  In this work a generic model of \emph{multi-photon optics} is studied. It is
considered $m$ pump and $n$ subharmonic fields. It shows how the multi-photon
system, with direct and homodyne detection, can be obtained rigorously in the
context of \emph{Measurement theory in Open Quantum System} (see
\cite{Bar-OQS}, \cite{Bar-1990-QO} and \cite{RCS-LAP}). This is done trough the
\emph{Quantum Stochastic Differential Equations} with unbounded coefficients
(see \cite{Fagnola-Wills}, and \cite{RCS-LAP}). In particular the existence of
the dynamics is proved, and a continuous measurements scheme is provided.
