Direct evidence for laser reabsorption as initial cause for self-pulsing in three-level fibre lasers

The temporal characteristics of a standing wave Tm3+-doped silica fibre laser are presented. For uni-directional pumping, the output displays the well-known self-pulsation phenomenon, but with bidirectional pumping, a significant increase in the laser stability is observed. A longitudinally uniform population inversion is shown to be of paramount importance to the stability of three-level lasers.