圆柱型光纤螺线圈轨道角动量模式

传统的沿z轴光纤传输光线的轨道角动量(orbital angular momentum,OAM)光束的制备方法共同之处都是从内部结构着想,光束的主光线基本上不变,只是波面在变.但要获得携带高mh的光有一定的难度.针对上述问题,本文建立以波面不变,光束主光线变化为基础的理论框架,利用微分几何理论验证不沿z轴圆柱型光纤螺线圈传输的光线可以携带高mh OAM的理论设想.研究发现:利用流动坐标(α,β,γ)计算光线在绕圆柱体的光纤中传输时光纤截面的衍射分布图呈现涡旋特征,有高阶OAM模式.当θ=θ0时,圆柱形轨道光纤过渡到直线轨道光纤.计算光线沿直线传输时光纤截面的衍射分布图是Airy斑,即圆孔衍射斑,无高阶OAM模式.     

Rapid and accurate analysis of surface and pseudo-surface waves using adaptive laser ultrasound techniques

A method has been developed to measure the phase velocity of laser generated and detected surface acoustic waves. An optical grating produced by an electronically addressable spatial light modulator (SLM) was imaged onto the sample surface to generate surface acoustic waves whose frequency and wavefront was controlled by the SLM. When the grating period matched the surface acoustic wavelength, the surface wave was strongly excited, thus the wavelength and, thereby the phase velocity was determined. We present results with this method that allows the phase velocity and the angular dispersion of the generalized surface wave as well as the pseudo-surface wave on the (100) nickel to be measured. Measurements on (111) silicon single crystals are also presented. The measurement precision is approximately 0.2%. Methods to further improve the measurement precision are also discussed.     

A spiral wave front beacon for underwater navigation: basic concept and modeling

A spiral wave front source produces an acoustic field that has a phase that is proportional to the azimuthal angle about the source. The concept of a spiral wave front beacon is developed by combining this source with a reference source that has a phase that is constant with the angle. The phase difference between these sources contains information about the receiver's azimuthal angle relative to the beacon and can be used for underwater navigation. To produce the spiral wave front, two sources are considered: a "physical-spiral" source, which produces the appropriate phase by physically deforming the active element of the source into a spiral, and a "phased-spiral" source, which uses an array of active elements, each driven with the appropriate phase, to produce the spiral wave front. Using finite element techniques, the fields produced by these sources are examined in the context of the spiral wave front beacon, and the advantages of each source are discussed.     

离心叶轮内三维湍流流场的实验研究

利用多普勒激光测速仪对闭式后弯离心叶轮内三维湍流流场进行了实验研究。叶轮在带有无叶扩压器的通风机内运行。对整个流道内各流面的测点进行了详细的数据采集和统计．由得到的测量结果,分析了叶轮内回转面、径向面上主流速度的分布及发展趋势,气流角由叶轮进口向出口、由压力侧向吸力侧的变化规律、以及叶轮出口处二次旋涡流动等流动特性。     

Analysis of the space-time dynamics of acoustic fields on the surface of a solid

The space-time dynamics of an acoustic field produced by a piezoelectric transducer in a pulsed mode is studied. The detection of acoustic fields is achieved using a Doppler laser interferometer. It is shown that, for a pictorial representation of the dynamics of a pulsed process, it is convenient to use the patterns of instantaneous spatial field distributions within the scanning area, the observation of which at successive instants makes it possible to trace the acoustic field variations on a time scale considerably smaller than the period of the ultrasonic wave. Experimental data demonstrating the process of phase propagation along the sample boundary as a function of time are presented. They are in good agreement with theoretical results obtained by using various methods of acoustic field calculation and different scalar potential distributions over the transducer surface. It is shown that the velocity of phase propagation along the sample boundary, which is mainly determined by the wave front curvature of the elastic wave incident on the sample surface, can considerably exceed the wave velocity in the unbounded medium.     

Restless rays, steady wave fronts

Observations of underwater acoustic fields with vertical line arrays and numerical simulations of long-range sound propagation in an ocean perturbed by internal gravity waves indicate that acoustic wave fronts are much more stable than the rays comprising these wave fronts. This paper provides a theoretical explanation of the phenomenon of wave front stability in a medium with weak sound-speed perturbations. It is shown analytically that at propagation ranges that are large compared to the correlation length of the sound-speed perturbations but smaller than ranges at which ray chaos develops, end points of rays launched from a point source and having a given travel time are scattered primarily along the wave front corresponding to the same travel time in the unperturbed environment. The ratio of root mean square displacements of the ray end points along and across the unperturbed wave front increases with range as the ratio of ray length to correlation length of environmental perturbations. An intuitive physical explanation of the theoretical results is proposed. The relative stability of wave fronts compared to rays is shown to follow from Fermat's principle and dimensional considerations.     

Suitability of laser Doppler velocimetry for the calibration of pressure microphones

The details of a new approach for absolute calibration of microphones, based on the direct measurement of acoustic particle velocity using laser Doppler velocimetry (LDV), are presented and discussed. The calibration technique is carried out inside a tube in which plane waves propagate and closed by a rigid termination. The method developed proposes to estimate the acoustic pressure with two velocity measurements and a physical model. Minimum theoretical uncertainties on the estimated pressure and minimum measurable pressure are calculated from the Cramer Rao bounds on the estimated acoustic velocity amplitude and phase. These uncertainties and the minimum measurable pressure help to optimize the experimental set up. Acoustic pressure estimations performed with LDV are compared with acoustic pressures obtained with a reference microphone. Measurements lead to a minimum bias of 0.006 dB and a minimum uncertainty of 0.013 dB on the acoustic pressure estimation for frequencies 1360 Hz and 680 Hz.     

水下低频声信号的激光探测及波的衰减

为了探测几十赫兹的低频水下声信号,建立了水下低频声信号的光学探测系统,得到了稳定、清晰的衍射图样.得到了衍射图样的宽度与声源距离的变化关系,声源距离越小,衍射图样越宽.当水下声波传至水表面后,实验上得到了表面声波的衰减特性,理论上得到了衍射图样的角宽度和液体表面波振幅的解析关系式.发现表面波振幅的衰减随距离是指数型衰减.并研究了衰减系数随频率的变化,频率不同衰减系数也不同,而且频率越大,衰减系数越小.     

Laser-Doppler Vibrometer measurements of acoustic-to-seismic coupling in unconsolidated soils

Measurements of acoustic-to-seismic coupling ratio, i.e. the ratio of the pressure exerted by an acoustic wave at a point on the surface to the acoustic particle velocity generated at the surface at that point, may be used to determine both elastic and structural properties of poroelastic materials. The sound pressure is measured using a microphone and, usually, the velocities are measured using geophones. Problems with geophone sensors have been shown to include both mass loading of the soil and coupling resonances within the frequency range of interest. The latter can lead to inaccurate amplitude and phase measurements. In an attempt to overcome these problems, the use of a Laser-Doppler vibrometer (LDV) has been investigated. Previous work with compacted plane soil surfaces has been extended to loosely consolidated soils. Good agreement has been found between geophone and LDV measurements of vertical particle velocity for a continuous wave sound source. Problems with poor LDV signal-to-noise ratio in unconsolidated materials have been overcome using local ground treatment. Subsequent modelling shows reasonable agreement between the data and the predicted values of material properties.     

浅海内波影响下的波导不变量变化特性分析

针对浅海内波引起波导不变量变化的问题,利用声场波导不变量的概率分布并结合声场简正波的理论,研究了内波活动下波导不变量的时变性,给出了波导不变量变化的机理和规律.具体结论是,在负跃层波导中,声场的波导不变量的最大概率取值具有明显的频变特性.内波环境下,当声传播方向与内波波阵面平行时,接收声场简正波的幅度变化不大,但是简正波的相慢度差和群慢度差的变化却能引起波导不变量最大概率取值的变化;而当声传播方向与内波波阵面垂直时,内波引起的简正波耦合同样会导致波导不变量的最大概率取值的明显变化.     

Acoustic source identification using a scanning laser Doppler vibrometer

This paper shows how a scanning laser Doppler vibrometer (LDV), an instrument designed to measure vibrations of structures or objects, can be used in a non-traditional fashion to identify acoustical sources. This is achieved by measuring the changes in the optical path induced by local fluctuation of the air refraction index to which the LDV is sensitive. The acoustical signal used is sinusoidal and may be recovered by scanning at a uniform rate over a subject area (continuous scan) parallel to the source axis and demodulating this signal. Due to the fact that the measured scan area is in fact a line integral over a measurement volume between the laser head and a rigid object needed to reflect the laser beam, multiple view planes around the axis of the acoustic source are usually measured. These are then passed through a tomographic algorithm, thereby reconstructing the full sound field. In this article however, only one view plane is measured, but the acoustic source is placed on a rotating surface with fixed rotational frequency, thereby imposing a modulation on the measured spectrum. Demodulation will allow reconstruction of the three-dimensional sound field.     

Experimental evidence for partial waves in the optical investigation of Lamb waves

An optical probing technique has been used to determine the propagation parameters and the acoustic field distribution of the longitudinal and shear partial waves coupled in Lamb modes. The partial wave momenta were determined from measurements of the light diffraction spectrum and partial acoustic fields analysed by combining spatial light filtering and polarization discrimination techniques. A method for the identification of the Lamb mode is presented.     

A study of how temperature field variations affect accuracy in measuring the distance to underwater objects

The paper presents the experimental testing results for an acoustic method that increases the accuracy of measuring underwater object positioning systems; the method is based on application of temperature field measurement data in a water area where an underwater object functions. Measurements conducted in September 2011 in Vityaz Bay of Possiet Bay have shown that application of the method permits a two- to threefold reduction in the positioning error related to the change in the sound propagation conditions.     

激光多普勒技术在固体测量方面的运用

介绍了激光多普勒测量技术的原理和后散射型固体表面测量方式。通过声光调制器的偏频后 ,能够确定物体的运动方向。对多普勒信号进行光学外差处理并对多普勒信号进行测相 ,可以提高测量的精度。用差动微分多普勒技术进行测量 ,可以消除由固体表面特性和激光光强的波动引起的误差。     

水声信号处理领域新进展

本文介绍近30年来水声信号处理领域理论研究的新进展和在声纳设计中的应用。包括水声信号建模、声场匹配、海洋波导和内波现象的探索和研究、声矢量场信息获取和处理,低频水声信道的时/空相关特性,水下目标辐射噪声的不变特征量提取和检测技术,水下语音、图像传输和抗干扰技术。同时概述,声纳设计的前沿领域:大孔径拖曳线列阵声纳、高分辨力合成孔径声纳、深海传呼机等的发展情况。     

Acoustic propagation from a spiral wave front source in an ocean environment

A spiral wave front source generates a pressure field that has a phase that depends linearly on the azimuthal angle at which it is measured. This differs from a point source that has a phase that is constant with direction. The spiral wave front source has been developed for use in navigation; however, very little work has been done to model this source in an ocean environment. To this end, the spiral wave front analogue of the acoustic point source is developed and is shown to be related to the point source through a simple transformation. This makes it possible to transform the point source solution in a particular ocean environment into the solution for a spiral source in the same environment. Applications of this transformation are presented for a spiral source near the ocean surface and seafloor as well as for the more general case of propagation in a horizontally stratified waveguide.     

The influence of optical properties of shallow shelf seas on temperature structure and acoustic propagation

The penetration of solar radiation below the sea surface is not always taken into account in numerical ocean models, even though its influence on the temperature (and therefore sound speed), structure of the water column can be significant, especially in shallow shelf seas where water turbidity tends to be high. Variations in sound speed structure have a consequent effect on the propagation of sound underwater which, in turn, influences the performance of sonar systems. A double exponential parameterization for the penetration of solar radiation with depth was implemented in an N × 1D turbulence closure model of the UK shelf seas. The model was run along a section through the Celtic Sea Front, with sets of optical extinction coefficients representing different water clarities, for one month to simulate the generation of the front, and the results were compared with a control model run in which all solar radiation was absorbed in the surface layer. Temperature structure and sound propagation were only affected by changes to the optical parameterization on the stratified side of the front, where consequent variations in acoustic propagation loss of up to 10 dB relative to the control were simulated using an acoustic model. Changes in propagation loss were greatest for acoustic sources placed in the stratified water above the thermocline. Similar changes to the optical parameterization made on the well-mixed side of the front had no discernible effect.     

Adaptive correction of vortex laser beam in a closed-loop system with phase only liquid crystal spatial light modulator

We propose and demonstrate the wave front correction of a vortex laser beam by using dual phase only liquid crystal spatial light modulators (LC-SLMs) and a stochastic parallel gradient descent (SPGD) algorithm. One phase only LC-SLM is used to generate vortex laser beam by loading spiral phase screen onto the wave front of input quasi-Gaussian beam. The other phase only LC-SLM under SPGD controller based on the subzone control method adaptively compensates the wave front of vortex laser beam. Numerical simulation and experimental results show that after correction, vortex doughnut like beam is focused into a beam with airy disk pattern distribution in the far field. The adaptive corrections of vortex laser beam with different optical topological charges are studied. The results show that the optical topological charge has little influence on adaptive correction. The powers in the main lobe of far field intensity distributions of vortex laser beams with different optical topological charges are all greatly improved by adaptive correction. The technique proposed in this paper can be used in optical communication, relay mirror and atmospheric turbulence correction.     

Medium characterization from interface-wave impedance and ellipticity using simultaneous displacement and pressure measurements

The interface-wave impedance and ellipticity are wave attributes that interrelate the full waveforms as observed in different components. For each of the fluid/elastic-solid interface waves, i.e., the pseudo-Rayleigh (pR) and Stoneley (St) waves, the impedance and ellipticity are found to have different functional dependencies on Young's modulus and Poisson's ratio. By combining the attributes in a cost function, unique and stable estimates of these parameters can be obtained, particularly when using the St wave. In a validation experiment, the impedance of the laser-excited pR wave is successfully extracted from simultaneous measurements of the normal particle displacement and the fluid pressure at a water/aluminum interface. The displacement is measured using a laser Doppler vibrometer (LDV) and the pressure with a needle hydrophone. Any LDV measurement is perturbed by refractive-index changes along the LDV beam once acoustic waves interfere with the beam. Using a model that accounts for these perturbations, an impedance decrease of 28% with respect to the plane wave impedance of the pR wave is predicted for the water/aluminum configuration. Although this deviation is different for the experimentally extracted impedance, there is excellent agreement between the observed and predicted pR waveforms in both the particle displacement and fluid pressure.     

Acoustic field imaging by light reflection at the free surface of a liquid

In this paper we present a novel technique for acoustic field imaging. This technique is based on reflection of a collimated laser beam at the free surface of a liquid. The reflected beam becomes phase modulated by the acoustic wave as in acoustical holographic systems. We do not use a reference acoustical beam for holographic reconstruction but we observe this phase modulation using dark-field techniques. It gives a measurement of the acoustic field power as a function of the position. The authors have built an optical imaging system and carried out experiments with piezocomposite transducers. The technique presented in this work is able to give fast quantitative information about the performance of individual ceramic rods of the piezocomposite.