| Nowadays,along with rapid development of information Internet technology innovation,artificial intelligence,high-definition video,live broadcast and other new application methods have attracted wide attention from the public.With the promotion of high-speed mobile communication interconnection network,the rapid development of information society,and the continuous innovation of optical communication technology,people's constant demand for the communication of information capacity,the limitation of the network capacity is becoming more and more obvious.As a new form of multiplexing,mode division multiplexing(MDM)based on orbital angular momentum(OAM)modes provides a new scheme for expanding channel capacity and improving communication quality.The key problem of MDM system application is the influence on the different communication links of the OAM modes,including optical fiber communication and free space wireless communication.Therefore,the transmission characteristics of OAM mode need to be studied deeply.Aiming at the problems above,the thesis focuses on the theme of key techniques in MDM communication system,and mainly studies two aspects.The first is the transmission characteristics of OAM modes in the optical fiber communication system,and the physical mechanism of external disturbance is deeply analyzed.A relatively perfect theoretical calculation and simulation model is established to deal with the stress and twist effect of the OAM fiber.On this basis,a novel of OAM photonic crystal fiber mode selective coupler is proposed.The other is for free space communication system,the OAM vortex electromagnetic wave space transmission model is established,and an adaptive compensation algorithm is proposed to alleviate the turbulence effect in space channel.The main research work of the thesis is as follows:(1)Research on stress and twist characteristics of OAM fiberThe transmission characteristics of OAM fiber under external disturbance such as stress and twist effect are studied.The mathematical model of stress-induced birefringence in OAM fiber with complex structure is established,and a circular OAM photonic crystal fiber with good performance is taken as an example to establish the simulation analysis model.Finally,the mode field quality and transmission characteristics of the fiber under the actual stress are analyzed,including intensity,phase,polarization,confinement loss,dispersion,nonlinear coefficient and stress-induced birefringence,etc.In addition,based on the theoretical analysis of fiber stress-induced characteristics theoretical analysis,a theoretical analysis model of the torsion effect of OAM fiber with complex structure was established.The composition of mode groups in the OAM fiber under different twist-induced effects is analyzed.A torsional OAM fiber simulation and analysis model is established.Finally,the mode composition and transmission characteristics of the twist OAM fiber under different torsional strength are analyzed and compared with the theoretical calculation results.(2)Design of photonic crystal OAM fiber mode selective couplerA novel double parallel structure photonic crystal OAM fiber mode selective coupler is designed.Firstly,the principle of double parallel structure mode selective coupler is studied,and the conversion of vector OAM mode is realized by the mode matching method.In the design scheme,a novel high performance photonic crystal fiber supporting OAM modes is used as the base fiber to make all-fiber coupler by side grinding method.Secondly,structural parameters were optimized according to the adjustable fiber distence and coupling length of the coupler to obtain the optimal mode purity and coupling efficiency.Finally,the performance indexes of the photonic crystal OAM fiber mode selective coupler are analyzed,including mode purity,coupling efficiency,loss characteristics and operation bandwidth.Over the C+L band,the coupler can excite the 3 orders OAM modes,the mode purity can reach 52%,the coupling efficiency is more than 51%,the insertion loss is greater than-1.73dB,and the excess loss is less than 0.175dB.(3)Research on spatial transmission characteristics of OAM modesAccording to the joint atmospheric turbulence model which is consistent with the actual atmospheric turbulence effect,the theoretical model of electromagnetic vortex wave carrying OAM modes transmission in the free space wireless communication channel is established.Aiming at the wavefront distortion and signal crosstalk caused by the turbulence channel,an adaptive compensation algorithm is proposed.It can alleviate the influence of atmospheric turbulence effect and realize the effective recovery of the wavefront disturbance of the OAM modes in the turbulent channel.Furthermore,the mode field quality and transmission performance of single OAM mode and multiple OAM mode multiplexed transmission before and after adaptive compensation are given.Finally,the changes of the signal-to-crosstalk ratio and channel capacity for the free space wireless communication system after adaptive compensation are studied to verify the effectiveness and feasibility of the proposed compensation scheme. |
PDF Full Text Request