Acousto-Optics
In 1922, Leon Brillouin began to analyze thermal acoustic
fluctuations in liquids and solids by the scattering of light or X-rays off of
changes in the refractive index of the medium. In his paper, Brillouin gave the
first look at the theoretical explanation of what has become known as
acousto-optic diffraction. He used an analogy to diffraction gratings to
determine binding equations on the output light. From his theory he was able to
predict some of the common phenomena known today regarding acousto-optic (AO)
devices, namely the deflection of light and the frequency shift of the
deflected light. An acousto-optic effect is produced by generating an
ultrasonic wave in an optically transparent material. As the acoustic wave
travels through the medium, it causes periodic variations in the index of
refraction. For instance, in a crystalline solid, the wave produces compressions
and refractions in the crystal lattice. Because the changes in the refractive
index are periodic, the material behaves much like a diffraction grating, with
site spacing equal to the wavelength of the ultrasonic wave.
The Acousto-Optic effect was first observed in 1932 by Lucas, Biquard, Debije
and Sears. The effect became extremely important after the invention of laser
light by Theodore Maiman in 1960, and found its way in electronic communication
systems and other fields of interest, very rapidly.
The early experiments have first been theoretically explained by Raman and
Nagendra Nath in 1935.
Typically the acoustic wave is launched into the medium by a piezo-electric
transducer. Application of an electric field to the transducer has the effect
of deforming the material, producing an internal strain. This strain is
transmitted to the acousto-optic medium which is mechanically coupled to the
transducer. The acousto-optic medium can be liquid (such as water), solid (such
as lead molybdate), or even gas (such as water vapor). Any strain in the
material should produce a change in the refractive index. Incident light on the
acousto-optic medium will scatter off variations in the refractive index. The
scattering of light can be tuned to create destructive interference between
certain wavelengths, thereby constructing an optical filter. Other common uses
of acousto-optic media include devices for modulating light for communication,
deflecting light, convolving or correlating signals, optical matrix processing,
analyzing the spectrum of signals, optical sources, laser mode lockers,
Q-switchers, delay lines, image processing, general and adaptive signal
processing, tomographic transformations, optical switches, neural networks,
optical computing, and much more.
In Raman-Nath scattering, also known as Debye-Sears scattering, the thickness,
L, of the cell is much less than a quarter wavelength. It is so thin that there
is no significant interaction between the even and odd waveguide modes. In this
case, the normally incident electromagnetic field will be spread out into a
number of grating modes, each of which is shifted in frequency either up or
down by an integer number of the acoustic wave frequency. This case is more
complicated to analyze since it involves all of the Fourier modes. In general
sidebands of like order are equal in amplitude and decrease as a function of n.
Bragg scattering can be accomplished by having a very wide slab of material,
the diffraction is said to be in the Bragg regime. The main characteristic of
Bragg diffraction is two output beams: the undiffracted beam or carrier beam,
and the principal diffracted beam or the sideband beam. The sideband beam will
be frequency shifted either up or down depending on the direction of the
incoming carrier beam. This case is similar to Bragg diffraction of X-rays in a
crystal lattice. It turns out that other orders are produced within the
acousto-optic device, but these orders destructively interfere since they are
not phase matched. The only order which is phase matched throughout the lattice
is the principal diffracted order.
Besides many AO devices, it is also possible to apply Acousto-Optics for
nondestructive testing. This can be done by Schlieren photography or by means
of Bragg imaging. The current trend in our team is to continue the work of the
A few examples:
Schlieren
photography to study sound interaction with highly absorbing materials
Very often, characteristics of a reflected beam are studied in order to obtain information about the material on which the reflection occurs. Nevertheless, in the exceptional case where the material is so highly sound absorbing, there is no reflected beam and therefore it is impossible to examine reflected beam properties. In such situations, Schlieren photography is a promising tool because it enables simultaneous visualization of the incident sound beam and the generation of heat on the surface. The technique can be used to study the extent of sound along the surface and to study the heat transformation rate. In addition, the technique does not require the installation of sensors, whence it is completely nondestructive.
Detection of fiber direction in composites by means of high frequency wide bounded beam and Schlieren photography
Fig.: This enlarged Schlieren image of a bounded ultrasonic beam, shows a fringing pattern that enables the determination of the fiber direction in composites.
There are very sophisticated techniques available to determine the fiber direction in composites. However, those techniques enable extraction of much more information than just the fiber direction. This makes them generally complicated to perform and require highly specialized staff. We have presented a very simple technique, applying Schlieren photography and a relatively high frequency wide bounded beam, to detect a spatially varying reflection coefficient on fiber reinforced composites. This spatial variation results in a fringing pattern that is straightforwardly related to the fiber direction.