A filtering device employed to modulate the wavelength and amplitude of illuminating laser light in the latest generation of confocal microscopes. The AOTF utilises sound waves to modulate the wavelength or intensity of light emitted by a laser or non-coherent illumination source (primarily arc-discharge lamps). The filter consists of a specialized crystal, such as tellurium oxide or quartz, sandwiched between an acoustic transducer and absorber to induce standing sound waves with alternating domains of high and low refractive index. As either polarised or non-polarised light passes through the filter, the crystal acts as a diffraction grating to deflect the incident beam. Specific wavelengths are selected by altering the frequency of the sound waves delivered to the crystal, thus changing the period of the diffraction grating.
These filters do not suffer from the mechanical constraints; speed limitations, image shift, and vibration associated with rotating filter wheels, and can easily accommodate several laser systems tuned to different output wavelengths. In addition, acousto-optic filters do not deteriorate when exposed to heat and intense light as do fluorescence interference filters.