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%.
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:
- They are used for Q switching of solid-state lasers. The AOM, called Q switch, then serves to block the laser resonator before the pulse is generated. In most cases, the zero-order (not diffracted) beam is used under lasing conditions, and the AOM is turned on when lasing should be prohibited. This requires that the caused diffraction losses (possibly for two passes per resonator round trip) are higher than the laser gain. For high-gain lasers (for example, fiber lasers), one sometimes uses the first-order diffracted beam under lasing conditions, so that very high resonator losses result when the AOM is turned off. However, the losses in the lasing state are then also fairly high.
- AOMs can also be used for cavity dumping of solid-state lasers, generating either nanosecond or ultrashort pulses. In the latter case, the speed of an AOM is sufficient only in the case of a relatively long laser resonator; an electro-optic modulator may otherwise be required.
- Active mode locking is often performed with an AOM for modulating the resonator losses at the round-trip frequency or a multiple thereof.
- An AOM can be used as a pulse picker for reducing the pulse repetition rate of a pulse train, e.g. in order to allow for subsequent amplification of pulses to high pulse energies.
- In laser printers and other devices, an AOM can be used for modulating the power of a laser beam. The modulation may be continuous or digital (on/off).
- An AOM can shift the frequency of a laser beam, e.g. in various measurement schemes, or in lasers which are mode-locked via frequency-shifted optical feedback.
- In some cases one exploits the effect that the diffraction angle depends on the acoustic frequency. In particular, one can scan the output beam direction (at least in a small range) by changing the modulation frequency.
Important Properties of Acousto-optic Modulators
Various aspects can be essential for the selection of an acousto-optic modulator for some application:
- The material should have a high transparency at the relevant wavelengths, and parasitic reflections should be minimized e.g. with anti-reflection coatings.
- In many cases, a high diffraction efficiency is important. For example, this matters when using the AOM as a Q switch in a high-gain laser, and even more so for cavity dumping.
- The required RF power influences both the electric power demands and cooling issues.
- The switching time is critical for some applications (e.g. Q switching and particularly cavity dumping). It is limited by the finite velocity of sound in the acousto-optic medium. This implies that an AOM switching a laser beam with large diameter is necessarily slow.
- For frequency shifters, the device often has to be used in a wide range of RF frequencies.
- High optical peak powers require a suitable material and a large open aperture, allowing for a high damage threshold.
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.
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