Theoretical And Experimental Research On Large Mode Area Gain Guided And Index Antiguided Fiber Laser

Single mode lasing on fiber laser with extremely large mode area is demonstratedin this dissertation using a specifically designed optical fiber, that is named gain guidedand index antiguided (GG IAG) fiber with a large active core to meet the forwardrequirement on the high output power. After introducing the mechanism of guidingmode on this weakly guided GG IAG fiber, its optical properties and working conditionon single mode operation are analyzed to evaluate the performance on amplificaiton ofGG IAG fiber amplifers and the transverse modes competition of GG IAG fiber lasers.With highly performed commercial devices, a practical diode laser side pumping GGIAG fiber laser system is designed to testify the physics of single mode operation withlarge mode area on the GG IAG fiber. Details on the following five parts:1. To analyze the characteristics of single mode operation and mode dispertion,by the method of complex-domain expansion of the refractive index, the Helmholtzequation and the mode dispersion equation are solved on the models of the GG IAGfiber with circular and elliptical cross section. The effective mode areas are calculated topresent large mode area.2. On the four cases of GG IAG fiber amplifiers where the pumping schemes areleft end pumping, right end pumping, two ends pumping and side pumping, the signalpower of the lowest two modes are numerically solved from the differential powertransferring equations and the steady ion rate equations, by the beam propagationmethod and Runge-Kutta method. The performance of the fiber amplifier onamplification is evaluated, and the relationship between the signal gain and fiberparameters is obtained to optimize the performance of single mode operated GG IAGfiber amplifier.3. On the theoretical model of side pumping GG IAG fiber laser, the physicalprocess of modes oscillation is numerically simulated after solving the differentialpower transferring equation and steady rate equation with the oscillator boundarycondition, by the iterated beam propagation method and Runge-Kutta method. Aphenomenon of transverse modes competition is observed between transverse modes because of their different lossless propopagation threshold, so that the lowest LP01mode can be successfully oscillated alone without too much strict condition on theoutput coupler or oscillator cavity. To achieve high efficient large mode area fiber laser,the output power and slope efficiency are the two major factors to be analyzed on thevariable fiber laser parameters such as fiber length, core diameter, numerical apture,cavity length, reflectivity of the output coupler, and so on. All of these parameters areoptimized to establish the theoretical research on the model of GG IAG fiber laser.4. On the issue of inevitable physical bending effect, the Helmholtz equation onbending step-index fiber is mathematically equivalent to an equation with a gradientrefractive index distribution on the cross section by the method of conformaltransformation. The mode deformation and bending loss of this large mode area GGIAG fiber are numerically calculated from the equivalent Helmholtz equation.5. With highly performed commercial devices, a practical diode laser sidepumping GG IAG fiber laser system is designed to be power scalable according to theoptimized prameters before. Meanwhile, in order to lower the thermal effect, thethermal dynamics of the fiber laser is simulated to design a high efficient coolingsystem before running the fiber laser. The physics of single mode operarion with largemode area on GG IAG fiber laser is testified by the good beam quality of M~2<2for allcases with core diameters of100μm,200μm,300μm, so that the effective mode areaachives35×10~3μm~2. The output power and slope efficiency achieve32.35W and26.4%,respectively.