An Acousto-Optical Tunable Filter (AOTF) is a crystal through which a light beam travels. Perpendicular to the light beam an acoustic beam is injected in the crystal. Depending on the frequency of the acoustic wave, the incoming light beam will be more or less diffracted.
By selecting the right frequency of the acoustic wave, a specific desired wavelength of the optical wave will be diffracted to the exit of the crystal. All other wavelengths will be blocked in the crystal.