Multimode emission in inhomogeneously broadened ring lasers

The threshold for multilongitudinal-mode emission in inhomogeneously broade ned ring lasers is analytically investigated. In the homogeneous limit the multimode instability corresponds to the classical Risken-Nummedal-Graham-H aken instability. It is found that by increasing the inhomogeneous linewidt h, the instability threshold is decreased and the growth of high-frequency side modes is favored. In the limit where the population-inversion decay ra te gamma (parallel to) is much smaller than the polarization decay rate gam ma (perpendicular to) (class B lasers), analytical expressions for the inst ability threshold are found, which are then generalized to three-level lase rs for a comparison with experimental results obtained with erbium-doped fi ber lasers. It is shown that even in class B lasers a full Maxwell-Bloch de scription (in opposition to a rate-equations approach) is necessary when th e free spectral range of the cavity is less than similar to(gamma (parallel to)/gamma (perpendicular to))(1/4)gamma (perpendicular to). (C) 2001 Optic al Society of America.