Photoacoustic probes for nondestructive testing and biomedical applications

Fiber-optic photoacoustic sources for nondestructive testing and biomedical applications are described. The photoacoustic sources consist of a pulsed laser, a fiber-optic cable, and a generation head. The generation head is a miniature hermetically sealed chamber, which can be embedded into solid structures or immersed in liquid media. The face of the chamber acts as a target for laser irradiation. Bulk ultrasonic waves generated inside of the target are transmitted into the medium. The proposed systems offer wide ultrasonic range (0.5-15 MHz), easy control over directivity of the ultrasonic beam, high efficiency of generation, and the ability to operate in a harsh environment. Sources with different radiation patterns with respect to the optical axis of the fiber, such as normal, sideways, as well as focused, have been devised. We present a proof-of-concept experiment using these sources in combination with fiber-optic ultrasonic receivers.     

激光致声技术的研究

从热弹与烧蚀机制两个方面，讨论了激光超声表面波的产生。通过对点光源激励声表面波的机理进行分析，出线光源激励的表达式。进行了表面波同线光源能量、方向角、位置、长度关系的实验研究。实验结果验证了激光超声表面波理论模型的合理性，证实了线光源产生的表面波具有信号强、方向性好等优点。     

激光声表面波检测铝板表面凹痕的数值研究

采用平面应变的有限元模型数值模拟了热弹机制下线型脉冲激光辐照金属铝板表面激发高频声表面波,及声表面波经过表面矩形凹痕时发生反射的过程.计算结果表明: 声表面波中的瑞利波经过表面凹痕时发生明显的反射,并产生两个相继出现且具有相同的传播速度的反射表面波成分;随着凹痕深度的增加,两个反射瑞利脉冲出现的时间间隔将增大;数值计算从理论的角度有力地证实了前一反射瑞利波产生于凹痕的顶端,而后者源于其底部的论断,从而为定量检测金属表面缺陷的深度提供了理论依据.     

Diffraction of elastic waves by defects in plates: Calculated arrival strengths for point force and thermoelastic sources of ultrasound

This paper applies geometrical ray theory to the calculation of the surface displacements generated by point force and thermoelastic sources of ultrasound in plates containing planar defects. The calculation includes direct wave arrivals, waves undergoing back-wall reflection with or without mode conversion, and waves diffracted by the crack-tip. Ultrasonic B-scan data are also simulated so that comparison can be made with experimental data. It is shown that the thermoelastic source, which can be generated by a pulsed laser, is particularly well suited to defect detection by the ultrasonic time-of-flight technique.     

Photoacoustic holographic imaging of absorbers embedded in silicone

Light absorbing objects embedded in silicone have been imaged using photoacoustic digital holography. The photoacoustic waves were generated using a pulsed Nd:YAG laser, λ=1064 nm, and pulse length=12 ns. When the waves reached the silicone surface, they were measured optically along a line using a scanning laser vibrometer. The acoustic waves were then digitally reconstructed using a holographic algorithm. The laser vibrometer is proven to be sensitive enough to measure the surface velocity due to photoacoustic waves generated from laser pulses with a fluence allowed for human tissue. It is also shown that combining digital holographic reconstructions for different acoustic wavelengths provides images with suppressed noise and improved depth resolution. The objects are imaged at a depth of 16.5 mm with a depth resolution of 0.5 mm.     

Ultrasonic inspection of adhesively bonded CFRP/aluminum joints using pulsed laser scanning

In this study, we propose an ultrasonic inspection technique for detecting disbonds in adhesively bonded carbon fiber-reinforced plastics (CFRP)/aluminum joints using pulsed laser scanning. A specimen with artificially induced square disbonds was scanned by a pulsed laser for ultrasound generation, and the propagating waves were received by a transducer placed on its surface. A series of images of the traveling waves were obtained by processing the received signals. An initial, quick inspection was performed using low-frequency Lamb waves. Changes in the propagation of the Lamb waves were observed at the disbond regions, and disbonds larger than 5 × 5 mm² were successfully detected. A second, detailed inspection of the detected disbond regions was performed using the high-frequency through-transmission ultrasonic method. The shape of the disbond was precisely imaged, and the evaluated size of the disbond matched well with the actual size. We thus demonstrated the efficiency and the feasibility of the proposed technique for the inspection of adhesively bonded CFRP/Al joints.     

Acoustic and flexural excitation of a floating structure by a single laser pulse

The acoustic and flexural vibrations of a small-scale floating structure following irradiation by a pulsed Nd:glass laser are compared with a radiated underwater sound field. A single subablative laser pulse of 600-μs duration was used both to bend and shock the floating structure at the irradiation site. The laser pulse caused the structure to flex at a frequency of approximately 1 kHz whereas relaxation oscillations in the laser output simultaneously excited ultrasonic Lamb waves within the material bulk. We present results to illustrate the broad bandwidth provided by this unusual form of excitation.     

Optical fiber array for the delivery of high peak-power laser pulses for fluid flow measurements

Fiber delivery of 64.7 mJ laser pulses (approximately 6 ns duration) from a Q-switched Nd:YAG laser operating at 532 nm is demonstrated. A custom diffractive optical element was used to shape the laser beam and facilitate coupling into a linear fiber array. This launch arrangement achieves an improvement in launch efficiency compared with a circular fiber bundle evaluated in previous work and the delivery of higher pulse energies is demonstrated. The bundle is capable of delivering light of sufficient pulse energy and, importantly, with suitable focusability, to generate a thin light sheet for the fluid flow measurement technique of particle image velocimetry (PIV). Fiber delivery offers an advantage, in terms of optical access, for the application of PIV to enclosed measurement volumes, such as the cylinder of a combustion engine.     

Sensitivity of point- and line-source laser-generated acoustic wave to surface flaws

Laser generation and air-coupled detection were combined as a hybrid ultrasonic technique for the inspection of surface flaws in rails. Narrowband acoustic signals were generated using a formed laser source by focusing the laser light to a point and to a line on the surface of the rail. The pulse energy, and therefore the intensity of the laser source, varied such that the generated signal transitioned from the weak thermoelastic to the strong ablative regime. The detection of flaws using a laser-generated surface acoustic wave, in the presence of surface flaws, was compared between both point and line laser sources operating under different pulse energy levels. The line source was found to be more sensitive to the presence of surface flaws than a point source. The sensitivity of the laser-generated acoustic signal appeared to be independent of the severity of the flaw and, within the ablative regime, independent of the laser-pulse energy. Theoretical analysis is provided to explain the underlying cause that influences the interaction of a formed laser-generated surface acoustic wave to surface flaws and how this sensitivity may vary between the thermoelastic and ablative regimes.     

Laser generation of Lamb waves for defect detection: experimental methods and finite element modeling

The propagation of Lamb waves generated by a pulsed laser beam in an aluminum sheet is modeled using finite element analysis, and the interaction with defects is studied and compared to experimental results. The ultrasonic Lamb waves are detected by an electromagnetic acoustic transducer (EMAT). The frequency content of the received wave is shown to be enhanced when the generation point is situated directly over the defect in both the modeled and experimental cases. Time-frequency analysis using a Wigner transform has enabled individual modes to be identified.     

Line-focused laser ablation for depth-profiling analysis of coated and layered materials

The performance features of line-focused laser ablation for the characterization of interfaces in layered materials by laser-induced plasma spectrometry (LIPS) have been compared with the point-focusing method in terms of signal precision, signal-to-noise ratio, ablation rates, and surface sensitivity. In both optical configurations a pulsed Nd:YAG laser beam operating at 532 nm, with a homogeneous energy distribution (flattop laser), is used to generate point and microline plasmas on the sample surface. Subsequent light from the plasma is spectrally resolved and detected with an imaging spectrograph and an intensified charge-coupled-device detector that is binned along the slit-height direction. Line-focusing LIPS permits much higher laser power input while maintaining relatively low laser fluence, thus yielding better surface sensitivity and improved detection power. Values of the signal-to-noise ratio are improved by a factor of 6. In addition the ablation rate is 9 nm/pulse with the microline approach compared with 23 nm/pulse obtained with the point-focusing method. The results demonstrate that the microline-focusing approach is suitable for the depth analysis of coated and layered materials.     

Modulated laser array sources for generation of narrowband and directed ultrasound

Patterned illuminating sources and appropriate time modulation may be used to enhance certain features of laser-generated acoustic waves in time or frequency as a function of direction. Steerable, narrowband, toneburst ultrasonic signals have been generated from a single laser pulse which was spatially modulated by transmission through a lenticular array. In addition, narrowband toneburst ultrasonic waves have been generated from a mode-locked laser pulse train providing spectral narrowing of the laser-ultrasonic signal. Both temporal and spatial modulation of the laser pulse improve the signal-to-noise ratio for laser ultrasonics.     

High-spatial-resolution sub-surface imaging using a laser-based acoustic microscopy technique

Scanning acoustic microscopy techniques operating at frequencies in the gigahertz range are suitable for the elastic characterization and interior imaging of solid media with micrometer-scale spatial resolution. Acoustic wave propagation at these frequencies is strongly limited by energy losses, particularly from attenuation in the coupling media used to transmit ultrasound to a specimen, leading to a decrease in the depth in a specimen that can be interrogated. In this work, a laser-based acoustic microscopy technique is presented that uses a pulsed laser source for the generation of broadband acoustic waves and an optical interferometer for detection. The use of a 900-ps microchip pulsed laser facilitates the generation of acoustic waves with frequencies extending up to 1 GHz which allows for the resolution of micrometer-scale features in a specimen. Furthermore, the combination of optical generation and detection approaches eliminates the use of an ultrasonic coupling medium, and allows for elastic characterization and interior imaging at penetration depths on the order of several hundred micrometers. Experimental results illustrating the use of the laser-based acoustic microscopy technique for imaging micrometer-scale subsurface geometrical features in a 70-μm-thick single-crystal silicon wafer with a (100) orientation are presented.     

Measurement of Interfacial Fracture Toughness of Surface Coatings Using Pulsed-Laser-Induced Ultrasonic Waves

This research develops a new technique for the measurement of interfacial fracture toughness of films/surface coatings using laser-induced ultrasonic waves. Using pulsed laser ablation on the bottom substrate surface, strong stress waves are generated leading to interfacial fractures and coating delamination. Simultaneously, a laser ultrasonic interferometer is used to measure the normal (out-of-plane) displacement of the top surface coating in order to detect coating delamination in a non-destructive manner. We can thus determine the critical laser energy for delamination, yielding the critical stress (that is, the interfacial strength). Subsequently, to examine the interfacial fracture toughness, additional pulsed laser irradiation is applied to a pre-delaminated specimen to show that the delamination area expands. This type of interfacial crack growth can be visualized using laser ultrasonic scanning. Furthermore, the calculation of elastic wave propagation was carried out using a finite-difference time-domain method) in order to accurately estimate the interfacial stress field. In this calculation, the stress distribution around the initial delamination is calculated to obtain the stress intensity factor. Based on the experimental and computational results, interfacial fracture toughness can be quantitatively evaluated. Since this technique relies on a two-laser system in a non-contact approach, it may be useful for a quantitative evaluation of adhesion/bonding quality (including both interfacial fracture strength and toughness) in various environments.     

Pulsed-Laser Ultrasound Generation in Graphite/ Epoxy Plate

A laser-based ultrasonic technique using a pulsed laser to stimulate ultrasonic waves in fiber-reinforced graphite/ epoxy composite plate is the subject of investigation. For convenience, the material is chosen to be homogeneous and transversely isotropic. The study is strictly limited to the laser power regimes that are suitable for nondestructive evaluation. An elastodynamic methodology is presented based on an integral formulation in order to develop a representation for the dynamic responses in terms of the characteristics of the source that originated the motion. This requires the computation of the elastodynamic Green's function which represents the displacement field from the idealized synthetic sources localized precisely in both space and time. A two-dimensional numerical analysis utilizing a finite difference method for computation of the Green function in a finite plate is developed which provides the basis for quantitative nondestructive evaluation of fiber-reinforced composite materials. Numerical results are presented for the surface displacement at the epicenter. Prediction based on numerical simulations are compared with experimental results.     

A Comparison of Q-Switched and Intensity Modulated Laser Pulses for Ultrasonic NDT

The purpose of this work was to investigate the potential of intensity modulated laser pulses for ultrasonic signal enhancement in NDT using a numerical model. Q-switched lasers produce broadband ultrasound which is susceptible to measurement noise. Intensity modulation can alleviate this problem by focussing the ultrasonic energy into a band centered around the modulation frequency. Ultrasonic waveforms and their DFTs are considered here in both aluminum and steel for a wide range of observation angles, for Q-switched laser pulses and for a 1 MHz intensity modulated laser pulse. These waveforms are useful to NDT researchers becauses they indicate what kinds of signals can be expected from unflawed materials. It is found that P waves are not as easily enhanced by modulation as are SV or Rayleigh surface waves. The time window which should be used to measure ultrasonic signals produced by intensity modulated laser pulses is determined.     

不同光源模式下位置敏感探测器响应特性分析

不同光源模式下 PSD 探测到的位置信息不同。从 Lucovsky 方程入手,提出四种光源模式下一维 PSD 的解析解,并分析 PSD 的输出响应特性。结果表明,稳态光生电势在光照射范围外呈直线分布,PSD 探测到的是光斑的能量重心位置,与光斑面积大小和能量分布无关。在脉冲光照射下,脉冲光生电流与光脉冲周期相等,若获取时间延迟 4.2 倍的时间响应常数,可以得到稳态输出值。     

Toward virtual biopsy through an all fiber optic ultrasonic miniaturized transducer: a proposal

The present generation of devices based on opto-acoustic and acousto-optic conversion lets us foresee the possibility of realizing complete miniaturized transmitting-receiving transducers, able to generate and detect wideband ultrasounds by laser light. In the present paper, a miniaturized ultrasonic transducer entirely based on fiber optic technology is proposed. Such a device springs from the conjunction between our research, which has produced a highly efficient fiber optic opto-acoustic source, with the results obtained by other researchers concerning the realization of an ultrasonic receiver based on optical interferometry. Making use of the thermo-elastic effect for ultrasound generation, a source of ultrasound can be obtained by coupling a fiber optic to pulsed laser, if a film capable of absorbing laser light is placed onto fiber end. Starting from these remarks, we propose an efficient opto-acoustic source, able to generate pressure pulses with amplitude of the order of 10(4) Pa and bandwidth extending up to 40 MHz and beyond by using graphite materials as absorbing film. This solution makes use of a low-power pulsed laser as optical source possible. An ultrasonic receiving element was realized placing a Fabry-Perot cavity over the tip of a fiber optic. The cavity thickness modulation induced by ultrasonic beam is detected by an interferometer optical technique. We have realized a prototype of a receiving device that exhibits a sensitivity comparable with that of piezoelectric devices (10-100 nV/Pa) and an almost flat bandwidth extending up to 20 MHz or more. The extreme miniaturization of the resulting ultrasonic transducer, together with its wide ultrasonic frequency bandwidth, is the first step toward ultrasonic tissue biopsy. In this paper, before discussing the problem of constructing a complete ultrasonic transducer composed by a transmitter and receiver, the results carried out in these fields during the last decade are reviewed.     

Fabry–Perot interferometer in pulsed illumination: a new approach and capabilities

An analysis of the spectral-time characteristics of a Fabry–Perot interferometer (FPI) in pulsed illumination is given. Processes that occur in the transit of light pulses of different widths through a FPI are considered in detail. A relationship between the baseline of a FPI and the width of the incident light pulse is established. The time structure of a pulse that has traversed a FPI is analyzed. The theoretical difference in the transit of a light pulse through a FPI and of a regular sequence of synchronized laser pulses is found. The maximum time resolution of a FPI is determined. A model that describes the transit of a regular sequence of synchronized laser pulses through a FPI functioning in the mode of an optical filter is determined.     

Low-cost,fast and easy production of germanium nanostructures and interfacial electron transfer dynamics of BODIPY–germanium nanostructure system

Germanium nanostructures are prepared by electrochemical etching of n-type Sb-doped (100) oriented germanium (Ge) substrates with resistivity of 0.01 Ω cm. Ge substrates are etched in an electrochemical double cell containing hydrofluoric acid and ethanol solution at room temperature. Although the use of illumination source is essential for etching of an n-type semiconductor material, the influence of illumination source type on the germanium surface morphology has not yet been investigated. In this work, the illumination effect is studied by halogen lamp, white LED, 470- and 405-nm pulsed diode laser. It is demonstrated that different Ge surface morphologies such as nanocone, nanorod, nanoplate and nanopyramid are obtained using different illumination source. The current density, anodization time and pulsed laser power density effects on Ge nanopyramid are investigated in order to optimize anodization conditions. The most uniform and continuous Ge nanopyramid array is obtained at the current density of 30 mA/cm² for 45 min under cathode side illumination with 470-nm pulsed diode laser. It is observed that the nanostructured Ge surfaces exhibit a broad photoluminescence band between 400 and 650 nm. Time-resolved fluorescence spectroscopy studies of electron transfer process between BODIPY dye and Ge nanostructures are reported. The obtained fluorescence lifetime data are analyzed in the light of the Marcus electron transfer theory to determine the conduction band energy level of nanostructured germanium substrates.