Vladimir I. Balakshy

Propagation of acoustic beams in a paratellurite crystal

A general solution to the problem of acoustic beam diffraction in an anisotropic medium has been obtained. The effect of acoustic anisotropy of paratellurite crystal on the structure of beams propagating in the XY crystallographic plane has been analyzed. Ray spectra are calculated for different propagation directions of ultrasound. The effects of beam focusing, defocusing, and autocollimation caused by acoustic anisotropy have been analyzed.

Optical image processing by means of acousto-optic spatial filtration

Results are presented of a theoretical and experimental study of a new method of optical image processing based on two-dimensional acousto-optic filtration of the spatial spectrum of images. Most attention is concentrated on the analysis of the acousto-optic cell transfer function form and its dependence on crystal cut, geometry of acousto-optic interaction and ultrasound frequency. Results of computer simulation of acousto-optic spatial filtering are illustrated by the example of an object in the form of an amplitude grating. Experiments investigating the transfer functions of acousto-optic cells with collinear and tangential geometry of acousto-optic interaction are described. Acousto-optic image processing is demonstrated experimentally using the example of the edge enhancement effect for some elementary images.

Acousto-optic properties of tellurium that are useful in anisotropic diffraction

The acoustic, optical and acousto-optic (AO) properties of tellurium crystals are examined here with the objective of developing AO devices in this crystal that operate in the 4–8 µm infrared region. Detailed analysis of anisotropic diffraction of light that is characterized by a change in the optical mode is carried out. The phase velocities of acoustic waves in three basic planes of the material, the directions of elastic polarization, and acoustic walkoff angles are calculated. Magnitudes of the AO figure of merit M2 in the crystal are evaluated to obtain optimum AO interaction geometries. Our results show that the slow shear elastic waves propagating in XZ and YZ planes of the crystal may offer the best AO interaction geometry applicable to AO deflectors and imaging filters, and predict a maximum value of M2 in tellurium to be 160 × 10−12 s3 kg−1. Measurement results for optical and AO parameters of tellurium are also included.

Acousto-optic collinear diffraction of a strongly divergent optical beam

Collinear diffraction of a strongly divergent optical beam by a monochromatic acoustic wave is investigated theoretically and experimentally. Three-dimensional patterns of acousto-optic (AO) phase matching are calculated and their transformation with varying acoustic frequency and ultrasound propagation direction is analysed. Two effects are discussed which appear at spectral collinear filtration of optical images, namely distortion of filtered image structure and spectral resolution deterioration. It is shown that the effects result in contradictory requirements on AO cell parameters. However, a compromise is possible which allows filtering of high-spatial-resolution images with an acceptable reduction in spectral resolution. Experimental results on diffraction of a strongly divergent He-Ne laser beam in a CaMoO4 AO cell are also presented in this paper.

Acousto-optic image processing

Acousto-optic processing of images is based on the angular selectivity of acousto-optic interaction resulting in spatial filtration of the image spectrum. We present recent theoretical and experimental investigations carried out in this field. Much attention is given to the analysis of the acousto-optic cell transfer function form depending on the crystal cut, the geometry of acousto-optic interaction, and the ultrasound frequency. Computer simulation results of the two-dimensional acousto-optic spatial filtration of some elementary images are presented. A new method of phase object visualization is suggested and examined that makes it possible to separate amplitude and phase information contained in an optical image. The potentialities of the acousto-optic image processing are experimentally demonstrated by examples of edge enhancement and optical wavefront visualization effects.

Polarization effects in acousto-optic interaction

Results of the theoretical investigation of light polarization changes during Bragg acousto-optic interaction are presented. It is shown that a linearly polarized light wave, entering an acoustic field, becomes elliptically polarized because of an additional phase shift effect. The polarization state can be controlled by varying power or frequency of the ultrasound. An effect of polarization nonreciprocity for two optical waves passing through the acoustic field in opposite directions is also studied.

Acousto-optic interaction in crystals with large acoustic anisotropy

The Bragg diffraction of light from a sinusoidal phase grating induced in an anisotropic medium by an acoustic wave has been studied theoretically for the case of a large acoustic energy walk-off. Based on the modified coupled-wave equations, the frequency and angular characteristics of an anisotropic diffraction in a paratellurite crystal have been calculated. It is shown that the acoustic beam walk-off significantly changes the angular and frequency ranges of acousto-optic interactions.

Acousto-optic interaction in a non-homogeneous acoustic field excited by a wedge-shaped transducer

The article is devoted to theoretical analysis of light diffraction in a non-homogeneous acoustic field created by a wedge-shaped piezoelectric transducer. Relationships describing electrical, acoustic and acousto-optic properties of the acousto-optic cell are derived in the approximation of a small thickness of the piezoelectric plate. Principal characteristics of acousto-optic interaction are examined such as dependences of the light diffraction efficiency on the phase mismatch and the acoustic wave amplitude. It is shown that the acoustic field has a complicated amplitude-phase structure changing with the acoustic frequency. It is demonstrated that the diffraction efficiency in the Bragg regime can approach 100% in spite of a noticeable phase mismatch. The appropriate optimal values of ultrasound power and incidence angles of light are found.

Compensation of thermal effects in acousto-optic deflector

Results on investigation of thermal effects which appear during operation of an acousto-optic deflector are presented in the paper. Influence of the effects on characteristics of the scanning device is evaluated. For a case of the tellurium dioxide deflector, it is proved that the most principal thermal effect consists in an undesirable spatial shift of a scanning optical beam. The shift leads to the optical beam positioning error which in a typical deflector may be of dozens of resolvable spots. A method of the thermal shifts compensation based on acoustic frequency tuning is proposed. For various regimes of the deflector operation, laws of the frequency tuning are found thus providing the compensation of the shifts with the required precision.

Application of acousto-optic interaction for holographic conversion of light fields

This paper represents a survey of research on the problem of acousto-optic conversion of optical images. This method of conversion permits us to retain all information that is contained in both amplitude and phase spatial modulation of a light wave. Particular attention is given to acousto-optic visualization of phase objects and registration of optical wavefronts. Information possibilities of the acousto-optic method are illustrated by numerical calculations as well as by some experiments recently performed in this area.