The physical layer of an optical network is vulnerable to a variety of attacks, including jamming, physical infrastructure attacks, eavesdropping, and interception. As the demand for network capacity grows dramatically, the issue of securing the physical layer of optical network cannot be overlooked. In this survey paper, we discuss the security threats in an optical network as well as present several existing optical techniques to improve the security. In the first part of this paper, we discuss various types of security threats that could appear in the optical layer of an optical network, including jamming, physical infrastructure attacks, eavesdropping, and interception. Intensive research has focused on improving optical network security, in the above specific areas. Real-time processing of the optical signal is essential in order to integrate security functionality at the physical layer while not undermining the true value of optical communications, which is its speed. Optical layer security benefits from the unique properties of optical processing-instantaneous response, broadband operation, electromagnetic immunity, compactness, and low latency. In the second part of this paper, various defenses against the security threats outlined in this paper are discussed, including optical encryption, optical code-division multiple access (CDMA) confidentiality, self-healing survivable optical rings, anti-jamming, and optical steganography.

We propose and demonstrate the use of real-time optical signal processing, implemented with integrated and packaged off-the-shelf commercial technology, to enhance the security of data networks. Optical steganography, based on spread spectrum, is used to inhibit the observation of data traffic and the analysis of traffic patterns. Using optical CDMA to modulate the data, service interruption is prevented in presence of infrastructure attacks. Unlike conventional wavelength and time multiplexing approaches, the soft blocking capability of optical CDMA provides protection paths without permanently allocating bandwidth. The large size of the optical CDMA code set also reduces the probability that the secure signal can be intercepted by an eavesdropper. Finally, high-speed all-optical data encryption is demonstrated, which prevents side-channel attacks and enables real-time encryption with no delay.