Researches On Several Key Techniques Of Multi-Dimensional Multiplexing Optical Fiber Communications Systems

With the higher requirement for information transmission and exchange,the amount of data transmission is increasing rapidly.However,Internet data traffic capacity is rapidly reaching limitation imposed by nonlinear effects of single mode fibers currently used in optical communications systems.Having almost exhausted available degrees of time,frequency,phase,amplitude and polarization in optical communications multiplexing technology,researchers are now exploring the possibility of using the spatial dimension of fibers,via multicore and multimode fibers,to address the forthcoming capacity crunch.Similar to single mode fiber communications technologies,the space division multiplexing(SDM)technologies are required to explore and study every module in the systems,including transmission fibers,mode conversion,mode multiplexing/demultiplexing,multi-mode erbium doped fiber amplifier,mode router etc.Moreover,the the performance criteria of devices,such as compatibility,integration,wavelength sensitivity and technical complexity etc,need to be considered.Aiming at the challenge and the unsolved problem of mode conversion and multiplexing/demultiplexing devices researches,the thesis proposes and experimentally demonstrates several all-fiber mode convertors and mode multiplexers.Firstly,the thesis presents the fiber modes characteristics,discusses the vector and scalar Helmholtz equations,and the corresponding solved fiber eigenmodes and scalar approximated modes,respectively.Secondly,the fiber vector modes in the weakly guided approximation and the relationship between vector mode and scalar modes are analyzed by using polarization perturbation theory.In order to solve the wavelength sensitivity problem of convertional mode selective coupler,a novel wideband tapered mode selective coupler is proposed in the thesis.Compared with the conventional mode selective coupler,the devices proposed in the thesis can work effectively in 200 nm wavelength band around 1550 nm with the coupling efficiencies of LP11?LP21 and LP02 modes are higher than 80%.In addition,the fabrication process and measured results are presented in the thesis.Currently,the photonic lantern is one of the most popular mode coversion and multiplexing devices,but it is not easy to fabricate due to the complex technique and the specific fibers and capillaries requirement.With the condition of our lab,we fabricate and test,for the first time in our country,the three-mode and six-mode mode selective photonic lanterns.The principles,fabricating methods and procedures,test results are presented in the thesis.The all-fiber mode control mechanism is introduced in the thesis,mainly including the mode orientation control for higher-order azimuthally asymmetric mode,the high-order mode evolution in the fiber and the exchange between linear-polarized and orbital angular momentum modes in the fiber.Firstly,the thesis designs a novel degenerate mode rotator with employing the asymmetric few mode fiber;analyzes the rotating performance and fabrication tolerance.A novel six-modes multiplexer is proposed based on this degenerate mode rotator and the coupling efficiencies of six modes are higher than 78%in 200 nm wavelength band around 1550 nm.Next,the degenerate mode rotation theory is expanded to arbitrary-angle mode rotation theory and the first-and second-order linear-polarized modes rotations with controllable angle are experimentally demonstrated by employing a few-mode polarization-maintaining fiber.In addition,a novel first-order orbital abgular momentum mode generator is proposed in the thesis and the generations of left-and right-handness first-order OAM modes are experimentally demonstrated by using a few-mode polarization-maintaining fiber.Finally,a novel all-fiber OAM mode multiplexer based on photonic lantern and coiled-type mode-polarization controller is proposed and experimentally demonstrated in the thesis.The multiplexer can be upgraded to any higher-order OAM modes and multiplex multi-order OAM modes with left-and right-handness simultaneouly.The high purity mode conversion is presently a diffcullt goal to achieve.In the thesis,a long-period fiber grating that generates vector OAM modes with high purity is experimentally demonstrated by using are-induced inscribing technique.According to the different applications,we fabricate two types of long period gratings that work in the telecom wavelength band(1550 nm)and the visible wavelength band(green,red,776.2 nm)and measure the transmission spectrum.In addition,we use the"ring interference analyzing method" to measure the purities of output vector OAM modes.The average purities of OAM mode output from fiber gratings in telecom wavelength band and visible wavelength band are higher than 20dB and 17dB,respectively.The OAM mode selective fiber gratings in the telecom and visible wavelength band can be widely applied in the SDM systems and super-resolution bio-image systems,respectively.For the second application,the all-fiber super-resolution image system is experimentally demonstrated in the thesis.