The temperature dependence of intensity and the function of the transition dipole moment for the induced optical absorption bands of oxygen in the region of the Herzberg photodissociation continuum (200–230 nm)

Application of coherent interaction of laser light with a focused ultrasonic wave to the technique of acoustooptic visualization in multiple-scattering media is discussed. By analyzing spatial distribution of the optical radiation modulated by ultrasound (the photocurrent at ultrasonic frequency), images of large-sized inhomogeneities embedded into the scattering medium have been obtained. A light-absorbing half-plane and a square with sides of 5 mm were used as the inhomogeneities. The visualization was performed under optimal conditions for measuring the alternating photocurrent calculated for the proposed model of coherent interaction between the laser and acoustic beams (the Raman-Nath diffraction). The alternating current at the ultrasonic frequency was obtained as a result of mixing the waves of the diffraction fields on the detector’s photocathode. All experimental values were obtained from a single measurement without averaging the alternating photocurrent at the ultrasonic frequency of 3 MHz, with the scattering parameter varying up to μL≤37.5, where μ is the extinction coefficient and L is the thickness of the scattering medium along the laser beam axis. The measured quantities varied in the course of the measurements by more than 10 orders of magnitude.     

Wideband light diffraction on bulk acoustic waves in uniaxial crystals

We study diffraction of light on ultrasonic waves, excited by a system of interdigitial transducers, in uniaxial crystals. We show that wideband acoustooptical diffraction in trigonal crystals in the presence of additional anisotropy differs significantly from the isotropy of diffraction on a shear ultrasonic wave in the absence of it. We establish that an exact Bragg angle correction in a wide frequency retuning range is achieved owing to electrically induced anisotropy in the external electric field.     

Acoustooptical interaction of light beams under conditions of internal conical refraction

The collinear acoustooptical (AO) interaction of light beams under conditions of internal conical refraction is considered. The dependences of the diffraction efficiency on the ratio of radii of light and ultrasonic beams, the AO coupling length, and the ultrasound intensity are found. An analytical expression is obtained describing the diffraction efficiency of collinear AO conversion of annular beams of internal conical refraction.     

Diffraction of light in intermediate regime of acoustooptical interaction at oblique incidence

The diffraction of light by ultrasound in an isotropic medium at arbitrarily small angles of incidence onto the acoustic layer is calculated. The intermediate regime of diffraction is considered, for which the conditions of observation of the Raman-Nath and Bragg diffraction are not satisfied in terms of the wave parameter. The effect of the acoustooptical interaction length and angle between the incident light beam and the acoustic wave on the light intensity distribution over the diffraction peaks is studied as a function of the acoustic power. Specific features of the transition from the Raman-Nath to the Bragg diffraction regime through the intermediate regime of diffraction are analyzed.     

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.     

Diffraction of light by an acoustic wave with three frequency components

Collinear light diffraction by three-frequency sound is investigated theoretically. The amplitude distributions of transmitted and diffracted light waves along the cell are calculated for different amplitudes of sound signals. The dependence of the intensity of principal diffraction peaks on the frequency difference between acoustic signal components is studied for different amplitude ratios of these components. It is shown that the character of this dependence for a wave being in synchronism differs substantially from that for two other waves characterized by detuning. The dependence of the amplitudes of principal and parasitic diffraction peaks on the efficiency of acoustooptical interaction is investigated. It is demonstrated that parasitic sideband components in diffracted light can play a considerable role if the diffraction efficiency is sufficiently high and exceeds 80%.     

The influence of progressive light diffraction in Debye-Sears phenomena

Two previous theories of Debye-Sears diffraction of light by plane ultrasonic waves are summarized. The theories assume that significant modulation of the light wave occurs in both phase and amplitude. In our own approach account is taken of both curvature of the light rays inside the ultrasonic beam and diffraction of the system alone at the exit plane, neglecting any diffraction effect in the liquid itself. In the Hargrove's method progressive diffraction in the medium is considered. Results from these theories are compared with experimental data obtained at a frequency of 5 MHz with ultrasonic waves having large amplitude and beamwidth (50 mm).     

Quasicollinear acoustooptical interaction in three-dimensionally optically inhomogeneous crystals

A theoretical model is given for the quasicollinear acoustooptical interaction of beams in a three-dimensionally optically inhomogeneous, anisotropic medium. Analytical solutions of the coupled wave equations are obtained for a regular linear inhomogeneity model in relation to the spatial profiles of the frequency spectrum of the diffracted field. The spatiospectral transfer functions are determined, and their selective properties are investigated for various orientations and magnitudes of the gradient of the optical inhomogeneity for both low and high diffraction efficiencies. Numerical simulation results are given.Tomsk State Academy of Control Systems and Radioelectronics. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 91–104, July, 1995.     

Normal Metal-High Tc Superconductor Point Contact Properties At Millimeter Waves

Experimental results of the response of a metal-high T_c superconductor (BSCCO-2212) point contact to millimeter wave radiation both at room and liquid nitrogen temperatures are presented. Measurements were carried out with and without bias current. An unexpectedly large response signal was observed when the bias current was applied. This effect is explained by the heating of the interface area below the tip of the metallic probe by the alternating current of the millimeter wave. The response signal was compared with that obtained using a Schottky diode.     

Raman-Nath acoustooptical diffraction in antiferromagnet α-Fe2O3 induced by modulation of the polarizations of optical normal modes

Raman-Nath acoustooptical diffraction was experimentally revealed in easy-plane antiferromagnet α-Fe₂O₃. The diffraction is due to linear modulation of the polarizations of the optical normal modes propagating along trigonal axis C₃ of the crystal due to oscillations of the antiferromagnetism vector L caused by magnetoelastic interaction. The dependence of the diffraction parameters on a dc magnetic field H _⊥ applied in the basal plane was studied. The dependence obtained is in good agreement with the field dependence of the exchange enhancement coefficient of magnetoelastic coupling in fields higher than that at which hematite transforms to a single-domain state (H _D ～ 1.8 kOe). for a sound flux power of about 1 W/cm² in a sample, the maximum intensity of the diffracted wave at H _⊥ = H _D is about 0.9% of the input light intensity and its polarization is perpendicular to that of the incoming linearly polarized wave. The results obtained agree qualitatively with the theory of acoustooptical diffraction in antiferromagnets and confirm the antiferromagnetic mechanism of diffraction in the experimental geometry under study.     

声光调Q激光器所需最小衍射效率的研究

以四能级系统为例, 研究了在输出脉冲能量一定的情况下,声光调Q激光器所需要的最小衍射效率如何确定的问题.通过对调Q反转粒子数密度的求解,获得了调Q激光器输出的一定脉冲能量与腔内损耗、所需最小Q开关的衍射效率和输出镜的反射率等的关系公式.这一结果为声光调Q激光器Q开关衍射效率的设计提供了简便的估算方法.     

Bragg diffraction of light beams in an ultrasonic field with an inhomogeneous amplitude-phase distribution in gyrotropic cubic crystals

We study Bragg diffraction of light beams by ultrasound with an inhomogeneous amplitude-phase distribution represented by nonlinear model functions in gyrotropic cubic crystals with strong acoustooptical interaction. Analytical solutions of the vector-matrix equations of coupled waves in partial derivatives with respect to the spatial distributions of light beams and their angular spectra are obtained. The transfer functions of acousto-optical interaction are determined. The solution asymptotics is considered for different modes and diffraction efficiencies.State Academy of Control Systems and Radio-Electronics, Tomsk. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 38, No. 9, pp. 939–960, September, 1995.     

超声场分布的演示实验

介绍了利用超声在液体介质中传播,使质点产生强烈振动而引起交变压力的原理,用微型探头测量,以演示超声场在平面辐射、聚焦系统下焦平面和焦点以内的横截面上的声场分布,使学生更深入地了解超声不皮传播的物理规律。     

Application of acoustic feedback to target detection in a waveguide: experimental demonstration at the ultrasonic scale

People are familiar with the acoustic feedback phenomenon, which results in a loud sound that is heard when a musician plays an electric instrument directly into a speaker. Acoustic feedback occurs when a source and a receiver are connected both acoustically through the propagation medium and electrically through an amplifier, such that the amplified received signal is continuously re-emitted by the source. The acoustic feedback can be initiated from a continuous sine wave. When the emitter and the receiver are in phase, resonance is obtained, which appears to be highly sensitive to any fluctuation of the propagation medium. Another procedure consists in initiating the acoustic feedback from a continuous loop of ambient noise. It then generates an unstable self-sustained feedback oscillator (SFO) that is tested here as a method for monitoring temperature fluctuations of a shallow-water oceanic environment. The goal of the present study is to reproduce and study the SFO at the laboratory scale in an ultrasonic waveguide. The experimental results demonstrate the potential applications of the SFO for the detection of a target in the framework of the acoustic-barrier problem in shallow-water acoustics.     

GaN发光二极管表观电容极值分析

利用正向交流(ac)小信号方法对GaN发光二极管的电容-电压特性进行测量,可以观察到GaN发光二极管中的负电容现象。正向偏压越大,测试频率越低,负电容现象越明显。测量到的负电容现象是表象,不存在负电容;提出GaN发光二极管p-n结的结电容在特定的正向电压范围内等效于可变电容。分析可变电容对正向交流小信号响应得到:特定参数的可变电容使结电容电流相位落后于交流小信号电压相位π/2,使得在测量中表现为负电容。发现表观电容-正向电压曲线的极值点与理论模型相吻合,证明了该理论模型的正确性。     

Diffraction effects on bulk-wave ultrasonic velocity and attenuation measurements

The loss and phase advance due to diffraction are experimentally observed by measuring the amplitude and phase of radio frequency (rf) tone burst signals in the VHF range, in an ultrasonic transmission line consisting of a buffer rod with an ultrasonic transducer on one end, a couplant of water, and a solid specimen of synthetic silica glass. The measured results agree well with the calculated results from the exact integral expression of diffraction. The diffraction effects on the velocity and attenuation measured in this frequency range and their corrections are investigated to realize more accurate measurements. It is shown that attenuation measurements are influenced by diffraction losses and can be corrected by numerical calculations, and that velocity measurements are affected by the phase advance caused by diffraction. This investigation demonstrates that, in complex-mode velocity measurements, in which the velocity is determined from the measured phase of the signals, the true velocity at each frequency can be obtained by correction using the numerical calculation of diffraction. Based on this result, a new correction method in amplitude-mode velocity measurements is also proposed. In this new method, the velocity is determined from the intervals of interference output obtained by sweeping the ultrasonic frequency for the superposed signals generated by the double-pulse method. Velocity may be measured accurately at frequencies in the Fresnel region, and diffraction correction is essential to obtain highly accurate values with five significant figures or more.     

宽光谱AOTF匹配网络对光谱衍射效率影响的研究

基于声光可调滤波器(acousto-optic tunable filter,AOTF)光谱成像分析仪在可见至红外光谱的多个谱区内广泛应用,对AOTF的光谱带宽及衍射效率提出了更高的要求。超声换能器作为AOTF的核心部件,其3 dB工作带宽决定AOTF的光谱衍射范围,故在同一声光介质上制作两片厚度不同的换能器来提高AOTF光谱带宽。由于超声换能器在不同频率下具有不同的输入阻抗,当驱动信号源输出阻抗与超声换能器输入阻抗失配时会产生能量损耗,导致无法把功率最大限度的传递给超声换能器,从而使AOTF光谱衍射效率降低,影响光谱成像清晰度。通过射频电路先进设计系统(ADS)仿真及实验测试,设计了一种新型宽带阻抗匹配网络,在60～200 MHz带宽范围内,阻抗匹配网络功率效率达到90％以上,光谱衍射效率最高达90％,提高了在420～1 150 nm波段内的光谱灵敏度。     

Testing for inhomogeneity of the nonlinear parameter in an acoustic medium by ultrasonic phase conjugation

The principle of applying a selective phase conjugation of the second harmonic of a focused ultrasonic wave to diagnosing inhomogeneity of the nonlinear parameter in an acoustic medium is considered. A solution to the three-dimensional problem of harmonic generation by phase-conjugated waves in a nonlinear medium with a localized isoechogenous inclusion is obtained. The signal amplitudes detected by a transmitting-receiving transducer at the second and forth harmonics of a probe wave are calculated for varying position of the inclusion relative to the focus.