Characteristic properties of the acoustooptic interaction in a thick acoustic grating at an orthogonal orientation of the interacting fields

The diffraction of light by ultrasound is calculated for the case of a normal incidence of a light wave on a sound layer in an isotropic medium. An intermediate case of an interaction region with a moderate length for which the Raman-Nath and Bragg diffraction conditions do not hold is considered. It is shown that a stable three-ray diffraction field of equal intensities with a high stability with respect to the intensity magnitude of the sonic wave can be obtained.     

Diffraction regimes of phase gratings

The occurrence of Raman-Nath regime and Bragg regime diffraction by planar phase gratings is rigorously analyzed. It is shown that the published indicators for a Raman-Nath regime are unsatisfactory. An attempt to redefine the Raman-Nath region is given in this paper.     

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.     

Polarization-Independent Ultrasonic Modulation of Light in Gyrotropic Cubic Bismuth Germanate Crystals

An intermediate regime of light diffraction by ultrasound propagating along the [001] and [110] axes of a gyrotropic cubic bismuth germanate crystal is investigated. The possibility of polarization-independent light modulation in the intermediate regime of diffraction close to the Bragg regime is shown. The dependences of the diffraction efficiency on the incident light polarization azimuth at different ultrasonic wave intensities are given.     

Optoacoustic interaction in uniaxial gyrotropic quartz crystals

The specific features are considered of the noncollinear optoacoustic interaction in uniaxial gyrotropic quartz crystals in an intermeadiate regime of diffraction close to the Bragg regime. A system of equations of coupled waves is presented which describes the optoacoustic interaction in uniaxial gyrotropic crystals in the Raman-Nath, intermedaite, and the Bragg regimes of diffraction. The dependences of the diffraction efficiency on the ultrasonic wave intensity, the optoacoustic interaction length, and on the incident light polarization are investigated. A multispike structure of the Bragg maximum is studied that appears in accounting for the crystal gyrotropy. A good agreement between the theoretical calculations and the known experimental results for a quartz crystal is shown. Mozyr State Pedagogical Institute, 28, Studencheskaya Str., Mozyr, 247760, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 2, pp. 278–281, March–April, 1999.     

Anisotropic light diffraction in crystals with a large acoustic-energy walk-off

The influence of energy walk-off in an acoustic beam on the characteristic of anisotropic Bragg diffraction of light has been investigated by the example of paratellurite crystal. The angular and frequency characteristics of acousto-optic diffraction have been calculated in wide ranges of ultrasound frequencies and Bragg angles using the modified Raman-Nath equations. It is shown that the walk-off of an acoustic beam may change (either widen or narrow) significantly the frequency and angular ranges. The calculation results have been experimentally checked on an acousto-optic cell made of 10.5°-cut paratellurite crystal.     

Use of the application of heterodyne laser interferometer in power ultrasonics

We are particularly interested in the measurement of nonlinear acoustic waves in air, within the range of the Raman-Nath diffraction regime. We used a heterodyne laser interferometric system with a Bragg cell frequency of 80 MHz. A harmonic power acoustic wave with a frequency in the range of 20 kHz was generated, but higher harmonics appear when nonlinear propagation occurs in restricted volumes or waveguides. This topic raises a number of interesting questions. The most complex problem arises when processing an extremely wide-band output signal from a photo-detector, where the signal contains a huge number of high amplitude harmonics in a measured acoustic signal. The wide-band output signal from nonlinear wave measurement was simulated by means of similar wide-band output signals obtained by measuring known dynamic mechanical displacements. Because the frequency band of common analog circuits used for signal processing is too narrow, the output signal from the interferometer was digitalized and processed. This paper discusses the results of these tests and provides estimates of the frequency restrictions of signal processing.     

Acousto-optic interaction in a non-homogeneous acoustic field excited by a wedge-shaped transducer

The article is devoted to theoretical analysis of light diffraction in a non-homogeneous acoustic field created by a wedge-shaped piezoelectric transducer. Relationships describing electrical, acoustic and acousto-optic properties of the acousto-optic cell are derived in the approximation of a small thickness of the piezoelectric plate. Principal characteristics of acousto-optic interaction are examined such as dependences of the light diffraction efficiency on the phase mismatch and the acoustic wave amplitude. It is shown that the acoustic field has a complicated amplitude-phase structure changing with the acoustic frequency. It is demonstrated that the diffraction efficiency in the Bragg regime can approach 100% in spite of a noticeable phase mismatch. The appropriate optimal values of ultrasound power and incidence angles of light are found.     

Reflection of light from a plane-parallel layer modulated by ultrasound

The features of Bragg reflection of light from a plane-parallel layer perturbed by a standing ultrasonic wave are investigated. It is shown that, with an increase in the thickness of the modulated layer, the reflection coefficient of the light wave diffracted by the ultrasound reaches the maximum value at smaller intensities of the ultrasound and is determined by the relation between the refractive indices of the adjoining media. The possibility of efficient acousto-optical diffraction of light in the Bragg reflection regime at superlattices of photoelasticity in the field of low-frequency ultrasonic waves with a moderate level of acoustic power is established.     

Conditions for the suppression of spatial harmonics at a two-frequency acoustooptic interaction in the Raman-Nath mode

It is demonstrated that the diffraction of a plane light wave by an acoustic wave of fundamental frequency in the Raman-Nath mode with admixture of the second acoustic harmonic may be accompanied by a considerable change in the higher diffraction orders with numbers ±2, ±3, etc. Conditions for the selective suppression of the orders are obtained in the weak and strong interaction modes. Results of a numerical simulation are presented.     

Angular splitting of the Bragg diffraction order in an acoustooptical modulator due to a frequency-modulated acoustic wave

Effects of angular splitting of the Bragg diffraction order arising in light acoustooptical diffraction by a frequency-modulated acoustic wave are considered. These effects occur when the size of the light spot in the acoustooptical interaction zone exceeds the characteristic spatial period of the modulating function. The Bragg diffraction order is found to be split into several beams. The directions of the additional beams, their number, and intensities are determined by the modulation parameters. In particular, there occurs a situation where the diffracted field consists of three beams of equal intensity spaced at a distance approximately equal to the diffraction divergence of the incident beam and the diffraction total efficiency is of the order of 100%. Therein lies the difference between this diffraction regime and the case where several independent acoustic waves are generated in the interaction domain and the diffraction total efficiency is limited to the intermodulation arisen. The effect is used in design of modulators for systems of image plotting with the help of high-power lasers.     

Polarization-independent modulation of light in gyrotropic cubic crystals

Polarization-independent modulation of light in gyrotropic cubic crystals is investigated in the Bragg and intermediate diffraction regimes. It is found that, for crystals with considerable specific rotation, the photoelastic anisotropy is suppressed by the circular anisotropy and the observed diffraction efficiency depends weakly on the polarization azimuth of the incident light. Good agreement is obtained between modulation parameters calculated theoretically on the basis of the coupled wave theory and experimental results for a bismuth silicate crystal in the intermediate diffraction regime.     

Two-coordinate diffusion of optical beams by an acoustooptic Raman-Nath modulator based on a paratellurite crystal

Acoustooptic Raman-Nath diffraction by a standing acoustic wave in a paratellurite crystal is investigated. An acoustic line is made in the form of a polished cube and serves as a high-Q acoustic resonator. A slow shear wave is excited by a single piezoelectric transducer. Multiple lossless sound reflections lead to two-coordinate light diffusion. When the acoustic intensity introduced into the crystal is about 2 W/cm² at a sound frequency of 7 MHz, there appears a diffraction pattern in the form of a homogeneous light spot with a solid angle of about 0.5 sr. An explanation for the features of the acoustooptic interaction is given. It is shown that this type of diffraction is helpful in designing acoustooptic two-coordinate diffusers of light beams.     

Numerical simulation of atomic diffraction in the Raman-Nath regime

The theory of atomic diffraction from a classical standing wave light field in the presence of spontaneous emission in the Raman-Nath regime was developed by Tanguy et al. [6]. We describe the basis of computationally efficient methods for performing calculations in this regime and show their agreement with recent experimental results of Gould et al. [4].     

Acoustic light diffraction, associated with the modulation of the polarization of the light, in an easy-plane antiferromagnet

The intensity of acoustic light diffraction by an easy-plane antiferromagnet in the Raman-Nath regime, due to a photoelastic interaction of antiferromagnetic origin, is calculated in the case when there is no modulation of the refractive index to first order in the acoustic deformations, and the entire effect is due to the linear modulation of the light polarization. Quantitative estimates are made for FeBO₃. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 4, 317–321 (25 February 1997)     

Intermediate regime of light diffraction by surface ultrasonic waves in planar gyrotropic optical waveguides

The intermediate regime of optoacoustic interaction, similar to the Bragg one, with Rayleigh surface ultrasonic waves in gyrotropic planar waveguides is studied. A system of equations of bound waves is presented that describes planar optoacoustic interaction in the intermediate, Raman-Natoh, and Bragg regimes of light diffraction by surface ultrasonic waves. It is shown that the optical activity of a wavequide film favors the incident light energy pumping to diffracted light for a TE-polarization wave and decelerates this process for an incident TM-polarization wave. Mozyr' State Pedagogical Institute, 28, Studencheskaya St., Mozyr', 247760, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 3, pp. 412–416, May–June, 1997.     

Intermediate regime for the diffraction of light on ultrasound in gyrotropic anisotropic crystals

An intermediate regime for the diffraction of light on ultrasound in gyrotropic anisotropic and cubic crystals in an external electric field is considered. A system of equations of the coupled waves, which describes acoustooptic diffraction in gyrotropic anisotropic crystals with consideration of the electrically induced optical anisotropy for a strong interaction between light and ultrasound, is presented. An intermediate regime for the diffraction of light on ultrasound in gyrotropic anisotropic crystals which is close to the Bragg regime for a weak acoustooptic interaction is studied. It is shown that the diffracted light is elliptically polarized and that the ellipticity and polarization azimuth of the diffracted wave depend on the anisotropy of the photoelasticity, the electrically induced anisotropy of the crystal in the external electric field, the gyrotropy, and the asymmetry of the diffraction structure. Zh. Tekh. Fiz. 67, 80–82 (September 1997)     

Double Bragg and Bragg/normal diffraction of two laser beams by ultrasound

An accurate theory is developed describing the diffraction of two light beams incident at various Bragg angles on an ultrasonic beam, taking into account the phase-difference between the two light waves. The general results are outside the range of the Raman-Nath approximation and a comparison is made with an alternative configuration in which one of the light waves is normally incident.     

Diffraction from a moving grating

A first-principles tutorial account is given for the diffraction of either light or free, non-relativistic, massive particles from a moving grating. Discussion on the properties of the moving grating including non-reciprocity is followed by considering both Raman-Nath (thin grating) diffraction and Bragg (thick grating) diffraction in either transmission or reflection geometry. Use of four-momentum conservation with dispersion relations yield the angles of diffraction. The increase in the angle of diffraction for grazing incident by using a moving grating relative to using a static grating is calculated. It is shown that this increase is possible only for Raman-Nath geometry.