Acousto-optics - AbsoluteAstronomy.com

Acousto-optics is a branch of physics

Physics

Physics is a natural science that involves the study of matter and its motion through spacetime, along with related concepts such as energy and force. More broadly, it is the general analysis of nature, conducted in order to understand how the universe behaves.Physics is one of the oldest academic...

that studies the interactions between sound waves and light waves, especially the diffraction

Diffraction

Diffraction refers to various phenomena which occur when a wave encounters an obstacle. Italian scientist Francesco Maria Grimaldi coined the word "diffraction" and was the first to record accurate observations of the phenomenon in 1665...

of laser

Laser

A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...

light

Light

Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...

by ultrasound

Ultrasound

Ultrasound is cyclic sound pressure with a frequency greater than the upper limit of human hearing. Ultrasound is thus not separated from "normal" sound based on differences in physical properties, only the fact that humans cannot hear it. Although this limit varies from person to person, it is...

or sound

Sound

Sound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.-Propagation of...

in general.

Introduction

Optics has had a very long and full history, from ancient Greece

Ancient Greece

Ancient Greece is a civilization belonging to a period of Greek history that lasted from the Archaic period of the 8th to 6th centuries BC to the end of antiquity. Immediately following this period was the beginning of the Early Middle Ages and the Byzantine era. Included in Ancient Greece is the...

, through the renaissance

Renaissance

The Renaissance was a cultural movement that spanned roughly the 14th to the 17th century, beginning in Italy in the Late Middle Ages and later spreading to the rest of Europe. The term is also used more loosely to refer to the historical era, but since the changes of the Renaissance were not...

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

Léon Brillouin

Léon Nicolas Brillouin was a French physicist. He made contributions to quantum mechanics, radio wave propagation in the atmosphere, solid state physics, and information theory.-Early life:...

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

Peter Debye

Peter Joseph William Debye FRS was a Dutch physicist and physical chemist, and Nobel laureate in Chemistry.-Early life:...

and Sears, and also by Lucas and Biquard.

The particular case of diffraction on the first order, under a certain angle of incidence

Angle of incidence

Angle of incidence is a measure of deviation of something from "straight on", for example:* in the approach of a ray to a surface, or* the angle at which the wing or horizontal tail of an airplane is installed on the fuselage, measured relative to the axis of the fuselage.-Optics:In geometric...

, (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

Refractive index

In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....

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

Modulation

In electronics and telecommunications, modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted...

, signal processing

Signal processing

Signal processing is an area of systems engineering, electrical engineering and applied mathematics that deals with operations on or analysis of signals, in either discrete or continuous time...

and frequency shifting of light beams. This is due to the increasing availability and performance of laser

Laser

s, which have made the acousto-optic effect easier to observe and measure. Technical progresses in both crystal growth

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 in the addition of new atoms, ions, or polymer strings into...

and high frequency piezoelectric

Piezoelectricity

Piezoelectricity is the charge which accumulates in certain solid materials in response to applied mechanical stress. The word piezoelectricity means electricity resulting from pressure...

transducer

Transducer

A transducer is a device that converts one type of energy to another. Energy types include electrical, mechanical, electromagnetic , chemical, acoustic or thermal energy. While the term transducer commonly implies the use of a sensor/detector, any device which converts energy can be considered a...

s have 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

Nondestructive testing

Nondestructive testing or Non-destructive testing is a wide group of analysis techniques used in science and industry to evaluate the properties of a material, component or system without causing damage....

, structural health monitoring

Structural health monitoring

The process of implementing a damage detection and characterization strategy for engineering structures is referred to as Structural Health Monitoring . Here damage is defined as changes to the material and/or geometric properties of a structural system, including changes to the boundary conditions...

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

Photoelasticity is an experimental method to determine the stress distribution in a material. The method is mostly used in cases where mathematical methods become quite cumbersome. Unlike the analytical methods of stress determination, photoelasticity gives a fairly accurate picture of stress...

, where there is a change of a material's permittivity

Permittivity

In electromagnetism, absolute permittivity is the measure of the resistance that is encountered when forming an electric field in a medium. In other words, permittivity is a measure of how an electric field affects, and is affected by, a dielectric medium. The permittivity of a medium describes how...

, due to a mechanical

Mechanics

Mechanics is the branch of physics concerned with the behavior of physical bodies when subjected to forces or displacements, and the subsequent effects of the bodies on their environment....

strain

Strain (materials science)

In continuum mechanics, the infinitesimal strain theory, sometimes called small deformation theory, small displacement theory, or small displacement-gradient theory, deals with infinitesimal deformations of a continuum body...

. Photoelasticity is the variation of the optical indicatrix coefficients

caused by the strain

given by
,

where

is the photoelastic tensor

Tensor

Tensors are geometric objects that describe linear relations between vectors, scalars, and other tensors. Elementary examples include the dot product, the cross product, and linear maps. Vectors and scalars themselves are also tensors. A tensor can be represented as a multi-dimensional array of...

with components,

= 1,2,…,6.

Specifically in the acousto-optic effect, the strains

are a result of the acoustic wave which has been excited within a transparent 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,

where

is the undisturbed refractive index,

is the angular frequency

Angular frequency

In physics, angular frequency ω is a scalar measure of rotation rate. Angular frequency is the magnitude of the vector quantity angular velocity...

is the wavenumber

Wavenumber

In the physical sciences, the wavenumber is a property of a wave, its spatial frequency, that is proportional to the reciprocal of the wavelength. It is also the magnitude of the wave vector...

, and

is the amplitude of variation in the refractive index generated by the acoustic wave, and is given as,

The generated refractive index, (2), gives a diffraction grating

Diffraction grating

In optics, a diffraction grating is an optical component with a periodic structure, which splits and diffracts light into several beams travelling in different directions. The directions of these beams depend on the spacing of the grating and the wavelength of the light so that the grating acts as...

moving with the velocity

Velocity

In physics, velocity is speed in a given direction. Speed describes only how fast an object is moving, whereas velocity gives both the speed and direction of the object's motion. To have a constant velocity, an object must have a constant speed and motion in a constant direction. Constant ...

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. This diffraction pattern corresponds with a conventional diffraction grating at angles

from the original direction, and is given by,

where

is the wavelength

Wavelength

In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...

of the optical wave,

is the wave length of the acoustic wave and

is the integer order maximum.

Light diffracted by an acoustic wave of a single frequency

Frequency

Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...

produces two distinct diffraction types. These are Raman-Nath diffraction and Bragg diffraction.

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 1cm. This type of diffraction occurs at an arbitrary angle of incidence,

.

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,

. 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,

where

is the wavelength of the incident light wave (in a vacuum),

is the acoustic frequency,

is the velocity of the acoustic wave,

is the refractive index for the incident optical wave, and

is the refractive index for the diffracted optical waves.

In general, there is no point at which Bragg diffraction 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 >> 1 and Q << 1 respectively, where Q is given by,

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 is preferable due to the lower optical losses. However, the acousto-optic requirements for Bragg diffraction 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

Three categories of acousto-optic devices will be discussed. They include the acousto-optic modulator, filter and deflector.

Acousto-optic modulator

By varying the parameters of the acoustic wave, including the amplitude

Amplitude

Amplitude is the magnitude of change in the oscillating variable with each oscillation within an oscillating system. For example, sound waves in air are oscillations in atmospheric pressure and their amplitudes are proportional to the change in pressure during one oscillation...

, phase

Phase (waves)

Phase in waves is the fraction of a wave cycle which has elapsed relative to an arbitrary point.-Formula:The phase of an oscillation or wave refers to a sinusoidal function such as the following:...

, 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 filter

The principle behind the operation of acousto-optic 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,

: the angle between the acoustic wave vector and the crystallographic axis z of the crystal;

: 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);

: the angle between the incident light wave vector and [110] axis of the crystal;

: the angle between the input face of the cell and acoustic wave vector;

: the angle between deflected and non-deflected light at the central frequency;

: the transducer length.

The incidence angle

and the central frequency

of the filter are defined by the following set of equations,

Refractive indices of the ordinary (

) and extraordinary (

) polarized beams are determined by taking into account their dispersive dependence.

The sound velocity,

, depends on the angle α, such that,

and

are the sound velocities along the axes [110] and [001], consecutively. The value of

is determined by the angles

and

The angle

between the diffracted and non-diffracted beams defines the view field of the filter; it can be calculated from the formula,

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

as a function of the change in frequency

is given as,

where

and

are the acoustic wavelength and velocity of the acoustic wave respectively.

AOD technology has made practical the Bose-Einstein condensation for which the 2001 Nobel Prize in Physics

Nobel Prize in Physics

The Nobel Prize in Physics is awarded once a year by the Royal Swedish Academy of Sciences. It is one of the five Nobel Prizes established by the will of Alfred Nobel in 1895 and awarded since 1901; the others are the Nobel Prize in Chemistry, Nobel Prize in Literature, Nobel Peace Prize, and...

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

Acousto-optic modulator

An acousto-optic modulator , also called a Bragg cell, uses the acousto-optic effect to diffract and shift the frequency of light using sound waves . They are used in lasers for Q-switching, telecommunications for signal modulation, and in spectroscopy for frequency control. A piezoelectric...

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

Some materials displaying acousto-optic effect include fused silica,lithium niobate, arsenic trisulfide

Arsenic trisulfide

Arsenic trisulfide is the inorganic compound with the formula As2S3. This bright yellow solid is a well known mineral orpiment , has been used as a pigment, and has played a role in the analysis of arsenic compounds. This chalcogenide material is a group V/VI, intrinsic p-type semiconductor and...

, tellurium dioxide

Tellurium dioxide

Tellurium dioxide 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 , α-TeO2...

and tellurite

Tellurite

Tellurite is a rare oxide mineral composed of tellurium dioxide .It occurs as prismatic to acicular transparent yellow to white orthorhombic crystals. It occurs in the oxidation zone of mineral deposits in association with native tellurium, emmonsite and other tellurium minerals. Its name comes...

glass

Glass

Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...

es, lead silicate, Ge₅₅As₁₂S₃₃, mercury(I) chloride

Mercury(I) chloride

Mercury chloride is the chemical compound with the formula Hg2Cl2. Also known as calomel or mercurous chloride, this dense white or yellowish-white, odorless solid is the principal example of a mercury compound...

, lead(II) bromide

Lead(II) bromide

Lead bromide is the stable salt of lead and hydrobromic acid. It is typically formed by precipitation from lead nitrate by the addition of a water-soluble bromide. As a chemical compound containing lead, it is categorized as probably carcinogenic to humans , by the International Agency for Research...

, and other materials.

Introduction

Acousto-optic effect

Acousto-optic devices

Acousto-optic modulator

Acousto-optic filter

Acousto-optic deflectors

Materials

See also