The impact of fiber core design and thermally-induced phase shifts on the threshold of mode instabilities

The output average power of fiber laser systems is currently limited by the effect of transverse mode instability (TMI) [1]. This effect is characterized by the sudden onset of beam fluctuations once that a certain average power threshold is reached [2], rendering the beam unfitting for most applications. Therefore, there is an urge in the fiber laser community to find suitable mitigation strategies for TMI. In order to do this, it is necessary to understand the underlying physics of this effect. In this context, the most important aspects of TMI are two: first TMI is the manifestation of energy coupling between different transverse modes in a fiber caused by a thermally-induced refractive index grating (RIG); second, for this energy transfer to take place, there must be a phase shift between the RIG and the modal interference pattern (MIP) in the fiber. Most of the mitigation strategies for TMI proposed up to now have focused on reducing the strength of the RIG (or of the MIP). However, a promising new path is to develop mitigation strategies that try to minimize the phase shift between the MIP and the RIG.