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Acousto-optic Modulators

Acronym: AOM

Definition: optical modulators based on the acousto-optic effect

German: akustooptische Modulatoren

Category: photonic devices

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An acousto-optic modulator (AOM) is a device which can be used for controlling the power, frequency or spatial direction of a laser beam with an electrical drive signal. It is based on the acousto-optic effect, i.e. the modification of the refractive index by the oscillating mechanical pressure of a sound wave.

The key element of an AOM is a transparent crystal (or piece of glass) through which the light propagates. A piezoelectric transducer attached to the crystal is used to excite a sound wave with a frequency of the order of 100 MHz. Light can then experience Bragg diffraction at the traveling periodic refractive index grating generated by the sound wave; therefore, AOMs are sometimes called Bragg cells. The optical frequency of the scattered beam is increased or decreased by the frequency of the sound wave (depending on the propagation direction of the acoustic wave relative to the beam) and propagates in a slightly different direction. (The change in direction is smaller than shown in Figure 1, because the wavenumber of the sound wave is very small compared with that of the light beam.) The frequency and direction of the scattered beam can be controlled via the frequency of the sound wave, whereas the acoustic power is the control for the optical powers. For sufficiently high acoustic power, more than 50% of the optical power can be diffracted – in extreme cases, even more than 95%.

acousto-optic modulator
Figure 1: Schematic setup of a non-resonant acousto-optic modulator. A transducer generates a sound wave, at which a light beam is partially diffracted. The diffraction angle is exaggerated.

The acoustic wave may be absorbed at the other end of the crystal. Such a traveling-wave geometry makes it possible to achieve a broad modulation bandwidth of many megahertz. Other devices are resonant for the sound wave, exploiting the strong reflection of the acoustic wave at the other end of the crystal. The resonant enhancement can greatly increase the modulation strength (or decrease the required acoustic power), but reduces the modulation bandwidth.

Common materials for acousto-optic devices are tellurium dioxide (TeO2), crystalline quartz, and fused silica. There are manifold criteria for the choice of the material, including the elasto-optic coefficients, the transparency range, the optical damage threshold, and required size. One may also use different kinds of acoustic waves. Most common is the use of longitudinal (compression) waves. These lead to the highest diffraction efficiencies, which however depend on the polarization of the optical beam. Polarization-independent operation can be obtained when using acoustic shear waves (with the acoustic movement in the direction of the laser beam), which however make the diffraction less efficient.

There are also integrated-optical devices containing one or more acousto-optic modulators on a chip. This is possible, e.g., with integrated optics on lithium niobate (LiNbO3), as this material is piezoelectric, so that a surface-acoustic wave can be generated via metallic electrodes on the chip surface. Such devices can be used in many ways, e.g. as tunable optical filters or optical switches.


Acousto-optic modulators find many applications:

Important Properties of Acousto-optic Modulators

Various aspects can be essential for the selection of an acousto-optic modulator for some application:

Due to various trade-offs, quite different materials and operation parameters are used in different applications. For example, the materials with highest diffraction efficiencies are not those with the highest optical damage threshold. A large mode area can increase the power handling capability, but requires the use of a larger crystal or glass piece and a higher drive power, and also increases the switching time, which is limited by the acoustic transit time. For fast acousto-optic beam scanners, a large mode area is required for achieving a high pixel resolution, whereas a smaller mode area is required for a high scanning speed.

See also: Q switches, optical modulators, pulse pickers, electro-optic modulators, Q switching, cavity dumping, active mode locking
and other articles in the category photonic devices

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