Acousto-optics or Acousto-optic effect, is a specific branch of physics that deals with the interaction between sound waves and light waves. It can more specifically be called as the diffraction of laser light by ultrasound or a sound wave with an ultrasonic grating. An ultrasonic grating is a kind of diffraction grating that is usually produced by disturbing ultrasonic waves in any medium which tends to alter the physical properties of that medium, and hence alter the refractive index in a grid-like pattern. The acoustic grating more specifically is a term that includes the operations at audible frequency ranges.
Optics have already been known to have a very long and full history since the time of ancient Greece and till modern times it has become a very essential part to be studied and gave rise to a new field of physics, the optics. If we compare the history of optics with the Acousto-optic effect, then we see that this acoustics branch has had a relatively shorter history. It began with Brillouin who predicted the diffraction of the light by an acoustic wave that is being propagated in a medium of interaction in the year around 1922. This fact was then confirmed further with experiments held in the year 1932 by Debye and Sears and then also by Lucas and Biquard.
If we talk about a particular case of diffraction, that too under a certain angle of incidence, this was observed by Rytow in 1935 and was also predicted by the famous scientist Brillouin. Raman and Nath (Indian) in around 1937, have also designed a general ideal model of interaction that take into account several other orders. This model was specifically developed by Phariseau in around 1956 for the kind of diffraction that includes only one diffraction order. In general terms, the Acousto-optic effects are primarily based on the change in the refractive index of a medium due to the presence of some kind of sound waves in that medium.
Sound waves produce a kind of refractive index grating into the material, and what is seen by the light wave is this grating only. These kinds of variations in the refractive index due to the pressure changes, can be easily detected optically by the processes of refraction, diffraction, and interference effects, in some cases reflection may also be used.
There are many uses of the Acousto-optic effect. It is extensively used in the measurement and the study of ultrasonic waves. There is a general growth in the principal area of interest in the Acousto-optical devices which is mainly in the field of deflection, modulation, signal processing and frequency shifting of various light beams. One possible reason for this may be due to the increasing availability and good performance of the lasers, which have made the Acousto-optic effect observations easier to not only observe them but also measure them.
If we look at the current scenario then we can see clearly that the Acousto-optics present us with a very interesting possible application. It may be used in case of nondestructive testing, structural health monitoring and other biomedical applications, where the waves that are optically generated and optical measurements of those ultrasound provides a non-contact method of imaging to us.
RELEVANCE WITH DIFFRACTION
The light that is diffracted by an acoustic wave having a single frequency usually produces 2 distinct diffraction types. These are Raman-Nath diffraction and Bragg’s diffraction. The Raman-Nath diffraction is observed primarily in low acoustic frequencies, which may be less than 10 MHz, and also with a small Acousto-optic interaction length which may be less than 1 cm.
Bragg’s diffraction on the other hand occurs mainly at a higher acoustic frequency that usually exceeds 100 MHz. The observed diffraction pattern by this method generally has two diffraction maxima but even these two diffraction maxima appear at definite angles of incidences which is close to the Bragg angle. Thus, it can be stated that as the acoustic frequency increases, the number of observed maxima gets reduced slowly owing to the fact of the angular selective nature of the Acousto-optical interaction. By changing the parameters of an acoustic wave, like the amplitude, phase, frequency and also polarization, the basic properties of an optical wave may be modulated or changed. The Acousto-optical interaction also makes it possible to change the optical beam by both temporal and spatial modulation.
A simple and easy method of modulation of the optical beam that is traveling through the Acousto-optic device can be simply done by just switching the acoustic field ON and then OFF. When turned OFF, the beam of the light is un-diverted. When it is switched ON and Bragg diffraction occurs, the intensity at the Bragg angle increases at such a phase. So, the Acousto-optical device is changing the output along with the Bragg diffraction angle when it is switched ON and OFF. The device is operated as a modulator when we keep the acoustic wavelength and the frequency fixed and we vary the driven power to vary with the amount of light in the deflected beam.
There are many limitations associated with the design and the performance of the Acousto-optical modulators. Most important is that the Acousto-optic medium must be designed carefully in a way that it provides maximum light intensity in a single diffracted beam. The time that the acoustic wave takes to travel across the whole diameter of the light beam gives another limitation on the switching speed of it and this in turn limits the bandwidth of the modulation.