An acousto-optic modulator is made to vary the intensity of the incoming laser beam (Amplitude Modulation, AM). The modulation can be digital (ON/OFF) or analog (sine, square, linear, random…).
The key parameter of a modulator is its rise/fall time, which defines the achievable « speed » of the modulation, or amplitude modulation bandwidth. The rise/fall time is proportional to the beam diameter inside the modulator. As a consequence, the laser beam must be focused inside the modulator to reach a fast rise time.