Fig. 1.

Conditions for a unidirectional propagation block due to a sharp increase of the coupling strength $D$. (A) The stepwise spatial variation of the parameter $D$, with $D_0=2$. (B) A right-traveling wave is blocked because it cannot overcome a large step in $D$, where $D/D_0$ is greater than the critical value. (C) A left-traveling wave propagates over the two steps in $D$, both of which are less than the critical value. (D) The critical value of the coupling strength $D_c/D_0$ at the inhomogeneity boundary as a function of the excitation threshold $\beta$.