Simulation And Realization Of Vector Beam In Optical Fiber Waveguide

Vortex beams are defined with phase singularity or polarization singularity in the center of optical field,which result in the doughnut shape intensity distribution.Vortex beams with phase singularity are called optical angular momentum(OAM)beams because of their helical phase wave-front which presents in an azimuthal phase term exp(ilφ)in its mathematical expression where l denotes the topological charge value and φ is azimuthal angle.And each photon in OAM beams has an angular momentum of 1h.OAM beam shows great promise in optical communication and quantum information processing due to the topological charge value l being any arbitrary integer.Vortex beams with polarization singularity are called cylindrical vector beams(CVBs)because of their cylindrically symmetric polarization including the most studied radially polarized beam and azimuthally polarized beam.And the special polarization leads to unique properties under focusing through lens with high numerical aperture,which makes CVBs an important role in many applications,like exciting surface plasmon resonance,lasing machining and so on.Besides,since both of OAM beam and CVBs have a dark spot in the center of their optical field,they have attracted great attention in optical trapping and particle manipulation.In all published studies,there are a lot of methods to generate OAM beams and CVBs.As for OAM beams,the most used optical elements are cylindrical lens pairs,spiral phase plates,spatial light modulators.As for CVBs,calcite crystals,segmented spatially variant λ/2 plates,spatially variant subwavelength grating and as well as spiral phase plates.Comparing to those spatial optical elements,OAM beams and CVBs generated in optical fiber can be more efficient and pure.Besides,the structure of all-fiber laser possesses the advantages of compactness and portability.We know that LP01 mode、LP11 mode and so on are eigenmodes of optical fiber under weakly-guiding approximation from mode theory of fiber.And there are four degenerate vector modes in linearly-polarized LP11 mode which are TE01 mode which is azimuthally-polarized mode、TM01 node which is radially-polarized mode and the even and odd HE21 modes which consist OAM with a phase difference of±π/2.Thus we can get OAM beams and CVBs by converting LP01 mode to LP11 mode through appropriate methods,and then controlling the polarization or phase of LP11 mode.In this dissertation,we introduced the mathematical expression,optical properties,applications,generation methods and detection methods of OAM beams and CVBs.Then we started off with fiber mode theory and mode coupling theory,and analyzed the coupling process of fiber gratings and mode-selective coupler.And we proposed an all-fiber laser with straight cavity emitting continuum OAM beam based on mode selective coupler and fiber Bragg grating.As for generating OAM beam,we simulated and proved that the high-order eigenmodes in optical rotatory fiber carry OAM.Then we demonstrated an all-fiber mode-locked laser outputting CVB pulses based on the home-made broad-band long period fiber grating.Finally,we theoretically proposed an air-core fiber with metal cladding realizing anomalous dispersion at 1.0μm band.The main work and research results of this dissertation are as follows:1.According to the mode coupling theory of mode selective coupler,we designed and fabricated a mode selective coupler realizing the mode conversion between LP01 mode and LP11 mode with theoretical coupling efficiency of 100%.Based on the mode selective coupler and a fiber Bragg grating working as output coupler,we demonstrated an all-fiber straight cavity laser at 1.0μm band outputting OAM beam with a slope efficiency of 15.7%and a threshold of 84 mW.And mode purity is 94.7%by the difference of modes power when twisting fiber。2.We analyzed optical rotatory fiber by finite element method,which proved that its vector eigenmodes in linearly-polarized LPmn(m≠0)modes carry m order orbital angular momentum,providing a new method of generating OAM beams.3.We designed and fabricated a long period fiber grating with a broad band width of 125 nm at 1.0μm band and a mode conversion efficiency higher than 93.7%.A gold deposited connector is chosen as output coupler avoiding the narrowing of spectrum.The mode locking mechanism of nonlinear polarization rotation is realized by a combination of a polarizer and a 3-port circulator and two polarization controller generating CVB pulses with pulse duration of 168 ps and repetition rate of 9.83 MHz.Besides the mode purity is over 95%.4.We theoretically designed an air-core fiber with metal cladding,and calculated group velocity dispersion of the fiber with defferent radius of air core,thickness of guiding layer and metal cladding.The total group velocity dispersion of TE01 mode reaches-700 ps2/km in 1.06μm band.As for TM01 mode,in considerasion of the loss caused by metal cladding,we calculated the group velocity dispersion without metal cladding which reaches-300 ps2/km in 1.06μm band.The innovation of this dissertation are as follows:1.OAM beam at 1.0 μm band with high efficiency and mode purity is realized with an all-fiber laser using mode selective coupler and few-mode fiber Bragg grating.2.We theoretically proved that the high order eigenmodes of optical rotatory fiber carry orbital angular momentum,offering a new method generating OAM beams.3.A high efficient broadband long period fiber grating is designed and fabricated.An all-fiber mode-locked laser based on NPR emitting CVB pulses is demonstrated.Besides,there are only two polarization controllers in the whole laser setup realizing NPR and CVB modes,so that it is easier to realize the output of CVB modes and pulses at the same time.4.A dispersion management fiber is theoretically proposed with large anomalous dispersion at 1.0 μm band for TE01 mode.Group velocity dispersion of TE01 mode and TM01 mode are calculated without metal cladding,pave the way of dispersion management to realize cylindrical vector pulse within femtosecond level.