An Acousto-Optic Frequency Shifter (AOFS) operates on the principle of light-sound interaction. Here's a detailed explanation:  

Acoustic Wave Generation:

• An RF signal is applied to a piezoelectric transducer bonded to a crystal medium (e.g., TeO2).  
• This generates a traveling acoustic wave within the crystal, creating periodic regions of compression and rarefaction.  

Light-Acoustic Interaction:

• An incident laser beam enters the crystal medium.
• As the light beam encounters the periodic density variations caused by the acoustic wave, it experiences a phase modulation.
• This phase modulation results in the diffraction of the light beam into multiple orders.

Frequency Shift:

Due to the Doppler effect, the diffracted light beams experience a frequency shift.

• The first-order diffracted beam experiences a frequency shift equal to the frequency of the acoustic wave.
• The direction of the frequency shift (up or down) depends on the relative directions of the light and acoustic waves.

Bragg Diffraction:

• For efficient frequency shifting, the light beam is typically incident at the Bragg angle.  
• The Bragg angle is the specific angle at which the light beam interacts most strongly with the acoustic wave, maximizing diffraction efficiency.

Output Beam:

• The desired frequency-shifted light beam is typically the first-order diffracted beam.  
• It can be separated from the undiffracted (zeroth-order) beam and other higher-order diffracted beams using appropriate optics.