A novel time-delay signature (TDS) concealing electro-optic (EO) chaotic system with multiply feedback nonlinear loops is proposed and analyzed by numerical simulation. The proposed system employs mutual injection structure implemented by two asymmetric branches named as multiply feedback nonlinear loop which introduces an extra nonlinear factor to the system dynamic equation. The complexity of the chaos system is increased by introducing this multiply feedback nonlinear loop. The permutation entropy (PE) of the proposed system is improved to higher than 0.96 when feedback strength (β) equals 5. The proposed system can enter to chaos regime with a small β (β = 0.8). The TDS is concealed effectively due to the extra nonlinear factor introduced by multiply feedback nonlinear loop. Meanwhile, key-space of the proposed system is about 10¹² times that of the classical EO system because more tunable time delay parameters are introduced. Furthermore, the performance of a secure communication system based on the proposed chaotic system is discussed, and the simulation results show that the system is sensitive to time delay parameters and robust to feedback strength, which proves the proposed system is suitable for secure communication.

In this paper, a novel chaotic secure communication system based on vertical-cavity surface-emitting lasers (VCSEL) with a common phase-modulated electro-optic (CPMEO) feedback is proposed. The security of the CPMEO system is guaranteed by suppressing the time-delay signature (TDS) with a low-gain electro-optic (EO) feedback loop. Furthermore, the key space is enhanced through a unique secondary encryption method. The first-level encrypted keys are the TDS in the EO feedback loop, and the second-level keys are the physical parameters of the VCSEL under variable-polarization optical feedback. Numerical results show that, compared to the dual-optical feedback system, the TDS of the CPMEO system is suppressed 8 times to less than 0.05 such that they can be completely concealed when the EO gain is 3, and the bandwidth is doubled to over 22 GHz. The error-free 10 Gb/s secure optical transmission can be realized when the time-delay mismatch is controlled within 3 ps. It is shown that the proposed scheme can significantly improve the system performance in TDS concealment, as well as bandwidth and key space enhancement, which has great potential applications in secure dual-channel chaos communication.

An optical conversion node scheme for direct detection of complex modulation format is proposed to bridge long-haul transmissions and short-reach interconnects. A noisy 10G Baud quadrature phase shift keying signal is converted into a 10G Baud normal 4-level pulse amplitude modulation (PAM4) signal by the node. The conversion node is realized mainly relies on four-wave mixing-based phase-sensitive amplifiers. The power ratio and constellation shape of the converted PAM4 both can be flexibly designed based on network demands and five kinds of uniform or non-uniform PAM4s are generated to verify the shaping functionality. With the input optical signal-to-noise ratio range of (10 dB∼30 dB), the key indicators of the signals went through every part are measured, includes constellations, eye diagrams, error vector magnitudes, bit error rates, normalized impact factors of phase and amplitude. The proposed node scheme has great application potential in intermediate nodes for bridging long-haul transmissions and short-reach interconnects, hierarchical modulation and flexible constellations design for advanced format signals.

A scheme is proposed for all-optical aggregation from four 10Gbps binary-phase-shift-keying (BPSK) signals into one 40Gbps 16-ary quadrature amplitude modulation (16QAM) signal by applying the coherent addition of optical vectors, and de-aggregation for the 16QAM signal to recover the data of four original BPSK signals in high nonlinear fiber (HNLF) without information loss or redundancy using self-phase modulation (SPM) and four-wave mixing (FWM). The simulation results show that the error-vector-magnitude (EVM) curves of BPSK signal before aggregation and the 16QAM signal almost coincide with each other, as well as after the de-aggregation it has about 20% degradation at most. The bit error rate (BER) performance of BPSK signals indicates that when the OSNR of BPSK before aggregation is greater than 21.0 dB, then the wrong code of BPSK signal after de-aggregation can be corrected at the receiving end. Additionally, the expanded methods are also proposed for the aggregation of 8PSK signal and 64QAM signal by employing the similar principle. This proposed method provides a new way for realizing the flexible optical transmission and reception of advanced modulation format signals, and suits for the future dynamically reconfigurable optical network at the gateway nodes between different transmission networks.

A simultaneous all-optical channel aggregation and de-aggregation scheme is proposed for two 8QAM modulation formats by employing four-wave mixing (FWM) in highly nonlinear fiber. The scheme consisting of two FWM processes is simple without photoelectric conversion, which is promising to improve efficient use of the fiber resources in elastic optical networking. The feasibility of the scheme has been confirmed by the input-output constellations and waveforms. System performance is also studied as a function of the optical signal-to-noise ratio of the two 8QAM signals. Finally, some potential application scenarios have also been discussed based on the same setup.

A novel chaos system with XOR operations and multi-bit PRBS is proposed to improve the sequence complexity and the security of the classic electro-optic intensity chaos system. Through the bifurcation diagram and permutation entropy analysis, the PE can be increased to 0.99. The key space is enlarged because additional DSP parameters and PRBS are introduced. The impacts of ADC/DAC characteristics and PRBS characteristics are analyzed in detail. The simulation results show that the time delay signature can be concealed with the appropriate DSP parameters.