Mode Theories And Simulations For Large Mode Area Doubly Clad Optical Fibers

In order to reduce nonlinear effects in high power fibers and amplifiers, largemode area doubly clad optical fibers are widely used. It is important and meaningfulto conduct researches on modal characteristics of large mode area doubly cladoptical fibers, for the purpose of obtaining high power and high quality output laserbeam.In this thesis, significant discrepancies are found when analyze the same fiberwith diverse methods. Researches in this thesis mainly include:(1)Modal characteristics of doubly clad optical fibers are analyzed under theweakly guiding approximation, by using singly clad model、normal doubly cladmodel and doubly clad model with complete boundary conditions. Modalcharacteristics equations are respectively given based on the electromagnetic theoryof light for these three models. Characteristic parameters are obtained by solvingthese characteristics equations, then the transverse filed distributions, the transverseintensity distributions, the power distributions, the dispersion characteristics and thepower confinements are analyzed. Differences between the results of these modelsare described.(2)Modified characteristics equations are derived under the semi-weakly guidingapproximation using the doubly clad model with complete boundary conditions.Characteristic parameters are obtained by solving these characteristics equations.Modal characteristics discrepancies between these models are described. Effects ofthe thickness of the inner cladding on modal characteristics and on the number ofguide modes are specified. The influences of the refractive index differences between inner and outer claddings are also described.(3)Giving up the weakly guiding approximation, this thesis also derived thevectorial characteristics equations for singly clad fibers and for doubly clad fibersrespectively. By comparing the results of these diverse models, the prerequisites forusing these methods are specified respectively. The thickness of the inner claddingand the refractive index differences between inner and outer claddings, aredemonstrated to be important in modal characteristics analysis.