All-optical modulation and switching have been demonstrated in various nonlinear directional couplers (NLDCs). These experiments were performed by varying the intensity of a single input light beam to change the relative transmission of the two channels. This single-beam method has some undesirable features: the total absolute output of the NLDC in the bar state is much higher than in the cross state, and the large detuning from the band edge needed for high transmission results in a decrease in nonlinearity. The authors propose and demonstrate a technique that uses the output of a single laser, allows a GaAs/AlGaAs NLDC to be operated at a wavelength where its transmission is reasonably high, and still maintains efficient switching. This technique is based on the anisotropic optical properties of quantum-well (QW) structures.