Acousto-optic Modulators | previous | next | feedback |
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(Acronym: AOM)
Definition: optical modulators based on the acousto-optic effect
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 high-frequency sound wave (with a frequency of the order of 100 MHz). Light can then experience Bragg diffraction at the periodic refractive index grating generated by the sound wave; therefore, AOMs are sometimes called Bragg cells. The scattered beam has a slightly modified optical frequency (increased or decreased by the frequency of the sound wave) and a slightly different direction. (The change of 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.
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, 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.
Applications
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.
- 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 requires an AOM for modulating the resonator losses with the round-trip frequency or a multiple thereof.
- An AOM can 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, AOMs are used for modulating the power of a laser beam.
- 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.
Important Properties
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.
- 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.
See also: Q switches, optical modulators, pulse pickers, electro-optic modulators, Q switching, cavity dumping, active mode locking
