Acousto-optic Cell Research Articles

Acousto-Optic Cell for Controlling Rotation of the Polarization Plane of Linearly Polarized Optical Radiation

Acousto-Optic Cell for Controlling Rotation of the Polarization Plane of Linearly Polarized Optical Radiation

Mathematical and Numerical Explanation of the Nonlinear Acoustic Wave Interaction in Acousto-Optical Cells

Mathematical and Numerical Explanation of the Nonlinear Acoustic Wave Interaction in Acousto-Optical Cells

Temperature impact on acoustic wave reflection in quasi-collinear acousto-optic devices.

Quasi-collinear geometry is a special configuration of acousto-optic (AO) diffraction that applies the acoustic wave reflection from the AO cell input optical face and provides an extremely large interaction length for achieving abnormally high spectral resolution of AO tunable filters. As a result, it becomes possible to implement the multifrequency diffraction which has found important applications for laser pulse shaping. The operation of quasi-collinear AO devices in the multifrequency diffraction regimen is accompanied by the appearance of the longitudinal and transverse temperature gradients in the crystal, mainly due to the acoustic power absorption. Temperature changes the AO cell material stiffness moduli, affecting the characteristics of the incident and reflected acoustic waves (propagation velocities and walk-off angles), and the reflection condition in general. On the example of paratellurite crystal is shown that the AO cell heating near the reflecting facet leads to a deviation of the reflected acoustic beam propagation direction from that specified during the AO cell manufacturing. The deviation magnitude depends on the reflection geometry choice and, in the paratellurite, may exceed several degrees, which adversely affects the AO diffraction characteristics, reducing the AO interaction efficiency and distorting the transmission function shape. The reflected beam deviation may be compensated by means of choosing the angle between the AO cell reflecting face and the piezoelectric transducer face, taking into account the operating AO device thermal regimen.

Polymer-dispersed liquid-crystal coatings for ultrasound visualization: Experiment and theory.

Polymer dispersed liquid crystal (PDLC) films are formed of droplets of liquid crystal (LC) held in a polymer matrix. Similar to aligned LC films, PDLCs exhibit the acousto-optic (AO) effect when excited by acoustic waves of sufficient amplitude, whereby the PDLC film becomes transparent in the excited regions (acoustic clearing). Despite decades of research there is still debate over the mechanisms of the AO effect for the case of LC films, with several competing theories, and AO effects in PDLC have not been studied theoretically. This paper explores the AO effect in PDLC both experimentally and theoretically, and attempts a theoretical description of the observed phenomena based on the theoretical approach by Selinger etal. for aligned LC films. The acousto-optic effect in PDLC is shown to be due to direct interaction of acoustic waves with LC droplets, rather than due to compression of the droplet itself. Polarizing microscopy revealed changes in droplet shape at excited points. This is consistent with reorientation as a contributing factor, possibly coexisting with flows at higher excitation powers. In previous experimental studies PDLC films were prepared with cover slides, in the same way as LC AO cells, significantly limiting applications by adding complexity to the design. Also, to exhibit AO clearing it was considered that the PDLC needed to be prepared with high LC concentrations (over 75% by weight). We demonstrate that no cover slide is necessary, and that PDLC coatings without a cover have improved sensitivity to acoustic waves. We demonstrate the AO effect for LC concentrations as low as 40% by weight. The ability to use standard composition PDLC, with no top cover, is paving the way to paint-on visual ultrasound sensors.

Collinear acousto-optic filtering of divergent optical beams in the presence of optoelectronic feedback

The divergence of a diffracting light beam affects the characteristics of acousto-optic (AO) interaction. This is due to the side components of the optical beam spatial spectrum propagating in the crystal along the directions different from the beam axis. The divergence causes the AO diffraction efficiency decrease and the distortion of the AO device transmission function — its bandwidth increases and the side lobes are smoothed. We suggest the method to compensate the optical beam effect on the collinear AO diffraction transmission functions shape based on optoelectronic feedback circuit application connecting the AO cell optical output and its piezoelectric transducer. It is shown that such method makes it possible to narrow the transmission function passband and increase the spectral contrast of the system. It is found out that feedback does not affect the AO diffraction transfer functions. Thus, the introduction of feedback makes it possible to increase the spectral resolution of AO devices without compromising the AO device angular aperture.

Laser beam Steering According to Linear Trajectories Using an Acousto-Optic Cell

Laser beam Steering According to Linear Trajectories Using an Acousto-Optic Cell

Examination of an acoustic field longitudinal power distribution in quasicollinear acousto-optic cells

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.

Terahertz imaging using optically controlled Fourier-basis structured illumination.

We demonstrate that a new type of structured-illumination imaging may be migrated from the optical to the terahertz domain. This Fourier-basis technique involves illuminating a target with rapidly moving sinusoidal fringes of controllable spatial frequency and orientation, while measuring the scattered radiation on a single fast detector. This initial proof-of-concept demonstration is purely one-dimensional since the fringe orientation is fixed, but the technique is readily extensible to two dimensions. The fringes are first generated in the near-infrared (808 nm) by passing a high-power laser beam through an acousto-optic Bragg cell driven by a superposition of two RF signals slightly offset in frequency, blocking the undeflected beam, and refocusing the two diffracted beams onto a metal-backed semiconductor wafer. The laser can be amplitude modulated to slow down the moving fringes to accommodate the semiconductor's temporal response. The semiconductor acts as an optically addressed spatiotemporal modulator for a THz beam illuminating the same area. The periodic optical fringes effectively transform the semiconductor into a reflective THz diffraction grating with a programmable period. The diffracted THz radiation is then imaged onto the remote target plane, where the diffraction orders interfere pairwise to create traveling THz fringes. Scattered radiation from the target is collected by a simple receiver operating in "light bucket" mode, which produces an output signal consisting of a superposition of sinusoidal tones, one for each spatial Fourier component of the target. We present measurements of the THz fringe projector's performance and compare with a model of the semiconductor modulator's operation. Finally, we present Fourier-reconstructed images of pairs of point targets as an initial demonstration of THz Fourier-basis agile structured illumination sensing imaging.

Acousto-optic interaction in LGS and CTGS crystals

Based on the matrices of elastic and elasto-optic coefficients, the extreme surfaces of the spatial distribution of the acousto-optic (AO) figure-of-merit М2 for langasite and calcium-tantalum gallosilicate crystals are constructed and analyzed. The determined maximal achievable values of М2 for these crystals are commensurate and twice the maximal value of М2 for quartz. The most effective AO interaction geometries and geometry of cuts for optimized AO cells are given. The investigated crystals, due to the temperature stability, excellent optical quality, resistance to environmental aggression and high optical damage threshold can be used for AO modulators of the ultraviolet spectral range.

Evaluation of Acoustic Waves in Acousto-Optical Devices by Ultrasonic Imaging.

The structure of the acoustic field defines the key parameters of acousto-optical (AO) devices. To confirm their compliance with the expected values in the presence of multiple real factors, AO crystalline cells require accurate experimental investigation of the acoustic field after being totally assembled. For this purpose, we propose to detect and quantify all the acoustic waves propagating in AO cells using an impulse acoustic microscopy technique. To validate this approach, we have analyzed both theoretically and experimentally the modes, amplitudes, propagation trajectories, and other features of the ultrasonic waves generated inside an AO modulator made of fused quartz. Good correspondence between theoretical and experimental data confirms the effectiveness of the proposed technique.

Determination of the paratellurite stiffness constants temperature coefficients by the acousto-optic method

Acousto-optic (AO) phase matching condition is determined not only by the optical and acoustic waves propagation direction and parameters, but also by the physical characteristics of the AO interaction medium. Wherein, these characteristics, mainly the acoustic wave velocity, are temperature dependent. The change of ultrasound velocity relies on the medium stiffness constants cij variation with temperature. However, even for the most popular AO material paratellurite (TeO2), the values of the stiffness constants temperature coefficients, given in various papers, vary significantly. The experimentally observed AO phase matching frequency temperature shift values do not coincide with those evaluated with literature data. In order to eliminate the observed discrepancy between theory and experiment, the AO phase matching frequency temperature dependencies for several AO cells based on TeO2 crystals with cut angles varying from 1.8∘ to 19∘ from the [110] axis in the (11¯0) plane were analyzed. The ultrasound velocity variation temperature coefficients were evaluated and the c11−c12 and c44 temperature coefficients that define the slow acoustic mode velocity in the selected plane were determined.

Aberration analysis of AOTF-based stereoscopic spectral imager using optical design software

Stereoscopic spectral imagers using acousto-optic tunable filters (AOTF) provide high-resolution narrow band images acquired from two viewpoints with different polarization in arbitrary spectral intervals, which allows obtaining three-dimensional hyperspectral models of inspected objects for many applications. We discuss modeling of acousto-optic (AO) cell for optical system design and introduce a program module for ray tracing through AO cell compatible with Zemax optical design software. A detailed study of the optical aberrations that limit the image quality in two AOTF-based stereoscopic systems implementing simultaneous AO diffraction of two differently polarized beams in single AO cell is presented. This approach may be used to design various AOTF-based imaging systems, but the limitations of ray tracing analysis should be considered.

Acousto-optical metasurfaces for high-resolution acoustic imaging systems

This work introduces a paradigm for acoustic imaging in which a metasurface converts the acoustic waves scattered by remote scenes into coherent light focused into images by conventional optical cameras. The metasurface is composed of acousto-optical unit cells that sense the local acoustic pressure and use the resulting signal to modulate the amplitude of the electric field produced by a laser on an optical aperture. We derive the general design requirements for the image reconstruction in the optical domain and validate the concept through acoustic field measurements of the ultrasound scattered from an object submerged in water followed by numerical simulations predicting how this field is processed by the metasurface and camera. We show that this approach has two main advantages compared to traditional acoustic imaging systems. First, the acoustic-to-optic wavelength down-conversion leads to effective acoustical apertures very large compared to the physical size. Second, the unit cells are not synchronized electronically and thus the complexity of the metasurface increases only linearly with the number of unit cells, which is a significantly slower increase compared to conventional synchronized arrays. This work shows that these advantages lead to compact acoustic cameras providing image resolutions higher than possible with conventional acoustic imaging methods.

Production of dichromatic light with a rotating polarization vector using a single Bragg cell

We propose an approach for producing a rotating polarization vector of dichromatic light controlled by the acoustic frequency. This approach is based on diffraction of one of the eigenmodes for each monochromatic component during two passes of dichromatic light through a gyrotropic acousto-optical cell and is demonstrated using rotation in the polarization of dichromatic light generated by an Ar laser at wavelengths of 0.488 µm and 0.514 µm using a paratellurite acousto-optical cell operating at 108 MHz.

Spectral Imaging Experiments with Various Optical Schemes Based on the Same AOTF.

Spectral image filtration by means of acousto-optical tunable filters (AOTFs) has multiple applications. For its implementation, a few different optical schemes are in use. They differ in image quality, number of coupling components, dimensions and alignment complexity. To choose the optical system of AOTF-based spectral imager properly, many factors have to be considered. Though various schemes of acousto-optic (AO) filtration have been tested and discussed, their comparative analysis has not been reported up to now. In this study, we assembled the four most popular schemes (confocal, collimating, tandem and double-path) using the same AO cells and experimentally compared their main features. Depending on the application, each scheme may be the basis of compact cost-effective spectral imaging devices.

Acousto-optic modulation of light polarisation in monoclinic crystals

A configuration of a laser polarisation modulator based on isotropic acousto-optic interaction is proposed. Polarisation control is performed by a 2D acousto-optic cell based on one monoclinic single crystal. A possibility of implementing this modulator on the basis of a KY(WO4)2 crystal is demonstrated.

Two-dimensional image edge enhancement using a spatial frequency filter of two-color radiation

Two-dimensional optical image edge enhancement is investigated using a spatial acousto-optical (AO) filter operating at two wavelengths of optical radiation, which significantly increases the reliability of measurements and also reduces the effect of noise. The filter operates basing on diffraction of light in two diffraction orders. It is shown that, in the general case, it is impossible to obtain a two-dimensional image contour simultaneously at two wavelengths at the same energy and frequency-angular parameters of the AO filter. A regime has been found that makes it possible to switch from one wavelength to another while maintaining the operation of edge enhancement by changing the acoustic power. This regime has been experimentally confirmed by the example of the formation of a two-dimensional image contour using two-colour radiation of an Ar laser emitting at wavelengths of 0.488 × 10−4 and 0.514 × 10−4 cm, and an AO cell made of TeO2, operating at a sound frequency of 57 MHz.

Acousto-Optic Cells with Phased-Array Transducers and Their Application in Systems of Optical Information Processing.

This paper presents the results of theoretical and experimental studies of anisotropic acousto-optic interaction in a spatially periodical acoustic field created by a phased-array transducer with antiphase excitation of adjacent sections. In this case, contrary to the nonsectioned transducer, light diffraction is absent when the optical beam falls on the phased-array cell at the Bragg angle. However, the diffraction takes place at some other angles (called “optimal” here), which are situated on the opposite sides to the Bragg angle. Our calculations show that the diffraction efficiency can reach 100% at these optimal angles in spite of a noticeable acousto-optic phase mismatch. This kind of acousto-optic interaction possesses a number of interesting regularities which can be useful for designing acousto-optic devices of a new type. Our experiments were performed with a paratellurite (TeO2) cell in which a shear acoustic mode was excited at a 9° angle to the crystal plane (001). The piezoelectric transducer had to nine antiphase sections. The efficiency of electric to acoustic power conversion was 99% at the maximum frequency response, and the ultrasound excitation band extended from 70 to 160 MHz. The experiments have confirmed basic results of the theoretical analysis.

Использование модуляционной акустооптической спектрометрии для определения местоположения спектральных особенностей в перекрывающихся спектрах

The spectrometer is not able to accurately determine the spectral position of the smaller maximum if it is located against the background of a powerful broad peak. The solution of the problem is known it is enough to differentiate the existing dependence, and the position of the smaller peak is determined with sufficient accuracy. Based on a quasi-collinear acousto-optic (AO) cell, a prototype spectrometer was created, which makes it possible to record both the spectrum of the optical signal and its derivative in real time. In the course of the work, a more detailed research of the work of the created model was carried out: the value of the phase shift was changed programmatically from zero to 360°, and the spatial modulation period - from zero to the length of the AO cell L. A neon lamp and all the data presented were used as a radiation source, were obtained for the same emission line. The maximum signal for the derivative corresponds to the phase values of 90° and 270°. In another series of experiments, the work of the model was investigated with a change in the modulation period (the phase shift is fixed, ψ=π/2). The maximum signal for the derivative corresponds to the values of the modulation period duration equal to L/2, half the length of the acousto-optic cell. The graphs of the results of "physical" and "mathematical" differentiation do not coincide. We assume that the differences are related to the shape of the instrumental function of the acousto-optical spectrometer. However, at the tops of the spectral peaks, in the regions of intersection of derivatives with a zero line, the results coincide, which makes it possible to use acousto-optic differentiation to reveal the "fine structure" of optical spectra, and in real time. Thus, as a result of the work carried out: a method was proposed for the precise determination of the position of spectral maxima in complex overlapping spectra in real time; an instrumental implementation of the proposed method was created, and it was shown that for the visible range (532 nm) the accuracy of determining the spectral position of the maxima is 0.2 nm.

Determination of the Paratellurite Stiffness Constants Temperature Coefficients by the Acousto-Optic Method

Acousto-optic (AO) phase matching condition is determined not only by the optical and acoustic waves propagation direction and parameters, but also by the physical characteristics of the AO interaction medium. Wherein, these characteristics, mainly the acoustic wave velocity, are temperature dependent. The change of ultrasound velocity relies on the medium stiffness constants cij variation with temperature. However, even for the most popular AO material – tellurium dioxide, the values of the stiffness constants temperature coefficients, given in various papers, vary significantly. The experimentally observed AO phase matching frequency temperature shift values do not coincide with those evaluated with literature data. In order to eliminate the observed discrepancy between theory and experiment, the AO phase matching frequency temperature dependencies for several AO cells based on TeO2 crystals with cut angles varying from 1.8° to 19° from the [110] axis in the (1‾10) plane were analyzed. The ultrasound velocity variation temperature coefficients were evaluated and the c11-c12 and c44 temperature coefficients that define the slow acoustic mode velocity in the selected plane were determined.