  We use a probabilistic method to describe the effect of laser noise on the
laser-atom interaction, in the case that the atom is a two level system without
spontaneous emission. The stochastic differential equation for the laser-atom
interaction is analyzed in the sense of perturbation approach, and we construct
a stochastic process corresponding to the time evolution of the atom's wave
function, whose extra randomness is induced by the laser noise. It also
provides the layout of a theory for the possible experiment of measuring the
laser line width by driving a narrow atomic transition.
