Research On The Applications Of Nonlinear Optical Parametric Effects In Free-Space Optical Communication Networks

Along with the extensions of the space exploration and the earth observation,capacity of spatial information,acquired by various aerocrafts and satellites,grows explosively and thus requires much larger data-bandwidth for transmission.Free-space optical communications?FSOC?become an outstanding candidate for the space massive data transmission because of its many advantages over the conventional microwave-based wireless communications,such as fiber-like bandwidth,free of electromagnetic interference and no need of spectrum licenses.Although there have many long-haul free-space optical communication experiments,from satellite on the geosynchronous orbit to the space station,or from the low earth orbit satellite to the optical ground station,been successfully demonstrated,the randomness of the atmospheric channel,however,is still an enormous challenge in the reliability of the practical FSOC systems.Nonlinear optical parametric effects,which can generate new frequencies?namely idlers?,be transparent to the modulation formats and data-rates,as well as own instantaneous response,are effective tools to realize all-optical processing of the high data-rate optical signals at the space nodes.This thesis mainly focus on the communication band selection as well as free-space network construction to seek for a better method which can alleviate or even settle the major difficulty.The main achievements are as the following:1.The physical principle of the 2^nd nonlinear parametric process is analyzed,and the application of such a process in the FSOC system is emphatically discussed.Meanwhile,the phase-matching conditions in a periodically poled lithium niobate?PPLN?crystal,to realize a high conversion efficiency of the parametric process,is also studied detailedly.Besides,the 3^rd nonlinear parametric process,namely four-wave mixing?FWM?,is theoretically researched because it plays an important role in the all-optical data-processing at the relay-nodes of the FSOC networks.The corresponding phase-matching conditions and the polarization sensitivity of the 3^rdd parametric process are particularly analyzed which are crucial issues in practical system realization.Theoretical analysis and corresponding simulation are offered concerning the D-FWM and the ND-FWM.This part is a theoretical foundation for the following experimental researches.2.Wide-range all-optical wavelength conversion based on DFG for high data-rate mid-infrared?MIR?optical signal generation and receiving is proposed.Then successfully design and construct an experimental system to realize the high data-rate MIR optical signal generation and receiving.The MIR-based FSOC,which is also an atmospheric window,owns many superiorities compared to the near-infrared?NIR?system.However,the immature or lack of key devices such as room-temperature worked laser source,high-bandwidth modulator and tens of gigahertz photo-detector,delay the comprehensive application of such a promising FSOC system.In this thesis,the NIR high data-rate optical signal is converted to the MIR and vice versa based on the DFG-process in the PPLN crystal.The 10Gbps,OOK and DPSK MIR optical signal is transmitted and received with the MIR wavelength tunable.The constructed experimental system,which can convert the high data-rate optical signal transparently from the NIR band to the MIR band and vice-versa,integrates the advantages of high devices maturity from NIR and excellent atmospheric transmission performance from MIR,thus offer a new and attractive option method for the FSOC system.3.Wavelength-address based all-optical switching in FSOC networks is proposed.After analyzing the characteristics of the FSOC network and its demands on the switching performance at spatial nodes,it is found that optical circuit switching?OCS?based on the wavelength-address can meet all the requirements.Based on the 3^rdd nonlinear FWM process,the wavelength-address based OCS are experimentally researched.The transparency to the modulation format and data-rate,the polarization insensitivity to the random incoming laser link,as well as the optical signal-to-noise ratio?OSNR?are especially analyzed.The obtained experimental results show that all-optical wavelength-address based switching can be realized for the OOK,BPSK and QPSK signals within the whole C-band?1535nm-1567nm?,while the receiving sensitivity loss caused by the switching nodes are less than 1dB for all the three formats.Also the polarization sensitivity of the FWM-based wavelength conversion are below 0.6dB,showing the feasibility of the proposed switching method for the space nodes.Finally a practical wavelength convertor is developed and the space adaptability of the crucial components are tested.4.All-optical information processing based on the 3^rd nonlinear FWM parametric effect is proposed for FSOC networks.Simulation and experiments on the optical amplify-and-forward relaying are carried out both for the OOK and DPSK formats.The receiving sensitivity loss caused by relaying node are less than 1.5dB for OOK and about 10dB for DPSK.It can be concluded from the obtained results that OAF relaying is not suitable for the PSK signals since it would introduce larger receiving sensitivity loss.Thus for PSK signals,the optical regenerative relaying?ORF?should be adopted based on the 3^rd nonlinear FWM process.Meanwhile,based on the phase erasure effect of the D-FWM processing,the format conversion from M-ary to M/2-ary PSK signals are also discussed and experimentally validated.The receiving sensitivity loss caused by the format conversion is less than 1dB from QPSK to BPSK,which can offer the path for data exchange between different networks and ensure the optical linkage unobstructed in the space optical networks.