All-optical encryption for links at 10 Gbps and above

Fiber and free-space optical transmission systems are the backbone of many critical data networks that require encryption. The traditional method of securing such communications is to electronically encipher the data stream. Electronic encryption technology is a mature and well-known technique; however, electronic encryption methods become very expensive as bit rates exceed 2.5 Gbps due to the limitations of the electronic-optical interfaces and accommodation of the various communication protocols. At any bit rate there is also concern over the general physical vulnerability at every optical-electrical-optical (OEO) conversion point. A need exists for a cost-effective method to adequately protect optical networks operating at any bit rate without introducing delays or altering the native WDM transmission, i.e., a method that is protocol agnostic. This paper describes such a system using an all-optical method of encryption with the recently patented optical tapped delay line (OTDL) channelizer. Using the OTDL technology, a band of information-carrying light is channelized into many narrow spectral subbands, and the phase of each subband is shifted according to a predetermined protocol or "key". The temporal waveform of this signal is therefore "encrypted". The encrypted signal is transmitted across standard optical media links. At the authorized point of destination, the original signal is recovered by inverting the phase shifts, thus returning the original temporal waveform. A transmission using this technique is protected from attack, even if sophisticated means are used. For example, a coherent attack would require coherent detection of a large bandwidth of optical analog data at a high precision digitization rate. Even if coherently intercepted, the properties of the signal are encrypted to the extent that decryption is virtually impossible. To make decryption even less likely, the encrypting phase shifts could be periodically changed.