Vladimir N. Parygin

Prolonged acousto-optic interaction with Lamb waves in crystalline plates

The propagation and acousto-optic interaction of Lamb modes in an anisotropic plate of tellurium dioxide (TeO2) are studied numerically and analytically. In the case of a Y-cut X-propagating TeO2 plate, the very high elastic anisotropy of the crystal greatly modifies the dispersion curves, giving rise to their multiple oscillations. The existence ranges of backward Lamb modes increase with the mode order contrary to the case of isotropic plates. The quasi-collinear light scattering by Lamb waves is considered. Owing to the structure of Lamb wave field, a simultaneous light diffraction at two different optical frequencies can take place while Lamb waves are excited only at the single frequency. It is demonstrated with the Z-cut (110)-propagating plate that a small change in the acoustic frequency can result in a significant shift in the frequency of the scattered light.

Collinear acousto-optic interaction of divergent beams in paratellurite crystal

Theoretical and experimental investigation of collinear diffraction of divergent optic beams by ultrasonic waves near by the direction [110] of paratellurite crystal has been carried out. The diffraction has been examined in direction [110] forbidden in TeO2 for acousto-optic interaction in the case of plane waves. It has been shown that the collinear diffraction along this direction exists only for divergent light and sound waves while efficiency of the diffraction increase with the growth of the divergence. The effect has been examined at the acoustic frequency f≈ 149 MHz and the wavelength of the non-collimated light λ=633 nm with the optical spreading up to 4°. A collinear acousto-optic filter on paratellurite applying the diffraction has the length of interaction l=2.7 cm. It has been proved the acoustic attenuation causes a decrease of efficiency of the diffraction and a broadening of a bandwidth of the device. The spectral resolution value as high as R~3000 has been obtained a λ=633 nm. The diffraction efficiency I1/I0≈0.8 and the wide angle aperture Δφ≈4° give perspectives to use the collinear diffraction in filters on the base of TeO2 crystals.

Improvement of the acousto-optic cell function by piezotransducer sectioning

Abstract The set of equations describing the acousto-optic diffraction problem and bounding the amplitudes of transmitted and diffracted light and their solution that determines the optical wave amplitudes at the acousto-optic cell output is presented. It is shown that when the piezotransducer is divided in sections, the non-uniform distribution of an acoustic amplitude can be obtained; this can lessen the acousto-optic cell transmission function side lobes maxima. Two methods of the piezotransducer sections connection-symmetrical and in series-are analysed. The calculation testified that the diminution of side lobe intensity is proportional to the number of sections. The affection of the phase shift between sections upon the form of the transmission function was investigated. It was shown that a small phase delay of electric signals feeding the neighbouring sections result in the asymmetry of the transmission function and in its main maximum shift.

Collinear interaction of Gaussian acoustic and light beams

Abstract A general expression for the amplitude of diffracted light under collinear interaction of Gaussian acoustic and light beams is obtained. It is shown that, if the radius of the acoustic beam has a finite value, the central frequency of the transmission curve shifts with respect to a point corresponding to the exact Bragg condition. The value of the light beam radius can increase or decrease this shift for upshifted or downshifted diffraction respectively. An optimum relationship between the interaction length and the acoustic beam radius is found. This optimum condition is related to a minimum of input acoustic power.

Collinear and quasi-collinear diffraction of bounded beams in crystals: acousto-optic interaction for the example of paratellurite

The equations in general form which permit us to solve the problem of collinear and quasi-collinear diffraction of three-dimensional optic and acoustic beams in an anisotropic medium are derived. These equations in the case of Gaussian optic and acoustic beams and under the conditions of the weak acousto-optic (AO) approximation are solved analytically. The optical beam spectra are described by the polynomial-Gaussian distribution and such an approach makes it possible to describe different prolonged AO interaction geometries. The possibility of collinear diffraction of bounded beams along the direction forbidden for plane waves is theoretically shown. The example calculation provided for the paratellurite crystal shows that the diffraction efficiency can reach a noticeable level at the usual magnitudes of input acoustic power.

COLLINEAR FILTER CONTROLLED BY VARIABLE ULTRASONIC PULSES

The experimental investigation of the problem of collinear acousto-optic tunable filters driven by variable acoustic pulses is carried out. It is shown that bandwidth and shape of transmission function can be altered in this case by varying the shape of a driving pulse. Experi- mental acousto-optic tunable filter transmission curves and their side wings are presented. It is shown that the utilization of pulses with a smooth temporal envelope results in a decrease of sidelobe level in the acousto-optic tunable filter transmission curves. It is found that the dif- fracted light can be generated continuously if the interpulse distance is equal to the crystal length. The transmission bandwidth in this case is determined by the length of each acoustic pulse. The impact of the phase difference between acoustic pulses on the diffraction efficiency and on the diffracted light form is investigated.

Strong collinear and quasi-collinear interaction of three-dimensional beams

The peculiarities of collinear and quasi collinear acoustooptical interaction in which the energy propagation directions of incident optical and acoustic beams coincide is theoretically investigated. The approximation of slightly divergent optical and acoustic beams in anisotropic medium is used. The problem of slightly divergent acoustic beam propagation in an anisotropic medium within the limits of parabolic approximation is discussed. Generalized equations describing the quasi-collinear interaction of light and sound in an anisotropic medium are obtained. An approximate method of solution of these equations in the case of the light beam from invariancy during diffraction is suggested. Both transmitted and diffracted beams are considered to have Gaussian distribution of amplitude with their radii slowly varying along the interaction coordinate. The system of first order equations that bound characteristics of transmitted and diffracted light beams and of acoustic beam is deduced. The solution of these equations in the case of strong acoustooptical interaction is analyzed.

Tunable acousto-optic filters and their applications in laser technology, optical communication, and processing of images

The paper presents results on research of tunable acousto- optic filters operating in ultraviolet, visible and infrared regions of spectrum. High spectral and spatial resolution, perfect diffraction efficiency and low driving power provide application of the filters in optics and spectroscopy as well as in optical information processing and laser technology. Precise and efficient electronic control of laser light intensity and fast electronic tuning of lasing wavelength was executed by means of collinear acousto-optic devices. Application of the collinear filters as selectors of arbitrary polarized optical signals in modern WDM waveguide communication lines has also demonstrated high capabilities of the acousto-optic instruments. Experiments with the acousto- optic devices on base of the wide-angular geometry of interaction confirmed the unique possibility to regulate spatial structure of convergent laser beams and optical rays forming images.

Quasi-collinear AOTF with improved resolution

A number of possible configurations of acousto-optical tunable filter (AOTF) with a quasi-collinear geometry of interaction in the paratellurite crystal are observed. A theoretical examination of the AOTF resolution improving problem in the case of quasi-collinear geometry of acousto-optical interaction is presented. The analysis of interaction of light and sound in different planes of the paratellurite crystal is carried out. Devices with spherical angles of sound equal to 85.5° and 88° in the plane (110) are calculated. Some new and original configurations of AOTF are described. Possible angle apertures of sound and light in all these kinds of devices are observed and the frequency dependence, figure of merit dependencies are presented.

Collinear and quasi-collinear acousto-optical diffraction: basic equations and two-crystal tunable devices

An approximate solution of the equations governing collinear acousto-optical interaction in terms of confluent hyper- geometric function is obtained under conditions of plane- wave approximation. However, the experimental acousto- optical devices deal with a beam of finite dimensions which imply a substantial transversal and longitudinal inhomogeneity altering considerably the results of diffraction. A theory of such quasi-collinear interaction is suggested. An acousto-optical tunable filter is one of most promising devices using acousto-optical diffraction and able to ensure high-resolution spectral analysis of natural optical radiations could further be improved if two instead of one consequently arranged crystals were used in the acousto-optical cell.