Abstract: An acousto-optic deflector system including first and second transducers for producing acoustic waves for interaction with an optic beam in an acousto-optic device. The transducers are driven by signals of differing frequencies (f.sub.1 +.DELTA.f, f.sub.1) in order to produce two deflected optical beams from an incident optical beam launched from a laser. The two deflected beams are detected by a common photodetector whose output is filtered whereby to separate a component which is dependent on the frequency difference. The phase of this component is the relative optical phase of the two deflected optical beams. By determining the phase of the component for different frequencies (f.sub.1, F.sub.1 =f.sub.T etc.), the optical angle of incidence (.theta..sub.i) may be obtained. Alternatively, by frequency modulating both transducer drive signals and appropriately electronically processing the detector output, a signal whose amplitude varies with the optical angle of incidence can be obtained.