  We study the 1:1 resonance for perturbed Hamiltonian systems with small
dissipative and energy injection terms. These perturbations of the 1:1
resonance exhibit dissipation induced instabilities. This mechanism allow us to
show that a slightly pumping optical cavity is unstable when one takes into
account the dissipative effects. The Maxwell-Bloch equations are the asymptotic
normal form that describe this instability when energy is injected through
forcing at zero frequency. We display a simple mechanical system, close to the
1:1 resonance, which is a mechanical analog of the Laser.
