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 amplified spontaneous emission (ASE) source based on an Yb-doped LMA 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-