  A study of the effects of absorption on the nonlinear process of parametric
down conversion is presented. Absorption within the nonlinear medium is
accounted for by employing the framework of macroscopic QED and the Green
tensor quantization of the electromagnetic field. An effective interaction
Hamiltonian, which describes the nonlinear interaction of the electric field
and the linear noise polarization field, is used to derive the quantum state of
the light leaving a nonlinear crystal. The signal and idler modes of this
quantum state are found to be a superpositions of the electric and noise
polarization fields. Using this state, the expression for the coincident count
rates for both Type I and Type II conversion are found. The nonlinear
interaction with the noise polarization field were shown to cause an increase
in the rate on the order of 10^{-12} for absorption of 10% per cm. This
astonishingly small effect is found to be negligible compared to the decay
caused by linear absorption of the propagating modes. From the expressions for
the biphoton amplitude it can be seen the maximally entangled states can still
be produced even in the presence of strong absorption.
