Investigation Of The Method For Specific Gaussian Beam Generation Based On Gain And Loss Distribution In Cavities

Gaussian beams are the fundamental beams with most applications in laser optics;their applications have gone deep into all walks of life.With the progress of society and the development of technology,Gaussian beams have aroused the widespread attention in recent years due to their unique optical properties.Gaussian beams play a growing role in the fields of particle manipulation,microscopy imaging,material processing,and free-space communication;they have a certain scientific value and social significance and their prospect has gained positive outlook.One of the key and basic research of Gaussian beams is how to obtain the beams with high mode purity.LD end-pumped solid-state laser is a common laser for generating Gaussian beams with high beam quality and has gained continuous and in-depth research due to their advantages of high pumping efficiency,good beam quality,and stable output.Based on the classical theory of transverse mode selection,in this paper a series of studies on the generation of Gaussian beams are carried out in models of LD end-pumped solid-state laser.At first,this paper introduces the applications of Gaussian beams in various fields and analyzes the methods for Gaussian beam generation,and then based on the correlation research both at home and abroad,a method for generating Gaussian beams with high beam quality is proposed,whose theoretical basis is the classical theory of transverse mode selection.After that,from the coupling effect of inhomogeneous medium the Gaussian beam generation is studied,and the oscillation modes in stable cavities is calculated by a matrix algorithm.Finally,a method for generating coherent multimode is proposed,which is based on the coupling effect of inhomogeneous medium and the Fresnel-Kirchhoff diffraction integral.The research contents of this paper are shown as follows:(1)A universally applicable model of LD end-pumped solid-state laser is proposed based on the angular spectrum diffraction.In this model,optical devices in cavities are seen as thin lenses with specific complex amplitude transmittance,and the transmittance distributions of resonator mirrors and gain medium are given.The laser field is expressed by plane wave superposition and the propagation in cavities is calculated by angular spectrum diffraction;we call this simulation method as angular spectrum calculation,which takes the amplitude and phase change into account,thus can reproduce the propagation process of lasers in cavities well.(2)A method for generating IG beams in LD end-pumped solid-state lasers is presented based on the transverse mode selection and the intensity distributions of IG beams.IG beams have several nodal lines with zero intensity,and these nodal lines are elliptical or hyperbolic shape,which divide an IG beam into many small spots.An aperture with several lines on it is placed into a cavity;the line width is certain.The line position and shape match with the nodal lines distributions of target IG beams.Passing through the aperture,the target IG beam is almost unaffected and other modes will suffer huge loss,thus the single mode oscillation of target mode is formed.This method is called “loss control”.Many IG beams with over 90% mode purity are obtained in the model of LD end-pumped solid-state laser.(3)The influence of the coupling effect of inhomogeneous medium is studied,and a matrix algorithm for calculating the oscillation modes in cavities is proposed.The complex amplitude transmittance of “loss control” aperture is inhomogeneous,which is an inhomogeneous medium.The coupling effect of inhomogeneous medium is introduced and based on it,oscillation modes in cavities are calculated by matrix algorithm.A model of LD end-pumped solid-state laser is presented,and the formation of IG beams under “loss control” is calculated.Many IG beams with over 90% mode purity are obtained.(4)From the coupling effect of inhomogeneous medium,the formations of laser beams are explained.Under the coupling effect of gain medium and “loss control” aperture,the energies and phases of ideal transverse mode components exchange with each other,and after many times coupling their energies and phases achieve dynamic balance,and ultimately superimpose forming a specific laser beam.This interpretation deepens our understanding of the formation of laser beams and benefit generating specific Gaussian beams with high quality.(5)On the basis of the coupling effect of inhomogeneous medium and the Fresnel--Kirchhoff diffraction integral,a method for generating coherent multimode is proposed.IG beams can be expressed as the linear superposition of ideal HG beams.Under the coupling effect of inhomogeneous medium,the amplitude ratio and phase relationship of these ideal HG components remain unchanged;the ideal HG components are coherent.If the coherence between several ideal modes can be realized,the coherent superposition of these modes possibly can be generated in cavities.We call the coherent superposition of laser modes as coherent multimode.The principle for coherent multimode generation in cavities is introduced in detail.A model of LD end-pumped solid-state laser with reflective pure-phase SLM as reflector is built,and many coherent multimodes are obtained using simulation in the model.