Acousto-Optic Modulators

A different approach to modulator technology uses the interaction between light and sound waves to produce changes in optical intensity, phase, frequency, and direction of propagation. Acousto-optic modulators are based on the diffraction of light by a column of sound in a suitable interaction medium.

When a sound wave travels through a transparent material, it causes periodic variations of the index of refraction. The sound wave can be considered as a series of compressions and rarefactions moving through the material. In regions where the sound pressure is high, the material is compressed slightly. This compression leads to an increase in the index of refraction. The increase is small, but it can produce large cumulative effects on a light wave passing some distance through the compressed material.
An acousto-optic device requires a material with good acoustic and optical properties and high optical transmission. Several types of material are available. We shall describe acoustic-optic materials in more detail later.
The sound wave is produced by a piezoelectric transducer. Piezoelectric materials exhibit slight changes in physical size when voltage is applied to them. An example of one such material is crystalline quartz. If the piezoelectric material is placed in contact with the acousto-optic material and a high-frequency oscillating voltage is applied to the piezoelectric material, it will expand and contract as the voltage varies. This in turn exerts pressure on the acousto-optic material and will launch an acoustic wave (sound wave) which will travel through the material. The frequency F of the acoustic wave will be the same as the frequency of the applied voltage. The acoustic wave will have a wavelength L given by:
FL = C Equation 3
where C is the velocity of sound in the material, typically of the order of 10⁵ cm/sec. Variation of the acoustic frequency of the driver will thus change the acoustic wavelength, which in turn changes the characteristics of the acousto-optic interaction.
A typical structure for an acousto-optic device is shown in Figure 5. The piezoelectric materials and metal layers are bonded or deposited on the acousto-optic material. A radio-frequency field is applied across the piezoelectric material using the metal layers as electrodes. The acoustic wave is then launched into the acousto-optic medium by the piezoelectric material. Acoustic waves propagating from a flat piezoelectric transducer into a crystal will form almost plane wavefronts traveling in the crystal. The opposite end of the material from the transducer should have an acoustic termination to suppress reflected acoustic waves.

Fig. 5
Typical structure of an acousto-optic device

The elasto-optic properties of the medium respond to the acoustic wave so as to produce a periodic variation of the index of refraction. A light beam incident on this disturbance is partially deflected in much the same way that light is deflected by a diffraction grating. The operation is shown in Figure 6. The alternate compressions and rarefactions associated with the sound wave form a grating that diffracts the incident light beam. No light is deflected unless the acoustic wave is present.

Fig. 6
Diagram showing the principles of operation of an acousto-optic light-beam modulator or deflector. The diagram defines the Bragg angle F and deflection angle F used in the text.

For a material with a fixed acoustic velocity, the acoustic wavelength or grating spacing is a function of the radio-frequency drive signal; the acoustic wavelength controls the angle of deflection. The amplitude of the disturbance, a function of the radio-frequency power applied to the transducer, controls the fraction of the light that is deflected. Thus, the power to the transducer controls the intensity of the deflected light. Modulation of the light beam is achieved by maintaining a constant radio frequency, allowing only the deflected beam to emerge from the modulator and modulating the power to the transducer. Thus the modulator will be in its off state when no acoustic power is applied and will be switched to its transmissive state by the presence of acoustic power.
The transmission T of an acousto-optic modulator is
T = T₀ sin²(p (M₂PL/2H)^0.5/l cos Q ) Equation 4
where P is the acoustic power supplied to the medium, l is the wavelength, L is the length of the medium (length of the region in which the light wave interacts with the acoustic wave), H is the width of the medium (width across which the sound wave travels), and the inherent transmission T₀ is a function of reflective and absorptive losses in the device. The quantity M₂ is a figure of merit, a material parameter that indicates the suitability of a particular material for this application. It is defined by
M₂ = n⁶p²/r v³ Equation 5
where p is the photoelastic constant of the material, r is its density, and v is the velocity of sound in the material. This figure of merit relates the diffraction efficiency to the acoustic power for a given device. It is useful for specifying materials to yield high efficiency, but in practical devices, bandwidth is also important, so that specifying a high value of M₂ alone is not sufficient.
The angle Q is the so-called Bragg angle. The diffraction of the light beam by the periodic array of acoustic waves satisfies the same relationship as the scattering of X rays by periodic planes of atoms (so-called Bragg scattering). Hence the diffraction of the light waves is also referred to as Bragg reflection. Bragg reflection of X rays occurs at the planes of atoms in a crystal which are spaced a regular distance apart. The so-called Bragg angle gives the angle at which the most efficient reflection occurs. The phenomenon of optical reflection from the regular wavefronts of the acoustic waves is exactly the same, with the provision that the acoustic wavelength replaces the distance between planes of atoms.
The Bragg angle Q is defined as the angle the beam makes with the reflecting waves. It is given by:
sin Q = l /2nL Equation 6
where n is the index of refraction of the material, l is the optical wavelength and L is the acoustic wavelength.
To use the acousto-optic device as a modulator, one should employ the deflected beam as shown in Figure 7. This will give higher values of the extinction ratio than using the undeflected beam. When the acoustic drive is off, the light in the direction of the deflected beam is zero. When the acoustic drive is on, light is diffracted into that direction. Thus, the acousto-optic device controls the light in that direction, turning it on and off at will. The device is operated as a modulator by keeping the acoustic wavelength (frequency) fixed and varying the drive power to vary the amount of light in the deflected beam. As we shall see later, the use as a light-beam deflector is somewhat different.

Fig. 7
Use of an acousto-optic device as a light-beam modulator

The design and performance of acousto-optic beam modulators have several limitations. The transducer and acousto-optic medium must be carefully designed to provide maximum light intensity in a single diffracted beam, when the modulator is in an open condition. The transit time of the acoustic beam across the diameter of the light beam imposes a limitation on the rise time of the switching and therefore limits the modulation bandwidth. The acoustic wave travels with a finite velocity and the light beam cannot be switched fully on or fully off until the acoustic wave has traveled all the way across the light beam. Therefore, to increase bandwidth, one focuses the light beam to a small diameter at the position of the interaction so as to minimize the transit time. Frequently the diameter to which the beam may be focused is the ultimate limitation for the bandwidth. If the laser beam has high power, it cannot be focused in the acousto-optic medium without damage.
General Comments of John Simcik
The rise time t_r for an acousto-optic modulator is given by the equation:
t_r = d/C Equation 7
where d is the diameter of the laser beam in the region of the interaction and C is the velocity of sound in the material. As an example, for a tellurium dioxide acousto-optic modulator, with an acoustic velocity of 8.03 � 10⁵ cm/sec and a laser-beam diameter of 100 m m, the rise time is 0.01 cm/8.03 � 10⁵ cm/sec = 1.245 � 10- ⁸ = 12.45 nsec. Thus acousto-optic devices are capable of high-frequency modulation.
Acousto-optic light-beam modulators have a number of important desirable features. The electrical power required to excite the acoustic wave may be small, less than one watt in some cases. High extinction ratios are obtained easily because no light emerges in the direction of the diffracted beam when the device is off. A large fraction, up to 90% for some commercial models, of the incident light may be diffracted into the transmitted beam. Acousto-optic devices may be compact and may offer an advantage for systems where size and weight are important. As compared to electro-optic modulators, they tend to have lower bandwidth, but do not require high voltage.
Table 2 presents the characteristics of some materials used in commercially available acousto-optic modulators. Several materials are available for use in the visible and near-infrared regions, and one material (germanium) is useful in the far infrared. The table also presents some values for the figure of merit M₂, at a wavelength of 633 nm, except for GaAs (1530 nm) and Ge (10.6 mm). The factors that enter into the definition of M₂ vary with crystalline orientation; the values in the table are for crystals oriented to maximize M₂. The values for bandwidth and for typical drive power are representative of commercially available acousto-optic devices.


Ricardo Monroy  C.I. 17646658

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