Transient modal instabilities in high power fibre lasers

Transverse modal instability (TMl) [1] is proving to be a major power-scaling limitation in high-power fibre lasers (HPFLs) [2]. The origin of TMI is predominantly thermal and a number of theoretical models have been introduced to explore its characteristics [3-9]. The majority of TMI reports so far deal with fibre amplifiers in MOPA configurations [3-14], and only a small numbers of recent reports refer to TMI in single-cavity HPFLs [15-16]. In addition, most of the experimental TMI work is carried out under steady-state pumping conditions and the transient TMI response is largely overlooked [17]. TMI threshold reduces in the presence of backreflections in amplifiers [18], and, therefore, TMI effects are expected to be more pronounced in single-cavity HPFL oscillators.