Ultrashort pulse propogation in saturable media: A simple physical model

We present a descriptive model of ultrashort pulse propagation in saturable amplifying and absorbing media, emphasizing a pictorial representation of the relevant parameters. The rate equation approximation is used and the earlier results of Frantz and Nodvik are both generalized and placed in a more intuitive form. A formal procedure is developed for analyzing multilevel systems that are more complicated than the basic two- and four-level models. Examples involving dye mixtures, linear, ring, and colliding pulse mode-locked lasers are presented, illustrating the ease with which the model can be applied.