An acousto-optical tunable filter is disclosed. The filter includes a medium having one or more indices of refractions alterable by the application of acoustic waves to the medium. The filter also includes an acoustic transducer coupled to the medium to generate the acoustic waves Altering the frequency of the acoustic wave selects among the different wavelengths in the incident light ray. The device also includes an optical mirror for reflecting a wave diffracted by the medium back to the medium so that it can again be diffracted. The first diffracted wave has its frequency shifted in one direction by the frequency of the acoustic wave while the second diffracted wave has its frequency shifted in the opposite direction, thereby compensating for the first shift. The result is an output ray with little or no frequency shift.