Application of refraction and diffraction of light to the measurement of the liquid surface shape deformed by ultrasonic radiation pressure

A method of measuring the liquid surface (hump) deformed by ultrasonic radiation pressure is proposed. The method is based on the usage of a hump as a liquid lens with an interference structure formed by sound waves falling on the surface. The hump shape is evaluated by means of shadowgrams obtained with laser illumination. The possibility of using the hump as a sensor of processes occuring in the liquid through which sound waves are passing is shown.     

Large acoustic transients induced by nonthermal melting of InSb

We have observed large-amplitude strain waves following a rapid change in density of InSb due to nonthermal melting. The strain has been measured in real time via time-resolved x-ray diffraction, with a temporal resolution better than 2 ps. The change from the solid to liquid density of the surface layer launches a high-amplitude strain wave into the crystalline material below. This induces an effective plane rotation in the asymmetrically cut crystal leading to deflection of the diffracted beam. The uniform strain in the layer below the molten layer is 2.0(+/-0.2)%. A strain of this magnitude develops within 5 ps of the incident pulse showing that the liquid has reached the equilibrium density within this time frame. Both the strain amplitude and the depth of the strained material in the solid can be explained by assuming a reduction in the speed of sound in the nonequilibrium liquid compared to measured equilibrium values.     

An Iteratively-Designed Binary Phase Grating for Flattop Beam Generation

A binary phase grating capable of shaping an incident Gaussian beam to a flattop beam in a Fourier domain is designed iteratively. It is essential in grating design to consider the phases of diffracted waves by the grating, in addition to their amplitude uniformity and diffraction efficiency. The optimized grating phase produces the in-phase diffracted waves that interfere constructively to form a uniform amplitude and phase profile. The dependence of the shaped profile on the illuminating amplitude distribution, the grating misalignment, and the lens focusing error is investigated by computer simulation and optical experiment.     

Solitary waves on vortex lines in Ginzburg-Landau models for the example of Bose-Einstein condensates

Axisymmetric disturbances that preserve their form as they move along the vortex lines in uniform Bose-Einstein condensates are obtained numerically by the solution of the Gross-Pitaevskii equation. A continuous family of such solitary waves is shown in the momentum (p)-substitution energy (Epsilon) plane with p-->0.09 rho kappa(3)/c(2), Epsilon-->0.091 rho kappa(3)/c as U-->c, where rho is the density, c is the speed of sound, kappa is the quantum of circulation, and U is the solitary wave velocity. It is shown that collapse of a bubble captured by a vortex line leads to the generation of such solitary waves in condensates. The various stages of collapse are elucidated. In particular, it is shown that during collapse the vortex core becomes significantly compressed, and after collapse two solitary wave trains moving in opposite directions are formed on the vortex line.     

Model equation for strongly focused finite-amplitude sound beams

A model equation that describes the propagation of sound beams in a fluid is developed using the oblate spheroidal coordinate system. This spheroidal beam equation (SBE) is a parabolic equation and has a specific application to a theoretical prediction on focused, high-frequency beams from a circular aperture. The aperture angle does not have to be small. The theoretical background is basically along the same analytical lines as the composite method (CM) reported previously [B. Ystad and J. Berntsen, Acustica 82, 698-706 (1996)]. Numerical examples are displayed for the amplitudes of sound pressure along and across the beam axis when sinusoidal waves are radiated from the source with uniform amplitude distribution. The primitive approach to linear field analysis is readily extended to the case where harmonic generation in finite-amplitude sound beams becomes significant due to the inherent nonlinearity of the medium. The theory provides the propagation and beam pattern profiles that differ from the CM solution for each harmonic component.     

Ultrasonic diffraction grating spectroscopy and characterization of fluids and slurries

The ultrasonic diffraction grating is formed by machining triangular grooves, 300 microns apart, on a stainless steel surface. The grating surface is in contact with the liquid or slurry. The ultrasonic beam, traveling in the solid, strikes the back of the grating and produces a transmitted m=1 beam in the liquid. The angle of this beam in the liquid increases with decreasing frequency and the critical frequency FCR occurs when the angle is 90 degrees. At frequencies below FCR, this m=1 wave does not exist and its energy is shared with other types of waves. The signal of the reflected m=0 wave is observed and an increase is observed at FCR. This information yields the velocity of sound in the liquid and particle size.     

Harmonic propagation of finite amplitude sound beams: experimental determination of the nonlinearity parameter B/A

We propose a method to determine the nonlinearity parameter B/A of a liquid from the spatial evolution of harmonic components. We describe an analytical model, in the parabolic and quasi-linear approximations, that predicts the continuous finite amplitude sound beam propagation radiated by a plane piston source. This model takes into consideration attenuation, diffraction and nonlinear effects. The fundamental and second harmonic ultrasonic fields are expressed as the superposition of Gaussian beams. Axial propagation curves are then compared with those obtained by direct numerical solution of the transformed beam equation using the finite difference method, and with experimental results. Accurate measurements of pressure levels for the nonlinearly generated harmonics in water are performed along and across the propagation axis for different pressure values delivered at the piston surface. Experimental results, for water and ethanol, are in agreement with those of our model, which allows us to obtain the expected value of the nonlinearity parameter B/A.     

基于轴棱锥产生近似无衍射Mathieu光束的新方法

提出了一种基于轴棱锥产生零阶近似无衍射Mathieu光束的新方法,利用轴棱锥聚焦具有椭圆高斯振幅调制的平面波,得到近似零阶无衍射Mathieu光束.根据椭圆高斯平面波经轴棱锥衍射的衍射积分公式,对光强分布进行了数值模拟,依据几何光学模型计算了近似无衍射Mathieu光束的最大无衍射距离,并设计了实验对理论模拟的结果进行了验证.实验采用柱透镜和准直扩束系统变换圆高斯光束产生具有椭圆高斯振幅调制的平面波,用轴棱锥聚焦该平面波后得到近似无衍射Mathieu光束,实验结果与理论模拟和计算相符.     

Modeling of an electrohydraulic lithotripter with the KZK equation.

The acoustic pressure field of an electrohydraulic extracorporeal shock wave lithotripter is modeled with a nonlinear parabolic wave equation (the KZK equation). The model accounts for diffraction, nonlinearity, and thermoviscous absorption. A numerical algorithm for solving the KZK equation in the time domain is used to model sound propagation from the mouth of the ellipsoidal reflector of the lithotripter. Propagation within the reflector is modeled with geometrical acoustics. It is shown that nonlinear distortion within the ellipsoidal reflector can play an important role for certain parameters. Calculated waveforms are compared with waveforms measured in a clinical lithotripter and good agreement is found. It is shown that the spatial location of the maximum negative pressure occurs pre-focally which suggests that the strongest cavitation activity will also be in front of the focus. Propagation of shock waves from a lithotripter with a pressure release reflector is considered and because of nonlinear propagation the focal waveform is not the inverse of the rigid reflector. Results from propagation through tissue are presented; waveforms are similar to those predicted in water except that the higher absorption in the tissue decreases the peak amplitude and lengthens the rise time of the shock.     

猫眼谐振腔氦氖激光器理论分析

猫眼逆向器作为谐振腔镜与一个凹面镜构成猫眼激光谐振腔.精确计算了三种猫眼逆向器(即理想猫眼逆向器和两种存在误差的猫眼逆向器)的逆向平行反射性，并对应用这三种猫眼逆向器的谐振腔中高斯光束的各项参数进行了深入分析，得出结论：对于高斯光束，凸透镜的焦距、凹面镜曲率半径、凸透镜与凹面镜间距三者相等的猫眼逆向器为理想逆向器，其逆向平行反射性最好，此时猫眼谐振腔处于理想状态，基模模体积最大，远场发散角最小，稳定性最好；在猫眼谐振腔的设计中，要尽量减小猫眼逆向器中凸透镜的焦距，增大凹面输出镜的曲率半径；猫眼逆向器存在误差时谐振腔的各项性能将受到影响.为猫眼谐振腔激光器的设计提供了理论依据.     

Acousto-optic interaction in laser vibrometry in a liquid

It is demonstrated that, when the classical method of laser vibrometry is used for measurements in a liquid, it gives erroneous results with measurement errors reaching 100% or more. The vibration pattern observed in this case exhibits a false structure with a spatial scale identical to the wavelength of acoustic waves in the liquid. In addition, the laser vibrometer shows displacements in the regions where they are actually absent. In the transient mode of operation, the image displays nonexistent surface waves, which propagate with the velocity of sound in the liquid. The origin of these distortions lies in the acoustooptic interaction that occurs in the condensed medium on the path of the probing laser beam. An analytic expression is obtained for the Green’s function characterizing laser vibrometry in the cases of harmonic and pulsed excitation of the surface under investigation. It is shown that this function explains all the artifacts observed in laser vibrometry in a liquid and can be used to correct the measurement data.     

高极化纯度的超表面透镜设计与应用

针对超表面在透镜方面的应用,基于各向异性超表面单元设计了一款高极化纯度的聚焦超表面透镜,并探讨了其在高增益高极化纯度天线方面的应用.设计了一款具有极化滤波特性的各向异性超表面单元,单元对x极化波保持高透性的同时,对y极化波保持近乎为零的透射率.利用该型单元设计了焦距为30 mm、阵列大小为105 mm×105 mm、单元数为21×21的聚焦超表面透镜.根据光路可逆原理,焦点处发出的球面波被超表面透镜有效转化为平面波,从而达到提高天线增益的目标.实验中分别用不同极化形式的球面波照射聚焦超表面来研究超表面对不同极化波的控制特性.结果表明,x极化波照射时,超表面工作于透镜模式,球面波转化为平面波,天线增益大大提高;y极化波照射时,超表面类似于金属板,将入射波全部反射;x/y极化混合波照射时,天线增益大大提高,且极化隔离度高于25 dB,充分说明设计的聚焦超表面在提高x极化波增益的同时可高效滤除y极化波,达到了高增益高极化纯度的目标.     

Laser Doppler anemometry and the acousto-optic effect

This paper deals with the effect of phase variations along the paths of the laser beams due to refractive index changes caused by the acousto-optic effect, when an LDA system is used to measure sound waves. Theoretical and experimental results will be discussed. A theoretical expression will be developed for the movement of the fringes in terms of the angle of the sound wave, the wavenumber and the distance of propagation of the laser beams. Results will show that as the wavenumber increases, the angle at which the maximum amplitude of the fringe movement occurs tends away from π/2 to the laser beam bisector, and towards the angle of the laser beams. It will also be shown that the phase modulations detected experimentally are in reasonable agreement with those predicted theoretically.     

A laser-induced fluorescence biosensor by using ellipsoidal reflector

A new and compact fluorescence biosensor is constructed. The distinguished feature of this setup is the use of an ellipsoidal reflector as an optical collector of the fluorescence light. A traditional collecting lens with numerical aperture of 0.5 can only collect a few percent of the available signal, while the incorporation of the ellipsoidal reflector enables the apparatus to provide a signal increase of at least 66 times. A very good linear relationship between the concentration of fluorescent dye and the fluorescence intensity is achieved with correlation coefficient R=0.99979. The 3σ detection limit is 5 nM for microliter volume sample. Meanwhile, it is found that laser light scattering by dirts and scratches on the sample cell's windows and by turbidity in the sample solutions is the dominant limitation to increase the detection sensitivity. Approach to solve this problem is suggested.     

COB光源的窄光束均匀照明设计

为了解决高功率白光LED光源输出高均匀度窄光束的问题,设计了一种由复合抛物面反射器、菲涅尔透镜和非球面透镜组成的照明系统。设计中以板上芯片型（COB）集成光源的配光曲线为依据构建光源仿真模型,由复合抛物面反射器实现大角度光线初次会聚之后,再由菲涅尔透镜控制溢散光,最后利用非球面透镜进行准直配光。采用TracePro进行蒙特卡洛光线追迹,根据仿真得到的系统性能指标,并研制出实物装置进行实验测试。最终测试结果表明:窄光束均匀照明系统可以输出±7.9°的光束,并且在距离系统出光面0.7 m左右的区域形成均匀度超过96%的圆形光斑,整体光效达到60%。     

光学薄膜测量时平顶光束激励的表面热透镜理论模型

推导了调制的平顶光束激励下基于Fresnel衍射积分的表面热透镜理论，通过数值模拟，比较了平顶光束和高斯光束激励下样品内部温度场、表面形变场和探测光衍射信号的径向分布，分析了影响表面热透镜信号的实验参数.结果表明，在最佳探测位置，平顶光束激励下的表面热透镜系统比相同情况下高斯光束激励下的灵敏度高，最高时约2倍，更有利于薄膜吸收测量. 关键词： 表面热透镜 光学薄膜 平顶光束     

Acoustic nonlinearity parameter tomography for biological tissues via parametric array from a circular piston source--theoretical analysis and computer simulations

The acoustic nonlinearity parameter B/A describes the nonlinear features of a medium and may become a novel parameter for ultrasonic tissue characterization. This paper presents a theoretical analysis for acoustic nonlinear parameter tomography via a parametric array. As two primary waves of different frequencies are radiated simultaneously from a circular piston source, a secondary wave at the difference frequency is generated due to the nonlinear interaction of the primary waves. The axial and radial distributions of sound pressure amplitude for the generated difference frequency wave in the near field are calculated by a superposition of Gaussian beams. The calculated results indicated that the difference frequency component of the parametric array grows linearly with distance from the piston source. It therefore provides a better source to do the acoustic nonlinearity parameter tomography because the fundamental and second harmonic signals both have a near field that goes through many oscillations due to diffraction. By using a finite-amplitude insert substitution method and a filtered convolution algorithm, a computer simulation for B/A tomography from the calculated sound pressure of the difference frequency wave is studied. For biological tissues, the sound attenuation is considered and compensated in the image reconstruction. Nonlinear parameter computed tomography (CT) images for several biological sample models are obtained with quite good quality in this study.     

Acoustic characteristics of pulse detonation engine with ellipsoidal reflector

Acoustic characteristics of a pulse detonation engine(PDE) with and without an ellipsoidal reflector are numerically and experimentally investigated. A two-dimensional(2 D) non-splitting unstructured triangular mesh Euler solver based on the space-time conservation element and solution element(CE/SE) method is employed to simulate the flow field of a PDE.The numerical results clearly demonstrate the external flow field of the PDE. The effect of an ellipsoidal reflector on the flow field characteristic near the PDE exit is investigated. The formation process of reflected shock wave and reflected jet shock are reported in detail. An acoustic measurement system is established for the PDE acoustic testing. The experimental results show that the ellipsoidal reflector changes the sound waveform and directivity of PDE sound. The reflected shock wave and reflected jet shock result in two more positive pressure peaks in the sound waveform. The ellipsoidal reflector changes the directivity of PDE sound from 20 to 0. It is found that the peak sound pressure level(PSPL) and overall sound pressure level(OASPL) each obtain an increment when the PDE is installed with a reflector. The maximum relative increase ratio of PSPL and OASPL are obtained at the focus point F2, whose values are 6.1% and 6.84% respectively. The results of the duration of the PDE sound indicate that the reflecting and focusing wave generated by the reflector result in the increment of A duration and B duration before and near focus point F2. Results show that the ellipsoidal reflector has a great influence on the acoustic characteristic of PDE sound. The research is helpful for understanding the influence of an ellipsoidal reflector on the formation and propagation process of PDE sound.