We propose and numerically investigate a chaotic optical coherent secure communication scheme, which supports long-haul secure transmission for signals in advanced modulation formats. A hybrid optical chaos system is designed with coordination of digital and analog signals. The hybrid entropy source provides a broadband analog optical chaos signal, which could serve as the carrier to load quadrature amplitude modulation (QAM) data. Simultaneously, a digital binary signal generated from the entropy source is transmitted to establish long-haul chaotic synchronization. Coherent detection is utilized at the receiver, and a digital signal processing (DSP) algorithm is adopted to reduce transmission distortion. A 5 Gbaud 16QAM signal is encrypted by a phase chaos carrier with the effective bandwidth of 5.8 GHz. A bit error rate (BER) below forward error correction (FEC) can be achieved after transmitting over 1600 km based on digital-signal-induced chaos synchronization technology. Optimal launch power is investigated to minimize nonlinear effects of transmission links. System security is guaranteed by the high dynamical complexity of the chaotic source and the sensitive time delay as the secret key.

We experimentally demonstrate a chaotic synchronization scheme based on the electro-optic hybrid entropy sources. The chaos synchronization method is experimentally validated that 200 km high-level chaos synchronization can be attained and the distance can be further extended. The digital signal induced chaos synchronization signal has good robustness against the potential distortions in long-haul optical transmission. On the basis of the digital signal induced chaos synchronization strategy, a correlated random bit generation (CRBG) scheme with bit rate of 1.25 Gb/s and distance of 200 km is proposed. The generated correlated random bit sequences (CRBSs) pass all the NIST tests. Meanwhile, a data encryption scheme based on the robust chaos synchronization method is proposed with low computation complexity. The synchronization signal, in which the encrypted data is embedded, maintains the strength in long-haul transmission. The strong robustness of the scheme under parameter mismatch is numerically analyzed. These propositions show great potential in the field of secure communication, especially the long-haul scenario.

We propose a secure key distribution (SKD) based on local polarization modulation driven by optical chaos in a reciprocal fiber link. A robust error-free SKD with a key generation rate of 4.3 Gbit/s over transmission of 10-km standard single-mode fiber is experimentally demonstrated. A chaotic laser system shared by legitimate users serves as an external wideband entropy source. The polarization reciprocity of the fiber channel provides fundamental safety against eavesdropping. The robustness of SKD resulted from local chaotic polarization modulation is also theoretically analyzed and then verified by practical performance. The proposed scheme is an alternative SKD strategy with high speed and strong security.

We propose a time delay signature extraction method for optical chaos systems based on a convolutional neural network. Through transforming the time delay signature of a one-dimensional time series into two-dimensional image features, the excellent ability of convolutional neural networks for image feature recognition is fully utilized. The effectiveness of the method is verified on chaos systems with opto-electronic feedback and all optical feedback. The recognition accuracy of the method is 100% under normal conditions. For the system with extremely strong nonlinearity, the accuracy can be 93.25%, and the amount of data required is less than traditional methods. Moreover, it is verified that the proposed method possesses a strong ability to withstand the effects of noise.

In this paper, a novel image encryption scheme based on analog-digital hybrid electro-optic chaotic sources and compressive sensing is proposed. Distinguished from other existing chaos-based encryption schemes, analog-digital hybrid electro-optic chaotic sources are utilized, in which the synchronization of chaotic systems can be achieved by transmitting binary digital bits. The robustness of the encryption scheme can be significantly improved and the theoretical transmission distance will be much longer because of digital chaos synchronization. Moreover, owing to the compression of compressive sensing and the sensitivity to initial values of chaotic sources, the proposed scheme can greatly reduce the amount of data and strengthen safety. Exhaustive simulations and analysis demonstrate that the proposed scheme is of good peculiarities such as high efficiency, good robustness and security.