| Cylindrical vector (CV) beams, including radially, azimuthally and hybrid polarized beams, have attracted greatly attention in recent years. Characterized by its polarization symmetric, CV beams have many attractive applications, such as particle acceleration, optical trapping, laser machining, and microscope.Various methods have been used to generate CV beams, such as using an intracavity birefringent lens, producing a space-variant polarization converters or sub-wavelength grating. All-fiber designs are also proposed, which enable the generation in an easy and flexible way. The degenerate LPn mode includes radially (TM01), azimuthally (TE01) and hybrid (HE21e/o) polarized beams. By exciting LP11mode in a few-mode fiber, both TM01and TE01modes become switchable using polarization controllers (PCs).In this work, we using different kinds of few-mode fiber gratings to generated CV beams in few-mode fiber laser, with the aim to maximize the generation of LP11mode while minimized the effect of fundamental mode. By proper design the grating structure, we get a wavelength tunable output of radially and azimuthally polarized beam with high polarization purity.This work is supported by Chinese Universities Scientific Fund.The main research works as follows:1. Generating CV beams in an Ytterbium-doped few-mode fiber laser by using a uniform few-mode fiber Bragg grating (FM-FBG). Since the two propagation modes have different intensity distribution over the cross-section of the Ytterbium-doped fiber (YDF), we analyze the modes competition based on three-dimensional rate equations. With the aim to minimize the generation of fundamental mode, we analyze the behaviors of propagation mode competition under different conditions, such as resonant wavelength, pump power, YDF length or doping concentration. We find a pure CV beam can be generated by producing a doughnut doping YDF or using an wavelength filter.2. Contribution of the transverse asymmetric refractive index change profile on the coupling efficiency of fiber grating. It found that transverse asymmetric index profile could be induced during the writing of fiber Bragg gratings with UV side-exposure techniques. We find the coupling efficiency of LP01â†'LP11can reach100%using this kind of grating, and a pure TE01mode output can be generated on a transverse asymmetric long-period fiber grating (LPFG). By design an M-fiber grating, we get a wavelength tunable output of radially and azimuthally polarized beam.3. Generating vortex beams by design a helical-few-mode fiber grating. Vortex beams can be generated by a helical-grating in a few-mode fiber due to the superposition of degenerated HE21e/o mode, with a Ï€/2relative phase shift. Since UV side-exposure can induced transverse asymmetric, the helical Bragg grating can be fabricated by rotation of the asymmetric index change along the fiber axis. By writing a helical fiber Bragg grating directly on an M-fiber, a high purity of the desired vortex beams can be generated on a few-mode fiber laser.The innovative work and results in this thesis are as follows:1. Based on three-dimensional rate equations, numerical analysis the modes competition in an YDF. A pure CV beam can be generated by producing a doughnut doping YDF or using an wavelength filter, while the generation of LP01mode is completely depressed, the slope efficiency can reach60%.2. Analysis the Contribution of the transverse asymmetric refractive index change profile on the coupling efficiency of fiber grating. A pure TE01mode output can be generated on a transverse asymmetric long-period fiber grating, and a wavelength tunable output of radially and azimuthally polarized beam can be generated by writing transverse asymmetric grating structure on a M-fiber.3. Design a method to fabricate helical Bragg gratings using a phase mask. Analysis the coupling theory on a helical fiber Bragg grating. A high purity of the desired vortex beams can be generated on a few-mode fiber laser by writing a helical fiber Bragg grating directly on an M-fiber. |
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