Recently, mode instability was observed in optical fiber lasers at high powers, severely limiting power scaling for single-mode outputs. Some progress has been made towards understanding the underlying physics. A thorough understanding of the effect is critical for continued progress of this very important technology area. Mode instability in optical fibers is, in fact, a manifestation of stimulated thermal Rayleigh scattering. In this work, a quasi-closed-form solution for the nonlinear coupling coefficient is found for stimulated thermal Rayleigh scattering in optical fibers. The results help to significantly improve understanding of mode instability.

Average powers from fiber lasers have reached the point that a quantitative understanding of thermal lensing and its impact on transverse mode instability is becoming critical. Although thermal lensing is well known qualitatively, there is a general lack of a simple method for quantitative analysis. In this work, we first conduct a study of thermal lensing in optical fibers based on a perturbation technique. The perturbation technique becomes increasingly inaccurate as thermal lensing gets stronger. It, however, provides a basis for determining a normalization factor to use in a more accurate numerical study. A simple thermal lensing threshold condition is developed. The impact of thermal lensing on transverse mode instability is also studied.

An empirical TMI threshold formula is derived based on a recently developed model and used to analyze the power-scaling performance of ytterbium-doped silica glass and YAG (Y₃Al₅O₁₂) and lutetia (Lu₂O₃) single-crystalline fiber amplifiers. Overall, the single-crystalline fiber lasers are found to scale potentially to higher average powers due to their higher thermal conductivities compared to silica glass. This work serves as a useful extension to earlier works and shines significant new light on optimal fiber and amplifier designs for maximum average output power with TMI considered.

Transverse mode instability is a key limit to power scaling of high-power fiber lasers. Accurate modeling efforts have, however, been hampered by a lack of experimental data to verify a model. Recently, there have been some good experimental studies, making it possible to validate a model. In this work, we developed a model by integrating a 3D fiber amplifier and stimulated thermal Rayleigh scattering. Since we are only interested in the regime where the fundamental mode dominates, our 3D amplifier divides the core into many cylindrical shells. This limits the model to situations where bend-induced mode distortion of the fundamental mode is negligible, but it is still applicable for most practical scenarios. The benefit of this model is high computational efficiency; it can run in minutes on a PC. This 3D amplifier model considers various pumping configurations and amplified spontaneous emission. It can simulate most experimental conditions. Excellent quantitative fit to experimental data was achieved. Additional studies were also conducted to show that gain saturation is a dominating effect in understanding the observed behaviors of transverse mode instability.

We present the first observation by transmission electron microscopy of glass densification induced by Bragg grating inscription in two germanosilicate single-mode optical fibers.