On the observability of the Risken-Nummedal-Graham-Haken multimode instability in erbium-doped fibre lasers

Departament d'Òptica, Universitat de València, Dr. Moliner 50, E-46100 Burjassot, Spain

Received: 16 December 1997
Accepted: 16 June 1998

Abstract

Recently the first experimental observation of the Risken-Nummedal-Graham-Haken (RNGH) multimode instability has been reported (Pessina E. M. et al., Phys. Rev. A, 56 (1997) 4086). In the present paper we propose a closed three-level model for erbium-doped fibre laser self-mode locking instabilities which explains some deviations of the experimental observations with respect to the standard RNGH instability theory. In particular a very low instability threshold is predicted in agreement with the experimental observations. Our study also provides predictions on the necessary conditions for the observability of the multimode instability, as the existence of an upper limit for the cavity losses rate. Preliminary numerical integrations show intensity pulsations that closely resemble those observed in the experiments.