Class A lasers with injected signal: bifurcation set and Lyapunov-potential function

We describe the bifurcation set for a class {\sl A} laser with an
external injected signal in terms of the amplitude and the
frequency of the applied field. We explain the dynamical behavior
of this kind of lasers in terms of a Lyapunov potential in the
case where such a description is possible. In particular, a full
description for the deterministic and non-deterministic dynamics
can be given by using the Lyapunov potential for some of the
external parameters. Depending also on the value of these
parameters, the phase of the electric field drifts with time in
the stochastic case.