Optimization of the characteristics of an acoustooptic cell based on a tellurium crystal

The problem of designing acoustooptic devices based on tellurium crystals intended for operation in the wavelength range λ > 5 μm is discussed. The optimum geometry of the isotropic acoustooptic interaction in this material is found. The problem of searching for the optimum geometry of a filter that uses anisotropic-acoustooptic interaction is analyzed.     

The anisotropy of elastic waves in a tellurium crystal

For acoustic waves propagating in an acoustooptic tellurium crystal, the dependence of their polarization on the propagation direction with respect to the crystal axes is discussed. The characteristic features of waves propagating in the crystal are considered; these features manifest themselves in an excess of the phase velocity of shear acoustic modes over the velocity of longitudinal modes. The change in the wave type from quasi-longitudinal to quasi-transverse as a result of the variation in the propagation direction of ultrasound is investigated. It is shown that such a behavior of bulk acoustic waves is caused by the specific relation between the elastic moduli, which differs from the corresponding relations observed in other acoustooptic materials.     

Dynamic optical transfer function of an acoustooptic modulator with a damped sound wave

Based on a spatially dependent dynamic optical transfer function of an acoustooptic modulator, a dynamic model of light modulation by a sound signal under conditions of acoustic damping is developed. A system of equations describing the dynamics of the acoustooptic interaction is given. Solutions of this equation for arbitrary power level and spatial-time structure of a sound signal are found. It is shown that acoustooptic damping has the strongest effect in a nonlinear modulation regime. Here, the dissipation of an acoustic signal suppresses the higher harmonic of the dynamic optical transfer function. An analytical model of the dynamic optical transfer function for a low level of acoustooptic coupling is given. It is shown that when the pump beam aperture is much greater than the spatial size of an acoustic signal, the time response of the acoustooptic modulator response is identical within a phase factor to the amplitude profile of a pump beam apodized by the exponential dependence of the amplitude of a damped sound wave. Otherwise, the sound damping produces almost no distortions of a plane top of the acoustooptic modulator response to a pulsed signal and shows itself only under phase mismatch conditions. Here, the asymmetry of overshoots at the edges of the acoustooptic modulator response is observed. Calculated plots are presented, which illustrate the aforementioned specific features of the transient process under conditions of acoustic damping in the acoustooptic interaction in paratellurite.     

Orientation phenomena in nematic liquid crystals under a periodic compressional deformation

The orientation behavior of homogeneous planar layers of nematic liquid crystals with open and closed ends in the field of compressional deformations caused by an acoustic effect is studied. The mechanisms determining the connection of the optical response of a nematic liquid crystal (the variable component of an optical signal and its spectrum) with the acoustic parameters (the oscillation amplitude and frequency, and the amplitude of sound pressure) and the layer thickness are revealed. The factors responsible for the mechanism and modes of acoustooptic conversion are considered. It is demonstrated that, by varying the layer thickness, it is possible to implement different modes of signal conversion. The possibility of designing a new modification of a sound receiver based on a nematic liquid crystal and the specific features of this design are discussed. Its advantages over conventional sound pressure receivers based on nematic liquid crystals are indicated, in particular, the absence of limitation of the frequency of the received signal in the low-frequency range.     

Effect of the spatial structure of an acoustic field on Bragg’s acoustooptic diffraction under strong acoustic anisotropy conditions

Bragg’s acoustooptic diffraction in an acoustically anisotropic medium is considered taking into account the two-dimensional spatial diffraction structure of the acoustic beam. The conditions are determined under which reverse transfer of optical power from the diffracted to the transmitted beam in the regime of 100% efficiency of diffraction is considerably suppressed. It is shown that this effect is due to diffraction bending of wave fronts of the acoustic beam in the acoustooptic diffraction plane. The problem of optimization of the piezoelectric transducer size and the spatial position of the input light beam is solved using the criterion of the minimal required power of the acoustic field. The results of simulation in a wide range of the acoustooptic interaction parameters for a Gaussian light beam are reported. The correctness of the model is confirmed experimentally. Recommendations for designers of acoustooptic devices are formulated.     

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%.     

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.     

Acoustooptic multiplication of the frequency shift of optical radiation on the basis of bragg polarization splitting

A technique for the acoustooptic multiplication of the frequency shift of an optical beam is proposed. The technique is based on the cascade diffraction of the beam by a single acoustic wave with the use of Bragg polarization splitting in a uniaxial crystal. The fundamental possibility of the practical realization of the technique is confirmed experimentally by using anisotropic acoustooptic diffraction in LiNbO₃.     

集成光学声光可调谐光谱仪的特性分析

提出了一种以集成光学声光可调谐滤波器作为分光元件的新型光谱仪 ,并详细介绍了这种光谱仪的具体特性。通过对这种光谱仪的特征方程、点扩展函数以及色散本领的详细分析 ,指出该光谱仪的分辨本领与器件的声光作用长度成正比 ,也与它的级联级数的平方根成正比。因此增加器件的声光作用长度比增加它的级联级数能更有效地提高光谱分辨率。另外还指出 ,准共线声光耦合是抑制侧瓣、提高分辨本领的有效途径。测试表明 ,当声光相互作用长度L =2 5mm ,声波导和光波导的夹角θ=0 .4 2°时 ,在中心频率 174MHz附近可获得带宽 1.4 4nm ,一级侧瓣 - 13.2dB ,模式转换效率大于 99%的结果。     

Optimization of a KDP-based acoustooptic tunable filter

The characteristics of an acoustooptic cell optimized for a tunable filter making it possible to analyze optical images in the UV range (λ = 200–400 nm) are studied. The attenuation of a slow shear acoustic wave in the (010) plane of the crystal is measured. It is shown that attenuation of ultrasound affects the filtering characteristics at frequencies f > 500 MHz. Analytical and experimental data are presented, and factors influencing the quality of images obtained with KDP-based acoustooptic filters are discussed.     

Efficiency of collinear acoustooptic interaction in an anisotropic medium

Collinear acoustooptic interaction in crystals with different values of birefringence is theoretically studied. The crystal medium is considered as a system of thin layers where the orientation of optical indicatrix varies from layer to layer due to the presence of an ultrasonic wave. For collinear diffraction in LiNbO₃, α-SiO₂, Te, CaMoO₄, and Tl₃AsSe₃ crystals, expressions for the transmitted and diffracted intensities are obtained by means of acoustooptic methods and a new method based on the Jones calculus.     

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.     

Light diffraction on ultrasonic Love waves

It is shown that layers based on acoustooptic crystals excited by ultrasonic Love waves can be used as light modulators in the reflection and transmission mode. It is established that various modes of Love waves can be detected by optical methods. For small ultrasound intensities, the diffraction efficiency is determined by the degree of homogeneity of deformations within the layer, and for large intensities, by the length of the acoustooptic interaction.     

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.     

Acoustooptic nonpolar light controlling devices and polarization modulators based on paratellurite crystals

A complex of investigations and projects aimed at solving problems of highly effective acoustooptic control of nonpolarized laser radiation is generalized on the basis of anisotropic Bragg’s diffraction in a TeO₂ crystal from a slow acoustic wave, which is characterized by extraordinarily high acoustooptic quality. Modulators of the zeroth diffraction order, modulators of the first Bragg’s order, and a polarization-insensitive deflector are considered. The developed systems ensure the control of the polarization state of optical radiation. These systems are employed in setups and devices based on high-power solid-state and fiber lasers with a wavelength of 1.06 μm.     

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.     

Acoustooptic and electrooptic guided wave conversion to leaky waves in an anisotropic optical waveguide

The possibility of effective acoustooptic and electrooptic guided wave conversion to leaky waves in an optical waveguide is shown experimentally. Conversion efficiency was 2.3% in an acoustic power of 24 mW for a TM₁?TE₁ process.     

Distinctive features of Raman-Nath scattering of light by sound in easy-plane antiferromagnets

This paper discusses the acoustic diffraction of light in the presence of an exchange-enhanced photoelastic interaction of aniferromagnetic origin resulting from acoustic modulation of the dielectric permittivity due to oscillations in the antiferromagnetism vector L. In the “easy-plane” type of antiferromagnet these oscillations arise from antiferromagnetoelastic interactions, and can be so large that the photoelastic interaction corresponding to them can be comparable in value to (or even exceed) the interaction in well-known nonmagnetic crystals actually used in acoustooptic devices. The advantage of antiferromagnets lies in the fact that both the diffraction angle and the amplitude of the diffracted light can depend on the magnitude and direction of a magnetic field in these materals. Here the Raman-Nath diffraction regime is discussed, which is probably more favorable from an experimental point of view for the antiferromagnets in question. It is shown that for these materials, the usual mechanism of photoelastic interaction associated with acoustic modulation of the index of refraction is accompanied by an additional mechanism arising from modulation of the polarization of the optical modes. Qualitative estimates are given for FeBO₃. Zh. éksp. Teor. Fiz. 112, 1464–1475 (October 1997)