High-power fiber lasers have emerged as a cornerstone in the realm of laser technology. Characterized by their exceptional efficiency, ruggedness, and versatility, fiber lasers are experiencing widespread use in manufacturing, medical, defense, science, and in long range sensing. Unfortunately, high-power applications require strict spatial and spectral performance characteristics to be maintained, which has yet to be perfected. This dissertation discusses the power scaling of ytterbium-doped fiber laser amplifiers, presenting three significant advancements. First, a novel photonic lantern-based method is introduced for real-time monitoring of laser beam modal content and beam quality. Initial tests highlight the photonic lantern's efficiency in predicting the onset of modal instability while simultaneously measuring the laser's output beam quality, M2. Second, this work achieved 2.2 kW single-mode narrow-linewidth laser delivery through a 5-tube nested antiresonant hollow core fiber, maintaining over 95% transmission efficiency and near diffraction-limited beam quality. Lastly, this research explores active-gain fiber designs to mitigate nonlinear effects for further power scaling. One design employing confined-doping strategies, achieving a 2.4x increase in the maximum output power before the onset of stimulated Brillouin scattering. Additionally, a second experiment employing a bend-insensitive fiber design demonstrated a transverse modal instability threshold nearly 3x that of its step-index counterpart. Collectively, this work presents a novel approach to power scale, deliver, and monitor multi-kW Yb-doped fiber laser amplifiers enabling the next-generation of applications requiring the strictest spatial and spectral performance.