Simulation of laser-generated ultrasonic wave propagation in solid media and air with application to NDE

Ultrasonic methods are well known as powerful and reliable tool for defect detection. In the previous decades focus and interest have been directed to non-contact sensors and methods, showing many advantages over contact techniques where inspection depends on contact conditions (pressure, coupling medium, contact area). The non-contact hybrid ultrasonic method described here is of interest for many applications, requiring periodic inspection in service or after manufacturing. Despite the potential impact of laser-generated ultrasound in many areas of industry, robust tools for studying the phenomenon are lacking and thus limit the design and optimization of non-destructive testing and evaluation techniques. Here a specific numerical method is presented to efficiently and accurately solve ultrasound wave propagation problems with frequencies in the MHz range traveling in relatively large bodies and through air. This work improves a previous numerical model where propagation of the acoustic waves through air had not been considered, allowing us to simulate the presence of a non-contact transducer in reception in order to simulate numerically the complete experimental setup. It is very important to limit the amount of air to be considered in the FE analyses; otherwise the computational cost would often exceed the resources available. A way to solve the problem is to implement non-reflecting boundary conditions. A non-reflecting boundary condition allows all outgoing waves to exit the domain at the boundary where they have been imposed without reflection; thus, it is possible to model only the portion of air between the non-contact transducer and the solid under testing. Several numerical and experimental analyses were conducted on a 136 lb AREMA rail; here we study in detail two fully non-contact testing configurations for the rail head and web. The information that can be acquired is very valuable for choosing the right setup and configuration when performing non-contact hybrid ultrasonic inspection.     

Numerical modeling of thermoelastic generation of ultrasound by laser irradiation in the coupled thermoelasticity

Laser-generation of ultrasound is investigated in the coupled dynamical thermoelasticity in the presented paper. The coupled heat conduction and wave equations are solved using finite differences. It is shown that the application of staggered grids in combination with explicit integration of the wave equation facilitates the decoupling of the solution and enables the application of a combination of implicit and explicit numerical integration techniques. The presented solution is applied to model the generation of ultrasound by a laser source in isotropic and transversely isotropic materials. The influence of the coupling of the generalized thermoelasticity is investigated and it will be shown, that for ultra high frequency waves (i.e. 100 GHz) generated by laser pulses with duration in the picosecond range, the thermal feedback becomes considerable leading to a strong attenuation of the longitudinal bulk wave. Moreover, the coupling leads to dispersion influencing the wave velocities at low frequencies. The numerical simulations are compared to theoretical results available in the literature. Wave fields generated by a line focused laser source are presented by the numerical model for isotropic and for transversely isotropic materials.     

激光激发超声波的新方法研究

介绍了激光激发超声波的原理，分别给出了热弹机制和烧蚀机制下的纵波和剪切波理论波形；基于排列因子作用，提出了一种新型激光激发超声波的方法——激光定相位排列激发超声波，激光源经过定相位排列后，在某一方向上产生的超声波幅度比传统单一源产生的超声波幅度要强很多，采用此种方法，可实现对超声信号方向和强度的有效激发控制，对下一步的超声无损检测有非常重要的意义；结合实际给出了该方法的实现方式.     

Schlieren visualization of ultrasonic wave fields with high spatial resolution

The visualization of ultrasonic wave fields in optically transparent liquids using the acousto-optic interaction is a well proven tool for the experimental investigation of wave propagation including wave field interaction effects with certain discontinuities and obstacles like reflection, refraction, and diffraction effects as well as for transducer testing and design. For high resolution visualization of wave fields including pulsed waveforms, pulsed light sources and sensitive optical imaging sensors with certain specifications are needed. In this paper the technical requirements of optical and electronic components for high resolution visualization of ultrasound wave fields will be presented. Also, specifications and operation results of a new designed, inexpensive Schlieren optical system will be presented, which is capable of pulsed wave field visualization in the MHz frequency range. The spatial resolution is high enough, not only for accurate beam shape and wave pattern visualization, but also for a gray-scaled display of wave amplitudes including amplitude zero crossings in ultrasound pulses. Consequently, ultrasonic wavelengths can be visualized quantitatively as well as wavelength changes of the ultrasound pulses while traveling through transparent media with different sound velocities. Results to be presented will include 2 MHz and 10 MHz experiments using single transducers as well as linear arrays of commercial medical scanners during their standard operation showing the system beamforming characteristics.     

Response of metal-insulator-metal point contact diodes to visible laser light

Video response and mixing behaviour of metal-insulator-metal point contact diodes have been investigated for visible laser light. Thermally enhanced tunneling is shown to dominate the dc detection behavior of those diodes, while mixing of frequencies being more than several MHz apart is a more complex phenomenon involving thermal, field-and photo-assisted tunneling. In further experiments the potential of point contact diodes for optical heterodyne spectroscopy was examined. Two green laser lines of 122 GHz frequency difference were mixed with the second harmonic of an appropriate microwave frequency, generated simultaneously on the diode. The modestS/N ratio achieved has to be assigned to the different behaviour of metal-insulator-metal diodes in the visible and rf range.     

Mode locking of argon ion laser by use of cylindrical ultrasonic waves

The phenomenon of light diffraction by a cylindrical ultrasonic wave has been investigated. The diffraction was applied in a new type of acousto-optic modulator used for laser mode locking. The paper describes the modulator and presents results on mode locking of argon ion laser. The mode locking was obtained by means of a cylindrical acoustic wave launched in water at the frequencies of ultrasound 33-34 MHz. A standing wave regime of operation of the modulator resulted in a reliable generation of laser pulses with the duration 720 ps.     

Analytical and numerical calculations of optimum design frequency for focused ultrasound therapy and acoustic radiation force

Focused ultrasound therapy relies on acoustic power absorption by tissue. The stronger the absorption the higher the temperature increase is. However, strong acoustic absorption also means faster attenuation and limited penetration depth. Hence, there is a trade-off between heat generation efficacy and penetration depth. In this paper, we formulated the acoustic power absorption as a function of frequency and attenuation coefficient, and defined two figures of merit to measure the power absorption: spatial peak of the acoustic power absorption density, and the acoustic power absorbed within the focal area. Then, we derived “rule of thumb” expressions for the optimum frequencies that maximized these figures of merit given the target depth and homogeneous tissue type. We also formulated a method to calculate the optimum frequency for inhomogeneous tissue given the tissue composition for situations where the tissue structure can be assumed to be made of parallel layers of homogeneous tissue. We checked the validity of the rules using linear acoustic field simulations. For a one-dimensional array of 4 cm acoustic aperture, and for a two-dimensional array of 4 × 4 cm² acoustic aperture, we found that the power absorbed within the focal area is maximized at 0.86 MHz, and 0.79 MHz, respectively, when the target depth is 4 cm in muscle tissue. The rules on the other hand predicted the optimum frequencies for acoustic power absorption as 0.9 MHz and 0.86 MHz, respectively for the 1D and 2D array case, which are within 6% and 9% of the field simulation results. Because radiation force generated by an acoustic wave in a lossy propagation medium is approximately proportional to the acoustic power absorption, these rules can be used to maximize acoustic radiation force generated in tissue as well.     

3D Gabor analysis of transient waves propagating along an AT cut quartz disk

Laser detection methods allow the investigation of ultrasonic transient phenomena in both space and time dimensions. Used for the experimental investigation of surface wave propagation along a 2D surface, laser ultrasonic leads to three dimensional (3D) space-time signal collections. The classical high resolution signal processing methods or 3D Fourier Transforms can be used in order to extract the wave propagation information, however these methods are not adapted for identifying where and when the waves are generated. In order to quantify these transient aspects in the space-time-wave number-frequency domains, the 3D Gabor transform is introduced. The 3D Gabor transform properties are presented. The potential of the 3D Gabor for the identification of the local and transient complex wave numbers is illustrated on the propagation of surface waves on a piezoelectric quartz (AT cut, 6 MHz). In this experimental study, the quartz is excited by a voltage pulse and the quartz surface is scanned by a laser vibrometer. The 3D Gabor analysis shows that the circular electrodes borders generate anti-phase surface waves that propagates outside the electrodes, with a strong energy contribution in the low frequency domain (<1 MHz). The transient analysis also points out, for higher frequencies, where the surface waves are generated and how they propagate with respect of both to the geometry of the electrodes and the crystallographic axis of the quartz. These results confirm the theoretical modal analysis and provide new knowledge about the key role played by the electrodes border. This will allow the optimization of the electrodes shape in order to design low frequency Lamb wave sensors.     

Ultrafast acoustics in the middle UV range: coherent phonons at higher frequencies and in smaller objects

We show that the propagation of coherent acoustic phonons generated by femtosecond optical excitation can be clearly resolved using a probe laser in the middle UV (MUV) range. The MUV probe is easily produced from a high-repetition-rate femtosecond laser and a homemade frequency tripler. We present various experimental results that demonstrate efficient and high frequency detection of acoustic phonons. Thus, we show that the MUV range offers a unique way to reach higher frequencies and probe smaller objects in ultrafast acoustics.     

多波长红外激光二极管峰值光谱热漂移研究

为了满足红外激光测试技术对多光谱集成光源在光谱范围和峰值精度等方面的要求,提出了一种高精度的多波长红外激光二极管,并设计了能够集成860 nm,905 nm和1064 nm(脉冲/单模)四种激光芯片的封装结构.建立了基于上述封装结构下中心热沉的温度场分布模型,并根据数学建模工具求解的中心热沉温度场数值分布规范了中心热沉的加工工艺.为了验证多波长激光二极管中心热沉对输出峰值光谱热漂移现象的抑制效果,制备了多波长激光二极管样机,并搭建了观察其峰值光谱热漂移现象的实验装置.实验结果显示,样机仅有两种芯片的峰值光谱发生了1—3 nm的微弱漂移,并未超出规定的峰值半宽.该现象证明了多波长激光二极管的输出光谱具备较高的精度和良好的稳定性.     

Parametric dependencies for photoacoustic leak localization

Unintended gas or liquid leaks from manufactured components or manufacturing systems may be detrimental to consumers, manufacturers, and the environment. Thus, leak testing is important for quality, safety, and environmental reasons. This paper describes parametric dependencies for photoacoustic leak localization. The technique is based on the interaction of 10.6-micrometer radiation from a carbon dioxide (CO2) laser and a photoactive tracer gas, sulfur hexafluoride (SF6). For the current investigations, acoustic signals are generated by scanning a laser beam at high speed through gas plumes formed above calibrated leaks. These signals are remotely measured with a four-microphone linear array and analyzed using Bartlett and minimum-variance-distortionless (MVD) matched-field processing (MFP) techniques to determine leak location. This paper extends prior work in photoacoustic leak testing through (i) use of more signal frequencies; (ii) parametric study of four different laser scan rates; and (iii) examination of mismatch between the actual acoustic environment and the propagation model used in the MFP; and (iv) presentation of leak localization results on a curved surface. For a 12-watt CO2 laser exciting the small SF6 gas plume produced by a one-cm3-per-day leak with microphones placed 0.41 m from the leak location, root-mean-square localization uncertainties as small as +/-0.5 mm on a line scan of 0.46 m can be achieved when the largest possible number of signal frequencies fall in a measurement bandwidth of approximately 70 kHz.     

Development of an amplitude-modulated Nd:YAG pulsed laser with modulation frequency tunability up to 60 GHz by dual seed injection

A pulsed, modulation frequency tunable, frequency-doubled Nd:YAG laser has been devised for use in target detection through turbid media. A modulated pulse laser radar system offers many advantages in target detection, such as significant signal contrast enhancement, compared with conventional remote-sensing systems. By implementation of the dual-longitudinal-mode seed injection technique, the modulation frequency of the designed Q-switched laser can be tuned from 250 MHz up to 60 GHz in steps of 250 MHz while maintaining a modulation depth of at least 75%. This provides the ability to explore propagation and scattering properties further at previously unattainable high RF modulation frequencies.     

Contrast harmonic imaging

The behavior of ultrasound contrast agents depends highly on the acoustic pressure of the insonified ultrasound wave. For low pressure the expansion and compression is linear to the pressure, for medium acoustic pressure nonlinear behavior starts to occur and for high pressures, but still in the diagnostic range transient scattering can be noticed, resulting in an enhanced scattering followed by a disappearance of the bubble. The nonlinear and transient regime can be utilized for imaging of the contrast agent in or nearby tissue. The magnitude of the nonlinear signal from the contrast has to compete with the nonlinear component of the ultrasound wave, which is generated during propagation. It is shown that contrast is superior to tissue when using low frequencies and imaging the third or fourth harmonic of the transmitted frequency.     

On the long-range detection and study of undamped directed temperature waves generated during the interaction between a cavitating water jet and targets

The features of the propagation of undamped thermal (temperature) waves in air are investigated. The presence of these waves is a consequence of solution of the heat equation taking into account the relaxation of local thermal perturbation. It is shown that such waves can exist only in media with a finite (nonzero) time of local thermal relaxation, and their frequencies are determined by this time. The time of relaxation in air depends on the gas composition, its temperature and increases with a decrease in pressure. Under normal conditions, the minimum frequency of undamped waves in air corresponds to 70–80 MHz. One of the methods for exciting these waves is associated with pulsed heating of the surface of a medium bordering air. Pulsed heating on account of the application of shock waves generated during water jet cavitation is used. It is shown for the first time that these waves with frequencies in the range of 70–500 MHz can propagate in air without damping over a distance of up to 2 m.     

Propagation of two longitudinal waves in a cancellous bone with the closed pore boundary

Ultrasound propagation in cancellous bone (porous media) under the condition of closed pore boundaries was investigated. A cancellous bone and two plate-like cortical bones obtained from a racehorse were prepared. A water-immersion ultrasound technique in the MHz range and a three-dimensional elastic finite-difference time-domain (FDTD) method were used to investigate the waves. The experiments and simulations showed a clear separation of the incident longitudinal wave into fast and slow waves. The findings advance the evaluation of bones based on the two-wave phenomenon for in vivo assessment.     

Evaluation of the contact resonance frequencies in atomic force microscopy as a method for surface characterisation (invited)

The combination of ultrasound with atomic force microscopy (AFM) opens the high lateral resolution of scanning probe techniques in the nanometer range to ultrasonics. One possible method is to observe the resonance frequencies of the AFM sensors under different tip-sample interaction conditions. AFM sensors can be regarded as small flexible beams. Their lowest flexural and torsional resonance frequencies are usually found to be in a range between several kHz and several MHz depending on their exact geometrical shape. When the sensor tip is in a repulsive elastic contact with a sample surface, the local indentation modulus can be determined by the contact resonance technique. Contact resonances in the ultrasonic frequency range can also be used to improve the image contrast in other dynamic techniques as, for example, in the so-called piezo-mode. Here, an alternating electric field is applied between a conducting cantilever and a piezoelectric sample. Via the inverse piezoelectric effect, the sample surface is set into vibration. This excitation is localised around the contact area formed by the sensor tip and the sample surface. We show applications of the contact resonance technique to piezoelectric ceramics.     

Precise control of micro-rod resonator free spectral range via iterative laser annealing

We demonstrate a novel method to control the free spectral range(FSR) of silica micro-rod resonators precisely. This method is accomplished by iteratively applying laser annealing on the already-fabricated micro-rod resonators. Fine and repeatable increasing of resonator FSR is demonstrated, and the best resolution is smaller than 5 MHz, while the resonator quality-factor is only slightly affected by the iterative annealing procedure. Using the fabricated micro-rod resonators, single dissipative Kerr soliton microcombs are generated, and soliton repetition frequencies are tuned precisely by the iterative annealing process. The demonstrated method can be used for dual-comb spectroscopy and coherent optical communications.     

Ultrasound generation with high power and coil only EMAT concepts

Electro-magnetic acoustic transducers (EMATs) are intended as non-contact and non-destructive ultrasound transducers for metallic material. The transmitted intensities from EMATS are modest, particularly at notable lift off distances. Some time ago a concept for a “coil only EMAT” was presented, without static magnetic field. In this contribution, such compact “coil only EMATs” with effective areas of 1–5 cm² were driven to excessive power levels at MHz frequencies, using pulsed power technologies. RF induction currents of 10 kA and tens of Megawatts are applied. With increasing power the electroacoustic conversion efficiency also increases. The total effect is of second order or quadratic, therefore non-linear and progressive, and yields strong ultrasound signals up to kW/cm² at MHz frequencies in the metal. Even at considerable lift off distances (cm) the ultrasound can be readily detected. Test materials are aluminum, ferromagnetic steel and stainless steel (non-ferromagnetic). Thereby, most metal types are represented. The technique is compared experimentally with other non-contact methods: laser pulse induced ultrasound and spark induced ultrasound, both damaging to the test object’s surface. At small lift off distances, the intensity from this EMAT concept clearly outperforms the laser pulses or heavy spark impacts.     

High frequency attenuation measurements of lipid encapsulated contrast agents

A number of recent studies have indicated the potential of ultrasound contrast agent imaging at high ultrasound frequencies. However, the acoustic properties of microbubbles at frequencies above 10 MHz remain poorly understood at present. In this study we characterize the high frequency attenuation properties of (1) BR14, (2) BR14 that has been mechanically filtered (1 and 2 microm pore sizes) to exclude larger bubbles, and (3) the micron to submicron agent BG2423. A narrowband pulse-echo substitution method is employed with a series of four transducers covering the frequency range from 2 to 50 MHz. For BR14, attenuation decreases rapidly from 2 to 10 MHz and then more gradually from 10 to 50 MHz. For 2 microm filtration, the attenuation peaks between 10 and 15 MHz. For 1 microm filtration, attenuation continues to rise until 50 MHz. The agent BG2423 exhibits a diffuse attenuation peak in the range of 15-25 MHz and remains high until 50 MHz. These results demonstrate a strong influence of bubble size on high frequency attenuation curves, with bubble diameters of 1-2 microm and below having more pronounced acoustic activity at frequencies above 10 MHz.