The physical principle of acousto-optical modulator

The physical principle of acousto-optical modulator
Acoustic Optical Modulator (AOM Modulator) is currently the most widely used passive modulation device outside the laser cavity. It’s core working logic is: after an external low-frequency RF electrical signal is input to a piezoelectric transducer, the transducer converts alternating electrical energy into high-frequency ultrasonic waves; Ultrasonic waves propagate periodically inside the acousto-optic crystal, forming a dynamic refractive index grating with alternating density. When the incident laser passes through the grating, Bragg diffraction occurs. The device modulates the beam intensity by switching between diffracted light and through light, ultimately cutting the continuous incident laser into a sequence of pulse lasers with controllable timing.

The physical basis of acousto-optic modulation includes two core physical laws: piezoelectric effect and Bragg acousto-optic diffraction.
1. Piezoelectric conversion of piezoelectric transducers: The transducer is made of piezoelectric ceramic chips and is powered by an external RF driver to output alternating RF electrical signals; Under the action of an alternating electric field, piezoelectric materials undergo periodic mechanical expansion and contraction, converting electrical oscillations 1:1 into ultrasonic vibrations of the same frequency. Ultrasonic waves propagate directionally along the interior of the crystal, forming periodic compressed and stretched dense layers in acousto-optic media such as tellurium dioxide, fused quartz, gallium phosphide, etc; The density variation of the medium can directly cause periodic fluctuations in the local refractive index, which is equivalent to a dynamic phase grating that moves synchronously with ultrasonic waves. The grating period is strictly equal to the wavelength of the ultrasonic waves.
2. Bragg diffraction optical law: When the incident laser enters the dynamic grating at a specific Bragg angle, the incident light that satisfies the Bragg interference condition will undergo directional diffraction, which is divided into first-order diffraction light and zero order through light; When the RF power supply is powered on and ultrasound is present, most of the light beams undergo diffraction deflection, and the optical path has no effective output; After RF power failure and disappearance of ultrasonic waves, the refractive index grating instantly dissipates, and the laser is directly output in the original direction. By relying on the on-off and amplitude changes of RF signals, diffraction efficiency can be precisely controlled, achieving continuous adjustment of output light intensity from fully off to fully on, and completing laser intensity modulation. According to the diffraction working mode, acousto-optic diffraction can be divided into Bragg diffraction and Raman Nas diffraction: almost all industrial grade acousto-optical modulator use Bragg configuration, with diffraction efficiency up to 85% or more, less stray light, and excellent extinction ratio; Raman Nas diffraction is mostly used in low-frequency, low-power scientific research experiments, and the diffracted beam has multi-level scattering, so it is rarely applied in industrial scenarios.