Acousto-optic methods are widely used not only in devices and systems for radio and optical signal processing, but also for studying physical properties of various materials.
The paper presents the results of the investigation of the "non-resonance" (due to native piezoelectric effect) generation of elastic waves at super high frequencies in the Bi12GeO20 crystal, using the acousto-optic method. Acoustic field patterns are visualized in various crystal cuts, including the "degenerated" acoustic mode in the [111] direction in the crystal.
Conditions are discussed under which acoustic modes and directions exist with the "self-collimation" effect (abnormally low power spread of acoustic wave). Quantitative estimation of collimation quality is made for various conditions.
Experimental characteristics of high-order harmonics generation processes appeared due to elastic non-linearity of the material are found. Basing on the experimental data, some parameters of the elastic non-linearity are evaluated.
The influence of elastic anisotropy on effects of power spread in crystals from the langasite family is under study. Conditions for self-collimation directions for which power spread is much less than that for the isotropic variant, are formulated. Directions with the quality of collimation close to the maximum possible value are found in langasite and langatate (LGT) crystals. Presented are computed spatial distributions of acoustic wave power at different distances from the source. Those distributions illustrate the advantages of self-collimation directions. Computed are the angular distributions of collimation coefficients for several planes and the optimal shape for the top electrode is found for resonator structures that uses self-collimation directions in the crystals from the langasite family.
The paper presents the analysis of applications and performance characteristics of one-crystal 2D acousto-optic deflectors used for optical signal switching in communication systems. It compares performance and prospects for acousto-optic deflectors made of different material and of different layouts. It shows that 2D light deflection requires higher uniformity of frequency responses of each acoustic channel. Prospects for a 2D deflector built using 'strong' elastic anisotropy in some crystals are estimated. In this case, acoustic beams can coincide, their wave normals being approximately orthogonal. It is shown that crystals with high acoustic velocities and moderate elastic anisotropy (like GaAs) look more promising than traditional for acousto-optics TeO2 in tasks requiring high operation speed (low switching time).
The comparative performance analysis for single-dimensional and two-dimensional acousto-optic deflectors is presented. A new generalizing parameter equal to the product between the number of resolvable spots and acousto-optic diffraction efficiency is proposed. The possibility to use self-collimating modes in crystals for two-dimensional deflectors is discussed. A new geometry for a two-dimensional deflector with coinciding acoustic beams is proposed. This geometry is based on the deviation of energy transmission direction from the wave normal. The experimental test results for the TeO2 two-dimensional deflector with coinciding acoustic beams are presented. The test results are in good agreement with estimations.
Direction [110] in the TeO2 crystal has unique acoustic and acousto-optic properties. This direction is widely used in many applications. But a slow shear acoustic wave traveling in this direction looks very unpromising for multichannel acousto-optic cells because of its low channel- to-channel isolation under reasonable channel package density. This low isolation is due to the abnormally high physical spread of this slow shear wave traveling in the [110] direction, and the therefore high value of the acoustic anisotropy coefficient b equals 26. During recent investigations, a direction exhibiting self-collimation with the coefficient b approximately equals 0.3 was found. This direction lies in the optical plane under the angle of 29 degree(s) with respect to the [110] axis. But the acousto-optic figure of merit M2 appears smaller for this direction. A number of directions in the TeO2 crystal's optical plane combining acceptable values of the acousto-optic figure of merit M2 and channel-to-channel insulation under high channel package density have been experimentally and theoretically studied. Discussed and analyzed are the features of these directions when they are used in multichannel acousto-optic cells with the wideband anisotropic diffraction.
A spectrum analyzer of optical signal based on an acousto- optic tunable filter (AOTF) is a sequential analyzing system where the scanning along the wavelengths is made by varying the control signal frequency. Traditionally, a chirp control signal is used in such systems, provided that the frequency variation rate in the cell's acoustical aperture is negligibly slow. In this case, the system spectral response replicates the amplitude frequency response of acousto-optic interaction. However, when the system speed are to be maximized, the frequency variation rate in the cell aperture has to be taken into account. Basing on the linear system theory, the paper presents the calculation of the spectrometer's output signal for a laser input optical beam, weak interaction approximation, and chirp RF control signal. The closed form equation is obtained using the stationary phase method. Basing on the obtained expression, the minimally possible time of analysis is obtained for the spectrometer based on the AOTF with the quasi-collinear interaction in the TeO2 crystal. The criterion was the 10 percent decrease in spectral resolution. The experimental results are in good agreement with the theoretical predictions.
KEYWORDS: Acousto-optics, Signal processing, Optical signal processing, Modulators, Spectrum analysis, Data processing, Electro optics, Radio optics, Bragg cells, Analog electronics
The history of acousto-optic is discussed. Numerous architectures of acousto-optic systems for spectral and correlation processing of electric and optical signals are analyzed. The results of experimental studies of signal processors and the parameters of experimental prototypes of acousto-optic cell prototypes are presented.
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