Research On All-fiber Laser With Flattop Beam Output

Flattop beams with uniform intensity distribution are differ from conventional Gaussian beams.They play a significant role in many practical applications,such as,laser materials manufacture,laser boring,optical data processing,biomedicine,and so on,which eliminate the negative effects of local high temperature led by non-uniform intensity distribution.Common methods for generating flattop beams can be divide into two categories:the first group converts the Gaussian beams to the flattop beams,including aperture method,beam integrators,binary optical elements,long-focal-depth method,liquid crystal spatial light modulator,birefringent lens system and aspherical lens system.The second group generates the flattop beans by focusing the specific beams.These flattop beams gained from the second kind method can be considered as a superposition of a donut-shape hollow beam and a solid center beam at the focal plane.In this dissertation,we propose a novel compact flattop beam shaping method with the all-fiber laser using few-mode fiber Bragg grating(FM-FBG),which is based on the second kind method.In order to make it come true,two precise modes of fiber lasers are adopted,which are the cylindrical vector mode with a donut-shape beam distribution and the fundamental mode gives rise to a solid central beam distribution.In experiment,two modes will oscillate in different resonant cavities and emit after the FM-FBG as a public output resonator mirror.After the FM-FBG,a flattop beam is generated as a superposition of two modes.What's more,two parameters are proposed for evaluating the quality of flattop beams.The major works in this dissertation:1.We propose a novel flattop beam forming system with the all-fiber laser.Flattop beams are generated by superposing the donut-shape hollow cylindrical vector mode and the solid central fundamental mode.Two modes are oscillating in disparate cavities and emitting from the public output resonator mirror.In this case,two modes already are incoherent in the laser cavity for nonfixed phase difference and larger wavelength difference.2.By using FM-FBG as the public resonant mirror and the mode-selective element,we demonstrate a novel flattop beam forming system with the all-fiber laser.In experiment,an offset splice spot(OSS)is adopted for generating the cylindrical vector beams,two polarization controllers(PCs)need to be adjusted for obtaining four cylindrical vector modes.A high quality flattop beam is generated for the normalized Root-Mean-Square(RMS)of the light intensity less than 5%and the edge steepness is 0.872.The main highlights are as following:1.We propose and demonstrate a novel compact flattop beam shaping method with the all-fiber laser.A high quality flattop beam obtained in experiment remains its transverse shape invariant along the propagation axis after passes through the output resonator mirror.2.We firstly propose two parameters for evaluating the quality of flattop beams,including flattop beams gained from other methods.