  We show that a useful connection exists between spontaneous parametric
downconversion (SPDC) and sum frequency generation in nonlinear optical
waveguides with arbitrary scattering loss, while the same does not hold true
for SPDC and difference frequency generation. This result deepens the
relationship between quantum and classical second-order nonlinear optical
processes in waveguides, and identifies the most accurate characterization of
their quantum performance in the presence of loss based solely on classical
measurements.
