Research On New Coding Modulation Technology In High-reliability Few-mode Multiplexing Optical Communication Syste

With the rapid development of emerging Internet technologies,the scale of optical communication networks is expanding rapidly and network data traffic is growing exponentially.Optical fiber communication systems carry most of the network data traffic in network communication.Therefore,it is important to improve system capacity and enhance communication security in the field of optical communication.This paper discusses the coding and modulation technology in the optical communication system of few-mode multiplexing,and studies the 3D constellation shaping design and chaotic encryption mechanism based on few-mode multiplexing.In order to improve the transmission performance of the system and enhance data transmission security,3D constellation shaping design and chaotic encryption based on few-mode multiplexing have been explored and studied.The specific research contents are as follows:（1）In order to improve the transmission capacity and performance of 3D constellation,a scheme of few-mode multiplexed optical transmission system based on the geometric shaping of 3D constellation of regular octahedron（3D-GS C-8）design,by studying and comparing the various basic components of 3D constellation.The 3D-GS C-8 is compared with the conventional 2D 16QAM constellation and 2D 8QAM constellation.The experiments show that the 3D-GS C-8 achieves the receiver sensitivity gain of 1.33 d B and 1.8 d B at the same total transmission rate and BER of 3.8×10^-3.On this basis,CCDM is used to optimize the constellation probability distribution,which obeys the Maxwell-Boltzmann distribution.It is experimentally verified that 3D-GS-PS C-8 achieves a SNR gain of 1.33 d B over 2D constellations with the same information entropy,and that the lower the information entropy,the better the constellation transmission performance.In addition,it was found that the higher the degree of GS of the 3D constellation,the lower the gain obtained by its PS.By transmitting different 3D constellations in the few-mode multiplexing system,the experimental results show that the 3D-GS-PS C-8 combined with MDM is significantly superior when transmitted over 5km of weakly coupled fibers in MPLC.At the BER of 3.8×10^-3 and the information entropy of 3.52 bit/symbol,the 3D-GS-PS C-8 achieves a 0.6 d B gain in receiver sensitivity compared to conventional 3D C-6,and this advantage becomes more apparent as the received optical power increases.（2）In order to secure the data transmission of a few-mode multiplexed optical communication system,an MDM chaotic encryption scheme based on key interleaving and concomitant transmission is proposed.The data and time-varying keys can be transmitted in two modes,LP01 and LP11,respectively,with the current key subject to step-by-step dissimilarity with the keys of all previous moments.In order to implement chaotic masks in the digital domain,the data is encrypted at the constellation phase,data symbol block and subcarrier level using three chaotic sequences corresponding to the new key.The transmission of an 8.89Gb/s encrypted 16QAM-OFDM signal over a 1 km weakly coupled FMF is demonstrated experimentally.Receivers with the correct key recover data normally,while the BER of illegal receivers remained at around 0.5.The cracking efficiency threshold of the time-varying key encryption scheme is 5.21×10⁶ times higher than that of the time-invariant key encryption scheme at a key transmission bit rate of 1 Gb/s.