| With the popularization of high-definition video services,evolution of mobile communication technology from 4G to 5G,and destination of the Internet of everything,the data traffic of information and communication networks continues to increase rapidly.However,as the most important component of information communication network,capacity growth rate of the optical network has gradually slowed down in recent years.At the same time,experiments has reached the fundamental Shannon limits of single-mode optical fiber.Optical networks will face a"capacity crunch" for the foreseeable future.Space division multiplexing(SDM)is a new form of optical multiple input and multiple output(MIMO)transmission,which is expected to be an important potential solution to further enhance the optical network capacity.Space division multiplexing based on few-mode fiber(FMF)is also commonly referred to as mode-division multiplexing(MDM).In this thesis,the author has built a numerical simulation platform of MDM transmission system based on Matlab,proposed a nonlinear compensation scheme for the MDM transmission based on the mid-link optical phase conjugation technology,and proposed a secure MDM transmission scheme based on digital chaotic function for physical layer security of optical transmission systems.1.The author have built a dual-polarization coherent optical transmission numerical simulation platform to simulate multi-channel signal transmission over few-mode fiber.The numerical platform includes flexible modular transmitter,few mode fiber link and coherent receiver,and it is convenience to simulate different transmission scenario.Compared with the traditional single-mode fiber transmission system,the simulation platform can simulate the unique physical damage in MDM transmission such as mode coupling,mode dispersion,and inter-mode nonlinearity.It can provide reference for the design of MDM transmission system,provide the basis for the analysis of the performance of MDM transmission,and provide a verification platform for the effectiveness of the damage compensation scheme and digital signal processing algorithm.2.In a MDM transmission system,the main physical impairments during signal transmission include fiber attenuation,mode coupling,chromatic dispersion,modal dispersion,and nonlinear effects.With the mid-link optical phase conjugation(OPC)technique,the accumulated chr-omatic dispersion in the first half of the few-mode fiber link can be completely cancelled in the second half;The Kerr effect is unlike the chromatic dispersion,a nonlinear impairment,dependent on the optical power of the signal.As a result,the degree in which the Kerr effect is compensated for is dependent on the design of the transmission link.In addition,the differential mode group delay in the few-mode fiber link also has a large impact on the nonlinear damage compensation performance.In the symmetric signal power evolution scenario,the simulation results in the absence of DMGD show that,the employment of OPC module would lead to an average Q-factor improvement of approximately 10dB.At the same time,in the presence of DMGD,an average Q-factor improvement would be 6.8dB and 2.8dB for single wavelength and WDM case,respectively.In the asymmetric signal power evolution scenario,in the absence of DMGD,an average Q-factor improvement would be 2.2 dB and 1.61 dB,respectively.However,by accounting the impacts of both DMGD and asymmetrical signal power map,the insertion of the OPC system will still lead to an average Q-factor improvement of 1.9dB and 0.9 1dB,respectively.3.The designed 3-D chaotic sequences are used to scramble therealand imaginary part of quadrature amplitude modulation(QAM)symbols,permute the symbolto-space mapping,and control iteration times between the two processes.At the same time,the security performance of digital chaotic MDM secure transmission system is analyzed in three aspects:security key,encryption system information entropy and initial value sensitivity.The results show that the eavesdropper cannot recover the original information with a part of the security keys or a part of the transmitted signals when the iteration times are beyond 5.We found the error spreading phenomenon can be combined with the mode-dependent loss(MDL)to realize the system security enhancement.When eavesdroppers coupled signals out from SDM waveguide by bending-induced fiber tapping,they need reduce the bend radius to receive more spatial leak modes.The bending-induced fiber tapping inherently causes MDL.Although the error spreading phenomenon leads to bit-error-rate increasing by 1.3 dB than conventional 16QAM transmission,it will be a promising solution to overcome the weakness that the huge amount of data can be compromised by tapping out the portion of the signal in single-mode transmission. |
PDF Full Text Request