Scientific area

"OPTO-ELECTRONICS AND OPTICAL INFORMATION PROCESSING"

Opto-electronics represents a novel area in physics and engineering which deals with problems of transmission, reception, processing and storage of information by applying both optical and electrical methods and means. Intensive development of opto-electronics began in the sixties of the past century just after appearance of sources of coherent light - lasers. The laser radiation, due to a high frequency of oscillations and coherence, gives absolutely unique capabilities for transmission and processing of information. The laser carrier can be modulated not only in time (as in radio-electronic systems), but also in space. Thanks to this peculiarity the information capacity of an optical communication channel can be in excess of many orders than that of radio-electronic systems. However, for the advantages of laser radiation to be realized, effective devices for optical beam control must be developed. The difficulty of this problem lies in the fact that optical beams cannot be acted upon by electric and magnetic fields directly, contrary to electronic beams, for example. Solely a non-direct change of optical wave parameters is possible through an action upon the medium where the optical beam propagates. Currently two effects are employed predominantly for laser radiation control, namely, electro-optic and acousto-optic effects. In the former case, peculiar crystals are applied, which change its refraction index under action of an electrical field. The acousto-optic effect consists in diffraction of light on a phase grating created in a medium by an acoustic wave. Varying parameters of the acoustic wave, we can change accordingly amplitude, frequency, phase and propagation direction of light.

Investigations being fulfilled in our scientific group are based on both the effects. There are two lines in these investigations. The first one has a fundamental character; it consists in study of these effects themselves, their regularities and peculiarities in various media and at different conditions of experiment. The second line of research is in the development of novel opto-electronic instruments and devices based on the electro-optic and acousto-optic effects. Theoretical and experimental investigations carried out to date allowed us to design and study new types of the devices with unique characteristics, which have no analogues in the world.

Acousto-optic interaction of wave beams having a complicated spatial and temporal structure (Prof. V.I.Balakshy; lab. 1-65)

Acousto-optic interaction has been studied in detail in the case of plane harmonic acoustic and optical waves. However, in real acousto-optic devices the finite dimension beams are employed, possessing complicated spatial and temporal spectra. The interaction of such beams is being investigated in our laboratory. As a rule, acousto-optic devices are linear relative to light and non-linear relative to sound. Only in the case of a weak acousto-optic interaction these systems become linear relative to both waves, this fact facilitates the solution of the diffraction problems considerably. Since 1988 the series of investigations have taken place devoted to the estimation of acoustic beam divergence influence on light diffraction. The diffraction by acoustic pulses of different duration was also studied. In this case the shape of the pulse can affect the acousto-optic cell transmission function form substantially. The pulses with a smooth temporal envelope decrease the transmission function side lobes considerably. If a number of short acoustic pulses is launched in the acousto-optic crystal simultaneously, then the apparatus function consists of a number of narrow peaks. The distance between them depends on the number of pulses in the crystal, while the number of peaks depends on the duration of each acoustic pulse.

Another method of regulation of the acousto-optic cell transmission function consists in apodization of piezoelectric transducer which serves for excitation of ultrasound. The technology of creation of the transducers achieved in the present time such a level that one can get almost any radiation diagram of the transducer and, consequently, any transmission function of the cell. The transducer apodization is fulfilled by means of its sectioning. The sections differ in sizes and thickness and may be excited by different electrical signals. As a result, the acoustic field created by the transducer has a very complicated structure and the calculation of acousto-optic interaction in this field can be carried out only by means of computer.

Recently, we paid attention to study of the light diffraction by a multi-frequency acoustic signal. The specific nonlinearity of acousto-optic interaction leads to the appearance of combinative frequencies in diffracted light spectrum. This results in the emergence of spurious signals at the acousto-optic spectrum analyzer output. A new method of selection of the spurious signals from the true signals is proposed by our collaborators.

Processing of optical images by means of acousto-optic spatial filtration (Prof. V.I.Balakshy; lab. 1-65)

Acousto-optic processing of images is an area of acousto-optics that has been originated and developed predominantly thanks to efforts of collaborators of our group. As early as 1983, it was proved that the angular selectivity of acousto-optic interaction makes it possible to act selectively upon the spatial spectrum of images, transforming it as necessary. Due to this action we can eliminate hindering details from the image (for example, dot structure in typographical pictures), change contrast of the image, produce edge enhancement in order to detect masked objects etc. Adjusting definitely the acousto-optic cell, one can fulfill with the optical signal such important operations as spatial differentiation and integration. The main advantage of the acousto-optic method of optical image processing consists in the capability of quick electronic tuning of the transfer function (by means of varying acoustic wave parameters) that makes it possible to process images in real time.

By applying the acousto-optic spatial filtration, invisible phase objects can be made visible, in other words, it means visualization of the wave front of a coherent optical wave. The problem of phase object visualization is widely met in optics, laser physics, biology, medicine etc. For their solution acousto-optic sensors worked out in our group can be effectively utilized. Developing this line of research, our collaborators have shown both theoretically and experimentally that on the basis of acousto-optic interaction the holographic process of a novel type can be realized, which differs essentially from conventional holography. In this process, full information (both amplitude and phase one) being present in an image is transferred into an electrical signal and thereafter, at the reception end of the information channel, the initial image is reconstructed from this electrical signal by means of an acousto-optic cell.

Spectral filtration of optical signals (Prof. V.I. Balakshy; lab. 1-65)

Tunable acousto-optic filters that provide spectral filtration of arbitrary polarized light were developed. These filters have extremely high spectral resolution and low RF power consumption. Compact and fast operating spectral devices for various applications were developed on the base of these filters.

Acousto-optic interaction in media having strong optical and acoustic anisotropy (Prof. Polikarpova N.V. lab. 1-62, Prof. S.N. Mantsevich, lab. 1-65)

Acousto-optic interaction in far infra-red and THz region (E.A. Dyakonov; lab. 1-65)

Acousto-optic interaction in biaxial crystals (E.A. Dyakonov; M.G. Milkov; lab. 1-65)

Dynamic processes in acousto-optic systems with feedback (Prof. V.I.Balakshy, Prof. S.N. Mantsevich and Prof. Y.I.Kuznetsov; lab. 1-65 and 1-63c)

The studies involve a number of problems that represent a novel avenue of research in acousto-optics. Acousto-optic systems with feedback are fundamentally nonlinear systems in which a signal delay in the feedback circuit is of primary importance. Very complicated dynamic phenomena, including both regular oscillations of various forms and chaos, are possible in the systems. On the basis of such systems a variety of effective instruments can be designed, for example, systems for stabilization of laser radiation parameters. In particular, we have designed and created a system which provides laser beam direction stabilization with the stabilization coefficient as high as 100. An acousto-optic generator based on the effect of optical heterodyning presents another example. This generator possesses unique properties. It has much in common with lasers, specifically, such effects as synchronization and competition of generated modes well known in laser physics have been observed in the acousto-optic generator as well. A high sensibility to varying acousto-optic interaction parameters permits us to forecast wide applications of these devices in measuring facilities.

Themes of scientific research suggested to students for graduation thesises imply both theoretical studies and experimental work at unique setups. Our students gain present-day notions of opto-electronics and optical information processing as well as of close domains of physics - optics, acoustics, crystallography and laser physics - that forms the basis for competent and fruitful work on stated problems. Since presently opto-electronic instruments are widely utilized not only in scientific research but also in everyday life, our graduates easily find job in firms where their knowledge is completely acceptable.