Effect of acoustooptic Bragg diffraction on the degree of spatial coherence of a light beam

The effect of acoustooptic (AO) Bragg diffraction on the degree of spatial coherence of an optical beam formed as the result of transmission of a Gaussian beam through a diffuser is studied. Experiments performed on the basis of the AO Bragg interaction in a single crystal of TeO₂ confirmed that the optical beam areas characterized by a higher spatial coherence diffract with a considerably higher efficiency than the low-coherence areas. In addition, the character of the speckle-structure distribution of the optical field of the diffracted beam is practically isotropic.     

Acoustically modified spatial coherence in optical fresnel diffraction region

Theoretical and experimental interpretations are given to the fact that in the optical Fresnel diffraction region acoustically modified spatial coherence is independent of the distance from the acoustic cell, but takes an identical expression across any planes parallel to the propagation direction of the acoustic wave.     

Isotropic acoustooptic Bragg diffraction in paratellurite by a slow shear wave

An analysis is made of acoustooptic Bragg diffraction in a paratellurite crystal in the case of a slow shear acoustic wave. It is shown that the isotropic acoustooptic interaction in crystals has characteristic physical features, which distinguish it from the well-known anisotropic interaction. The isotropic interaction under study is shown to be a more complicated process than diffraction in optically isotropic media. A theoretical and experimental study of the dependences of Bragg angle of incidence on the ultrasound frequency was made. Expressions for the calculation of this dependence are presented, which give results that agree well with the experimental data. The dependences of the isotropic-diffraction efficiency on the acoustic power were studied. For the first time, it was found that the corresponding dependences for anisotropic and isotropic interactions are substantially different. It is shown that the isotropic interaction under study is weaker than the anisotropic interaction. However, the isotropic-diffraction efficiency has a noticeable value and can reach tens of a percent.     

Change in the spatial coherence function at Bragg reflection of an X-ray beam

The influence of coherent properties of an X-ray source (X-ray tube, synchrotron radiation source, free-electron X-ray laser) on the spatial field distribution and the spatial coherence function of this field has been theoretically considered for Bragg reflection from a single crystal.     

Efficient multiple-beam Bragg acoustooptic diffraction with phase optimization of a multifrequency acoustic wave

The subject of investigation is the formation of a highly efficient multiple-beam diffraction field resulting from acoustooptic diffraction by a periodically modulated acoustic wave and acoustic signal composed of a set of independently generated equidistant frequency components. Conditions for minimization of optical losses associated with higher diffraction orders are analyzed. A highly efficient multiple-beam diffraction field is formed by optimizing the phases and amplitudes of signal independent components. A technique of acoustooptic measurements at a high laser radiation intensity is developed, and the basics of the theory are verified experimentally. An attempt to split the power of a laser beam propagating through an optical fiber into seven channels of equal intensities is realized with a net efficiency of 80%.     

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.     

Acoustooptic interaction in two-dimensional photonic crystals: Efficiency of Bragg diffraction

We obtain equations for the Bragg regime of acoustooptic diffraction of light in two-dimensional photonic crystals. We determine applicability conditions of the single-wave approximation, in which it is sufficient to take into account only one Fourier component of each of Bloch waves involved in the acoustooptic interaction. In the single-wave approximation, we obtain formulas that make it possible to estimate the acoustooptic figures of merit of a photonic crystal. We show that, in a photonic crystal, higher acoustooptic figures of merit can be achieved than in the materials that make up the crystal.     

Effects of spatial coherence and dispersive diffraction on spectral changes

This paper describes the peak shift in a Gaussian-like spectrum caused by the effects of spatial coherence and dispersive diffraction. The peak shift depends greatly on the ratio of the coherence areaA _c produced by a primary incoherent source to the area лр² of a circular aperture in the secondary source plane. The measured results are in reasonably good agreement with the numerical computational results. The peak shift is caused by a mixture of spatial coherence and dispersive diffraction in the region 0<A _c/лр²<1. In the coherent limitA _c/ лр²→∞, dispersive diffraction is most dominant and results in the maximum peak shift, whereas in the incoherent limitA _c/л р²～0 no spectral change takes place.     

Measurement of spatial coherence through diffraction from a transparent mask with a phase discontinuity

We describe a robust method by which the spatial coherence from all pairs of points along a line may be simultaneously acquired from an image formed by diffraction from a phase discontinuity. In contrast to other methods, this approach can accurately reveal weak correlations in the tails of a coherence function.     

Experimental study of small signal amplification in Bragg acoustooptic bistable system

The external small signal amplification ability of the Bragg acoustooptic system has been studied in this paper. It has been proven by experiment that there are saturation and resonance phenomena in the amplification. Bifurcation parameters at the bifurcation points are decreased by external simple harmonic signals. At the period-2 bifurcation point, small signal amplification energy is mainly from the period-2 component. External signals have the ability of frequency pulling and synchronising period-2 frequency. These phenomena have great significance concerning bifurcation and chaos.     

Energy and spatial redistributions of the particle flux resulting from its interaction with the scattering center

Energy and intensity distributions of both projectiles and recoil atoms on the plane behind the scattering center at finite distance are calculated for different projectile-to-target mass ratio and different projectile energies. The projectile energy and intensity have been found to be double-valued or three-valued functions of the distance from the collision epicenter. At the same time, the recoil energy and intensity have proved to single-valued and double-valued functions of the distance from the collision epicenter for parallel and divergent projectile fluxes, respectively.     

基于布拉格声光衍射场的光强非均匀空间分布研究

为了保证合成孔径激光雷达测试中信号光的光束质量,从波动方程出发,推导出传统的衍射光源布拉格声光作用的耦合波方程。根据测试对光束质量的不同要求,借助有限差分方法分析衍射场光强的空间三维分布,发现声场振幅的不均匀分布会影响声光衍射场的光强分布,光强分布不均造成实验中信息光源的探测难度。通过光强空间分布研究增大有效通光孔径到3 mm,解决了声光调制器光束衍射质量问题,改进后的光束质量达到实验要求。该研究方法适用于改进声光器件工作参数。     

Acousto-optic Bragg diffraction in paratellurite by the sidelobes of the spatial radiation spectrum of an acoustic transducer

Acousto-optic Bragg diffraction in paratellurite is investigated within the two first diffraction orders for the case of diffraction by the sidelobes of the spatial radiation spectrum of an acoustic transducer. One of the diffraction orders is due to anisotropic diffraction, and the other, to isotropic diffraction. Such a diffraction regime is achieved when the diffraction plane is inclined toward the optical axis of the crystal. For light with a wavelength of 0.63 × 10^–4 cm diffracted by a “slow” sound wave with a frequency of 26 MHz, the effect manifests itself when the angle between the acousto-optic diffraction plane and the optical axis of paratellurite is ~3°. The effect is experimentally verified. The diffraction efficiency is 20% for each of the diffraction orders for a microwave signal of 8 V at the transducer.     

Triple Bragg diffraction in paratellurite crystal

Triple Bragg diffraction in a paratellurite crystal has been considered for the case when the plane of diffraction is oblique to the optical axis of the crystal. It has been shown that effective photoelastic constants for isotropic and anisotropic diffraction depend on the inclination of the plane of diffraction insignificantly. Triple Bragg diffraction of 0.63-μm coherent radiation in paratellurite at a 47.3-MHz slow acoustic wave has been experimentally demonstrated. For an optical power of 0.69 W delivered to a piezoconverter, the relative intensities of diffraction orders equal ~0.4, 0.4, 0.1, and 0.1, respectively.     

Bragg diffraction of surface magnetostatic waves from magnetic lattices

We present a theoretical and experimental investigation of Bragg scattering of surface magnetostatic waves (SMSW) by a time-independent, spatially periodic magnetic field when the wave orientation is arbitrary with respect to the magnetization field. In the theoretical section the theory of single-mode Bragg diffraction is generalized to the case of waves with arbitrary dispersion propagating through an anisotropic medium. The calculated results are, on the whole, supported by experimental measurements on SMSW. We demonstrate that a geometry which in isotropic media leads to a sinusoidal distribution of diffraction order amplitudes as a function of penetration into the differing lattice, can lead to a nearly exponential distribution of such amplitudes in anisotropic media. The anisotropy of the interaction between SMSW and the magnetic diffracting lattice is manifested by anomalously high scattering efficiencies for certain cases of relative orientation.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 76–85, November, 1988.     

Surface acoustic wave distribution and acoustooptic interactions in silica waveguide Bragg devices

The efficiency of acoustooptic interaction in single-mode strip silica waveguide is analyzed theoretically for the first time by determining the overlap integral between the optical and acoustic field distributions. The results show that there is a good overlap of the optical and SAW fields in the low SAW frequency range. At high acoustic frequencies, the overlap integral decreases with increasing acoustic frequency. At 216 MHz, the maximum of 0.8544 for the overlap integral is obtained provided that the H/Λ equals 0.02.     

Nuclear Bragg diffraction of synchrotron radiation from the a-sites of YIG

Time differential measurements of nuclear Bragg diffraction of synchrotron radiation were applied to⁵⁷Fe nuclei at the a-sites of yttrium iron garnet (YIG). The hyperfine parameters were determined by evaluation of the time spectra using the dynamical theory of Mössbauer optics. The observation of nuclear Bragg diffraction allows site selective spectroscopy.     

Shift of the Bragg position in grazing-incidence diffraction

The intensity distribution in grazing-incidence diffraction along the grazing exit angle, a f, has been generally studied assuming the fulfilment of the Bragg condition, both experimentally and theoretically. We consider deviations of the incidence angle and detector position (θ) from the exact Bragg angle, θ _B, as well as variation of the incidence angle with respect to the surface, α _i, and lattice mismatching of layered structures. The so-called surface peak is caused by refraction and appears at the fixed angular position of the critical angle of the total external reflection, α _c. Beside it an additional peak occurs, that is explained by fulfilling the Bragg condition of the lateral wave vector components of incident and diffracted beams. This corresponds to the intersection of the truncation rod and the Ewald sphere. Therefore its angular position in the diffracted scattering fan depends on both α_i and θ ? θ _B. This additional peak is only visible if α _i is below α _C or θ > θ _B. These considerations have been verified experimentally on an InP layer.