This paper investigates how an approach alternative to the canonical quanti
zation schemes may be used to describe Quantum Optics phenomena. By utilizi
ng the approach pioneered by Keldysh, we derive equations for the time depe
ndent correlation functions of the quantized optical fields. These contain
the coupling to matter in linear and nonlinar response functions which repl
ace the material parameters of phenomenological macroscopic theories. We pr
esent these results as alternatives to existing theoretical methods in Quan
tum Optics. The paper presents die general formulation of the theory, deriv
es the equations in some specific cases relating to non-linear optics, and
solves some illustrative special cases. We regain known results but also so
me additional terms deriving from the quantum fluctuations of the material
media.