Acousto-optic Cell Research Articles

Spatio-temporal operator formalism for holographic recording and diffraction in a photorefractive-based true-time-delay phased-array processor.

We present a spatio-temporal operator formalism and beam propagation simulations that describe the broadband efficient adaptive method for a true-time-delay array processing (BEAMTAP) algorithm for an optical beamformer by use of a photorefractive crystal. The optical system consists of a tapped-delay line implemented with an acoustooptic Bragg cell, an accumulating scrolling time-delay detector achieved with a traveling-fringes detector, and a photorefractive crystal to store the adaptive spatio-temporal weights as volume holographic gratings. In this analysis, linear shift-invariant integral operators are used to describe the propagation, interference, grating accumulation, and volume holographic diffraction of the spatio-temporally modulated optical fields in the system to compute the adaptive array processing operation. In addition, it is shown that the random fluctuation in time and phase delays of the optically modulated and transmitted array signals produced by fiber perturbations (temperature fluctuations, vibrations, or bending) are dynamically compensated for through the process of holographic wavefront reconstruction as a byproduct of the adaptive beam-forming and jammer-excision operation. The complexity of the cascaded spatial-temporal integrals describing the holographic formation, and subsequent readout processes, is shown to collapse to a simple imaging condition through standard operator manipulation. We also present spatio-temporal beam propagation simulation results as an illustrative demonstration of our analysis and the operation of a BEAMTAP beamformer.

Prise en charge des lombalgiques

The quasicollinear geometry of acousto-optic (AO) diffraction is notable as makes it possible to achieve an extremely high AO interaction length and, consequently, an anomalously high spectral resolution for AO devices. This geometry is especially convenient for the implementation of multifrequency AO diffraction, which has found wide application for solving the problems related to the laser pulse shaping. Since acoustic beams propagate over long distances in quasicollinear AO devices, and optical radiation spectral components diffract in the acoustic field in different parts of the AO crystal, accurate calculation of the characteristics of such devices requires knowing the distribution of the acoustic field amplitude inside the AO cell. The acoustic beam structure is affected by several factors in quasicollinear AO cells: the dimensions of the piezoelectric transducer, the geometry of acoustic wave propagation in the AO cell, acoustic anisotropy and the acoustic energy absorption along the chosen direction in the crystalline material used. In this paper, we propose a generic method to measure the acoustic beam power distribution along the direction of its propagation in the quasicollinear AO cell in the presence of ultrasound power absorption and media acoustic absorption. The measurements were carried out for the ultrasound frequency range from 72 to 176 MHz, for the case when the wave vector of the acoustic beam is directed at an angle of 1.58∘ to the [110] axis in the (11¯0) plane of the paratellurite crystal. The ultrasound attenuation coefficients were obtained for frequency interval between 87 and 176 MHz and their linear dependence on ultrasound frequency was confirmed.

Dynamic demonstration of diffractive optic analog-to-digital converter scheme.

Dynamic behavior of an analog-to-digital converter (ADC) based on diffractive optical element(s) (DOE)(s) was studied and found to be in agreement with predictions. The analog signal was translated to an angular deflection of a laser beam by means of an acousto-optic (AO) cell. The number of bits in this experimental demonstration was three, using an eight-element DOE array. The maximum sample rate was found to be 2.5 MS/s, the limiting factor being the transit time for the acoustic wave across the width of the laser beam in the AO cell. The study is intended as a first dynamic demonstration of a proposed ADC scheme previously demonstrated in a quasi-static version. The full potential of the ADC scheme will require the use of a fast tunable diode laser to replace the AO deflection scheme used here.

Collinear diffraction of restricted light beams by nonmonochromatic sound

The collinear diffraction of Gaussian light beams by a two-frequency sound beam is investigated theoretically. A set of equations relating the amplitudes of transmitted and diffracted waves is derived in the case of third-order nonlinearity. The distributions of light intensities along an acousto-optical cell are calculated at different ratios between the radii of the light and sound beams. The transmission curves for detuned light waves and combination components of the diffracted waves are constructed at different ratios between the radii of the light and sound beams. The changes in the dynamic range of an acousto-optical device are calculated for different diffraction efficiencies.

Nonlinear dynamics of a Bragg cell under intensity feedback in the near-Bragg, four-order regime.

A detailed examination of the nonlinear dynamical behavior of an acousto-optic Bragg cell in the near-Bragg regime of operation for the case of four scattered orders under intensity feedback is carried out. This problem is an extension of the standard ideal-Bragg feedback model whereby traditionally bistability, hysteresis, and chaotic oscillations are observed under zeroth- or first-order feedback of the scattered light. For the present case, the closed-loop equations are developed from a priori knowledge of the open-loop analytical solutions for four-order near-Bragg scattering. The results, obtained via computer simulation, reveal a variety of interesting dynamics, including bistability, bifurcation, hysteresis, chaotic oscillations (including in this case the relatively uncommon period-three behavior, in addition to the more usual period-doubling phenomenon en route to chaos), and potentially useful parametric dependence of these features. The observed results are interpreted in terms of system behavior for varying feedback gain and bias, the so-called Klein-Cook parameter Q, and time delay, and are compared with earlier work based on the ideal Bragg regime.

Reviews Of Acoustical Patents

The purpose of these acoustical patent reviews is to provide enough information for a Journal reader to decide whether to seek more information from the patent itself. Any opinions expressed here are those of the reviewers as individuals and are not legal opinions. Printed copies of United States Patents may be ordered at $3.00 each from the Commissioner of Patents and Trademarks, Washington, DC 20231. Patents are available via the Internet at http://www.uspto.gov.

Polarization-independent acoustooptically tuned spectral filter with frequency shift compensation

A new polarization-independent tunable spectral filter based on the combination of a single isotropic acoustooptic cell and a pair of conventional diffraction gratings is suggested and investigated. The filter features narrow bandwidth, frequency shift compensation, and low polarization dependence. Using standard AMTIR-1 chalcogenide glass acoustooptic modulator with frequency 40-77 MHz and ruled diffraction gratings with 600 grooves/mm, a 3-dB bandwidth of 1.1 nm, and a tuning range of 16 nm were obtained. Tuning range can be increased by the use of acoustooptic cell with increased center frequency or diffraction grating of a lower dispersion. Inserting the filter inside a laser cavity, continuous-wave tunable generation of Er-doped fiber laser with maximum output power 3 mW was realized. When a Fabry-Perot etalon (0.166-nm free spectral range, 40% reflectance) was inserted into laser cavity, the linewidth was observed to be less than 0.05 nm.

Analysis of the AO-FDPC optical heterodyne technique for microwave time delay and phased array beamsteering applications

This paper presents a theoretical analysis of the application of the acousto-optic frequency-dependent phase compensated (AO-FDPC) optical heterodyne technique for microwave time delay applications and phased array beamsteering. A primary goal of the paper is to resolve open questions that have been associated with this interesting and highly referenced technique. The work presented here quantifies, for the first time, the fundamental time delay performance bounds of this technique in terms of the parameters associated with the AO-FDPC acousto-optic (AO) cell and the signal bandwidth. The theory presented in this paper is used to interpret previously reported experimental results that have been subject to debate. Much of the theoretical approach is general and may, therefore, be modified to address the design of new FDPC approaches. Finally, the wide-band beamsteering performance that can be achieved with this technique is quantified in terms of the AO cell parameters and phased array specifications.

Two-dimensional selection of optical spatial frequencies by acousto-optic methods

We theoretically and experimentally examine the selection of spatial frequencies of optical beams carried out by acousto-optic meth- ods. Control of the optical rays is provided in two orthogonal directions with the help of a single acousto-optic cell fabricated of a birefringent paratellurite single crystal. The investigation is performed using a cell of a tunable acousto-optic filter illuminated by coherent radiation of a He-Ne laser at optical wavelength l5633 nm. The filter is capable of a 2-D selection of optical beams propagating all over the range of spatial fre- quencies un xu5un yu5<300 cm 21 in the spectral passband Dn550 to 100 cm 21 . Optimal regimes of the acousto-optical interaction character- ized by high efficiency of the filtration are considered during the re- search. New results on processing of optical images formed by divergent optical rays are presented and regular trends of the processing are dis- cussed. © 2002 Society of Photo-Optical Instrumentation Engineers.

Optimization of the transfer function of an acoustooptic cell with an apodized piezoelectric transducer

It is shown that an apodized piezoelectric transducer can significantly reduce the side lobe level of the acoustooptic cell transfer function. Series and symmetric connections of the transducer sections and measures to suppress the effect of spurious elements arising in the electric circuit are proposed. In particular, the effect of spurious capacitances and inductances on the frequency response of the transducer is studied. It is shown that they violate the optimal condition for the suppression of the transfer function side lobes, especially at high frequencies. It is shown by calculations that the effect of spurious elements can be eliminated by the insertion of additional capacitors at 80 and 150 MHz.

Submicrosecond speed variable optical attenuator using acoustooptics

A novel variable optical attenuator (VOA) is introduced that can provide high dynamic range and high-resolution all at fast submicrosecond speeds. The VOA design exploits an in-line one-to-one imaging optical architecture operating with Bragg-mode acoustooptic Bragg cells. Optical attenuation is achieved via frequency and drive voltage control of the Bragg cells, leading to flexible and precise attenuation-level settings. VOA demonstrated a 45-dB dynamic range with a 175-ns switching time. Other measured parameters include 1.14-dB/MHz resolution, 1.56-dB polarization dependent loss, and an excess loss of 3.55 dB.

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.

Optical packet synchronization device based on acousto-optic interaction: characterization and performance

An optical packet synchronizer for telecommunication networks at 1.55 µm is presented. The device carries out the phase alignment of packet flows at the input of a switching node. It is based on packets switching by means of an acousto-optic cell towards various optical paths achieved with optical fibres. The characterization of an acousto-optic cell providing four diffraction directions is presented. From this cell we have implemented a synchronizer device with four discrete delays (400 ns delay resolution) and we have experimentally evaluated its performance in terms of configuration time (130 ns), insertion loss (< 18 dB), transparency to data bit rate and cross-talk (< -22 dB).

Acousto-optic interactions and devices in a spherical waveguide

A spherical surface was previously shown experimentally to be capable of simultaneously guiding, collimating and focusing a light beam, and thus serving as an alternative substrate for realization of acousto-optic (AO) Bragg cell modulators and related device modules. In this paper we report, for the first time, a theoretical treatment for such spherical guided-wave AO interactions together with new experimental results at significantly higher surface acoustic wave centre frequency (500 MHz) using a LiNbO3 hemisphere block. The theoretical analysis for the optical and the acoustic waves is carried out using the lowest-order asymptotic expansions and the resulting AO Bragg diffraction calculated using the Green function technique.

Transmission function of acousto-optic cell with apodized piezotransducer

The problem of transmission function side-lobe suppression is discussed. The set of equations describing the acousto-optic (AO) diffraction problem and bounding the amplitudes of transmitted and diffracted light and their solution that determines the optical wave amplitudes at the AO cell output is presented. Two schemes of piezotransducer section connection are presented and two methods of piezotransducer section connection - symmetrical and in series - are analysed. The acoustic amplitude distribution along the apodized piezotransducer was investigated. The optimal relationships between the piezotransducer section lengths were calculated. The possibility of making the AO filter with a pair of identical apodized cells is considered. The influence of spurious network parameters on the acoustic amplitude distribution is analysed.

Acousto-optic modulator for IR diode laser radiation

A new amorphous Si20Te80 glass alloy with high optical transparency has been synthesized and investigated. A modulator based on the amorphous Si-Te acousto-optic cell and operating in the mid-IR region has been manufactured and tested using CO2 (10.6 µm), He-Ne (3.39 µm) gas lasers and 3.3 µm (based on InGaAsSb/InAsSbP double heterostructure), and 1.87 µm (based on GaInAsSb) diode lasers. The modulator response time appeared to be not more than 0.3 µs which together with the high modulation efficiency (up to 90%) make possible use in many applications, e.g. for modulation of CW laser radiation, for optical gas sensor applications, for diode laser spectroscopy to make mode selection of wavelength selection within a lasing pulse.

A new method of acousto-optic image processing and edge enhancement

The paper presents detailed results of research devoted to the development of a new acousto-optic method of spatial frequencies filtration. The filtration of coherent optical beams is examined theoretically and basic statements of the investigation are confirmed experimentally. A selection of partial rays forming optical images and control of their propagation directions are provided by the application of acousto-optic cells based on paratellurite single crystals. Efficient regulation of the beams was executed in two orthogonal directions by means of the wide-angular regime of Bragg diffraction in the birefringent materials. The advantages of this new method of image processing have been demonstrated during edge enhancement performed electronically in real time.

Acoustooptical Spectral Processing of Radar Signals Taking into Account the Signal Spectra of Local Heterodyne and Optical Radiation

We describe the formation of a radio signal designed for acoustooptical processing based on the radar signal analyzed which is characterized by a spectrum, a heterodyne signal with a spectrum of known structure, and a mixer. The signal formed fits the working frequency of the acoustooptical cell. In the cell, the signal produces a traveling acoustic wave due to propagating inhomogeneities of the dielectric constant or refractive index. Within the framework of physical optics, we consider how the acoustooptical cell used for processing the signal works in the regime of the Raman–Nath diffraction. The cell is illuminated by quasimonochromatic radiation of known arbitrary spatial and spectrum structures. One works in the first and higher diffraction orders. Using an original method proposed earlier, we analyze the functioning of the acoustooptical cell in the Bragg diffraction regime at small radiation transformation coefficients. The description is done for a probing signal of known spatial and spectrum structures. The method of acoustooptical analysis of the spectrum structure of a radio signal or radar signal is considered under the decreased influence of the spectra of the probing optical radiation and heterodyne signal. The effect of the spectrum structure of the probing radiation used when processing the radio signal which propagates within the acoustooptical cell is studied in complete analogy with taking into account the exciting radiation spectrum in spectroscopy. For signals with a low level of noise, we use the method based on the Fourier transformation of the intensity distribution registered and on that of the intensity distribution of the initial probing radiation spectrum. Effects of the width and spectrum structure of the heterodyne can also be taken into account similar to the method of excluding the spread function used in investigating spectrograms. The processing uses Fourier transforms of the registered spectrum intensity distribution and of the known spectrum intensity distribution of the heterodyne signal.

Acousto-optic interaction of diverging Gaussian beams in anisotropic media

A strong acousto-optic interaction of bounded light and sound beams of a Gaussian shape is considered for different geometries permitting long-term interaction. The anisotropic spreading of an acoustic beam in the course of its propagation is taken into account. The spectra of light beams are described by a Gaussian-polynomial expression, and a set of differential equations is derived which allows one to describe various geometries of acousto-optic diffraction. It is demonstrated that the transmission function of an acousto-optic cell essentially depends on the ratio between the dimensions of the light and sound beams and on the angle between the propagation direction of the diffracted beam and that of the incident beam. The degree of spreading of the acoustic beam noticeably affects the suppression of the transmission side lobes.

Improvement of the acousto-optic cell function by piezotransducer sectioning

Improvement of the acousto-optic cell function by piezotransducer sectioning