Abstract: The self-deflection of a bright solitary beam can be controlled by a dark solitary beam via a parametric coupling effect between the bright and dark solitary beams in a separate bright－dark spatial soliton pair supported by an unbiased series photorefractive crystal circuit. The spatial shift of the bright solitary beam centre as a function of the input intensity of the dark solitary beam ($\hat{\rho}$) is investigated by taking into account the higher-order space charge field in the dynamics of the bright solitary beam via both numerical and perturbation methods under steady-state conditions. The deflection amount ($\Delta s_0$), defined as the value of the spatial shift at the output surface of the crystal, is a monotonic and nonlinear function of $\hat{\rho}$. When $\hat{\rho}$ is weak or strong enough, $\Delta s_0$ is, in fact, unchanged with $\hat{\rho}$, whereas $\Delta s_0$ increases or decreases monotonically with $\hat{\rho}$ in a middle range of $\hat{\rho}$. The corresponding variation range ($\delta s$) depends strongly on the value of the input intensity of the bright solitary beam (r). There are some peak and valley values in the curve of $\delta s$ versus r under some conditions. When $\hat{\rho}$ increases, the bright solitary beam can scan toward both the direction same as and opposite to the crystal's c-axis. Whether the direction is the same as or opposite to the c-axis depends on the parameter values and configuration of the crystal circuit, as well as the value of  $r$. Some potential applications are discussed.

Key words: spatial optical solitons, photorefractive effects, photovoltaic effects, self-deflection