任意声场中非规则形状Rayleigh散射体的声辐射力研究

为满足声辐射力更广泛的应用,克服传统辐射力理论仅适合简单理想声场及规则形状散射体的局限性,建立了任意入射声场中非规则形状Rayleigh散射体的声辐射力计算理论.针对任意声场中流体质点沿曲线轨迹振动以及非规则散射体空间姿态随机取向的特点,在散射场计算中同时计入散射体质心甲移与姿态转动的影响,得到了更为普适性的散射场速度势函数.在此基础上,推导出适于任意声场中非规则散射体的声辐射力计算公式.实例研究表明,本文方法不仅完全满足简单声场规则散射体的辐射力计算,而且还适合于任意声场非规则散射体辐射力的应用,规则散射体的计算结果与传统方法完全一致,而对于非规则散射体证实了其旋转角速度不为零,且声辐射力随姿态不同而变化.     

自由空间中球形粒子的负向声辐射力

针对一定声场作用下自由空间中的球形粒子,首先分析了声散射过程中的吸收声功率、散射声功率和损失声功率以及三者之间的关系,并通过计算发现了由于参数选取不当导致的负吸收现象。接着从动量守恒定律出发推导了声辐射力的一般表达式,阐释了声辐射力与声能流之间的关系,并从理论和计算两方面验证了负向声辐射力的存在。当负向声辐射力产生时,声波的背向散射被抑制。在此基础上,进一步研究了粒子的偏心特性和流体的黏度这两种常见因素对负向声辐射力的影响。利用球函数的加法公式推导了偏心球的散射系数和声辐射力公式,结果显示偏心距离、粒子的材料等都会显著改变负向声辐射力的产生条件。在低频近似下,由于流体黏度附加的正向声辐射力是否能完全抵消原来的负向声辐射力将决定最终的声辐射力方向。该结果对利用负向声辐射力制成单行波声学镊子来实现对特定粒子的操控有着理论指导意义。      

基于声粒子分布积分的无网格声场计算方法

小尺度封闭空间内部声场的数值计算是声学设计、噪声控制等领域的关键技术。由于波动声学及几何声学方法计算频率上的限制,中频段声场计算问题一直是个难点。本文以声学无网格法为基础,提出了一种基于声粒子分布积分的无网格声场数值计算方法。文中利用声线跟踪理论计算声场中的声粒子分布,并以某个时间点上的声粒子作为蒙特卡罗法中的积分点,将其应用于无网格法中,从而获得声场中的节点声压。利用该方法对一个矩形封闭空间的中低频声场进行了计算,并与模态叠加法、商用声场计算软件、经典无网格法的结果进行了对比,证明基于声粒子分布积分的无网格声场数值计算方法在中低频段相较于传统基于网格的方法具有更高的精度。     

利用k-Wave计算超声在腹壁组织的声辐射力*

随着科学技术的发展,声辐射力在生物医学领域得到了更为广泛的应用,尤其是在弹性成像领域。为了使弹性成像技术更加精准,对声辐射力的预测至关重要。该文基于腹壁组织图像,利用k-Wave对超声波在腹壁组织区域传播时的声场进行数值模拟,获得了其声场分布,进而求得了组织中声辐射力分布情况,同时对面阵换能器的阵元宽度、间距、阵元个数以及工作频率等参量对声辐射力的影响进行了计算与分析。结果表明,腹壁组织的声场分布受其非均匀性的影响,声辐射力的分布情况依赖于换能器参量的选择。该研究为声辐射力在弹性成像技术中的应用奠定了基础,为其技术的改进提供了重要依据。     

零阶Bessel驻波场中任意粒子声辐射力和力矩的Born近似

声辐射力和声辐射力矩的计算是实现粒子精准操控的重要基础.基于经典声散射理论的偏波级数展开法较难直接用于复杂模型的研究,而纯数值的方法则不利于进行系统的参数化分析.基于Born近似的基本原理,推导了低频情况下零阶Bessel驻波场中心任意粒子的声辐射力和力矩表达式.在此基础上,以球形粒子、椭球形粒子和柱形粒子为例进行详细地计算,并考虑声参数的非均匀性对声辐射力和力矩的影响.仿真结果表明,在低频范围内Born近似具有很高的精度,随着频率的增加和粒子与流体的阻抗匹配变差,Born近似的精度逐渐下降.对于倾斜放置于零阶Bessel驻波场中的椭球形粒子和柱形粒子,非对称性会导致其受到声辐射力矩的作用.在粒子尺寸远小于波长的情况下,声辐射力特性与粒子的具体形状几乎无关,但声辐射力矩不然.最后,引入周围流体的黏滞效应并对声辐射力的表达式进行了修正.该研究预期可以为生物医学、材料科学等领域利用驻波场声镊子实现微小粒子的精准操控提供一定的理论指导.     

共轴双涡旋声束形成离轴多涡旋的定位

涡旋声束具有螺旋的相位波前,中轴线上形成声强为零的相位奇点,其所携带的轨道角动量在粒子操控领域有着广阔的应用前景。传统声涡旋只在传播轴线上形成一个拓扑荷可控的涡旋波束,这限制了声涡旋的应用灵活性。基于环形点声源阵列和相位编码技术,利用奇偶声源分别产生共轴双涡旋声束的声场叠加,在传播截面上形成了具有中心涡旋和子涡旋的离轴多涡旋声场;研究了双涡旋拓扑参数对离轴涡旋的个数、位置及拓扑荷的影响,基于声涡旋的径向声压和相位分布,确定了离轴涡旋的离轴半径,并结合声源位置推导子涡旋中心方位角的计算公式,实现离轴涡旋的精确定位。本研究突破了沿轴分布的涡旋声场只能形成单点涡旋势阱的操控局限,为利用离轴多涡旋实现多点粒子捕获提供了理论依据,促进涡旋声场在精确粒子操控和传输方面的高效应用。      

空气中一维声栅对微粒的声操控

本文对一维空气声栅表面微粒受到的声辐射力进行了详细的理论研究.首先采用有限元方法研究一维声栅的透射性质及表面声场分布,然后将有限元与动量张量积分结合研究处于一维声栅表面微粒受到的声辐射力特征.声栅共振透射增强是表面周期衍射波与狭缝Fabry-Perot共振耦合形成的,并且与声栅周期和厚度密切相关.研究发现,当共振波长与声栅周期相当时,微粒在其表面可受到指向声栅板面的声吸引力;当共振波长为声栅周期的二倍及以上,微粒可受到指向狭缝中的吸引力,且强度远小于第一种情况的吸引力.因此,在声栅处于共振波长与周期相当的共振模式时,可以在空气中利用声栅表面操控、吸引和排列微粒.     

非傍轴双光束势阱产生的辐射力

分析了两束相对传输的非傍轴高斯光束相干叠加形成的双光束势阱对瑞利粒子产生的辐射力,并作了数值计算,结果表明,与傍轴双光束势阱相比,非傍轴双光束势阱的辐射力有明显的不同,纵向辐射力和横向辐射力都增大,y方向平衡点数目由一个增加到多个,且势阱更深,横向辐射力变化趋势更陡,更有利于微粒的精确定位,与非傍轴单光束势阱相比,势阱更深,所产生的辐射力更大,因而更利于控制瑞利粒子。     

椭球粒子声辐射力计算及分析

针对水下椭球粒子,以声散射理论为基础,采用分波序列的方法,建立了椭球粒子声辐射力的理论计算模型。进而根据声辐射力计算公式,以刚性椭球粒子和液体椭球粒子为例,计算并分析不同Bessel波束作用下椭球粒子的轴向声辐射力函数特征。数值仿真计算结果表明,对于刚性椭球粒子,扁平椭球粒子相对于细长椭球粒子更有助于激发负声辐射力；对于液体椭球粒子,细长椭球粒子相对于扁平椭球粒子更加容易产生负声辐射力；对于不同介质的椭球粒子,不同的入射波束激发的负声辐射力的效果也存在明显的差异。该结果为复杂的尺寸和介质粒子声操控技术提供了理论的可行性。     

混响环境下目标声场稀疏分解多域声场重放方法*

对于封闭空间内的声场重放,传统的多点声压匹配方法（Cov-PM）直接依据在目标声场测得的声压直接使用最小二乘来计算扬声器权重。然而这种方法要求较多的目标声场采样点以实现足够的精度。对于一类特殊的声场景,也就是目标声场是由少量声源辐射产生时,提出一种对目标声场稀疏分解的方法来进行混响环境下多域声场重构以降低对目标声场采样数量的要求。本文给出基于目标声场等效源稀疏分解多域重构方法（Sparse-ESM）理论推导,通过数值计算以及实验测试两种方式对比所提方法与最小二乘等效源分解方法以及Cov-PM的声场重放性能。数值结果表明,在600Hz 以上的频段,Sparse-ESM 方法的重构误差性能提升明显。实验结果也得出了与数值计算相同的结论。同时,还通过数值计算和实验测试两种方法证明了当目标声场声源方向波动时,Sparse-ESM 仍然可以保持与其余两类方法相近的声对比度同时实现较高的亮区重构精度。     

Theoretical analysis and simulation of conjugate heights for dual-conjugate AO system in lidar

A multi-conjugate adaptive optics （MCAO） can offer a possibility of widening field of view （FOV） characterized by the isoplanatic angle, and the choose of conjugate height becomes a basic problem for MCAO, which influences the size of iosplanatic angle. Considering the application of lidar, the isoplanatic angle＇s expressions of two deformable mirrors （DMs） MCAO for uplink and downlink are deduced. The effects of conjugate heights for dual-conjugate AO are thoughtfully discussed, and the isoplanatic angles are further analyzed. The results show that the isopanatic angle varies with the conjugate height and reaches the maximum as the conjugate height is at the optimal altitude. Moreover, the optimal conjugate height changes with the propagation distance.     

Quick single-photon detector with many avalanche photo diodes working on the time division

Due to the limit of response speed of the present single-photon detector, the code rate is still too low to come into practical use for the present quantum key distribution (QKD) system.A new idea is put up to design a quick single-photon detector.This quick single-photon detector is composed of a multi-port optic-fiber splitter and many avalanche photo diodes (APDs).Au of the ports with APDs work on the time division and cooperate with a logic discriminating and deciding unit driven by the clock signal.The operation frequency lies on the number N of ports, and can reach N times of the conventional single-photon detector.The single-photon prompt detection can come true for high repetition-rate pulses.The applying of this detector will largely raise the code rate of the QKD, and boost the commercial use.     

Characterization of PolyA and PolyC mismatches by Raman spectroscopy

A.C mismatches are studied by Raman spectral characterization of PolyA, PolyC, and their equimolar complex in solution of 0.14 mol/L Na ,pH7.0.Experimental results show that A·C mismatches occur to be A/B (mainly A) conformers, and unlike Watson-Crick base pairing, this kind of mismatches is stabilized by only one hydrogen bond involving cytosine N4H2 and adenine N7.The formation of A·C complex makes the base stacking interactions much stronger, and conformation of the backbone more ordered, which leads to obvious Raman hypochromic effect with some shifts in corresponding bands.     

Improvement of four-wave mixing-based wavelength conversion efficiency in dispersion shifted fiber by 40-GHz clock pumping

40-GHz clock modulated signal as a pump to improve the efficiency of four-wave mixing (FWM)-based wavelength conversion in a 26.5-km dispersion shifted fiber (DSF) is investigated. The experimental results demonstrate that the conjugated FWM component has higher intensity with the clock pumping than that with the continuous-wave (CW) light pumping. The improvement of FWM-based wavelength conversion efficiency is negligible when the pump power is less than Brillouin threshold. But when the pump power is greater than Brillouin threshold, the improvement becomes significant and increases with the increment of pump power. The improvement can increase up to 9 dB if pump power reaches 17 dBm.     

Multi-pulse operation of a Kerr-lens mode-locked femtosecond laser

Our experimental results show that the presence of a proper amount of negative group velocity dispersion is essential to multi-pulse operation of a Kerr-lens mode-locked femtosecond laser. We demonstrate that the pulse separations and the number of pulses contained within a cavity round trip are strongly dependent on the initial perturbations. The results allow us to get a better understanding on the influences of the convoluted self-phase modulation and intra-cavity dispersions on the stable multi-pulse oscillation in a Kerr-lens mode-locked femtosecond laser.     

Propagation of Helmholtz-Gauss beams in weak turbulent atmosphere

Based on the Rytov approximation of light propagation in weak turbulent atmosphere,the closed-form expressions of field and average irradiance of each one of the four fundamental families of Helmholtz-Gauss (HzG)beams:cosine-Gauss beams,stationary Mathieu-Gauss beams,stationary parabolic-Gauss beams,and Bessel-Gauss beams,which are propagating in weak turbulent atmosphere,are obtained.The results show that the field and average irradiance can be written as the product of four factors:complex amplitude depending on the z-coordinate only,a Ganssian beam.a factor of complex phase perturbation induced by atmospheric turbulence,and a complex scaled version of the transverse shape of the non-diffracting beam.The effect of weak atmospheric turbulence on irradiance distribution of the HzG beam can be ignored.     

An effective method for reducing speckle noise in digital holography

An effective method for reducing the speckle noise in digital holography is proposed in this paper.Different from the methods based on classical filtering technique,it utilizes the multiple holograms which are generated by rotating the illuminating light continuously.The intensity images reconstructed by a series of holograms generated by rotating the illuminating light possess different speckle patterns.Hence by properly averaging the reconstructed intensity fields,the speckle noises can be reduced greatly.Experimental results show that the proposed method is simple and effective to reduce speckle noise in digital holography.     

Influence of purity of HfO2 on reflectance of ultraviolet multilayer

The impurities in two kinds of HfO2 materials and in their corresponding single layer thin films were determined through glow discharge mass spectrum technology and secondary ion mass spectrometry(SIMS) equipment respectively.It was found that ZrO2 was the main impurity in the two kinds of HfO2 either in the original HfO2 materials or in the electron beam deposited films.In addition,the difference of Zr content in the two kinds of HfO2 single laver films was much laxger than that of the other impurities such as Ti and Fe.which showed that it was just ZrO2 that made the difference between the optical performance of the film products including the two kinds of HfO2.With these two kinds of HfO2 and the same kind of SiO2.we deposited HfO2/SiO2 multilayer reflective coatings at the wavelength of 266 nm.Experimental results showed that the reflectances of these two mirrors were about 99.85% and 99.15% respectively,which agreed well with the designed results what were based on the optical constants obtained from the corresponding single layer thin films.     

Speech enhancement by array crosstalk resistant ANC and spectrum subtraction

Microphone array-based speech enhancement has great importance for speech communications and speech recognition. To reduce the aperture of the microphone array and to increase the effect of the speech enhancement will greatly broaden the application areas of the microphone array. An array crosstalk resistant adaptive noise cancellation method is therefore presented. And then an improved spectral subtraction algorithm is further cascaded to obtain better enhancement results. Theoretic analysis and experiments indicate that the proposed scheme needs only a very small microphone array while it simultaneously achieves a higher SNR improvement. Besides, the proposed scheme can be used in many noisy environments and is easy for real-time implementation.