HOME

TheInfoList



OR:

Acousto-optics is a branch of
physics Physics is the natural science that studies matter, its fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of knowledge which r ...
that studies the interactions between sound waves and light waves, especially the diffraction of
laser A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The fi ...
light Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 te ...
by
ultrasound Ultrasound is sound waves with frequencies higher than the upper audible limit of human hearing. Ultrasound is not different from "normal" (audible) sound in its physical properties, except that humans cannot hear it. This limit varies ...
(or
sound In physics, sound is a vibration that propagates as an acoustic wave, through a transmission medium such as a gas, liquid or solid. In human physiology and psychology, sound is the ''reception'' of such waves and their ''perception'' b ...
in general) through an
ultrasonic grating An ultrasonic grating is a type of diffraction grating produced by Interference (wave propagation), interfering ultrasonic waves in a medium altering the physical properties of the medium, and hence the refractive index, in a grid-like pattern. Th ...
.


Introduction

Optics Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behaviour of visible, ultrav ...
has had a very long and full history, from
ancient Greece Ancient Greece ( el, Ἑλλάς, Hellás) was a northeastern Mediterranean Sea, Mediterranean civilization, existing from the Greek Dark Ages of the 12th–9th centuries BC to the end of Classical Antiquity, classical antiquity ( AD 600), th ...
, through the
renaissance The Renaissance ( , ) , from , with the same meanings. is a period in European history The history of Europe is traditionally divided into four time periods: prehistoric Europe (prior to about 800 BC), classical antiquity (800 BC to AD ...
and modern times. As with optics, acoustics has a history of similar duration, again starting with the ancient Greeks. In contrast, the acousto-optic effect has had a relatively short history, beginning with Brillouin predicting the diffraction of light by an acoustic wave, being propagated in a medium of interaction, in 1922. This was then confirmed with experimentation in 1932 by
Debye The debye (symbol: D) (; ) is a CGS unit (a non- SI metric unit) of electric dipole momentTwo equal and opposite charges separated by some distance constitute an electric dipole. This dipole possesses an electric dipole moment whose value is g ...
and
Sears Sears, Roebuck and Co. ( ), commonly known as Sears, is an American chain of department stores founded in 1892 by Richard Warren Sears and Alvah Curtis Roebuck and reincorporated in 1906 by Richard Sears and Julius Rosenwald, with what began a ...
, and also by Lucas and Biquard. The particular case of diffraction on the first order, under a certain angle of incidence, (also predicted by Brillouin), has been observed by Rytow in 1935. Raman and Nath (1937) have designed a general ideal model of interaction taking into account several orders. This model was developed by Phariseau (1956) for diffraction including only one diffraction order. In general, acousto-optic effects are based on the change of the
refractive index In optics, the refractive index (or refraction index) of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium. The refractive index determines how much the path of light is bent, or ...
of a medium due to the presence of sound waves in that medium. Sound waves produce a refractive index grating in the material, and it is this grating that is "seen" by the light wave. These variations in the refractive index, due to the pressure fluctuations, may be detected optically by refraction, diffraction, and interference effects, reflection may also be used. The acousto-optic effect is extensively used in the measurement and study of ultrasonic waves. However, the growing principal area of interest is in acousto-optical devices for the deflection, modulation,
signal processing Signal processing is an electrical engineering subfield that focuses on analyzing, modifying and synthesizing ''signals'', such as sound, images, and scientific measurements. Signal processing techniques are used to optimize transmissions, ...
and frequency shifting of light beams. This is due to the increasing availability and performance of
laser A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The fi ...
s, which have made the acousto-optic effect easier to observe and measure. Technical progress in both
crystal growth A crystal is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. Crystal growth is a major stage of a crystallization process, and consists of the ...
and high frequency
piezoelectric Piezoelectricity (, ) is the electric charge that accumulates in certain solid materials—such as crystals, certain ceramics, and biological matter such as bone, DNA, and various proteins—in response to applied mechanical stress. The word '' ...
transducer A transducer is a device that converts energy from one form to another. Usually a transducer converts a signal in one form of energy to a signal in another. Transducers are often employed at the boundaries of automation, measurement, and cont ...
s has brought valuable benefits to acousto-optic components' improvements. Along with the current applications, acousto-optics presents interesting possible application. It can be used in nondestructive testing,
structural health monitoring Structural health monitoring (SHM) involves the observation and analysis of a system over time using periodically sampled response measurements to monitor changes to the material and geometric properties of engineering structures such as bridges an ...
and biomedical applications, where optically generated and optical measurements of ultrasound gives a non-contact method of imaging.


Acousto-optic effect

The acousto-optic effect is a specific case of
photoelasticity Photoelasticity describes changes in the optical properties of a material under mechanical deformation. It is a property of all dielectric media and is often used to experimentally determine the stress distribution in a material, where it gives ...
, where there is a change of a material's
permittivity In electromagnetism, the absolute permittivity, often simply called permittivity and denoted by the Greek letter ''ε'' (epsilon), is a measure of the electric polarizability of a dielectric. A material with high permittivity polarizes more in ...
, \varepsilon, due to a
mechanical Mechanical may refer to: Machine * Machine (mechanical), a system of mechanisms that shape the actuator input to achieve a specific application of output forces and movement * Mechanical calculator, a device used to perform the basic operations of ...
strain Strain may refer to: Science and technology * Strain (biology), variants of plants, viruses or bacteria; or an inbred animal used for experimental purposes * Strain (chemistry), a chemical stress of a molecule * Strain (injury), an injury to a mu ...
a. Photoelasticity is the variation of the optical indicatrix coefficients B_i caused by the strain a_j given by, : (1) \ \Delta B_i = p_ a_j, \, where p_ is the photoelastic
tensor In mathematics, a tensor is an algebraic object that describes a multilinear relationship between sets of algebraic objects related to a vector space. Tensors may map between different objects such as vectors, scalars, and even other tensor ...
with components, i,j = 1,2,...,6. Specifically in the acousto-optic effect, the strains a_j are a result of the acoustic wave which has been excited within a
transparent Transparency, transparence or transparent most often refer to: * Transparency (optics), the physical property of allowing the transmission of light through a material They may also refer to: Literal uses * Transparency (photography), a still, ...
medium. This then gives rise to the variation of the refractive index. For a plane acoustic wave propagating along the z axis, the change in the refractive index can be expressed as : (2) \ n(z,t)=n_0+\Delta n \cos (\Omega t - Kz), \, where n_0 is the undisturbed refractive index, \Omega is the
angular frequency In physics, angular frequency "''ω''" (also referred to by the terms angular speed, circular frequency, orbital frequency, radian frequency, and pulsatance) is a scalar measure of rotation rate. It refers to the angular displacement per unit tim ...
, K is the
wavenumber In the physical sciences, the wavenumber (also wave number or repetency) is the '' spatial frequency'' of a wave, measured in cycles per unit distance (ordinary wavenumber) or radians per unit distance (angular wavenumber). It is analogous to te ...
of the acoustic wave, and \Delta n is the amplitude of variation in the refractive index generated by the acoustic wave, and is given as, : (3) \ \Delta n = - \frac \sum_j n_0^3p_ a_j, The generated refractive index, (2), gives a
diffraction grating In optics, a diffraction grating is an optical component with a periodic structure that diffracts light into several beams travelling in different directions (i.e., different diffraction angles). The emerging coloration is a form of structur ...
moving with the
velocity Velocity is the directional speed of an object in motion as an indication of its rate of change in position as observed from a particular frame of reference and as measured by a particular standard of time (e.g. northbound). Velocity i ...
given by the speed of the sound wave in the medium. Light which then passes through the transparent material, is diffracted due to this generated refraction index, forming a prominent
diffraction pattern Diffraction is defined as the interference or bending of waves around the corners of an obstacle or through an aperture into the region of geometrical shadow of the obstacle/aperture. The diffracting object or aperture effectively becomes a ...
. This diffraction pattern corresponds with a conventional diffraction grating at angles \theta_n from the original direction, and is given by, : (4) \ \Lambda \sin (\theta_m) = m\lambda,\, where \lambda is the
wavelength In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, t ...
of the optical wave, \Lambda is the wavelength of the acoustic wave and m is the integer order maximum. Light diffracted by an acoustic wave of a single
frequency Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...
produces two distinct diffraction types. These are Raman–Nath diffraction and
Bragg diffraction In physics and chemistry , Bragg's law, Georg Wulff, Wulff–Bragg's condition or Laue–Bragg interference, a special case of Laue diffraction, gives the angles for coherent scattering of waves from a crystal lattice. It encompasses the superposit ...
. Raman–Nath diffraction is observed with relatively low acoustic frequencies, typically less than 10 MHz, and with a small acousto-optic interaction length, ℓ, which is typically less than 1 cm. This type of diffraction occurs at an arbitrary angle of incidence, \theta_0. In contrast, Bragg diffraction occurs at higher acoustic frequencies, usually exceeding 100 MHz. The observed diffraction pattern generally consists of two diffraction maxima; these are the zeroth and the first orders. However, even these two maxima only appear at definite incidence angles close to the Bragg angle, \theta_B. The first order maximum or the Bragg maximum is formed due to a selective reflection of the light from the wave fronts of ultrasonic wave. The Bragg angle is given by the expression, : (5) \ \sin \theta_B = - \frac\left 1+\frac \left( n_i^2 - n_d^2 \right) \right where \lambda is the wavelength of the incident light wave (in a vacuum), f is the acoustic frequency, v is the velocity of the acoustic wave, n_i is the refractive index for the incident optical wave, and n_d is the refractive index for the diffracted optical waves. In general, there is no point at which
Bragg diffraction In physics and chemistry , Bragg's law, Georg Wulff, Wulff–Bragg's condition or Laue–Bragg interference, a special case of Laue diffraction, gives the angles for coherent scattering of waves from a crystal lattice. It encompasses the superposit ...
takes over from Raman–Nath diffraction. It is simply a fact that as the acoustic frequency increases, the number of observed maxima is gradually reduced due to the angular selectivity of the acousto-optic interaction. Traditionally, the type of diffraction, Bragg or Raman–Nath, is determined by the conditions Q \gg 1 and Q \ll 1 respectively, where Q is given by, : (6) \ Q = \frac, which is known as the Klein–Cook parameter. Since, in general, only the first order diffraction maximum is used in acousto-optic devices,
Bragg diffraction In physics and chemistry , Bragg's law, Georg Wulff, Wulff–Bragg's condition or Laue–Bragg interference, a special case of Laue diffraction, gives the angles for coherent scattering of waves from a crystal lattice. It encompasses the superposit ...
is preferable due to the lower optical losses. However, the acousto-optic requirements for
Bragg diffraction In physics and chemistry , Bragg's law, Georg Wulff, Wulff–Bragg's condition or Laue–Bragg interference, a special case of Laue diffraction, gives the angles for coherent scattering of waves from a crystal lattice. It encompasses the superposit ...
limit the frequency range of acousto-optic interaction. As a consequence, the speed of operation of acousto-optic devices is also limited.


Acousto-optic devices


Acousto-optic modulator

By varying the parameters of the acoustic wave, including the
amplitude The amplitude of a periodic variable is a measure of its change in a single period (such as time or spatial period). The amplitude of a non-periodic signal is its magnitude compared with a reference value. There are various definitions of am ...
, phase, frequency and polarization, properties of the optical wave may be modulated. The acousto-optic interaction also makes it possible to modulate the optical beam by both temporal and spatial modulation. A simple method of modulating the optical beam travelling through the acousto-optic device is done by switching the acoustic field on and off. When off the light beam is undiverted, the intensity of light directed at the Bragg diffraction angle is zero. When switched on and Bragg diffraction occurs, the intensity at the Bragg angle increases. So the acousto-optic device is modulating the output along the Bragg diffraction angle, switching it on and off. 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. There are several limitations associated with the design and performance of acousto-optic modulators. The acousto-optic medium must be designed carefully to provide maximum light intensity in a single diffracted beam. The time taken for the acoustic wave to travel across the diameter of the light beam gives a limitation on the switching speed, and hence limits the modulation bandwidth. The finite velocity of the acoustic wave means the light cannot be fully switched on or off until the acoustic wave has traveled across the light beam. So to increase the bandwidth the light must be focused to a small diameter at the location of the acousto-optic interaction. This minimum focused size of the beam represents the limit for the bandwidth.


Acousto-optic tunable filter

The principle behind the operation of acousto-optic tunable filters is based on the wavelength of the diffracted light being dependent on the acoustic frequency. By tuning the frequency of the acoustic wave, the desired wavelength of the optical wave can be diffracted acousto-optically. There are two types of the acousto-optic filters, the collinear and non-collinear filters. The type of filter depends on geometry of acousto-optic interaction. The polarization of the incident light can be either ordinary or extraordinary. For the definition, we assume ordinary polarization. Here the following list of symbols is used, \alpha: the angle between the acoustic wave vector and the crystallographic axis ''z'' of the crystal; \gamma: the wedge angle between the input and output faces of the filter cell (the wedge angle is necessary for eliminating the angular shift of the diffracted beam caused by frequency changing); \varphi: the angle between the incident light wave vector and 10axis of the crystal; \alpha_\ell: the angle between the input face of the cell and acoustic wave vector; \beta: the angle between deflected and non-deflected light at the central frequency; \ell: the transducer length. The incidence angle \varphi and the central frequency f_i of the filter are defined by the following set of equations, : (7) \ n_\varphi = \frac : (8) \ f_i(\varphi)=\frac\left _\varphi\cos(\varphi+\alpha)\pm\sqrt\right/math> Refractive indices of the ordinary (n_0) and extraordinary (n_e) polarized beams are determined by taking into account their dispersive dependence. The sound velocity, v, depends on the angle α, such that, : (9) \ \nu (\alpha) = \nu_ \sqrt v_ and v_ are the sound velocities along the axes 10and 01 consecutively. The value of \alpha_1 is determined by the angles \varphi and \alpha, : (10) \ \alpha_\ell = \varphi + \alpha The angle \beta between the diffracted and non-diffracted beams defines the view field of the filter; it can be calculated from the formula, : (11) \ \beta = \arcsin \left( \frac \sin \alpha + \varphi \right) Input light need not be polarized for a non-collinear design. Unpolarized input light is scattered into orthogonally polarized beams separated by the scattering angle for the particular design and wavelength. If the optical design provides an appropriate beam block for the unscattered light, then two beams (images) are formed in an optical passband that is nearly equivalent in both orthogonally linearly polarized output beams (differing by the Stokes and Anti-Stokes scattering parameter). Because of dispersion, these beams move slightly with scanning rf frequency.


Acousto-optic deflectors

An acousto-optic deflector spatially controls the optical beam. In the operation of an acousto-optic deflector the power driving the acoustic transducer is kept on, at a constant level, while the acoustic frequency is varied to deflect the beam to different angular positions. The acousto-optic deflector makes use of the acoustic frequency dependent diffraction angle, where a change in the angle \Delta \theta_d as a function of the change in frequency \Delta f is given as, : (12) \ \Delta \theta_d = \frac\Delta f where \lambda is the optical wavelength of the beam and \nu is the velocity of the acoustic wave. AOD technology has made practical the
Bose–Einstein condensation Bose–Einstein may refer to: * Bose–Einstein condensate ** Bose–Einstein condensation (network theory) * Bose–Einstein correlations * Bose–Einstein statistics In quantum statistics, Bose–Einstein statistics (B–E statistics) describe ...
for which the 2001
Nobel Prize in Physics ) , image = Nobel Prize.png , alt = A golden medallion with an embossed image of a bearded man facing left in profile. To the left of the man is the text "ALFR•" then "NOBEL", and on the right, the text (smaller) "NAT•" then " ...
was awarded to Eric A. Cornell, Wolfgang Ketterle and Carl E. Wieman. Another application of acoustic-optical deflection is optical trapping of small molecules. AODs are essentially the same as
acousto-optic modulator An acousto-optic modulator (AOM), also called a Bragg cell or an acousto-optic deflector (AOD), uses the acousto-optic effect to diffract and shift the frequency of light using sound waves (usually at radio-frequency). They are used in lasers ...
s (AOMs). In an AOM, only the amplitude of the sound wave is modulated (to modulate the intensity of the diffracted laser beam), whereas in an AOD, both the amplitude and frequency are adjusted, making the engineering requirements tighter for an AOD than an AOM.


Materials

All materials display the acousto-optic effect.
Fused silica Fused quartz, fused silica or quartz glass is a glass consisting of almost pure silica (silicon dioxide, SiO2) in amorphous (non- crystalline) form. This differs from all other commercial glasses in which other ingredients are added which ch ...
is used as a standard to compare when measuring photoelastic coefficients. Lithium niobate is often used in high frequency devices. Softer materials, such as
arsenic trisulfide Arsenic trisulfide is the inorganic compound with the formula . It is a dark yellow solid that is insoluble in water. It also occurs as the mineral orpiment (Latin: auripigmentum), which has been used as a pigment called King's yellow. It is produ ...
,
tellurium dioxide Tellurium dioxide (TeO2) is a solid oxide of tellurium. It is encountered in two different forms, the yellow orthorhombic mineral tellurite, β-TeO2, and the synthetic, colourless tetragonal (paratellurite), α-TeO2. Most of the information regard ...
and
tellurite The tellurite ion is . A tellurite (compound), for example sodium tellurite, is a compound that contains this ion. They are typically colorless or white salts, which in some ways are comparable to sulfite. A mineral with the formula TeO2 is ...
glasses, lead silicate, Ge55As12S33,
mercury(I) chloride Mercury(I) chloride is the chemical compound with the formula Hg2Cl2. Also known as the mineral calomel (a rare mineral) or mercurous chloride, this dense white or yellowish-white, odorless solid is the principal example of a mercury(I) compound ...
, lead(II) bromide, with slow acoustic waves make high efficiency devices at lower frequencies, and give high resolution.


See also

*
Acousto-optic modulator An acousto-optic modulator (AOM), also called a Bragg cell or an acousto-optic deflector (AOD), uses the acousto-optic effect to diffract and shift the frequency of light using sound waves (usually at radio-frequency). They are used in lasers ...
*
Acousto-optic deflector An acousto-optic deflector (AOD) spatially controls the optical beam. In the operation of an acousto-optic deflector the power driving the acoustic transducer is kept on, at a constant level, while the acoustic frequency is varied to deflect the be ...
*
Nonlinear optics Nonlinear optics (NLO) is the branch of optics that describes the behaviour of light in ''nonlinear media'', that is, media in which the polarization density P responds non-linearly to the electric field E of the light. The non-linearity is typic ...
*
Sonoluminescence Sonoluminescence is the emission of light from imploding bubbles in a liquid when excited by sound. History The sonoluminescence effect was first discovered at the University of Cologne in 1934 as a result of work on sonar. Hermann Frenzel an ...
* Schaefer–Bergmann diffraction


References

{{Authority control Diffraction Light Acoustics Nonlinear optics Waves