The Research Of Key Technologies On Free-space Secure Optical Communication

There is a long-standing history of people using light to communicate. Particularly in special environments such as satellite and submarine that are not suitable to lay ca-bles and fibers, free-space optical communication (FSO) has attracted wide attentions because of its flexible features. Compared with the conventional microwave wireless communication technique, FSO has advantage in terms of communication confidential-ity owing to the small width, good direction,and strong ability of anti-interception and anti-electromagnetic interference nature of laser beams. When the light is attenuated to the scale of single photons, it is possible to encode information into quantum states and realize quantum communication with theoretically guaranteed security.In this thesis, we focus on two key applications of free-space secure optical com-munication, including the practical demands of satellite-ground quantum communica-tion and submarine secure laser communication, space-borne quantum optical source and submarine optical communication.Quantum communication is hitherto the only mathematically-proven absolutely secure communication scheme. Satellite-ground quantum key distribution (QKD) re-layed via satellite platforms is the most promising and most feasible solution towards a global quantum-secured network. Being as one of the first four scientific satellites from the Chinese Academy of Sciences’Strategic Priority Program, the Quantum Sci-ence Satellite will use the onboard space-borne quantum optical source to constitute the space-ground integrated quantum experiment platform together with the ground station, and complete the first satellite-ground high-speed QKD experiment internationally.Towards the demand for satellite-ground quantum communication, this thesis has designed and developed a space-borne quantum optical source with excellent perfor-mance and high reliability in the context of satellite-ground link’s large attenuation and outer space’s harsh environment. The functionality and performance of the opti-cal source meet the strict requirements of future satellite-ground QKD experiments.Firstly, according to the principles of quantum key distribution and the space appli-cation background of high reliability, the requirements for the optical source in satellite-ground QKD experiment have been summarized and the relatively simple, mature and highly-reliable multi-laser-based method is chosen to realize the quantum optical source. Secondly, during the design and implementation of the optical source, the candidate has united the Agilecom Fiber Solution (Zhongshan) Inc to develop the laser products dedicated to space applications. After the simulation and analysis of the dynamic char-acteristics when modulating the lasers based on the gain-switching method, the weak coherent optical source with excellent performance is achieved. The output quantum states of the lasers are indistinguishable on the dimensions of time, intensity, frequency, phase and so on. The repetition frequency of the optical source is 100MHz, the polar-ization extinction ratio (PER) and signal to noise ratio (SNR) are better than 25dB, the time duration of the output optical pulses is about 123p.s and the long-time intensity stability of the quantum state is better than 99.4%. Finally, being as one part of the payload from the Quantum Science Satellite, the optical source has passed a series of space environment verification tests and participated in the in-orbit QKD experiments simulated at ground. All these tests and experiments have achieved good results. Cur-rently the payload has been successfully delivered and the satellite is expected to launch in July 2016.Since the conventional submarine microwave communication is limited in per-formance and security, using laser to realize the confidential communication between satellite (aircraft) and submarine has also possessed strategic value and important re-search significance. Towards the realization of submarine secure laser communica-tion, it is an essential approach by studying the inherent optical properties (IOPs) of the complex atmosphere-ocean channel theoretically, establishing the channel parameters model based on the real environment, and finally combining the parameters of practi-cal devices to study and analyze the performance of submarine optical communication systems.Concerning the demand for submarine secure laser communication, this thesis has established the Monte Carlo simulation algorithms for the submarine optical communi-cation system based on the parameters model of real atmosphere-ocean channel. Then based on the model, the performances of the aircraft-submarine high-speed duplex com-munication system and the satellite-submarine optical communication system have also been studied and analyzed.Firstly, according to the real atmosphere-ocean channel, the height-based atmo-sphere parameters hierarchical model, the fluctuant atmosphere-ocean interface model and the depth-based seawater parameters hierarchical model have been established. The parameters of seawater are determined by the chlorophyll concentration distribution at a particular depth of different water types, which can describe the IOPs of real seawater better. Secondly, based on the Monte Carlo simulation method of photon transmission, the simulation algorithms of submarine optical communication system comprising com-plex atmosphere-ocean channel have been established. Thirdly, towards the scheme of aircraft-submarine high-speed duplex communication, the simulation algorithms of the uplink channel and downlink channel have been established respectively. And the performance of aircraft-submarine communication system has also been studied and analyzed. The results show that, under the condition that system communication ca-pacity is about 1Gbps, the attainable submarine communication depth is deep as 110m in JWT I seawater and the potential communication capacity will experience significant improvements as the detector bandwidth becoming better. Finally, towards the scheme of satellite-submarine optical communication, the simulation algorithms of signal ir-radiance and background solar irradiance have also been established. The candidate has also studied and analyzed the performance of satellite-submarine communication system combined with the parameters of practical devices. The results show that, at the bit error rate (BER) limit of 10-4,the attainable submarine communication depth is about 140m in JWT I seawater. The maximum communication data rate can reach up to 8kbps.The key innovation points are listed as follows,1. In the context of satellite-ground link’s large attenuation and outer space’s harsh environment, a space-borne quantum optical source with excellent performance and high reliability has been designed and developed. The optical source has passed a series of space environment verification tests and participated in the in-orbit QKD experiments simulated at ground. All these tests and experiments have achieved good results, which lay a solid foundation for the realization of the Quantum Science Satellite’s scientific objectives. Currently the payload has been successfully delivered and the satellite is expected to launch in July 2016.2. Based on the atmosphere-ocean channel parameters model of real environments, the performances of the submarine optical communication systems have been s-tudied and analyzed using the Monte Carlo method. The simulations and theoreti-cal analysis towards the submarine optical communication of complex atmosphere-ocean channel in this thesis take a significant step forward towards submarine s-trategic secure laser communication, and provide a valuable design guidance and theoretical basis for the final realization of satellite(aircraft)-submarine secure laser communication.