The diffraction of light by an acoustic wave generated by an oppositely phased transducer array is studied. Special attention is paid to the parasitic second-order diffraction which takes place in the usual deflector design at the frequency-band centre, and which results in a decrease in the first-order diffraction efficiency. On the basis of a simplified three-coupled-wave analysis, a modified deflector design is suggested which minimizes the diffraction efficiency dip in the deflector frequency-band centre.