All AO devices apply a frequency shift to the diffracted output beams. Acousto-optic frequency shifters (AOFS) are compact devices designed specifically for this task. Depending on the selected Bragg angle, these devices will either up-shift or down-shift the laser light by the frequency of the applied RF signal. Two or more devices can be cascaded in order to achieve sum or difference frequency combinations. Compound absorber angles are incorporated to minimize acoustic reflections.