Study On The Generation And Transmission Properties Of Vortex Light

The wavefront phase of a vortex beam that has a phase singularity and zero intensity in its center is a helical structure which introduces orbital angular momentum(OAM).With the help of vortex beams the optical tweezers can realize the rotation of tiny particles,avoiding the damage to them caused by the traditional operation mode.This new type of tweezers is also called "optical wrench".Moreover,due to the orthogonality of the vortex modes of different topological charges the new degree of freedom of the spiral phase can be used to developing,the next generation of multiplexing technology for optical communication to improve the transmission capacity.Because of its novel phase characteristics,vortex light has a wide range of applications in optical imaging,biomedicine and optical communication.Therefore,it is of great significance to study the method of generating high-quality vortex beam and the transmission mechanism of vortex beam in various media.Because most of the vortex beams are realized in the field of volume optics,the generation and transmission of vortex mode in optical fiber need further study.This thesis mainly studies the preparation and transmission of vortex light at a small scale of dozens of microns.Here,a novel method of generating vortex light by etching spiral phase plate on the optical fiber end face was proposed.The transmission process of Laguerre Gauss beam in gradient index multimode fiber was investigated,and the excitation mechanism of vortex mode in a fiber was analyzed.A theoretical model of excitation of Bessel vortex mode has also been developed,and verified by numerical simulation.Finally,the propagation of ultra-short vortex pulse in nm-scale ultrathin plasma plate was studied.The main contents are as follows:1.The generating method and theoretical model of vortex light are introduced.This thesis summarizes the equation description,and the basic characteristics of vortex light are analyzed,and several commonly used methods of vortex light generation are introduced:spiral phase plate method,mode conversion method,holographic method and spatial light modulator method.2.A method for fabricating spiral phase plate by etching on the end face of optical fiber is presented.By the effective refractive index principle,it is selected that the width of the etching is grooved from the target index of refraction distribution within a certain thickness scale of the optical fiber end face,and the effective refractive index has an increase or decrease uniformly with the angle.Different orders of vortex beams are obtained in optical fiber by using the spiral phase plate.The thesis analyzes the principle of etched spiral phase plate,and gives the design method of specific parameters.It can be seen from the simulation results that the etched spiral phase plate can generate up to 4th order vortex mode in the multi-mode fiber.3.The basic form of Bessel vortex mode in multimode optical fiber is given,and the basic principle of Bessel vortex mode excitation in optical fiber is analyzed when Laguerre Gaussian beam enters the multimode optical fiber from the optical fiber end face by air.The energy distribution ratio of Bessel vortex mode in optical fiber given Laguerre vortex mode is analyzed theoretically.It is found that the energy ratio of Bessel vortex mode is related to the size of focal spot,and most of the energy can be coupled into a single Bessel vortex mode in a specific focal spot size.The proposed model coupling theory is simulated by FDTD algorithm.The case of the focal spot size ?0=8?m,?0=9.2?m and ?0=4?m is simulated.It is shown that the numerical simulation results agree well with that the results are predicted by the theoretical model.4.The propagation of ultra-short vortex optical pulse in plasma film medium is studied.The results show that the transmitted optical pulse can keep the vortex structure well,so the film can be used as a ultra-high speed optical switch to filter the prepulse of the vortex light.The film shortens the time width of the transmitted pulse and reduces its spatial distribution,but the main pulse still retains more energy.Compared with the propagation process of a Gaussian beam without vortex phase,the electrons captured by the optical pulse with vortex have a net angular momentum,while the gaussian beam has almost no net angular momentum.In the case of vortex field,the electron beam with a clear angular momentum structure can travel forward with the light pulse.