High-speed physical key distribution is diligently pursued for secure communication. In this paper, we propose and experimentally demonstrate a scheme of high-speed key distribution using mode-shift keying chaos synchronization between two multi-longitudinal-mode Fabry–Perot lasers commonly driven by a super-luminescent diode. Legitimate users dynamically select one of the longitudinal modes according to private control codes to achieve mode-shift keying chaos synchronization. The two remote chaotic light waveforms are quantized to generate two raw random bit streams, and then those bits corresponding to chaos synchronization are sifted as shared keys by comparing the control codes. In this method, the transition time, i.e., the chaos synchronization recovery time is determined by the rising time of the control codes rather than the laser transition response time, so the key distribution rate is improved greatly. Our experiment achieved a 0.75-Gbit/s key distribution rate with a bit error rate of 3.8 × 10⁻³ over 160-km fiber transmission with dispersion compensation. The entropy rate of the laser chaos is evaluated as 16 Gbit/s, which determines the ultimate final key rate together with the key generation ratio. It is therefore believed that the method pays a way for Gbit/s physical key distribution.

Chaotic external-cavity semiconductor laser (ECL) is a promising entropy source for generation of high-speed physical random bits or digital keys. The rate and randomness is unfortunately limited by laser relaxation oscillation and external-cavity resonance, and is usually improved by complicated post processing. Here, we propose using a physical broadband white chaos generated by optical heterodyning of two ECLs as entropy source to construct high-speed random bit generation (RBG) with minimal post processing. The optical heterodyne chaos not only has a white spectrum without signature of relaxation oscillation and external-cavity resonance but also has a symmetric amplitude distribution. Thus, after quantization with a multi-bit analog-digital-convertor (ADC), random bits can be obtained by extracting several least significant bits (LSBs) without any other processing. In experiments, a white chaos with a 3-dB bandwidth of 16.7 GHz is generated. Its entropy rate is estimated as 16 Gbps by single-bit quantization which means a spectrum efficiency of 96%. With quantization using an 8-bit ADC, 320-Gbps physical RBG is achieved by directly extracting 4 LSBs at 80-GHz sampling rate.

Time delay signature (TDS) of a semiconductor laser subject to dispersive optical feedback from a chirped fibre Bragg grating (CFBG) is investigated experimentally and numerically. Different from mirror, CFBG provides additional frequency-dependent delay caused by dispersion, and thus induces external-cavity modes with irregular mode separation rather than a fixed separation induced by mirror feedback. Compared with mirror feedback, the CFBG feedback can greatly depress and even eliminate the TDS, although it leads to a similar quasi-period route to chaos with increases of feedback. In experiments, by using a CFBG with dispersion of 2000ps/nm, the TDS is decreased by 90% to about 0.04 compared with mirror feedback. Furthermore, both numerical and experimental results show that the TDS evolution is quite different: the TDS decreases more quickly down to a lower plateau (even background noise level of autocorrelation function) and never rises again. This evolution tendency is also different from that of FBG feedback, of which the TDS first decreases to a minimal value and then increases again as feedback strength increases. In addition, the CFBG feedback has no filtering effects and does not require amplification for feedback light.

Optical chaos communication has advantages of high speed and long transmission distance. Unfortunately, the key space of the traditional transceiver, i.e. semiconductor laser with mirror feedback, is limited due to the time delay signature. In this paper, we propose and numerically demonstrate a key space enhancement by using semiconductor laser with optical feedback from a chirped fiber Bragg grating (FBG). The chirped FBG feedback can make feedback delay a key parameter by eliminating the time delay signature. Moreover, the grating dispersion and center frequency can also be used as new keys. As a result, the dimension of key space is increased. By taking a bidirectional communication scheme as an example, numerical results show that the key space is raised by 2⁴⁴ times as against mirror feedback with a data rate of 2.5 Gb/s and a coupling strength of 0.447. As the coupling strength decreases, the key space increases due to the fact that chaos synchronization becomes more sensitive to parameter mismatch.

To achieve complete security of communication, there is a need for “real-time” ultrafast physical random bit generators (RBGs). In the available physical RBGs, random bit extraction is effected in the electrical domain and, therefore, cannot directly function beyond the frequencies of 10 GHz or so in real time. Here, we present a fully photonic strategy for real-time random number generation and report a proof-of-concept experimental demonstration of an integrated 10 Gb/s RBG system (prototype) based on laser chaos. The use of an ultrastable mode-locked laser, together with ultrafast optical interactions, gives our method the capability to develop super-high-speed RBGs in practice. The photonic implementation is fully compatible with optical communications so that well-established optical multiplexing techniques can be used to realize Tb/s real-time random bit generation.

Common-signal-induced synchronization of semiconductor lasers have promising applications in physical-layer secure transmission with high speed and compatibility with the current fiber communication. Here, we propose an ultra-long-distance laser synchronization scheme by utilizing random digital optical communication signal as the common drive signal. By utilizing the long-haul optical coherent communication techniques, high-fidelity fiber transmission of the digital drive can be achieved and thus ultra-long-distance synchronization is expected. Experiments were implemented with distributed feedback lasers injected by a random-digital phase-modulated drive light. Results show that high-quality synchronization can be achieved as the drive signal rate is larger than the laser relaxation frequency and the transmission bit error ratio is below a critical value. Chaos synchronization over 8191-km fiber transmission was experimentally achieved. Compared to traditional common-signal-induced synchronization using analog drive signal such as chaos, the distance is increased by 8 times, and complicated hardware devices for channel impairment compensation are no longer required. In addition, the proposed method does not sacrifice communication capacity like traditional methods which need a channel to transmit analog drive signal. It is therefore believed that this common-digital-signal induced laser synchronization paves a way for secure backbone and submarine transmission.