An acousto-optic modulator is a device that can be used to control the power, frequency, or spatial direction of a laser beam using an electronic drive signal. It exploits the acousto-optic effect, whereby the refractive index is changed by the mechanical oscillation of pressure with an acoustic wave.

The key element of an AOM is a transparent crystal (or glass) in which the light propagates. A piezoelectric transducer in contact with the crystal is used to excite acoustic waves, which have a frequency of the order of 100 MHz. The light propagating in the periodic refractive index grating is diffracted by Bragg to produce acoustic waves, and AOMs are sometimes called Bragg cells.

The frequency of the scattered light increases or decreases by an amount equal to the acoustic frequency (which is related to the direction of propagation of the acoustic wave relative to the beam), and the direction of the scattered light changes slightly. (The change in direction is small, as shown in the figure, because the wave number of the acoustic wave is very small compared to the optical wave.) The frequency and direction of the scattered light can be controlled by controlling the frequency of the acoustic wave, but the acoustic power is limited by the optical power. When the acoustic power is high enough, more than 50% of the optical power is diffracted, and in extreme cases more than 95% of the light is diffracted.

The acoustic wave may be absorbed at the other end of the crystal. This traveling wave structure allows a wide modulation bandwidth to be achieved. Other devices are resonant with the sound wave, taking advantage of the strong reflection of the sound wave at the other end of the crystal. The resonance effect can significantly increase the modulation depth (or reduce the required sound wave power), but it will reduce the modulation bandwidth.

Common materials for AO modulators are tellurium dioxide (TeO2), quartz crystal and fused silica. There are many criteria for material selection, including electro-optic coefficient, transparency range, optical damage threshold and required size. Different sound waves can also be used, the most common of which is longitudinal wave (compression). This gives the highest diffraction efficiency, which is also related to the polarization of the beam. When using acoustic shear waves (acoustic vibration direction is the same as the laser beam), it is independent of the polarization direction, but this will reduce the diffraction efficiency.

There are also integrated optical devices that contain multiple AO modulators on a chip. Optical devices can be integrated on lithium niobate (LiNbO3), which is piezoelectric, so metal electrodes on the surface of the chip can generate surface acoustic waves. Such devices have many uses, for example, as tunable optical filters or optical switches.

Acousto-optic modulators have many applications:

Acousto-optic modulators (AOMs) can be used as optical shutters (cycle light on and off at a set frequency) or variable attenuators (dynamically control the intensity of transmitted light). Under the action of Bragg diffraction, only the first-order diffracted light appears in this product. According to the structure type, Xinte Optoelectronics' AOMs can be divided into free-space AOMs and fiber-coupled AOMs.

AOMs can also be used as tilted cavities in solid-state lasers to generate nanosecond or ultrashort pulses. In the latter case, the speed of the AOM can only meet the requirements when the resonant cavity is relatively long, or an electro-optic modulator is required. Active mode locking can be achieved by adjusting the resonant wave loss of the round-trip light in the resonant cavity using an AOM. AOMs can be used as pulse pickers to reduce the pulse repetition rate of a pulse train in order to obtain high pulse energy for subsequent amplification of the pulses. In laser printers and other devices, AOMs can be used to modulate the power of laser beams. The modulation can be continuous or digital (on/off). AOMs can shift the frequency of a laser beam, for example for use in various measurement devices or in lasers that use mode locking via frequency-shifted optical feedback. In some cases it is desirable to exploit the effect that the diffraction angle is related to the acoustic frequency. In particular, the direction of the outgoing beam can be scanned (at least over a small range) to vary the modulation frequency.

An important factor in choosing a modulator is the required speed. This affects the choice of materials, the modulator design and the RF driver to be used. The speed of a modulator is described by the rise time, which determines how fast the modulator can respond to the applied RF driver and limits the modulation rate. The rise time is proportional to the time it takes for the acoustic wave to travel through the beam and is therefore affected by the beam diameter within the modulator.