175kHz高频空气超声换能器研制及其特性测量

本文报道了一种用于复杂空气旋涡场参量测量的高频空气超声换能器．在用超声-激光方法研究流场时，它是关键的换能元件。该换能器工作频率为175kHz。由于采用了管状压电元件及特制的伞状反射器，该换能器具有优越的指向性（士2．5°）     

基于单个BBO晶体载波包络相位稳定的高效率光参量放大器

基于单个BBO非线性晶体,利用非共线光参量放大技术,研究了载波包络相位稳定的高效率可调谐近红外脉冲产生.以载波包络相位稳定的飞秒激光放大系统产生的白光作为种子光,注入一个二类匹配的二级光参量放大器,在1350 nm波段获得抽运-信号光34%的转换效率.利用f—2f光谱相干测量技术,放大脉冲载波包络相位的抖动30 min内小于137 mrad.该方法提供了一种简单高效的载波包络相位稳定的红外脉冲产生技术.     

莫尔测量法中相位物体位置对结果的影响研究北大核心CSCD

莫尔测量法被广泛用于各类流场的结构显示和关键参量测量中,关于相位物体在光路中的放置位置已有不同的报道,针对该问题进行了理论和实验研究。理论分析表明,当相位物体放置于2个光栅前时,偏移量和相位主要取决于2个光栅之间的距离,偏折角取决于相位物体引起的相位扰动与2个光栅之间的间距的比值;而当相位物体放置于2个光栅之间时,偏移量和相位主要取决于相位物体到后面一个光栅之间的距离,偏折角则取决于相位物体引起的相位扰动与其到后面一个光栅之间的距离的比值。为了验证理论结果的正确性和合理性,实验中选取燃烧蜡烛火焰作为对象,对比发现在两种情况下,只要相应的距离选取适当,则相位物体的位置并不影响实验中重建的相位和偏折角等结果。相关研究成果可为莫尔测量法更好地用于各类流场的检测提供一定的参考。     

基于单晶体的可调谐参量超荧光的产生

研究了一种基于单晶体的可调谐超荧光产生机理，在一个偏硼酸钡(BBO)晶体中实现了飞秒脉冲倍频过程和光参量产生过程.实验中采用kHz高功率钛宝石激光系统输出的飞秒脉冲光倍频后的蓝光作为抽运光，获得了可调谐范围为480—530 nm参量超荧光光谱输出.理论上分析了这种超荧光产生机理，并利用放大传递函数模拟出参量超荧光环的产生过程.结果表明，在一个BBO晶体中，当抽运光源输出光入射晶体角度同时满足倍频相位匹配角和非共线光参量产生相位匹配角时可产生参量超荧光环，通过微调相位匹配角可控制参量超荧光光谱调谐输出.该理论和实验研究为控制参量超荧光和量子纠缠态的产生提供了理论依据，对于量子成像和量子通讯等领域的发展具有重要意义.     

一种液晶相位调制特性的测量方法

提出一种基于衍射光栅的液晶相位调制特性测量方法.该方法利用液晶构建相位分别为0和φ的二值光栅,通过傅里叶光学的方法推导衍射光栅第0级衍射光斑光强和调制相位φ之间的关系,然后实测光强和液晶驱动电压之间的对应关系来得到相位和液晶驱动电压之间的对应关系,即液晶相位光栅的相位调制特性.最后利用测量相位调制特性结果构建液晶相控阵,用光束偏转误差验证调制特性测量结果,相位测量误差小于1×10~(-3)rad.     

利用预失真条纹校正PMP系统投影仪畸变的研究

依据摄像机畸变模型提出了一种投影条纹相位畸变校正方法来简化相位-高度映射关系.该方法首先通过投两套互相垂直的相移正弦条纹,以相位值代替投影仪像素坐标,将投影仪当成摄像机看待,标定出投影仪的内参量.然后根据标定出的投影仪镜头畸变参量对理想相位施加反向的畸变,在投影时反向畸变的光栅通过镜头畸变,又成为理想的光栅,从而简化了相位-高度映射关系.实验中,虽然由于测量误差的影响,校正后的投影仪径向畸变系数还是比原来小了1个数量级.     

受激旋转喇曼散射效应对强紫外激光聚焦特性的影响

采用Maxwell-Bloch-Langevin 方程,建立了强紫外激光在空气传输中的瞬态受激旋转喇曼散射（SRRS）效应模型；利用相位均方根（RMS）梯度描述的随机相位屏表示低频相位畸变,用扰动幅度表征的随机相位屏描述中、高频相位畸变,建立了相位畸变模型.讨论了强紫外激光在空气长程传输中所产生的受激旋转喇曼散射（SRRS）效应对其聚焦光斑形态的变化,定量分析了焦斑半径随入射强紫外激光的空间相位畸变及传输距离的变化.研究结果表明：当传输距离超过阈值条件时,焦斑半径随传输距离的增大而明显增大,且相位畸变越大,焦斑半径也越大.     

利用预失真条纹校正PMP系统投影仪畸变的研究*

依据摄像机畸变模型提出了一种投影条纹相位畸变校正方法来简化相位-高度映射关系.该方法首先通过投两套互相垂直的相移正弦条纹,以相位值代替投影仪像素坐标,将投影仪当成摄像机看待,标定出投影仪的内参量.然后根据标定出的投影仪镜头畸变参量对理想相位施加反向的畸变,在投影时反向畸变的光栅通过镜头畸变,又成为理想的光栅,从而简化了相位-高度映射关系.实验中,虽然由于测量误差的影响,校正后的投影仪径向畸变系数还是比原来小了1个数量级.     

超短脉冲激光光束在一维反射型体全息光栅中的衍射

基于Kogelnik的耦合波理论,研究了在色散效应的影响下,超短脉冲激光光束在反射型体全息光栅中衍射的性质.研究给出了衍射光及透射光在频谱域及时间域的振幅及强度分布、光栅的光谱宽度及衍射效率随光栅参量及入射条件的变化.数值研究的结果表明,在光栅记录介质色散效应的影响下,衍射光束的光谱宽度减小,脉冲展宽,衍射效率降低.通过适当的选取光栅参量及入射条件,可以控制衍射和透射光束的频谱和时间强度分布,得到满意的衍射和透射光束的带宽和波形,从而可以将其应用于脉冲整形等技术中.     

基于光束偏转的扫描式宽带光参量啁啾脉冲放大

光参量啁啾脉冲放大(OPCPA)是超短激光脉冲领域的重要技术之一,增大增益带宽对提高OPCPA的转换效率、实现宽带光参量放大具有重要的意义.本文将光束偏转和非共线OPCPA有机结合,提出了基于光束偏转的扫描式宽带OPCPA模型.分析了通过光束偏转来时刻改变非共线角,以保证各频率成分的相位匹配,从而增大增益带宽的基本原理.采用提出的扫描式宽带OPCPA,针对800 nm中心波长、带宽约为100 nm信号光的光参量放大进行了数值计算.结果表明:经过扫描式OPCPA后,信号光的带宽与放大之前几乎相同,光谱没有窄化;扫描式OPCPA比固定非共线角方式的放大极大地增加了增益带宽和转换效率,实现了宽带的光参量放大;要满足信号光各频率成分的相位匹配,达到最大的增益带宽和转换效率,需要尽量减小加载到钽铌酸钾(KTa_(1-x) Nb_xO_3, KTN)电光晶体上的电压抖动和电压延时.     

The application of laser reflective tomography in near-field measurements of ultrasonic transducers

With Laser Reflective Tomography（LRT）,the near fields of ultrasonic transducers were measured and analyzed.The principle of LRT measurement of ultrasonic field distribution was introduced and an experimental system was set up.Acoustic pressure of a multiple element piston transducer was measured by using of a laser vibrometer.Its distribution in amplitude and phase was obtained.The acoustic pressure in the same region was measured with a needle hydrophone to validate the LRT method.Furthermore,through reconstruction of acoustic fields,it indicated that LRT method is suitable for predicting the distribution on transducers＇surface and conditions of active elements.     

Characterization of airborne transducers by optical tomography

This paper describes the application of an acousto-optic method to the measurement of airborne ultrasound. The method consists of a heterodyne interferometric probing of the pressure emitted by the transducer combined with a tomographic algorithm. The heterodyne interferometer measures the optical phase shift of the probe laser beam, proportional to the acoustic pressure integrated along the light path. A number of projections of the sound field, e.g. a set of ray integrals obtained along parallel paths, are made in moving the transducer to be tested. The main advantage of the method is its very high sensitivity in air (2 x 10(-4) Pa Hz-1/2), combined with a large bandwidth. Using the same principle as X-ray tomography the ultrasonic pressure in a plane perpendicular to the transducer axis can be reconstructed. Several ultrasonic fields emitted by wide-band home made electrostatic transducers, with operating frequencies between 200 and 700 kHz, have been measured. The sensitivities compared favorably with those of commercial airborne transducers.     

反常氧化碲声光偏转器控制理论研究

根据晶体的声光特性以及反常布喇格衍射的Dixon方程,得出了氧化碲声光偏转器在反常布拉格衍射条件下的参量矢量关系.分析了超声波方向、入射光角度、衍射光角度与声波频率之间的关系.针对卫星光通信中衍射角固定的应用情况,通过数值求解和参量拟合,建立了一个基于平面方程的简单控制模型.利用该模型可以很快得到超声控制频率,使衍射角固定不变.     

Calibration of high-frequency hydrophone up to 40 MHz by heterodyne interferometer

A calibration technique for high-frequency hydrophone utilizing a heterodyne interferometer is presented in this article. The calibration system is mainly composed of optical and signal processing modules. In the displacement measurement, a pellicle is mounted at the surface of water to avoid acousto-optical interaction. The phase modulated carrier signal is digitized and transferred to the computer, then processed by digital phase demodulation. A phase unwrapping algorithm is employed to remove ambiguity of the arctangent function and has proven effective in large displacement measurements. Pellicle displacement and voltage output of the hydrophone in focused ultrasonic field are processed by DFT to determine the amplitudes of the fundamental and harmonic components. Experiments show that the heterodyne technique can provide hydrophone calibration up to 40 MHz, with a slightly smaller sensitivity compared with the National Physical Laboratory (NPL) calibration results for most frequency ranges. Since the heterodyne technique is independent on assumptions about the geometry of the ultrasonic field and the performance of the transducer, it can be easily extended to high frequency and high power ultrasound measurement applications.     

基于超声相控阵的环焊缝缺陷检测方法研究

针对传统超声探头焦距固定,检测位置的改变就要更换相应焦距的探头而影响检测效率的问题,提出一种基于超声相控阵换能器的环焊缝缺陷检测方法。而超声相控阵具有电子偏转和电子聚焦特性,能在不移动的情况下发射偏转聚焦超声束,有效地解决了上述问题。首先基于超声相控线阵换能器的声场特点,采用数值分析方法,研究了影响声束偏转聚焦性能的几个主要参数。然后给出了与超声相控阵换能器相连接的多通道数据采集系统结构。介绍了单通道声信号的硬件结构及相应的信号处理方法,实现了对换能器中单个阵元的精确延时的控制。实验结果表明,优化设计的超声相控线阵换能器具有较高的检测精度和检测效率。     

Measurement of elastic nonlinearity using remote laser ultrasonics and CHeap Optical Transducers and dual frequency surface acoustic waves

A nonlinear ultrasonic technique for evaluating material elastic nonlinearity has been developed. It measures the phase modulation of a high frequency (82MHz) surface acoustic wave interacting with a low frequency (1MHz) high amplitude stress inducing surface acoustic wave. A new breed of optical transducers has been developed and used for the generation and detection of the high frequency wave. The CHeap Optical Transducer (CHOT) is an ultrasonic transducer system, optically activated and read by a laser. We show that CHOTs offer advantages over alternative transducers. CHOTs and nonlinear ultrasonics have great potential for aerospace applications. Results measuring changes in ultrasonic velocity corresponding to different stress states of the sample are presented on fused silica and aluminium.     

激光反射全息技术在超声换能器近场测量中的应用

研究了基于光学反射层析法的超声换能器近场测量方法。介绍了激光反射层析法测量超声场声压分布的原理,建立了一套实验测量系统,利用激光测振仪完成了对一多基元平面活塞换能器近场声压的测量,得到了该换能器辐射面附近声压的幅度和相位分布。使用探针式水听器对相同位置处的声压进行了直接测量,验证了激光层析法测量结果的正确性;声场反演结果进一步表明使用激光反射层析法适用于对换能器辐射面声压分布和阵元工作状况进行预报。      

Radiation Acoustic Field of a Linear Phased Array on a Cylindrical Surface

A new linear ultrasonic phased array fixed on a cylindrical surface is designed. This kind of the cylindrical phased array can meet the specific requirements of the application in testing pipe quality inside pipes. Using the transducer, we can not only avoid mechanical rotating but also test the quality of any point in a pipe by ultrasonic phase array technology. The focused acoustic field distributions in the axial, radial and tangential directions of the transducer are investigated theoretically by numerical simulation. The energy flux density, the width of the main lobe, the imaging resolution, the grating lobe elimination and other characteristics are analysed. The effect of the focal distance, effective aperture, transducer radius, number of total element, and steering angle on the acoustic field distribution is also studied.thoroughly. Many important results are obtained.     

声表面波在声栅上的Bragg衍射

本工作揭示了声表面波在声栅上的Bragg型衍射效应。应用简正波理论近似处理了在强Bragg条件下,声表面波在声栅上衍射的一些规律:栅阵内简正模式,偏转角和偏转效率与入射声波工作频率和声栅参数的关系。实验上,我们在y切割LiNbO₃压电晶体表面,用金属栅阵构成声栅,用光学法观察了声表面波(Rayleigh波)在上述声栅上的Bragg型衍射现象,并测量了偏转角和偏转效率随工作频率的关系。实验与理论结果比较表明,二者基本上是符合的。 关键词：     

Acoustic streaming in an ultrasonic air pump with three-dimensional finite-difference time-domain analysis and comparison to the measurement

The direct finite-difference fluid simulation of acoustic streaming on a fine-meshed three-dimensional model using a graphics processing unit (GPU)-based calculation array is discussed. Airflows are induced by an acoustic traveling wave when an intense sound field is generated in a gap between a bending transducer and a reflector. The calculation results showed good agreement with measurements in a pressure distribution. Several flow vortices were observed near the boundary layer of the reflector and the transducer, which have often been observed near the boundary of acoustic tubes, but have not been observed in previous calculations for this type of ultrasonic air pump.