| This thesis presents theoretical modeling of three fiber laser structures designed to emit high-power, high-brightness coherent radiation with diffraction-limited beam quality. The three structures are arrays of coherently phased rare-earth doped fiber lasers, Raman fiber laser arrays, and bent large mode area leakage channel fibers,;For discretely coupled rare-earth doped fiber laser arrays, a novel model is proposed for studying its beam combining mechanism, spectral and temporal dynamics, the role of nonlinearity, and the power scaling issues. The model accounts for the multiple longitudinal modes of individual fiber lasers and shows directly the formation of the composite-cavity modes. Detailed output power spectra and their evolution with increasing array size and pump power are also explored for the first time. In addition, it is the only model that closely resembles the real experimental conditions in which no deliberate control of the fiber lengths (mismatch) is required while highly efficient coherent beam combining is still attained. The important question of the scaling of combining efficiency and power fluctuations with array size is also investigated systematically and good agreement between experimental and theoretical results is obtained for up to 16-channel fiber laser arrays.;We also propose and analyze discretely coupled fiber laser arrays based on Raman gain as high-power coherent sources with good beam quality at wavelengths inaccessible by rare-earth doped fiber lasers. It is shown that under single-mode operation the nonlinear phases inherent in the stimulated Raman scattering process contribute to the phase locking mechanism in Raman fiber lasers arrays in the weak coupling regime. The more realistic condition of multimode lasing is also studied and it is found that the intrinsic nonlinear refractive index leads to substantial spectral broadening that disrupts beam combining and phase locking.;Finally, a different route to high power is investigated in large mode area fibers by accurately analyzing the modes of curved leakage channel fibers with proper coordinate transformation of Maxwell's equations and setup of perfect matched layers. The bend performance of this microstructured fiber is explored for various design parameters and its implication is also discussed. |
