  In this paper we show that the dissipative version of the laser model
proposed by Alli and Sewell can be obtained by considering the stochastic limit
of the (open system) hamiltonian introduced by Hepp and Lieb in their seminal
work. We also prove that the Dicke-Haken-Lax hamiltonian produces, after the
stochastic limit is considered, the generator of a semigroup with equations of
motion very similar to those of Alli-Sewell, and coinciding with these under
suitable conditions.
