Anisotropic Light Diffraction Research Articles

Complex source point theory of paraxial and nonparaxial elliptical Gaussian beams.

Using the operator transformation technology, we extend the circular Gaussian beam based virtual (complex) sources method to investigate the paraxial and nonparaxial propagation properties of the elliptical Gaussian beams (EGBs) with planar or cylindrical wavefronts travelling in free space. The paraxial approximation analysis reveals the self-reappearance and self-focusing propagation features for the EGBs with cylindrical wavefront under proper parameter conditions. We further introduce the nonparaxial theory to derive the analytical expressions for the field distribution of an EGB in free space, and confirm that these intriguing propagation features can still be observed with added nonparaxial correction. Comparing with the paraxial approximation results, it is worth noting that there is a clear deviation of the on-axial intensity and phase distributions near the self-focusing position on the basis of nonparaxial correction solution. Our results reveal that the anisotropic diffraction of light propagating through homogeneous medium or free space is possible. The approach in this work can easily be generalized to other beam models with elliptical geometry, which allows us to correctly predict some important information about their near field propagation characteristics for various applications.

Light Diffraction on Ultrasound in a Spatially Periodic Acoustic Field

Characteristic features of anisotropic light diffraction in a spatially periodic acoustic field created by a sectioned piezoelectric transducer with antiphase excitation of adjacent sections are studied theoretically. The numerical calculation is carried out for a cell of a paratellurite crystal with a shear acoustic mode propagating at an angle of 3° to the (001) plane of the crystal. It is shown that this type of acoustooptic interaction has some interesting features that can be used when developing acoustooptic devices, in particular, modulators of nonpolarized light.

Дифракция света на ультразвуке в пространственно периодическом акустическом поле

AbstractCharacteristic features of anisotropic light diffraction in a spatially periodic acoustic field created by a sectioned piezoelectric transducer with antiphase excitation of adjacent sections are studied theoretically. The numerical calculation is carried out for a cell of a paratellurite crystal with a shear acoustic mode propagating at an angle of 3° to the (001) plane of the crystal. It is shown that this type of acoustooptic interaction has some interesting features that can be used when developing acoustooptic devices, in particular, modulators of nonpolarized light.

Anisotropic light diffraction in a biaxial crystal of alpha-iodic acid

A program for calculating characteristics of acousto-optic interaction in biaxial crystals is developed and used to analyze light diffraction by ultrasound in an iodic acid crystal. Crystal cuts with maximum acousto-optic figures of merits are found. It is shown that in the region of the optical axes the frequency dependences of the Bragg angles under anisotropic diffraction are substantially different from those in uniaxial crystals.

Anisotropic Light Diffraction by Ultrasound in Crystals with Strong Acoustic Anisotropy

In modern acousto-optics, crystalline materials are used predominantly for manufacturing acousto-optic instruments. Among these materials, such crystals as paratellurite, tellurium, calomel, TAS and some others occupy a prominent place, which are distinguished by exceptionally large anisotropy of acoustic properties. In this work, the influence of acoustic beam energy walk-off on characteristics of Bragg diffraction of light is studied by the example of tellurium crystal. It is shown that the walk-off can substantially change angular and frequency ranges, resulting in their narrowing or broadening subject to position of the operating point in the Bragg angle frequency characteristic. Coefficients of broadening are introduced for characterization of this effect.

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.

Features of using multi-element converters in anisotropic acousto-optic Bragg cells

The geometry of anisotropic light diffraction in ultrasound waves excited by a multi-element converter in an optically uniaxial negative crystal of lithium niobate is analyzed. An expression for the refractive index of a diffracted wave was found on the basis of a study of the optical indicatrices of the refractive indices of ordinary and extraordinary light waves in the crystal. A method for calculating the angular frequency characteristics of anisotropic acousto-optic Bragg cells with multi-element converters is proposed.

Characteristic features of wideband anisotropic light diffraction in lithium-niobate crystal by a longitudinal acoustic wave

Two modes corresponding to wideband acousto-optic interaction have been revealed based on the analysis of Bragg conditions. The moduli of the electric fields of the incident and diffracted light beams are calculated for one of these modes. The half-power frequency bandwidth of the control signal at a level of diffracted beam was found to be 495 MHz for the light wavelength λ0 = 632 nm at a diffraction efficiency of 2.2% on an average ultrasonic frequency of 3.27 GHz.

Diffraction of IR radiation by ultrasound in tellurium single crystals

The optic, acoustic, and acousto-optic properties of tellurium single crystals are considered to analyze the possibilities of applying this material in acousto-optic filters to control mid- and far-IR radiation. The results of measuring the optic and acousto-optic parameters of a tellurium crystal are reported. The isotropic and anisotropic light diffraction by ultrasound in the crystal is investigated. The wide-aperture geometry of interaction in tellurium is implemented and an acousto-optic figure of merit M 2 of a tellurium crystal exceeding 10 × 10−15 s3 g−1 is experimentally obtained.

Investigation into the acoustooptic properties of tellurium crystals by anisotropic diffraction of light

The optical, acoustic, and acoustooptic properties of tellurium crystals viewed as a candidate material for mid-and far-IR acoustooptic devices are considered. The phase velocities, polarization, and drift angles of the acoustic energy in different crystal planes are calculated. The acoustooptic figure of merit for tellurium under the anisotropic diffraction conditions is estimated, and light-sound interaction geometries promising for acoustooptic applications are discussed. Measuring data for the optical and acoustooptic parameters of tellurium crystals are given.

Acousto-optic properties of tellurium that are useful in anisotropic diffraction

The acoustic, optical and acousto-optic (AO) properties of tellurium crystals are examined here with the objective of developing AO devices in this crystal that operate in the 4–8 µm infrared region. Detailed analysis of anisotropic diffraction of light that is characterized by a change in the optical mode is carried out. The phase velocities of acoustic waves in three basic planes of the material, the directions of elastic polarization, and acoustic walkoff angles are calculated. Magnitudes of the AO figure of merit M2 in the crystal are evaluated to obtain optimum AO interaction geometries. Our results show that the slow shear elastic waves propagating in XZ and YZ planes of the crystal may offer the best AO interaction geometry applicable to AO deflectors and imaging filters, and predict a maximum value of M2 in tellurium to be 160 × 10−12 s3 kg−1. Measurement results for optical and AO parameters of tellurium are also included.

Anisotropic acousto-optic diffraction by leaky wave radiation in ZX-LiNbO3

The anisotropic light diffraction by bulk acoustic waves (BAWs) radiated by an interdigital transducer (IDT) operating at the leaky surface acoustic wave resonance in ZX-lithium niobate has been experimentally studied at an optical wavelength of 633nm in the range of acoustic frequencies from 35to190MHz. The possibility of optical probing of the IDT-radiated BAW properties is demonstrated. The experimentally determined values of the BAW propagation angle and velocity are 25° and 4050m∕s, respectively.

Spatial structure of acousto-optic phase matching in uniaxial crystals

The spatial structure of Bragg angles and the transfer functions of an acousto-optic cell are calculated for the cases of isotropic and anisotropic light diffraction in a uniaxial crystal. Their change with ultrasound frequency is traced. The possibility of image processing by acousto-optic filtration of the spatial spectrum of an image is considered. The results of experimental visualization of the transfer functions of a calcium molybdate crystal cell are presented.

Anisotropic diffraction of light by volume holographic grating in birefringent photorefractive crystals with extended wavelength range

The properties of anisotropic diffraction of light by volume holographic grating in birefringent photorefractive crystals are discussed. This diffraction takes place when the refractive index for diffracted light is different from the refractive index for incident light. It is found that in some special geometry of wavevector diagram the diffraction becomes less sensitive to the wavelength mismatch of Bragg condition. The wavelength range may extent in several times the range of ordinary isotropic diffraction on the grating of the same spacing and thickness. Theoretical explanation of this phenomena and experimental results of widerange diffraction in BaTiO3 photorefractive crystal are also presented.

Acousto-optical stabilisation of the laser beam direction

An investigation is reported of an acousto-optical system with optoelectronic feedback, ensuring stabilisation of the laser beam direction. Various modifications of the system based on isotropic and anisotropic diffraction of light are considered. An analysis is made of the influence of the acousto-optical interaction selectivity on the angular operating range and on the stabilisation coefficient. The results of an experimental study of a system with a paratellurite crystal cell are given.

Anisotropic light diffraction on ultrasound in a tellurium dioxide single crystal

Various cases of Bragg anisotropic diffraction of an optical beam by ultrasound propagating in a TeO 2 single crystal are examined. Special attention is devoted to non-traditional diffraction with interaction lengths of light and ultrasound up to several centimetres. Possible applications of the examined cases of the diffraction for spectral filtration of collimated and non-collimated optical beams are discussed in the paper.

Theoretical study of the anisotropic diffraction of light waves by acoustic waves in lithium niobate crystals

The anisotropic diffraction of light by high frequency longitudinal ultrasonic waves in the tangential phase matching configuration may present some definite advantages over the same interaction using transverse acoustic waves. A systematic search for favorable crystal cuts in lithium niobate was worked out. The main results of this study are reported here; they enable the choice of the best configuration for a given operating center frequency.

Acoustooptic device for conversion of images into electrical signals

Theoretical and experimental investigations were made of an acoustooptic light–signal converter in which the image is line-scanned by a traveling acoustic train. The maximum resolution was obtained for anisotropic diffraction of light at the Bragg angle close to zero. A description is given of a prototype of a converter made of a paratellurite crystal which can resolve an image with 510 pairs of elements in a line at the television scanning rate.

Rotational Striations and Diffusion in Barium Sodium Niobate Crystals

AbstractThe possibility of inducing rotational growth striae with a small period in barium sodium niobate crystals (BSN) is examined. Crystals grown had a period stability up to 1 percent and were suitable material for anisotropic light diffraction by ferroelectric domains. However intense striae were produced only for periods λ > 5 m̈m. With the help of Bragg optical diffraction by growth striae the diffusion coefficient D in solid phase responsible for striae smoothing during annealing was measured. The magnitude D ≈ 10‐11 cm2 s‐1 at the melting point was obtained. Such D value leads to the vanishing of striae with λ < 5 m̈m in BSN crystals during growth.

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.