  Propagation losses in micro-ring resonator waveguides can be determined from
the shape of individual resonances in their transmission spectrum. The losses
are typically extracted by fitting these resonances to an idealized model that
is derived using scattering theory. Reflections caused by waveguide boundaries
or stitching errors, however, cause the resonances to become asymmetric,
resulting in poor fits and unreliable propagation loss coefficients. We derive
a model that takes reflections into account and, by performing full-wave
simulations, we show that this model accurately describes the asymmetric
resonances that result from purely linear effects, yielding accurate
propagation loss coefficients.
