CONNECTION BETWEEN APPROXIMATE FOKKER-PLANCK EQUATIONS AND THE WIGNERFUNCTION APPLIED TO THE MICROMASER

We present a derivation of a Fokker-Planck equation for the micromaser based on the Wigner function and valid in the semiclassical limit. Th e resulting equation is shown to correspond to the conventionally used approximate Fokker-Planck equations obtained for a continuous approxi mation to the discrete Fock representation of the density matrix of th e light field. We thus establish a connection between the continuous a pproximation and the Wigner function. Then expressing expectation valu es as properly symmetrized products, we can extend in an exact way the validity of the Fokker-Planck approach to the micromaser down to the zero pump off-state of few photons and purely thermal statistics.