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.

This paper presents an all optical fiber based implementation of a hybrid analog-digital computational primitive that provides a basis for complex processing on high bandwidth signals. A natural implementation of a hybrid analog/digital photonic processing primitive is achieved through the integration of new nonlinear fiber, and exploitation of the physics of semiconductor device to process signals in unique ways. Specifically, we describe the use of a semiconductor optical amplifier to implement leaky temporal integration of a signal and a highly Ge-doped nonlinear fiber for thresholding. A straightforward correspondence between our computational primitive and leaky-integrate-and-fire neurons permits leveraging of a large body of research characterizing the computational capabilities of these devices and the emerging pulse processing computational paradigm as a means to implement practical signal processing algorithms in hybrid computing platforms. An experimental demonstration of the behavior of the pulse processing primitive is presented.

In this paper, we demonstrate for the first time an ultrafast fully functional photonic spiking neuron. Our experimental setup constitutes a complete all-optical implementation of a leaky integrate-and-fire neuron, a computational primitive that provides a basis for general purpose analog optical computation. Unlike purely analog computational models, spiking operation eliminates noise accumulation and results in robust and efficient processing. Operating at gigahertz speed, which corresponds to at least 10⁸ speed-up compared with biological neurons, the demonstrated neuron provides all functionality required by the spiking neuron model. The two demonstrated prototypes and a demonstrated feedback operation mode prove the feasibility and stability of our approach and show the obtained performance characteristics.

An optical analog self-interference cancellation system for radio-frequency communications is proposed and experimentally demonstrated. The system uses two electro-absorption modulators, an optical attenuator and delay, and a balanced photodetector to subtract strong self-interference from a corrupted received signal and recover a weak signal of interest. The system achieves >; 65 dB narrowband cancellation and >; 30 dB broadband cancellation across more than 40 MHz bandwidth for interference in both the 900 MHz and 2.4GHz bands. Both narrowband and broadband cancellation are frequency tunable, and can be combined with other forms of interference cancellation to achieve even higher levels of cancellation.

We propose and experimentally demonstrate an optical steganography method in which a data signal is transmitted using amplified spontaneous emission (ASE) noise as a carrier. The ASE serving as a carrier for the private signal has an identical frequency spectrum to the existing noise generated by the Erbium doped fiber amplifiers (EDFAs) in the transmission system. The system also carries a conventional data channel that is not private. The so-called “stealth” or private channel is well-hidden within the noise of the system. Phase modulation is used for both the stealth channel and the public channel. Using homodyne detection, the short coherence length of the ASE ensures that the stealth signal can only be recovered if the receiver closely matches the delay-length difference, which is deliberately changed in a dynamic fashion that is only known to the transmitter and its intended receiver.

A wideband co-site co-channel interference cancellation system (ICS), based on hybrid electrical and optical techniques, is proposed and is experimentally demonstrated. The demonstrated cancellation system subtracts the in-band wideband interfering signal from the received signal, such that the weak signal of interest (SOI) can be recovered. Our system utilizes a broadband radio frequency (RF) Balun transformer to invert the phase of the interfering signal, while electro-absorption modulated lasers are used for converting the RF signals into the optical domain to enable fine adjustment with the hybrid ICS. We experimentally achieve 45 dB of cancellation over a 220 MHz bandwidth, and over 57 dB of cancellation for a 10 MHz bandwidth, at a center frequency of 900 MHz. The proposed system also experimentally shows good cancellation (30 dB) over an enormously wide bandwidth of 5.5 GHz. To the best of our knowledge, this is the first demonstration of such a wide bandwidth cancellation with good cancellation depth. This property is extremely useful when there are multiple interference signals at various frequency bands. The proposed hybrid ICS has a spurious-free dynamic range of $93.2\mathrm{dBm}/{\mathrm{Hz}}^{2/3}$. Moreover, a $10\mathrm{Gb}/\mathrm{s}$ SOI is recovered from a strong interfering signal, sweeping over 3 GHz bandwidth. A widely open eye diagram, as well as error-free bit-error rate measurements, is experimentally achieved, with the use of the hybrid ICS. The approach also works well for various frequency bands that are within the bandwidth of the Balun transformer and electro-absorption modulated lasers.