The Research About Mode Analysis And Beam Quality In Large Mode Area Fiber Laser

Compared with bulk lasers, fiber lasers have attracted significant attentionsbecause of their potential advantages including simple structure, high beam quality,good heat dissipation, high conversion efficiency and so on. Especially the high powerfiber lasers are strongly demanded owing to their wide applications in defense, medicaland industry fields. Therefore, the key issue is to achieve high power output as well ashigh beam quality. Employing large mode filed fiber is an effective way to increase theoutput laser power resulting from the increasing damage threshold and decreasing fibertemperature, but it will unavoidably lead to multimode operation and reduced beamquality. Therefore, it is necessary to study a method to suppress the high-order modestheoretically and experimentally. In this dissertation, our theoretical and experimentalresearches can be divided into the following three parts:（1） An analytical method of calculating elements, characteristic equations of theguided mode LPmnis presented by deducing Maxwell equations. Moreover, the numberof these modes, the intensity distribution related to different modes and effective modearea in fiber are simulated. Based on the structure of Yb³⁺-doped fiber laser and modecoupling theory, the multimode competition and coupling rate equations and evolutionequations of large-mode-field double-cladding Yb³⁺-doped fiber lasers are established.（2） Multilayered method was introduced to simplify the relative power evolutionequations indicating formation of multimode propagation theory. The distributions ofpower filling factors related to different modes were calculated. The method ofrestraining high-order modes by controlling gain and loss was studied separately. Theinfluence of fiber coiling and various dopant profiles in fiber core on mode competitionwas analyzed numerically. A new method of combining of coiling and dopantdistribution to suppress the high order modes was proposed for the first time. Theresults show that this method can suppress the high order modes more effectively withmaximum decreasing of mode field deformation resulting from the coiling.（3） Based on the constructed model, laser output as a function of coiling radius intwo Yb³⁺-doped fibers lasers with core/cladding diameter of10/125μm and25/250μm are calculated. The practical fiber lasers corresponding to the above two fibers andrelative measurement system for high order modes suppression were constructed.Experimental results show that the power ratio of fundamental mode increased withreduced coiling radius, and thus the beam quality was increased. The error betweentheoretical and experimental results was obtained and the relative reason was given.