  Light propagation in a nonlinear optical medium is nonreciprocal for
spatially asymmetric linear permittivity. We here examine physical mechanism
and properties of such nonreciprocity (NR). For this, we calculate transmission
of light through two models of a nonlinear optical isolator consisting of (a) a
two-level atom and (b) a driven $\Lambda$-type three-level atom coupled
asymmetrically to light inside open waveguides. We find a higher NR in the
model (b) than in the model (a) due to a stronger optical nonlinearity in the
former. We determine the critical intensity of incident light for maximum NR
and a dependence of the corresponding NR on asymmetry in the coupling.
Surprisingly, we find that it is mainly coherent elastic scattering compared to
incoherent scattering of incident light which causes maximum NR near the
critical intensity. We also show a higher NR of an incident light in the
presence of an additional weak light at the opposite port.
