Acousto-optic diffraction requires momentum matching between light and sound (acoustic) waves in the crystal material. For the AO laser beam modulator, momentum matching is maximized when the input beam is aligned at the Bragg angle. AOTF applications are different. Unlike the modulator case, AOTF’s are frequently used with non-coherent, poorly collimated light and require large apertures with wide acceptance angles. To accommodate these demands, AOTF’s utilize non-linear properties of birefringent crystals. The crystal orientation and acoustic launch angle are designed such that diffraction occurs over a very narrow wavelength range for a very specific RF frequency. In fact it is so specific that multiple simultaneous frequencies can be applied to an AOTF without excessive intermodulation. This allows controlled wavelength mixing.
The acoustic launch angle and incident angle are related by design and together define the AOTF characteristics.