This paper discusses measurements of non-stationary optical signals using an acousto-optic spectrometer. Any
spectral measurements should take non-stationary forms of optical signals into account. The paper presents the
results of experiments where a non-stationary optical beam from an electric spark was used. An acousto-optic
tunable filter based on off-axis diffraction in the TeO2 crystal was used. The total analysis time was approximately
1 sec for an optical signal cycle of 100 Hz. The spectrum had a "black body" shape with distinguished fragments at
572 nm and 471 nm. The paper also proposes a method to increase the accuracy of measurement.
The paper presents theoretical and experimental investigations of the structure of the spatial distributions of strain
amplitudes for an oscillating surface using an optical interferometer. A conventional Michelson interferometer and
an interferometer containing an acousto-optic cell have been studied. It has been shown that the maximum possible sensitivity of the device is 0.001 Å, with the minimum strain amplitude reaching 0.001 Å. Practical advantages of the interferometer with an acousto-optical cell have been realized.
Acousto-optical tuning has been implemented for an α-Al2O3:Ti3+ laser pumped by a 30 W Cuvapor laser, which is a generator-amplifier system operating at 15,625 Hz, with a pulse duration of 50 nsec. The system can select an arbitrary laser wavelength in the IR range for approximately 64 μsec. Experimentally the transformed IR beam has been varied in the visible and UV range.
When a new element base is created for information and telecommunication systems, the properties of the materials used must be well understood. In particular, a thorough investigation of
the elastic and elasto-optical properties is required for materials used in optical telecommunication systems, optical display panels, acousto-electronic information devices, etc. For example, the elastic
nonlinearity for various crystals must be known. If the materials under study are optically transparent, acousto-optics can be used to obtain such information. It is known that elastic nonlinearity is one of the most important factors distorting and limiting the performance of acousto-optic information systems. Due to such nonlinearity, the
attenuation coefficient of acoustic waves increases; this effect can be strong in some materials and depends on the acoustic power density.
Using the coupled-mode method, new solutions describe the spatial distributions of the acoustic wave amplitude, taking into account the initial power of the wave at the generation edge and assuming the square law of the nonlinearity. The current method approximates real distributions more accurately than previous solutions, and we have confirmed this experimentally. The experimental results of a specific nonlinearity in an acoustic resonator are shown. The higher harmonics sharply reduce their amplitudes after reflecting from the rear face of the sample, then the amplitudes steadily increase. The decrease in the amplitude of the higher harmonics is related to the decrease of the attenuation coefficient for the main acoustic mode. Using the measured data, an approximate synthesis has been made for the shape of the acoustic signal deformed by the elastic nonlinearity of the medium. Evaluations have been made for a 500 MHz longitudinal acoustic mode and an initial power level of 200 W/cm2 at various traveling points of that wave in LiNbO4 crystal.
This paper discusses some ways to use acousto-optic devices for controlling optical beams in high-power laser systems. As a medium of interaction, TeO2 crystals grown using a specially developed technology are suggested. Some examples of the use of the developed acousto-optic devices in high-power laser systems are presented.
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 global and versatile problem of fire and environmental safety is formulated. It is pointed out that one of the main ways to solve this problem is the development of equipment for early fire detection. The results of the development and study of a smoke fiber optic fire detector are presented. Such detector is absolutely explosion-safe and immune to increased radiation level and aggressive chemical environment.
The devices entering an input information signal into an optical processing scheme are the most important elements in optical information and measurement systems. For a number of applications, acousto-optic cells are the most preferable such devices. The parameters of the devices entering the initial signal into optical processing schemes determine the maximum and operation characteristics of the entire system. The questions of the design, manufacturing, and characterization of acousto-optic cells for information and measurement systems are discussed.
The paper discusses the mechanism of cross-produce component generation connected with the acoustic parameters of the medium and acoustic power density for the fundamental frequencies. The expressions for the spatial distribution of harmonic components for the fundamental and cross-produce frequencies are given. The paper presents some experimental results of spatial distributions for the fundamental and higher acoustic harmonics for several widely used acousto- optic crystals under different power density levels of fundamental harmonic. Basing on these experimental data, are analyzed the variations of the spectrum spread function for the space-integrating acousto-optic spectrum analyzer and spatial optical distribution of cross-product components.
Three architectures of acousto-optic deflectors performing both spatial and angular multiplexing for holographic memory systems are discussed. The main parameters of such deflectors are determined and analyzed. It is found that increase in one parameter inevitably results in decrease in one or two other parameters. The highest information capacity can be achieved in the two coordinate multichannel variants.
An acousto-optic method for spectral processing of rf signals is proposed. This method is based on a multichannel cell with frequency separated channels within a given band. The optimum structure of such a system is a multichannel cell with the slow shear mode in the (110) direction in TeO2 and far- axis anisotropic diffraction. A system with 12 channels covering the frequency band of 84 - 96 MHz with the bandwidth of each channel of approximately 0.5 MHz and frequency separation of approximately 1 MHz is experimentally studied. An optical beam which spreads in the plane orthogonal to that of the acousto-optic interaction must be used in this system. The influence of the transducer electrode shape on the acoustic crosstalk in the adjacent channels is studied. The experimental results are in good agreement with the calculated data. The expansion of acousto-optic processing requires that multichannel acousto-cells be used. Narrow-band acousto-optic interaction regimes can be used for frequency-domain filtering of rf signals in multichannel cells. This scheme can be used for the parallel analysis of an rf signal spectrum. This paper describes the process of the design and manufacturing of a multichannel acousto-optic filter for an rf signal with a narrow bandwidth of each channel and estimates its possible parameters. Each channel of the filter is tuned to its own frequency different from those of the adjacent channels within a given overall bandwidth of the whole device.
A review of physical properties of the materials used in manufacturing real acousto-optic cells for optical and RF signal processing is presented. The overall design of acousto-optic cells requires the knowledge of not only the elastic modulii of the second order, but those of higher orders also, as they determine the acoustic nonlinearity and the dynamic range of any device. The parameters of elastic anisotropy determine the near zone of cell's transducer and directly affect cell's time aperture and number of resolvable spots, and interchannel cross-talks in multichannel devices. The requirements to the properties of crystal materials used in cells with anisotropic and collinear interaction are discussed.
A review of physical properties of the materials used in manufacturing real acousto-optic cells for optical and RF signal processing is presented. The overall design of acousto-optic cells requires the knowledge of not only the elastic modulii of the second order, but those of higher orders also, as they determine the acoustic nonlinearity and the dynamic range of any device. The parameters of elastic anisotropy determine the near zone of cell's transducer and directly affect cell's time aperture and number of resolvable spots, and interchannel cross-talks in multichannel devices. The requirements to the properties of crystal materials used in cells with anisotropic and collinear interaction are discussed.
The increase in information capacity of acousto-optic systems can usually be achieved by increasing the bandwidth (Delta) f of their acousto-optic cells. At the present time, acousto-optic cells can have (Delta) f of 1.2 GHz. During the process of manufacturing a system with maximum possible frequency characteristics, some problems could arise, the main one is the problem of electric power feeding to the top electrode of a piezoelectric transducer. This problem is caused by small dimensions of the top electrode (tens or hundred micrometer). Due to the same reason, the acoustic power density drastically increases in the acousto-optic cells (up to 0.5 - 1.0 kW/cm2), which could produce nonlinear effects in the medium of propagation, and thus considerably restrict the dynamic range of the acousto-optic systems. Moreover, high acoustic power localized in a small volume can result in some thermoacoustic effects and even destroy the acousto-optic material. High electric strength (up to 30 - 50 kV/cm) in the material of the piezoelectric transducer can also be a problem. These problems are very hard to solve, and their solutions usually decrease several parameters of the acousto-optic cell, and in some cases, there are no solutions at all. To some extent, feeding the electric power is similar to the common problems of UHF transistor manufacturing, and such experience can be used, but the other problems are very specific and have nothing common in the modern electronics. That is why new alternate solutions are very important.
The results of experimental investigations of the regime of acoustic interference in acousto- optic cells for optical beam control are represented. The crystals TeO2, LiNbO3, Al2O3 and KRS-6 are used as the materials of interferometers. The forms of frequency responses of interferometers verify sharp changes of input impedance. The transitory regime of interferometer is investigated and the changing of input impedance of interferometer in transient process is shown. The extension of frequency bandwidth approximately 2 times more is observed. The gain of peak power for constructive interference was reached approximately 4 divided by 30 times for different conditions. The results of experimental investigations of quasi- collinear acousto-optic filter using degenerated shear mode in (001) direction of LiNbO3 crystal is presented.
Among contemporary well-known acousto-optic rf signal processing systems, space integrating acousto-optic spectrum analyzers (SIAOSA) are most adapted for commercial applications. This is because their laboratory prototypes achieve high parameters and their elementary base is well-developed. SIAOSA can process rf signals directly at carrier in the metric and decimetric bands, has a wide band of simultaneously processed frequencies, high parameter stability, and small weight and dimensions. Due to this unique parameter combination, SIAOSA are already used in warfare, rf monitoring, and radio astronomy. A method for SIAOSA design is required in order to broaden the area where those systems can be used. In its turn, such a method requires a SIAOSA mathematical model for simulating its operation. The most general mathematical model is a statistics model. But such a model requires statistic characteristics of separate SIAOSA elements, and their connections and joint operation. The latter information can be obtained from a deterministic model based on the study of how a known signal passes through SIAOSA. Such a deterministic model is used in most works on acousto-optic spectrum analyzers. This paper is based on an experimental model and devoted to some ways of SIAOSA performance enhancement.
Acousto-optic devices are used for the control of laser radiation in different branches of science and technology, and for real-time information processing. Optical systems with two-dimensional architectures make it possible to realize space-multichannel information processing systems. We provide a historical excursus, a presentation of the contemporary state of research, and examples of experimental studies carried out at the Leningrad Institute of Aviation Instrument Making. Some realizations of acousto-optic systems for spectral, correlation, and filtering analysis of electrical signals are discussed. The parameters of several tunable acousto-optic filters are presented. Some problems connected with the manufacturing of acousto-optic elementary base and potentially feasible parameters of acousto-optic systems are discussed.
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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