Transverse Mode Instability Analysis in Fibre Amplifiers

Mode instabilities in fiber amplifiers are analyzed by a new approach, considering the stability of the steady state FM amplification in the presence of transverse amplitude and/or phase perturbations, taking into account the effects of population inversion and thermal loading due to quantum-defect heating. Population inversion contribution to instability is shown to be dominant at low powers and high inversion. Under high powers and low inversion (high amplifier saturation) the thermal effects dominate the instability behavior. A simple and easy to interpret TMI power threshold formula is derived for the first time.