitter that generates an optical signal containing payload data and security data. The transmitter double modulates an optical signal where payload data is in-band and security data is out-of-band. If a man-in-the-middle attack occurs and the intrusion device is unable to detect the out-of-band signal, the intrusion device will not replicate the out-of-band signal thereby allowing the presence of the intrusion device to be detected.