Design Of Special Optical Fiber For 2?m Thulium-Doped Fiber Laser Beam Shaping

With the rapid development related technologies of high power thulium doped fiber lasers(TDFL)and the wide application in the field of laser radar,laser medicine,biology,remote sensing,multi core TDFL has attracted extensive attentions of many researchers.How to pump laser effectively into the doped multi-core fiber and rare earth ions able to obtain pump excitation to produce effective particle number reversal is an important way to improve the efficiency of the pump.In this paper,several kinds of elliptical core fiber and double cladding fiber which can be used for beam shaping are designed,and the properties of the two kinds of optical fibers are analyzed theoretically and simulated by finite element method.The main works of this paper are as follows:1.The beam propagation models of double-clad fiber and elliptical-core fiber are established and the influence of the parameters of double-clad fiber and elliptical-core fiber on the beam propagation characteristics is studied.2.A double-clad fiber structure with a large mode field area is designed,and the different sizes of hollow beams can be obtained by changing the structure parameters of the fiber.The finite element method was used to analyze the parameters such as effective mode area,limiting loss,nonlinearity,dark spot size and annular beam width of the designed fiber.After shaping the 793 nm pump light,the energy distribution is more uniform.After the injection of multi-core TDFL,so Tm3+ can effectively absorb the pump light,and improve the pump efficiency and output power.3.An elliptic core fiber with simple structure is designed,which can produce flattened single polarized beam,and the guidance mode of the elliptic core fiber is numerically simulated and analyzed by using the finite element method.By adjusting the elliptical core minor axis size,elliptic ratio and the refractive index of the elliptical core,the fiber structure with the optimal parameters was obtained.This type of fiber is used to shape the 2 ?m TDFL outgoing laser to have flat top single polarization characteristic.The combination of the flattened mode field and the polarization maintaining property can effectively improve nonlinear threshold and thermal damage threshold of fiber amplifier,Which is favorable for high power laser output of 2?m band TDFL.