Experimental observation of the Risken-Nummedal-Graham-Haken multimode laser instability

The unstable regime, experimentally observed with different reflectivity coefficients in a ring Er³⁺doped fiber laser, drives the field to generation of periodic pulses, with a period equal to the round-trip time of the cavity, via a self-mode locking mechanism. We investigate this instability using the standard Maxwell-Bloch equations for a resonant two-level laser with a ring cavity under conditions of homogeneous broadening. The results of the linear stability analysis and the numerical simulations are strongly suggestive that the observed pulsations correspond to those of the classic scenario in the multimode laser instability predicted by Risken and Nummedal, Graham and Haken.