Theoretical Design Research On M-profile Fiber And Its Fiber Laser

Nowadays, compared with the traditional solid-state laser and gas laser, fiber laser owns the best beam quality. And high power fiber lasers will show very important application prospect in industry, aerospace and military application fields, however, high power output of this kind of laser is severely limited for the too small core diameter of the gain fiber, thus various Large Mode Area Fibers (LMAF) emerge at the right moment, and the mode field diameter of fiber is substantially increased, the power density is lowered, consequently, lots of nonlinear effects (such as stimulated Raman scattering and stimulated Brillouin scattering) are eliminated and the limitation of higher output power for conventional single-mode fiber laser is broken through.M-profile fiber (MPF) investigated in this thesis is one kind of LMAF. Generally, the refractive-index distribution of cross-section for LMAF is very complicated, leading to high preparation cost. However, M-profile fiber is much simpler and not difficult to manufacture, it adds only one layer cladding based on the traditional double-cladding fiber, and the high refractive index part turns into the annular cladding close to the undoped fiber core, so laser resonates in an annular waveguide actually. By designing the width of the annular waveguide appropriately, single mode operation can be maintained. With the very large cross-section area, it is suitable for high pump power and laser power output for fiber laser using MPF.The main content of the thesis can be summarized as following four points:(1) The introduction of transverse structure and refractive-index distribution of MPF, and the advantage and application prospect about it.(2) Establish the mathematical theory model of M-profile fiber mode. And derivate MPF’s cut-off condition of different modes from maxwell’s equations.(3) Simulate the single-mode operation condition about the fiber size structure and refractive index distribution and the distribution of amplitude and phase for various modes in MPF using Rsoft simulation software.(4) Design a kind of resonant cavity to get a single fundamental mode or high-order mode output according to the MPF’s high-order model characteristics.