Transverse modes in large-mode-area fibers

This thesis is devoted to the study of transverse modes in large-mode-area (LMA) fibers. It highlights the importance of transverse spatial-hole burning (TSHB) effect in LMA multimode fibers through experiments and simulations. The measured beam-quality factor decreases until the gain becomes saturated in an ASE source based on an ytterbium-doped, large-mode-area, multimode fiber. At saturation, the beam-quality factor reaches a minimum, beyond which it increases again. Numerical simulation trends based on a model using spatially resolved gain and transverse-mode decomposition of the optical field agree with the experimental results. A simplified model without TSHB is shown not fit to predict the observed behavior of beam quality in LMA fibers, especially at high powers. A comparison of both models shows that TSHB is also critical for properly modeling beam quality in LMA fiber amplifiers.;New precise modal decomposition methods for field distribution with phase information and intensity distribution without phase information respectively, are presented and extended to single-mode fiber characterization. In these new methods, different mode sets are calculated using varied sets of fiber parameters, and the modal power weights of each set are calculated using the experimentally extracted field or intensity distributions of the beam. The remaining residue is then minimized amongst the mode sets to extract the corresponding modal power weighting, phase differences, and even experimental fiber parameters. Experiments are carried out for both single-mode fiber characterization and modal decomposition in few-mode fibers. The experimental results of the parameters obtained by the modal decomposition method agree well with the nominal or measured values.;The thesis also investigates helical-core fibers. An improved semi-analytic bend-loss model is derived that allows for the propagation of radiated fields outside the plane of the fiber bend. This allows for the modeling of small-bend radii for which the waveguide condition for total internal reflection is violated in a large angular spread of incident angles at the interface of the fiber core. This improved model is applied to large-mode-area helical-core fibers (which require small-bend radii) for use as high-power fiber lasers and amplifiers, which enable bending loss to be utilized for mode selection without the deleterious effects of coiling straight-core fibers. A comparison of the fundamental-mode propagation loss between theory and experiments shows that this improved model is well matched using parameters of the fabricated helical-core fiber, but that more accurate measurements are needed for all parameters.