Transverse mode instability thresholds in fiber laser oscillators

A first theoretical study of Transverse Mode Instability (TMI) in oscillators based on a Stimulated Thermal Rayleigh Scattering (STRS) model is conducted. Higher Order Mode (HOM) lasing is found to happen at high powers. Further Fundamental Mode (FM) growth is limited once HOM lasing takes place, with further increase of pump power mostly going to HOM growth, a fundamentally different phenomenon from that in fiber amplifiers. TMI thresholds defined as when the HOM lasing condition is met is studied. The results are consistent with the measured TMI thresholds and their dependence on pumping configurations and pump wavelengths.