Power Scaling Limits of Diffraction-Limited Fibre Amplifiers Considering Transverse Mode Instability

High power diffraction-limited fibre lasers have found many applications. It is now well understood that transverse mode instability (TMI) is the major limit to further average power scaling. A study of power scaling limit was performed in 2008 for Yb silica fibre lasers considering a number physical, thermal, and optical limits [1]. TMI was not known at the time and was not included thereby significantly over-estimating powers. Recently, a simple and approximate TMI threshold formula was developed based on an instability analysis. It is almost entirely based on fibre parameters, ignoring amplifier parameters known to have significant impact on TMI. The formula was used in a recent study of power scaling limit [2].