Unraveling complex ultrafast behavior in high-power fiber oscillators

High-power continuous-wave (CW) fiber lasers are pivotal in manufacturing, medicine, and research because of their structural simplicity, high brightness, and operational flexibility. Their output performance, shaped by intricate interactions among gain, loss, dispersion, nonlinearity, and cavity dynamics, reflects complex evolution and diverse physical phenomena. However, systematic studies on the complex dynamical evolution of optical fields within fiber oscillators remain limited. To bridge this gap, this work presents a comprehensive analysis of the output characteristics of an ytterbium-doped CW fiber oscillator through integrated simulations and experiments: simulations based on the generalized nonlinear Schrödinger equation (GNLSE), and experiments employing spectral slicing and high-speed temporal measurements. The results reveal that the temporal and statistical characteristics of the fiber oscillator evolve with output power, exhibiting not only power dependence but also spectral-position dependence. In particular, instability in the longitudinal mode spacing was observed at the far wings of the spectrum. This study provides valuable insights into the nonlinear dynamics of continuous-wave fiber lasers and offers guidelines for optimizing oscillator performance in practical applications.