Coupled-mode theory of a fiber laser-amplifier

The behavior of a fiber laser-amplifier is considered, including the effects of mode coupling, radiation losses, and noise. In the linear regime, a competitive effect between mode coupling and losses is present, in the sense that mode coupling tends to equalize the gains of different modes, while radiation losses favor low-order modes. Thus, single-mode amplification and collective amplification are predominant, respectively, in the weak and strong coupling case. In the saturated regime a mode coupling effect is found that is independent of fiber imperfections and relies on the nonlinear nature of the process.