Theoretical and experimental investigation of the photoacoustic effect in solids with residual stresses

Modern experiment and theory in the field of residual stress detection by the photoacoustic method are summarized and analyzed. A multimode approach based on the simultaneous application of several photothermal and photoacoustic methods is proposed for the study of thermal and thermoelastic effects in solids with residual stress. Some experimental results obtained within the framework of this approach for Vickers indentation zones in ceramics are presented. The effect of annealing on the photoacoustic, piezoelectric signal for ceramics and the influence of the given external loading on the behavior of the photoacoustic signal near the radial crack tips is investigated. It is experimentally shown that both compressive and shear stresses contribute to the photoacoustic signal near the radial crack tips. The model of the photoacoustic, thermoelastic effect in solids with residual stress is proposed. It is based on the modified Murnaghan model of non-linear elastic bodies, which takes into account a possible dependence of the thermoelastic constant on stress. This model is further developed to explain the photoacoustic signal behavior near the radial crack tips. It is demonstrated that this model of the photoacoustic effect agrees qualitatively with the available experimental data.     

光声光谱检测装置中光声池的数值计算及优化

利用光声光谱技术进行痕量气体的检测具有独特的优势,光声池是系统装置中最为重要的核心部件,它决定着整机性能的优劣.以一圆柱形共振型光声池为研究对象,基于声学与吸收光谱学的基本理论,建立了光声池声场激发的数学模型;利用数值模拟方法对光声池空腔结构进行了声学模态仿真,获得了前8阶声学模态值以及声压可视化振型;在考虑热黏性声学损耗的作用下,对光声池进行了热-声耦合多物理场仿真计算;将仿真结果与解析计算和实验结果进行对比,明确了利用数值模拟方法来计算光声池有关指标的可靠性与可行性;针对光声池的优化问题,提出了一种将响应面代理模型与遗传算法相结合的优化算法,在将原光声池中的谐振腔两端形貌更改为喇叭口形的情况下,通过优化算法获得了以光声池品质因数Q及池常数C_(cell)为最大值寻优的Pareto最优解集;选取一组解进行考察,结果表明,代理模型预测值与数值模拟值指标最大误差仅为1.3%,优化后的新型光声池Q较之前增长了48.9%, C_(cell)增长了34.4%.研究方法可为光声光谱中光声池的优化设计提供参考借鉴.     

阶梯复合形光声池声-流特性计算与评估

为了提高光声池的检测性能,提出并分析了一种阶梯复合形光声池.以传统圆柱共振型光声池为基准模型,通过对比解析与模拟计算结果,验证了所采用模拟方法的可靠性,基于模拟方法求解并获得了阶梯腔半径、阶梯腔长度和阶梯腔数量对阶梯复合形光声池声-流特性的影响规律.结果表明:减小阶梯复合形光声池中阶梯腔半径,光声信号相对增强,阶梯腔长...     

圆柱形光声池结构及环境因素对声学本征频率的影响

以典型的圆柱形光声池为研究对象,建立光声池声学仿真有限元模型,并在此基础上,研究了光声池中谐振腔、缓冲腔、进出气孔结构参数以及温度、湿度因素对其声学本征频率的影响规律.研究结果表明:圆柱形光声池的进、出口孔对其声学本征频率影响极不敏感,设计计算中可以忽略不计,谐振腔的长度影响最为敏感,其次为谐振腔的直径.此外缓冲腔的长度与直径对其亦有一定影响,因而在准确计算时需要加以考虑.温度与湿度对光声池声学本征频率的影响均呈现正线性增长规律,温度的影响随着谐振腔长度的增大而减小,湿度的影响随着温度的升高而增大,仅计算光声池的声学本征频率时,湿度的影响在室温环境下且湿度变动较小的条件下可以忽略.     

基于光声光谱联合主成分回归法的血糖浓度无损检测研究

利用可调谐脉冲激光器激发联合聚焦超声探测器前向探测模式搭建了一套血糖光声无损检测实验装置。为了测试该装置的可靠性,实验中利用532 nm泵浦Nd∶YAG调Q脉冲激光器激发不同浓度的葡萄糖水溶液产生实时光声信号;采用脉冲激光在近红外波段1 300～2 300 nm内固定间隔波长10 nm扫描方式激发不同浓度的葡萄糖水溶液,获取了不同波长下的葡萄糖光声峰峰值,利用差谱方法筛选出了多个葡萄糖的特性波长;然后采用主成分回归算法优选了三个特性波长,并建立了浓度梯度与对应三个优选波长光声峰峰值之间的数学校正模型。实验表明,葡萄糖水溶液的光声信号符合弱吸收介质的柱状光声源模型;利用建立的校正模型对校正集和预测集的葡萄糖浓度预测结果表明,葡萄糖浓度的校正和预测均方根误差均小于10 mg·dl-1,相似系数为0.993 6。     

Effect of an external mechanical load on elastic stresses near radial cracks in Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-SiC-TiC ceramics: Photoacoustic study

The variation of the photoacoustic signal near the mouths of radial cracks in Vickers-indented externally loaded Al₂O₃-SiC-TiC ceramics is studied. A theoretical model of a photoacoustic signal that is recorded near the mouths of vertical cracks is suggested. Indentation zones in Al₂O₃-SiC-TiC ceramics are visualized by laser scanning photoacoustic microscopy. The sensitivity of the photoacoustic method with piezoelectric detection of signals to both normal and shear stresses acting on a crack is demonstrated. Experimental and theoretical data for the effect of external stresses on the photoacoustic signal near the mouths of radial cracks are compared. For the ceramics under study, agreement is fairly good. It is shown that the strain coefficients near the mouths of vertical cracks can be determined from photoacoustic experimental data.     

The photoacoustic effect and non-radiative transitions OF F-centers in alkali halides

The common photoacoustic spectrum (PAS) is wider than the optical absorption spectrum in KCl crystals. A photoacoustic experiment was performed with a solid KCl sample that was additively colored according to the Van Doorn technique. The sample had about 10 17 v [F/cm 3 ]. It was determined that the PAS is composed of many bands of the F and aggregate centers. It is explained here, using the configuration coordinate model, that the photoacoustic effect in alkali halides with F-centers shows experimental broadening of the photoacoustic band relative to the F -band. Furthermore, experimental data is added resolving the photoacoustic bands of the agglomerates and also the bands of higher excited states of the F-center. Finally, assuming a configuration coordinate model and the F-centers as sources of heat, an estimation is made for the increase of the sample temperature.     

Design, simulation and structural optimization of a longitudinal acoustic resonator for trace gas detection using laser photoacoustic spectroscopy (LPAS)

The design of the acoustic resonator is critical for the optimization of the sensitivity of laser photoacoustic spectroscopy (LPAS) in trace gas detection applications. In this paper, an LC circuit model is used for the simulation of a 1D acoustic resonator. This acoustic resonator is designed for CO photoacoustic spectroscopy. The effects of the structural parameters, quality factor and resonant frequency on the performance of the device are theoretically analyzed. The role of the buffer volume as an acoustic filter is investigated and optimized dimensions of the buffer volume, to achieve minimum noise transmission coefficient, are calculated. The effects of the ambient temperature, variety of pressure and gas flow velocity on the resonant frequency of photoacoustic resonator and PA signal are simulated. The temperature dependence of the microphone sensitivity is also introduced.     

一种实现光声光谱的导数光谱的新方法

提出了一种实现光声光谱的导数光谱的新方法,为了获得光声光谱的导数光谱,利用一台单色仪和一个分光棱镜研制成一个波长-光强分束器,并用这个波长-光强分束器同时获得两束光强相等而波长有微小差别的两束光,这两束光经过一个互补调制器调制后在光声探测器内叠加,可以实现光声信号的差分,扫描单色仪的波长,就可以获得光声吸收光谱的导数光谱。实验证明这种方法可以准确实现光声光谱的一阶导数光谱,而且导数光声光谱比光声吸收光谱具有更高的光谱分辨率。     

k-space propagation models for acoustically heterogeneous media: application to biomedical photoacoustics

Biomedical applications of photoacoustics, in particular photoacoustic tomography, require efficient models of photoacoustic propagation that can incorporate realistic properties of soft tissue, such as acoustic inhomogeneities both for purposes of simulation and for use in model-based image reconstruction methods. k-space methods are well suited to modeling high-frequency acoustics applications as they require fewer mesh points per wavelength than conventional finite element and finite difference models, and larger time steps can be taken without a loss of stability or accuracy. They are also straightforward to encode numerically, making them appealing as a general tool. The rationale behind k-space methods and the k-space approach to the numerical modeling of photoacoustic waves in fluids are covered in this paper. Three existing k-space models are applied to photoacoustics and demonstrated with examples: an exact model for homogeneous media, a second-order model that can take into account heterogeneous media, and a first-order model that can incorporate absorbing boundary conditions.     

Open photoacoustic cell x-ray detection

A simple open-cell configuration photoacoustic x-ray detector is experimentally demonstrated. The front air chamber of a commercial electret microphone is used as the transducer medium of conventional photoacoustics. The observed signal is well described by the thermal diffusion model for the photoacoustic signal.     

Photoacoustic express diagnostics of lubricating oil: Model investigations

The possibility of evaluating the size of metallic inclusions in lubricating oil by photoacoustic measurements in a gas-microphone cell is investigated. The photoacoustic spectra of suspensions of microscopic spherical steel particles with diameters from 5 to 70µm in an oil layer with a thickness of 20 µm are measured. A semiquantitative model of the photoacoustic effect in a suspension layer is proposed for inclusions of both small and large size in comparison with the layer thickness. It is demonstrated that the body of information obtained from the measurements considerably increases when the thickness of the oil layer is close to the size of inhomogeneous inclusions.     

基于柱弥散光源体内辐照的前列腺扫描光声成像仿体实验

光声成像技术是一种非常有前景的前列腺早期检测与成像的新技术. 在现有前列腺光声成像技术中, 通常采用的腺体外光源辐照方式由于受外围组织的影响, 容易减弱深部前列腺组织的光能量吸收, 导致辐照范围减小, 引起光声成像深度不足, 难以实现无损前列腺癌的检测. 本文基于前列腺组织的结构特征, 依据一种以柱状弥散光纤在尿道对前列腺实施光辐照、并利用外置于直肠内的长焦区聚焦式超声换能器检测光声信号的前列腺光声扫描成像技术, 构建了光声成像实验系统, 开展了仿体实验. 测试结果表明, 系统能够实现样品中吸收体的定位和成像, 结合柱弥散光源体内辐照可使成像深度提高, 同时侧向成像范围也较大. 初步结果表明, 借助柱弥散光源进行体内光辐照的光声信号新激发方式结合长焦区超声探头在前列腺癌早期无创诊断上具有潜在的应用前景.     

Energy efficiency of near infrared cobalt luminscence in ZnSe:Co determined by a photoacoustic method

The paper presents results of computations of the energy efficiency of the cobalt luminescence in ZnSe:Co determined by the photoacoustic method. The transmission spectra, photoacoustic experimental and theoretical spectra, and the frequency dependence on the photoacoustic amplitude characteristics are presented. From them, the energy efficiency of Co²⁺ the near infrared luminescence (3200 nm) was computed in the frame of new proposed photoacoustic model of computations of the luminescence energy efficiency.     

超声换能器带宽对光声成像的影响

研究了不同尺寸吸收体产生的光声压的频谱特性：对于厘米量级、毫米量级和几百个微米量级的吸收体,产生光声压频谱的主要范围分别约为20～300kHz、70kHz～2．5MHz和400kHz～20MHz;讨论了不同频率范围的光声信号对重建图像的影响,低频段的光声信号能反映物体的非边界区域,而高频段的光声信号能突出物体的细微结构,尤其是物体的边界特征。提出了不同尺寸的吸收体要选用或设计不同带宽范围的探测器进行检测的方法．当探测器的带宽范围与光声压频谱范围基本吻合时,损失的频率成份较少,重建的光声图像效果较好,这一结论在仿真和实验结果中都得到了证明。实验用的光源为YAG激光器,波长为532nm,重复频率为30Hz,脉宽为7ns,探测器为针状的PVDF膜水听器,接收面积的直径为1mm。     

Photoacoustic spectroscopy of photoactive matter — Applications to photographic emulsions

A new technique — the photoacoustic spectroscopy with microphone detection has been successfully applied to the studies of spectral sensitivity of silver halide emulsions. Regardless of the fact that the sensitivity of photographic emulsions is greater or comparable with the detectivity of real photoacoustic transducers (i.e. the photoacoustic cell with microphone or piezoelectric transducer), absorption-like spectra of strongly diffusive and nontransparent silver halide emulsions can be readily recorded. Applications of this new technique for the studies of silver halide emulsions bears full potential advantages of photoacoustic spectroscopy. Particularly we demonstrate priority of the new technique over the conventional photographic sensitometry or reflectance spectroscopy both in qualitative and quantitative manner. Photoacoustic measurements of photochemical decomposition rates are shown and suggestions for the measurements of excitation energy transfer efficiencies are given. The results clearly show a unique applicability of the technique in fundamental as well as industrial research and process control of the photographic emulsion production.Dedicated to Professor Miroslav Trlifaj on the occasion of his sixtieth birthday.The autors would like to acknowledge the ORWO co. (German Democratic Republik) for supplying the model silver halide emulsions, the ILFORD co. for the supply of the sample of commercial films materials and FUJI co. for sending the sensitogram of their film NEOPAN SS).     

Photoacoustic investigations of the optical absorption spectra of porous silicon layers on a silicon backing

This paper presents a series of experimental photoacoustic spectra of porous silicon layers on crystalline silicon and their numerical analysis performed in the proposed two-layer model. The goal of the analysis was to calculate the optical absorption spectra of porous silicon from the photoacoustic spectra of porous silicon layers on a silicon background. The experimental character of the observed absorption band associated with the porous silicon was revealed. This is the first attempt at a theoretical interpretation of the photoacoustic spectra of porous silicon on a silicon backing.     

血管内光声图像的建模与仿真

为了给血管内光声(intravascular photoacoustic,IVPA)成像的图像重建算法和图像后处理算法提供数据源,为培训医师提供图像库,提出一种二维IVPA图像的建模与仿真方法。建立包含斑块的血管横截面模型,并对成像导管发射激光脉冲照射血管壁组织、血管壁组织产生光声信号、利用得到的一组光声信号重建IVPA图像整个过程进行仿真,得到血管横截面模型的二维IVPA灰阶图像,同时根据冠状动脉血管随心脏搏动的规律,实现序列图像的仿真。利用仿真模型得到的IVPA图像与原电磁吸收分布图接近程度较高。仿真模型的物理意义清晰,可得到质量稳定的IVPA图像。      

Theoretical analysis of off beam quartz-enhanced photoacoustic spectroscopy sensor

Off beam quartz-enhanced photoacoustic spectroscopy (OB-QEPAS) sensors are based on a recently developed approach to off-beam photoacoustic (PA) detection which employs a quartz tuning fork (QTF) as an acoustic transducer. A microresonator (mR) with a side slit in the middle is used to enhance PA signal. This paper describes a theoretical model of an OB-QEPAS-based sensor. By deriving the acoustic impedances of the mR at two ends and the side slit in the middle in the model, we obtain a formula for numerically calculating the optimal mRs' parameters of OB-QEPAS-based sensor. We use the model to calculate the optimal mRs' lengths with respect to the resonant frequency of the QTF, acoustic velocities inside mRs, inner diameters of mRs, and acoustic conductivities of the mRs' side slits, and found out that the calculated results closely match experimental data. We also investigated the relationship between the mR selected in “on beam” QEPAS, OB-QEPAS, and an acoustic resonator (AR) excited in its first longitudinal mode used in conventional photoacoustic spectroscopy (PAS).     

Evaluation of thermal conductivity of porous silicon layers by a photoacoustic method

 The paper shows that it is possible to evaluate the thermal conductivity of porous silicon layers by a conventional photoacoustic gas-microphone technique, on the basis of a simple interpretative model for stratified samples. Samples produced by electrochemical etching of crystalline wafers of different type and thickness are considered. The frequency variation of photoacoustic signal, in amplitude and phase, is studied and interpreted by means of a four-layers (air, porous silicon, crystalline silicon, air) model. Thermal conductivity values in the range 2.5–31.2 W/m K are obtained. Received: 4 July 1996/Accepted: 13 August 1996