Scanning Light with Sound
An acousto-optic deflector steers a laser beam by electronically controlling the frequency of sound waves in a crystal. Since the Bragg diffraction angle is proportional to the acoustic frequency, sweeping the RF drive across a bandwidth Δf sweeps the beam across an angular range Δθ = λΔf/v_s. Unlike galvanometric mirrors which are limited by mechanical inertia, AOD scanning speed is limited only by the speed of sound — enabling microsecond-scale random access to any position.
The Time-Bandwidth Product
The key performance metric of any AOD is the number of resolvable spots: N = Δf × τ_a. This time-bandwidth product represents a fundamental trade-off — more spots require either wider bandwidth (better transducer and crystal) or longer aperture time (larger beam, slower switching). The angular spacing between adjacent spots equals the beam divergence, so each spot is just resolved from its neighbors, analogous to the Rayleigh criterion in imaging.
Crystal Selection and Design
Tellurium dioxide in the slow-shear acoustic mode provides an extraordinary combination of high figure of merit and low acoustic velocity (617 m/s), yielding large scan angles per MHz of bandwidth. For a 10 μs aperture, the acoustic aperture is only 6 mm, yet 1000+ spots are achievable. Faster materials like lithium niobate (v_s ≈ 6570 m/s) require proportionally larger apertures for the same spot count but offer advantages at high acoustic frequencies where TeO₂ attenuation becomes prohibitive.
Two-Dimensional and Cascaded Systems
For 2D scanning — essential in laser displays, confocal microscopy, and optical tweezers — two AODs are cascaded with orthogonal acoustic propagation directions. The total addressable positions scale as N_x × N_y. Advanced configurations use chirped acoustic waves to implement cylindrical lens effects, enabling simultaneous scanning and focusing. Multi-frequency drive signals can even address multiple spots simultaneously, a capability impossible with mechanical scanners.