Particular features of acousto-optic visualization in scattering media upon detection of nonballistic light

Images of optically inhomogeneous objects immersed into a cell with a scattering liquid through which orthogonal laser and ultrasonic beams pass are obtained upon measuring the scattered light modulated at the ultrasonic frequency. The alternating current from a photodetector recording the optical radiation emerging from the cell was used as a parameter of the acousto-optic visualization. The quality of the visualization is analyzed in relation to the registration conditions and the scattering parameter magnitude, as well as the size and shape of three-dimensional and planar objects. Under the experimental conditions used, the positions of objects immersed into the scattering liquid are detected fairly precisely, with a satisfactory contrast and sharpness of images in the plane perpendicular to the laser beam axis, whereas, along this axis, the object positions are not determined. The experiment was performed under conditions of transition from a regime without scattering to a regime of multiple scattering.     

Time resolved three-dimensional acousto-optic imaging of thick scattering media

Acousto-optic imaging is based on light interaction with focused ultrasound in a scattering medium. Thanks to photorefractive holography combined with pulsed ultrasound, we perform a time-resolved detection of ultrasound-modulated photons in the therapeutic window (780 nm). A high-gain SPS:Te crystal is used for this purpose and enables us to image through large optical thickness (500 mean free paths). We are able to generate three-dimensional (3D) acousto-optic images by translating a multielement ultrasound probe in only one direction. A 3D absorbing object is imaged through a 3 cm thick phantom.     

Pulsed acousto-optic imaging in dynamic scattering media with heterodyne parallel speckle detection

We present a new detection scheme for acousto-optic tomography based on pulsed-wave ultrasound and illumination combined with heterodyne parallel speckle detection. This setup can perform tomographies inside several-centimeter-thick scattering samples. Test experiments confirm the suitability of this method for performing tomographies inside various types of optically scattering media, including liquids.     

Particular features of the acoustic wave propagation in condensed media in different conditions

The light spectrum of molecular scattering is used for studying the particular features of the hypersound propagation in a guaiacol—glycerol binary solution with two critical points. The kinetics of the spectrum of scattered light in liquid salol is studied when the viscosity of the latter varies over ten orders of magnitude. Acoustic singularities are revealed in the critical region at the phase transition and at a large viscosity.     

Polarization visualization of scattering media with CW laser radiation

The method of polarization visualization of a multiply scattering medium containing macroinhomogeneities based on analysis of polarization spatial distribution of a scattered linearly polarized light is discussed. The treatment is based on statistical properties of the effective optical path distribution of scattered field components. The influence of media scattering properties and the geometry of the experiment on the inhomogeneity image contrast obtained with use of polarization degree and of normalized scattered intensity of radiation as visualization parameters are discussed, as well as spatial resolution achieved in these both cases. Using the results of theoretical analysis and of the experimental model, the relationship between the shapes of spatial distributions of polarization degree and the intensity of the scattered light is considered as a function of the position of the visualized object (an absorbing half-plane immersed in a plane layer of the scattering medium). The opportunities for enhancing the quality of the images formed in this way are also discussed.     

Polarization changes and beam profile deformation of light during acousto-optic interaction in isotropic media

A spatial Fourier transform approach is proposed to investigate the effects of polarization changes and beam profile deformation of light during acousto-optic (AO) interaction in isotropic media. The behaviour of the total scattered optical fields inside the AO cell can be properly described by a vector wave equation of which the permittivity is perturbed by an acoustic wave propagating inside the medium. In the Bragg regime, using a spatial Fourier transform approach, two coupled differential equations can be derived from the wave equation to depict AO interaction in the spatial frequency domain. Analytic solutions, which comprise the effects of changing polarization, beam deformation and propagating diffraction, can be found from the coupled equations. Detailed numerical simulations, including Fourier transforming the incident light profile to calculate the spectra of the scattered light beams and, hence, their profiles in space using the inverse transform, are presented.     

散射介质中层间杂质检测效率的影响因素及分析

散射介质中层间杂质检测是一个非常重要的研究课题.改进现有的Monte Carlo方法,模拟大量光子在散射介质中的传输,得到入射光强、杂质埋藏深度、介质折射率、介质散射系数和各向异性因子对光学透反射成像法检测层间杂质效率的影响规律.结果表明,入射光强、杂质埋藏深度和介质折射率对透反射成像检测结果均有影响,且影响规律相似.增加入射光强、减小杂质埋藏深度或减小介质折射率均可提高反射光成像的检测效率;增大入射光强、减小介质折射率、减小介质散射系数或增大各向异性因子均可提高透射光成像的检测效率.这些规律对散射介质中层间杂质检测具有一定指导意义.     

Wide-aperture acousto-optic interaction in birefringent crystals

Regularities of the wide-aperture mode of light diffraction by acoustic waves in crystals with different values of birefringence are considered. The diffraction has been investigated theoretically and experimentally with the aim of applying it in acousto-optic filters used in processing uncollimated optical beams and images. It is proved that the main filtration parameters depend on the birefringence of crystals used in processing luminous fluxes. By the example of wide-aperture filters based on paratellurite, magnesium fluoride, and potassium dihydrogen phosphate crystals, it is shown that the choice of crystals with high birefringence may increase the maximum attainable angular aperture and throughput of devices. It is also proved that application of crystalline compounds of mercury and tellurium (characterized by high optical anisotropy) in filters increases spectral resolution and improves the quality of filtered images.     

Measurement of acousto-optic interaction in Germanium in the far infrared

Acousto-optic interaction in a Germanium Bragg cell, which was operated as acoustic resonator, was measured at 119 m relative to 10.6 m. The figure of merit of the material was found to be approximately 20% higher in the far infrared. The performance of various acousto-optic materials in the far infrared is briefly discussed.     

Features of acousto-optic interaction in uniaxial gyrotropic crystals

The features of noncollinear acousto-optic interaction in gyrotropic crystals in the intermediate diffraction regime and the regime of Bragg diffraction were investigated. The dependence of the efficiency of diffraction in gyrotropic paratellurite, tellurium, and quartz crystals on the intensity of the ultrasonic wave, acousto-optic interaction length, and incident-light polarization was investigated. It is shown that the gyrotropy of the crystal is responsible for the appearance of a multiple-peak structure of the Bragg maximum. It has been established that in the case of propagation of incident and diffracted waves in the vicinity of the optical axis of the crystal, the diffraction efficiency is independent of the polarization state of the incident light. The results of theoretical calculations are in agreement with the experimental results obtained for uniaxial crystals. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 67, No. 1, pp. 105–110, January–February, 2000.     

Optimal conditions of the acousto-optic interaction in crystals with gyrotropy

The conditions of the acousto-optic interaction in the crystals with gyrotropy are considered. The optimal length of the acousto-optic interaction and the diffraction efficiency in the gyrotropic direction are calculated. The diffraction efficiency is determined by the coupled wave method which allows calculation of the field interaction in the gyrotropic crystal [1]. It is theoretically shown and experimentally confirmed that the diffraction efficiency in the gyrotropic direction is independent of incident light polarization, when the polarization plane rotates by 45° along the interaction length.     

Effect of parametric refraction upon acousto-optic interaction

The interaction of optical and acoustic beams is studied for the case of a violated Bragg matching condition. The optical beam experiences self-action due to a cascade nonlinearity. Acousto-optic interaction can result in total reflection of the optical rays from the ultrasound beam.     

Detection of the nonballistic component of scattered light in the process of acousto-optic visualization

For determining the optimum conditions of detecting the signal during acousto-optic visualization, the modulation characteristics of radiation of a laser beam crossed by a focused acoustic beam in a medium and scattered by this medium were studied. The characteristics were obtained by measuring the amplitude of the alternating-photocurrent component at the ultrasonic frequency as a function of the geometry of the experiment, the parameters of the scattering medium, and the laser radiation power. Special attention was given to the magnitude of the signal-to-noise ratio. A photodetector recording the field of the nonballistic component of the scattered radiation was positioned outside the geometrical sizes of the laser beam. The photocurrent component corresponding to this radiation at the ultrasonic frequency was used as a parameter of the acousto-optic visualization. Images of optically opaque objects immersed into a medium whose scattering parameters corresponded to the transition from the regime without scattering to the multiple-scattering regime were obtained.     

Collinear acousto-optic interaction in a single tellurium crystal

The problem of designing acousto-optic imaging filters based on tellurium crystal is discussed. The measurement and calculation results of acousto-optic properties of tellurium are presented. The collinear acousto-optic interaction in tellurium crystal is experimentally implemented for the first time at the light wavelength of 10.6 μm.     

Certain features of radiation transport in spatially bounded media with anisotropic scattering

The problem of radiation transport (RT) in media with a nonsymmetric scattering index is examined under multiple scattering conditions. The governing influence on the nature of the RT is exerted by the integral index parameters that take account of the forward-backward radiation. Nonsymmetry of the index /, equal to three produces a relative energy redistribution through the side faces of the medium within 20% limits, where the maximum redistribution is observed for small transverse dimensions _y=_z=5. For large transverse dimensions the effect of index nonsymmetry does not appear. It is shown that the degree of anisotropy is less in spatially bounded media as compared with an unbounded layer of equal optical thickness.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 69–72, December, 1989. Original article submitted January 11, 1988.     

Intermediate and Bragg acousto-optic interaction in elastically anisotropic medium

The Raman-Nath, intermediate and Bragg regimes of acousto-optic interaction in an acoustically anisotropic medium are theoretically examined in the paper. The general model of the interaction is extended over the case of an elastically anisotropic medium. Basic results of computer modeling of diffraction processes taking place in crystals are presented in the paper. The interaction of light and ultrasound is investigated in media with arbitrary elastic anisotropy in a wide range of the Klein-Cook and Raman-Nath parameters. The influence of the acoustic anisotropy on the process of light diffraction, especially on the light intensity in diffraction maxima, is also discussed.     

Prolonged acousto-optic interaction with Lamb waves in crystalline plates

The propagation and acousto-optic interaction of Lamb modes in an anisotropic plate of tellurium dioxide (TeO2) are studied numerically and analytically. In the case of a Y-cut X-propagating TeO2 plate, the very high elastic anisotropy of the crystal greatly modifies the dispersion curves, giving rise to their multiple oscillations. The existence ranges of backward Lamb modes increase with the mode order contrary to the case of isotropic plates. The quasi-collinear light scattering by Lamb waves is considered. Owing to the structure of Lamb wave field, a simultaneous light diffraction at two different optical frequencies can take place while Lamb waves are excited only at the single frequency. It is demonstrated with the Z-cut (110)-propagating plate that a small change in the acoustic frequency can result in a significant shift in the frequency of the scattered light.     

Overlap integral dependence on the penetration depth in abnormal guided acousto-optic interaction

A numerical analysis about the overlap integral for TE-TM polarized light is presented about abnormal guided acousto-optic interaction using mode conversion. Overlap integral dependence on the penetration depth is obtained in abnormal acousto-optic interaction.