Generation And Conversion Of Optical Vortices In Optical Fiber

Structured light beams have received great attention in recent years and optical vortices (OVs) is a typical structured light. Optical vortices with a helical wave fronts, annular intensity profiles, phase singularities and a well-defined orbital angular momentum (OAM) have attracted considerable interests in recent years. There are many applications of OVs, such as optical trapping, optical communication, microscopy and material processing. Recently, generation and propagation of OVs in optic fiber has become an attractive approach, such as stress-induced chiral fiber gratings and helically twisted photonic crystal fibers. However, in these works the schematic setups for fabricating are complex, only helical long period gratings have been achieved. As we all know, higher order modes are composed of two fiber eigenmodes with different propagation constant. In this dissertation, we propose a scheme of fabricating the helical fiber Bragg gratings (H-FBGs) written directly on a multimode fiber (MMF) by using single-side UV exposure method. And the helical index modulation is obtained by rotating the fiber during single-side UV exposure. Because it is single-side exposure, an asymmetric index distribution is formulated in the cross-section of the fiber. As a result, the powers can transfer between different modes in the MMF which have OVs, and we can control the generation of OVs in the fiber. With the help of mode selective coupler (MSCs), the properties of MSCs are studied in detail. Through optimization the MSCs, OVs can be generated in the fiber.The main research works are listed as follows:1. Generation and conversion of higher order optical vortices in optical fiber with helical fiber Bragg gratings. H-FBGs induced by U V-side exposure exhibits wavelength-dependent coupling between different modes. We propose an effective method to generate high order OVs using H-FBG written in a multimode fiber. We simulate the conversion of OVs with different topological charges:0→±1,±1→0,0→±2,0→3, and the conversion efficiency is as high as 97%. And higher order topological charge of OVs can be generated as well, if the number of modes in the fiber is increased. We also analyze the conversion of H-FBG under various conditions of the induced asymmetric index profile and the length of H-FBG.2. An effective method to fabricate orthogonal-dislocated gratings (ODGs) by using UV-side-exposure method is proposed. Simulation results show that, the power of the fundamental mode can be coupled to higher order modes, which have OVs equal to 1 or-1. And even the distance and the written direction of the gratings is not accurate or orthogonal, OVs can also be generated. What is more the vortex can be adjusted easily by tuning the resonant wavelength of the gratings.3. The property of mode selectively coupler (MSC) is analyzed in detail and an effective method using mode selective coupler which composed of three-core fiber is presented to generate OVs. The conversions of OVs with different topological charges: 0→±1 and 0→±3 are simulated in detail.And higher order topological charge can be get if the parameters of the fiber is changed and the number of modes in the fiber is increased.The innovative and major works are listed as follows:1. We analysis the coupling behavior of the mode field distribute during the UV-side exposure and propose an effective method to fabricate H-FBG, which can generate higher order OVs.2. An effective method to fabricate ODGs by using UV-side-exposure method is proposed. Based on the coupling theory, OVs with topological charge equal to 1 or-1 can be generated. And even the distance and the written direction of the gratings is not accurate or orthogonal, OVs can also be generated as well.3. The property of MSCs are analyzed in detail and we design a new MSC which is composed of three-core fiber. Using this MSC, OVs can be generated easily. The conversions of OVs with different topological charges:0→±1 and 0→±3 are simulated in detail. And we can generated higher order topological charge if the number of modes in the fiber is increased. And the polarization of the OVs which generated in the MSC, can be controlled as well.