Acousto-optic Crystal Research Articles

An ultra-fast distance sensor based on dynamic speckles generated by acousto-optic deflection

In this paper, we analyse the applicability of dynamic speckles for distance measurement to any rough surface. The technique is based on spatial filtering of a speckle pattern generated when the object surface is scanned by a laser beam deflected from an acousto-optic device. An extremely short response time was achieved because of the high scanning speed provided by the acousto-optic deflector. We have developed a prototype of the distance-measuring system and studied its performance. The measurement setup has a very simple configuration. The distance can be measured within as short a time as 50 ns, but with rather high inaccuracy caused by the stochastic nature of the speckle effect. The data averaging is easily implemented within a single scan and allows us to achieve an accuracy of 110 µm within the time window of 2.5 µs. An analysis of the factors affecting the performance of the proposed technique is carried out. It is shown that the response time can be diminished by an order of value using an acousto-optic crystal with higher acoustic speed. The proposed technique may be useful for monitoring the geometrical parameters of fast moving or rotating surfaces in various industrial applications.

Refractive index nonuniformities in acousto-optic devices due to heat production by ultrasound

The effect of heat in acousto-optic devices is a persistent problem with respect to the electric, acoustic and optical behavior of the device. The temperature differences across the acousto-optic crystal driven at acoustic power levels up to 1W, common for operation of these devices, can reach values of 90–100°C that cause significant refractive index nonuniformities, severely influencing optical performance of these devices. In this paper, we present a simple and effective physical model of the heat generation due to acoustic absorption in the device and calculate the temperature distribution that influences the optical parameters. We compare the modeling results with temperature distribution measurements on acousto-optic crystals of different configurations. The changes in refractive indices are also calculated.

Nearly backward reflection of bulk acoustic waves at grazing incidence in a TeO2 crystal

Reflection of bulk acoustic waves in a TeO2 acoustooptic single crystal is studied for the case of a grazing incidence on the free crystal-vacuum boundary. The propagation and reflection of elastic waves is considered in the XOY plane of the material cut out in the form of a rectangular prism. An extraordinary case of reflection at the grazing incidence, when the energy flow of one of the two reflected waves in the crystal is directed opposite to that of the incident wave, is studied. It is shown that the transformation of the incident elastic energy into the energy of the backward-reflected wave can occur with an efficiency close to 100% and can be observed in a wide range of crystal cut angles. An abrupt change of the reflection coefficients in the vicinity of the critical angle is predicted.

Dispersion compensation in high-speed optical coherence tomography by acousto-optic modulation

We report studies of the analyses of and compensation for group dispersion to improve the axial resolution of high-speed optical coherence tomography (OCT) by acousto-optic modulation (AOM). Theoretical modeling and experiments reveal that the high-order group dispersion induced by acousto-optic crystals broadens the measured coherence length (Lc) and thus degrades the axial resolution of OCT imaging. Based on our experimental studies, we can compensate for the dispersion to less than 50% broadening of the source Lc by adjusting the grating-lens-based optical delay in the reference arm and can further eliminate it by inserting like acousto-optic crystals in the sample arm of the OCT system. The results demonstrate that this AOM-mediated OCT system permits high-performance OCT imaging at A-scan rates of as much as 4 kHz by use of a resonant scanner. Because of its ultrastable direct frequency modulation, this AOM-mediated OCT system can potentially improve the performance of high-speed Doppler OCT techniques.

Diffraction of light by ultrasound under the conditions of Fresnel reflection

The features of the Bragg diffraction of light by ultrasound under the conditions of Fresnel reflection from the boundaries of a perturbed layer have been investigated. It has been shown that layers based on acousto-optical gyrotropic crystals perturbed by ultrasound can be used as efficient light modulators. The relative intensities of diffracted waves have been found to be determined by the relation of the refractive indices of adjacent media and by the ultrasound intensity. With an increase in the thickness of the modulated layer, the reflection and transmission coefficients of the first-order diffracted waves achieve their maxima at a lower ultrasound intensity.

音響光学レーザ偏向器を用いた高速現象の可視化

An acousto-optic laser deflector was used for visualization of high-speed phenomena, such as shock waves and density perturbations accompanying an impulse discharge, or shock waves generated by laser-induced breakdown in air. Using a continuous wave laser as the light source, shadowgraphs of shock waves and density perturbations were obtained at shutter speeds down to 1µs. Results showed that shock waves propagated at a speed of 417 m/s in the case of an impulse discharge, and 485 m/s in the case of laser-induced breakdown. Prebreakdown phenomena such as leaders progressing from the high-voltage electrode were also visualized. Compared to conventional high-speed imaging techniques, this method is useful when using a laser light source, since the acousto-optic crystal can accommodate high-intensity laser light. © 2005 Wiley Periodicals, Inc. Electr Eng Jpn, 154(3): 9–15, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20279

Visualization of high-speed phenomena using an acousto-optic laser deflector

An acousto-optic laser deflector was used for visualization of high-speed phenomena, such as shock waves and density perturbations accompanying an impulse discharge, or shock waves generated by laser-induced breakdown in air. Using a continuous wave laser as the light source, shadowgraphs of shock waves and density perturbations were obtained at shutter speeds down to 1µs. Results showed that shock waves propagated at a speed of 417 m/s in the case of an impulse discharge, and 485 m/s in the case of laser-induced breakdown. Prebreakdown phenomena such as leaders progressing from the high-voltage electrode were also visualized. Compared to conventional high-speed imaging techniques, this method is useful when using a laser light source, since the acousto-optic crystal can accommodate high-intensity laser light. © 2005 Wiley Periodicals, Inc. Electr Eng Jpn, 154(3): 9–15, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20279

Nearly Backward Reflection of Elastic Waves in an Acousto-Optic Crystal of Paratellurite

Some features of the propagation and reflection of acoustic waves in the XY plane of a paratellurite crystal have been studied. It is established that the acoustic waves exhibit an unusual backward reflection from the side crystal facet in the case of glancing incidence onto the crystal-vacuum interface. The experimental results are in good agreement with theoretical predictions.

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.

Acousto-optic photonic crossbar switch Part I: design

We present the design of a 12 × 12 photonic crossbar interconnection network constructed using a single three-dimensional acousto-optic crystal. Previous crossbars based on bulk acousto-optic cells require multichannel deflectors with one deflector per optical input; in contrast the design presented here angularly multiplexes these independent deflectors into a single-transducer acousto-optic device. A Fourier-optics analysis of an acoustically lossy Bragg deflector is coupled to a momentum-space analysis that permits the derivation of complete design equations for the switch. As a concrete example, the complete design of a 12 × 12 crossbar is presented. Finally, a coupled-mode analysis of the first- and second-order diffractions in the angularly multiplexed Bragg cell reveals the fundamental efficiency bounds of the switching network.

Electronically tuned Ti:sapphire laser

We have applied a birefringent acousto-optic crystal for wavelength tuning of a Ti:sapphire laser. A continuous-tuning range of 12 nm, which was limited by wavelength-dependent angle deviation of the beam def lection in the crystal, was extended to over 100 nm by optical correction. Fast and random access of lasing wavelengths by direct electronic tuning was demonstrated at a pulsed operation of 10 Hz.

Time-resolved Fourier transform spectroscopy with 100-ps acquisition time

As a step toward the realization of high-information time-resolved Fourier-transform spectroscopy, in a first experiment we have achieved time-resolved spectra with our modified high-information Connes-type interferometer as we explored its time-resolution capabilities. To this purpose, a well-defined test source made of a He–Ne laser intensity modulated by an acousto-optic crystal was built. Recorded spectra demonstrate that time resolutions ranging from 40 ns to 100 ps are possible. The spectra showed a 2-order-of-magnitude improvement over the best previous experimental results.

Unidirectional device for a ring laser using an acousto-optic modulator

The behavior of the first-order diffracted beam of an acousto-optic modulator is investigated under rotation of the acousto-optic crystal about an axis perpendicular to the plane of the incident and diffracted beams. It is found that the diffraction efficiency as a function of this rotation is not symmetric about the Bragg angle of incidence. A unidirectional device for a ring dye laser that uses this effect is demonstrated.

High‐speed photodamage cell selection using bromodeoxyuridine/Hoechst 33342 photosensitized cell killing

One of the major drawbacks of droplet sorting in a flow cytometer is the relatively low sorting speed. Thus, we have developed an alternative, faster sorting technique: photodamage cell sorting. In a photodamage cell sorter all unwanted cells, as detected with the first, measuring laser, are killed with the second, damaging laser. Thus, the cells need to be photosensitive to the second laser. In addition, a mechanism is needed to switch this laser on and off based on the sorting criteria. In our photodamage cell sorter, the ZAPPER, we use an acousto-optic crystal to switch the laser beam. Cells are made photosensitive by vital staining with photosensitizers. With cells grown in the presence of 5-bromo-2'-deoxyuridine (BrdUrd) and stained with Hoechst 33342 (H42) at least a 5-decade cell reduction is accomplished after irradiation with 400 mW UV light. With this system, sorting rates have been achieved of 30,000 cells per second. Due to the selection based on photodynamic killing, this sorting technique is restricted to the selection of viable cells. Photodamage cell sorting seems well suited for isolating viable cells occurring in low percentages or for the sorting of large numbers of cells. Another application can be the sorting of large or fragile cells.

Demonstration of bistability and multistability in wavelength with a hybrid acoustooptic device

Bistability in wavelength is demonstrated using an acoustooptic deflector driven by an RF oscillator whose frequency is a non-linear function of optical wavelength. Multistable operation featuring distinct stable states in wavelength is also described. It is shown that the use of wavelength as the switching parameter in an acoustooptic deflector with a <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\tau \cdot \deltaf</tex> time-bandwidth product permits one to obtain up to <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2\tau \cdot \deltaf</tex> steady states in the multistable regime. A number of stable states between 100 and 1000 times greater than those previously achieved using hybrid multistable devices in intensity are expected using this method. The switching time is given by the access time of the acoustooptic crystal. A possible application is discussed with a two-channel wavelength switch suitable for wavelength division multiplexing.

Liquid crystal cell for acoustical imaging

First Page

Anisotropic diffraction of light in a gallium phosphide crystal with induced anisotropy

Experimental and theoretical studies were made of the anisotropy of the Bragg diffraction of light by ultrasound in gallium phosphide crystals. An external static pressure produced a controlled optical anisotropy in the initially isotropic GaP samples. In this way a birefringence of ~10−4 was induced and it corresponded to acoustic frequencies of hundreds of megahertz. This should make it possible to use effectively acoustooptic GaP crystals in anisotropic diffraction devices for the control of laser radiation.

Comparison of CW Acoustooptical and Pulse-Echo Techniques for Measuring Ultrasonic Attenuation in TeO2and TiO2

Ultrasonic attenuation measurements obtained on the same samples by a CW acoustooptical technique and the standard pulse-echo technique are presented. For the two acoustooptical crystals studied, the results of the two methods are in good agreement. The frequency dependence of the attenuation along (100) and (1 10) directions in Ti02 is pre- sented here for the first time. Our results show that both materials are useful at least up to 600 MHz.