Theoretical Study On High Power Fiber Lasers

The high-power Yb3+-doped double-clad fiber lasers (HPYDDCFLs) develop very fast in recent years for its advantages such as excellent beam quality, high efficiency, compact structure and good heat dissipation. The fiber lasers have been widely applied in the fields of optical communication, laser processing, medical treatment and military etc. In this dissertation, HPYDDCFLs has been studied in numerically and theoretically. These works provide a theoretical basis to the optimum design of HPYDDCFLs.(1) Based on the steady state rate equations of HPYDDCFLs, an approximate analytic function of distributed laser along the whole fiber is obtained. The distribution of pump power and laser power, and the dependence of the performance of HPYDDCFLs on the parameters are discussed.(2) The principal of angle side-coupling between LD and the double-clad fiber is studied. The relationship between coupling efficiency and angle is derived. The numerical results show there is an angle to maximize the coupling efficiency.(3) A theoretical model of HPYDDCFLs based on different pump modes is presented. An approximate analytic function of distributed laser along the whole fiber is obtained. The influence of pump mode on the performance of HPYDDCFLs is discussed. The numerical results show that laser power increase uniformly by utilizing distributed pump and optimizing the arrangement of pump powers.(4) According to the heat conduction equation, the expression about the distribution of temperature in HPYDDCFLs is derived. The axial and radial temperature distributions are numerically simulated. It has been shown that the distributed pumping scheme is best to dissipate heat uniformly in fibers and reduces operating temperature without significant output power degradation. The heat dissipation issue in HPYDDCFLs can be effectively solved by optimizing the arrangement of pump powers and pump absorption coefficients.(5) The theoretical analyses of stimulated Raman scattering (SRS) and stimulated Brillouin scattering (SBS) in HPYDDCFLs are presented by solving the steady-state rate equations. Numerical results show that the SRS and SBS threshold power can be improved by shortening the cavity length, using large mode area fiber and lowering the Yb3+ concentration.(6) The outputs from an n-element HPYDDCFLs array have been combined into a single beam in the plane of the junction. This beam combining is achieved by use of a common external cavity containing a blazed grating, which simultaneously forces each array element to operate at a different, but controlled, wavelength and forces the beams from all the elements to overlap and propagate in the same direction. The grating diffraction efficiency is >99%.(7) Diffraction of the divergent beams with certain spectral width on VBG in inorganic photo-thermo-refractive glass is described. Design principles and technical approach for spectral combining of two HPYDDCFLs by VBG are developed. A mathematical model that reveals the critical parameters for high efficiency spectral beam combining by means of VBG has been presented. A general expression for the system efficiency can serve as a guideline when considering design issues of VBG.(8) The theory of the master-oscillator power amplifier (MOPA) is described. Numerical analysis of the transient-state of MOPA has been preformed.