Laser Generation Of Ultrasound Research Articles

Ultrathin materials for wide bandwidth laser ultrasound generation: titanium dioxide nanoparticle films with adsorbed dye.

Materials that convert the energy of a laser pulse into heat can generate a photoacoustic wave through thermoelastic expansion with characteristics suitable for improved sensing, imaging, or biological membrane permeation. The present work involves the production and characterization of materials composed of an ultrathin layer of titanium dioxide (<5 μm), where a strong absorber molecule capable of very efficiently converting light into heat (5,10,15,20-tetrakis(4-sulfonylphenyl)porphyrin manganese(iii) acetate) is adsorbed. The influence of the thickness of the TiO2 layer and the duration of the laser pulse on the generation of photoacoustic waves was studied. Strong absorption in a thin layer enables bandwidths of ∼130 MHz at -6 dB with nanosecond pulse laser excitation. Bandwidths of ∼150 MHz at -6 dB were measured with picosecond pulse laser excitation. Absolute pressures reaching 0.9 MPa under very low energy fluences of 10 mJ cm-2 enabled steep stress gradients of 0.19 MPa ns-1. A wide bandwidth is achieved and upper high-frequency limits of ∼170 MHz (at -6 dB) are reached by combining short laser pulses and ultrathin absorbing layers.

Mid-IR Laser Generating Ultrasound in a Polyetheretherketone Polymer

We demonstrate laser ultrasonic generation in polyetheretherketone (PEEK). A middle infrared ZnGeP2 optical parametric oscillator (ZGP-OPO) pumped by a Q-switched Ho:YAG laser is employed as the ultrasonic excitation source. The ZGP-OPO has a spectral range of 3.2–3.4 μm. At an output wavelength of 3.4 μm, the maximum average output power of ZGP-OPO is 3.05 W with a pulse width of 24.3 ns, corresponding to a peak power of approximately 127.5 kW. The ultrasound is generated by the laser converted from 3.2 to 3.4 μm in the PEEK composite. The maximum ultrasonic signal amplitude in PEEK is 33 mV under the condition of thermoelastic excitation at 3.4 μm. Ablation occurs in the CPRF sample when the energy fluence is over 122.45 mJ/cm2. PEEK has a stronger absorption at 3.4 μm and laser-ultrasound generation is influenced by the wavelength of the laser.

High-repetition-rate laser ultrasonic generation in carbon-fiber-reinforced plastics excited by a 3.2-3.4 μm ZGP master oscillator power amplifier system.

In this paper, we demonstrate 1-kHz-repetition-rate laser ultrasound in carbon-fiber-reinforced plastics (CFRPs). A ZGP master oscillator and power amplifier (MOPA) system was used to generate high-repetition-rate laser radiation in the spectral range 3.2-3.4μm. At the output wavelength of 3.4μm, the maximum output energy of the ZGP MOPA system is 5.62mJ with a pulse width of 24ns, corresponding to a peak power of approximately 233.9kW. The ultrasound was generated by the laser converted from 3.2 to 3.4μm in the graphite/epoxy composite. The maximum ultrasonic signal amplitude in the CFRP sample was 36.2mV in the condition of thermoelastic excitation at 3.4μm. Ablation occurred in the CFRP sample when the energy fluence was over 90 mJ/cm2. Compared in different samples, laser-ultrasound generation was influenced by the wavelength of the laser.

Multipoint fiber-optic laser–ultrasound generation along a fiber based on the core-offset splicing of fibers

Fiber-optic laser–ultrasound generation is being used in an increasing number of applications, including medical diagnosis, material characterization, and structural health monitoring. However, most currently used fiber-optic ultrasonic transducers allow effective ultrasound generation at only a single location, namely, at the fiber tip, although there have been a few limited proposals for achieving multipoint ultrasound generation along the length of a fiber. Here we present a novel fiber-optic ultrasound transducer that uses the core-offset splicing of fibers to effectively generate ultrasound at multiple locations along the fiber. The proposed laser–ultrasonic transducer can produce a balanced-strength signal between ultrasonic generation points by reasonably controlling the offsets of the fibers. The proposed transducer has other outstanding characteristics, including simple fabrication and low cost.

Ultrasonic signal analysis according to laser ultrasound generation position for the detection of delamination in composites

Carbon-fiber-reinforced plastic should be inspected in the fabrication process to enhance quality by preventing defects, such as delamination and voids. Conventional ultrasonic evaluation methods cannot be applied during the fabrication process because they require contact measurement by a transducer. Thus, an optical method using a laser was employed in this study for non-contact ultrasonic evaluation. Ultrasonic signals were generated by a pulsed laser and received by using a laser interferometer. First, an ultrasonic signal was generated from the back side of a material sample with artificial internal defects in the composite. The ultrasonic signal directed through the interior of the specimen was then detected at the front side. After determining the locations of the internal defects, the defects were quantitatively evaluated from the front side of the composite by using ultrasonic signal generation and reception.

Study of Laser-Array-Source-Generated Narrow-Band Acoustic Waves Interacting with Surface-Breaking Cracks Using Finite Element Method

Based on the classical theory of laser ultrasound generation in the thermoelastic regime, the generation of ultrasound by the irradiation of a laser array source is simulated by the finite element method. The interaction of ultrasound with a surface-breaking crack is studied. By changing the space distribution of a simultaneously triggered laser array source, narrow-band acoustic waves can be obtained. The characteristics of the narrow-band acoustic waves in the frequency domain are analyzed. By comparing with the spectrum of wide-band reflected acoustic waves generated by a single laser source, the mechanism of the interaction of a surface-breaking crack on the reflected narrow-band acoustic waves is studied. According to the variation of a reflected narrow-band acoustic wave spectrum on cracks with different depths, the influence of the crack-depth variation on different components of the narrow-band acoustic waves is obtained. Results suggest that the narrow-band acoustic waves could also be sensitive to the variation of the crack depth so that it can provide an alternative option for crack-depth gaging using all-optical methods.

Numerical Simulation Study of Defect Detections by Using Laser Array Generated Giant Acoustic Waves

Based on the classical theory of laser ultrasound generation in a thermoelastic regime, the generation of ultrasound by laser array source irradiation is simulated by the finite element method. The propagation and interaction of the laser-generated ultrasound with a bulk defect are studied. By changing the trigger time delay of a spatially evenly distributed laser array source, giant bulk waves with specific wavefronts and amplitudes several times larger than those of waves generated by a single laser source can be obtained. The interaction between giant acoustic waves and a bulk defect is compared with that in the model without the defect obtained under the same conditions. Results show that compared with a single-pulse generated acoustic wave, the detectability of multiple-source generated giant acoustic waves to defects can be improved significantly.

Flaw Inspection of Aluminum Pipes by Non-Contact Visualization of Circumferential Guided Waves using Laser Ultrasound Generation and an Air-Coupled Sensor

Our group had previously proposed a generation laser scanning system for visualizing ultrasound propagation on an object as an animate image, which provided visible and quick flaw inspection. Recently, we improved this system to make it completely non-contact by employing an air-coupled ultrasound transducer as a receiver instead of a contact transducer, and demonstrated the successful visualization of Lamb waves propagating on aluminum and carbon fiber reinforced plastic plates, as well as the detection of flaws. In this research, we applied this system to the non-contact visualization of circumferential guided waves on aluminum pipes. It was shown that circumferential guided waves propagating in opposite directions could be visualized separately, and that a flaw such as a slit or thinning on the inside surface of the pipe could be successfully detected even when it existed outside the scanning area.

Cracks measurement using fiber-phased array laser ultrasound generation

A phased array laser ultrasound generation system by using fiber optic delivery and a custom-designed focusing objective lens has been developed for crack inspection. The enhancement of crack tip diffraction by using phased array laser ultrasound is simulated with finite element method and validated by experiment. A non-contact and non-destructive measurement of inner-surface cracks by time-of-flight diffraction method using fiber-phased array laser ultrasound generation and electromagnetic acoustic transducer detection has been studied.

Distributed fiber-optic laser-ultrasound generation based on ghost-mode of tilted fiber Bragg gratings

Active ultrasonic testing is widely used for medical diagnosis, material characterization and structural health monitoring. Ultrasonic transducer is a key component in active ultrasonic testing. Due to their many advantages such as small size, light weight, and immunity to electromagnetic interference, fiber-optic ultrasonic transducers are particularly attractive for permanent, embedded applications in active ultrasonic testing for structural health monitoring. However, current fiber-optic transducers only allow effective ultrasound generation at a single location of the fiber end. Here we demonstrate a fiber-optic device that can effectively generate ultrasound at multiple, selected locations along a fiber in a controllable manner based on a smart light tapping scheme that only taps out the light of a particular wavelength for laser-ultrasound generation and allow light of longer wavelengths pass by without loss. Such a scheme may also find applications in remote fiber-optic device tuning and quasi-distributed biochemical fiber-optic sensing.

Optical MEMs transducers with enhanced efficiency and sensitivity

We introduce a highly sensitive laser ultrasonic detection MEMs transducer and an efficient laser ultrasound generation MEMs transducer. The detection transducer consists of a series of cantilevers with the same dimensions; any two cantilevers next to each other are separated by a solid with the same width as finger's. When ultrasound is incident upon this transducer, as long as there is a vibration component perpendicular to each finger's top surface and with a frequency the same as the finger's first resonance in the ultrasound, each finger will resonate upon the ultrasound. The moving fingers and the still solid separations form an optical phase grating, and therefore the ultrasound can be readout by a detection laser remotely. Because the ultrasound amplitude is amplified many times by the transducer's resonance before detection, the sensitivity of this transducer is much higher than that of traditional transducers. The generation transducer consists of a micro-disk seated upon a micro-stem. When a suitably focused laser pulse illuminated on the center of the disk, a certain order of flapping motion of the disk is mainly actuated, while other orders are just slightly, or not excited. This flapping motion couples a very narrow bandwidth of longitudinal wave propagating along the axis of the stem and into a sample. Because all absorbed optical energy is concentrated into this narrowband ultrasound, its amplitude is much higher than that of normal thermoelastic generation. It is possible to use these MEMs generation and detection transducers to form a simple but highly efficient laser ultrasound generation and detection system in the near future.

Flaw Inspection of Aluminum Plates with a Non-contact Ultrasound Visualization System Using Laser Ultrasound Generation and an Air-coupled Sensor

Flaw Inspection of Aluminum Plates with a Non-contact Ultrasound Visualization System Using Laser Ultrasound Generation and an Air-coupled Sensor

The applicability of a material-treatment laser pulse in non-destructive evaluations

A practical optodynamic study was performed to determine the usability of different lengths of laser pulses for the generation of ultrasonic transients in a solid material. The aim of the study was to evaluate the possibility of a dual use for a laser pulse—for laser material processing, on the one hand, and for the ultrasonic wave generation on the other—with both processes being combined on the same production line. The propagation of the laser-generated ultrasonic waves is evaluated by detecting and measuring with a PID-controlled stabilized interferometer. Thus, both systems provided the basic tools, the generation and detection of ultrasonic waves, for an ultrasonic, laser-based, non-destructive material evaluation. The ultrasonic transients generated by ‘classical’ nanosecond laser pulses were compared with the transients generated by industrial laser pulses with a duration of a few tenths of a microsecond. The experimental results are compared with the results of a time-of-flight analysis that also involved part of a mode-conversion analysis for both regimes in a layered material structure. The differences between the two waveforms were assessed in terms of their visibility, wavelength and resolution. The limit values were calculated and estimated for the laser-pulse parameters, when such pulses are intended for use in an ultrasonic, laser-based, non-destructive evaluation. The possibility of using an industrial marking laser for laser ultrasound generation is thus demonstrated.

Development and Verification of a Confocal Fabry-Perot Interferometer for Broad Band Ultrasound Detection

This paper describes the development of a confocal Fabry-Perot interferometer (CFPI) for non-contact and non-destructive detection of broadband ultrasound generated by a pulsed laser. The operation theory of CFPI is introduced. The transmission and reflection modes of operation were investigated theoretically and verified experimentally. For the present study, a CFPI cavity of 50cm with 95.4% reflectivity spherical mirrors was constructed with associated resonant cavity control and signal detection electronics. The design is capable of providing detection frequency bandwidth from 140kHz to 50MHz. For the first step of verification, the input signal simulated by an electro-optical modulator (EOM) was used for verifying the feasibility of surface wave measurement. Signals obtained from an avalanche detector were compared with the results through theoretical analysis of the CFPI transfer function in a transmission mode. The results show a favorable agreement between the two. Furthermore, transmitted ultrasound signals from a 5MHz contact ultrasound transducer were detected and compared between the CFPI system and a Michaelson interferometer. Patterns of ultrasound arrival and reflection were clearly detected by both. Because an intrinsic transfer function is embedded in the operation of CFPI, the output signal will be distorted when measuring surface displacement. A digital filtering process was considered for compensation for the surface displacement signal. From the comparative results, it was further concluded that the present CFPI design has a displacement resolution of 0.05nm. Future studies will be focused on the reflection mode operation for fully utilization of non-contact laser ultrasound generation and detection.

Experimental verification of the effects of optical wavelength on the amplitude of laser generated ultrasound in polymer-matrix composites

Laser ultrasound is now integrated into the manufacturing process of some of the most modern aircraft for the inspection of composite parts. Unfortunately, for some material and process combinations, laser-ultrasound suffers from a lack of sensitivity. In laser-ultrasound generation, optical penetration depth plays a very important role. It was shown that changing the generation wavelength from the 10.6 μm of the CO 2 laser to the 3–4 μm range can significantly improve generation efficiency. In this paper, ultrasonic displacements are compared to measurements of optical penetration depth in different polymer-matrix composites. Ultrasonic waves were generated using an optical parametric oscillator operating in the 3.0–3.5 μm band and optical penetration depth spectra were evaluated using quantitative photoacoustic spectroscopy. The relative amplitudes of the generated ultrasonic waves track closely the optical penetration depth spectra. These results experimentally demonstrate the importance of optical penetration in the laser-ultrasound generation process.

Experimental comparison between optical spectroscopy and laser-ultrasound generation in polymer-matrix composites

Laser ultrasound is a technique based on lasers to generate and detect ultrasound. The generation mechanism involves several parameters among which one of the most important is the optical penetration depth. This letter presents a comparison between the amplitude of ultrasonic waves generated by an optical parametric oscillator and optical penetration depth spectra measured by photoacoustic spectroscopy in the 3.0 to 3.5 μm wavelength range for three different composite samples. The laser-ultrasound amplitude spectra closely track the photoacoustic spectra. The results presented in this letter experimentally demonstrate why the 3.0–3.5 μm wavelength range generates more efficiently generates ultrasonic waves in the ultrasonic frequency range of interest for the inspection of polymer-matrix composites than the 10.6 μm wavelength of the CO2 laser.

Optoacoustic sources: a practical Green function-based model for thin film laser-ultrasound generation

The theory of optoacoustic generation of ultrasound in fluids through the thermoelastic effect has been addressed, considering the case of a thin absorbing film used as a target for the laser light. A novel theoretical method, based on the use of the Green function and of the image theorem, has been developed and discussed. The model is mainly carried out in time and space domains, and connects the sound field directly to the laser pulse intensity through equations that show the relevant underlying phenomena. With respect to standard double transform (Laplace + Hankel) techniques, it ensures a simpler physical understanding and appears to be more robust from the computational point of view. Moreover it allows us to identify the approximations that may simplify many practical problems. Many approximate formulae are deduced, in addition to the most significant ultrasonic pressure field parameters, such as amplitude and bandwidth, which are explicitly evaluated in terms of laser excitation and of the materials characteristics.

Efficient laser-ultrasound generation by using heavily absorbing films as targets.

An efficient all-fiber optic source is presented; it adopts absorbing films, deposed directly over the fiber tip, as targets. It is demonstrated that the use of absorbing films made of pure graphite, or graphite powder mixed with epoxy resin, has produced a conversion efficiency increase of two orders of magnitude with respect to metallic materials. It is observed that the conversion efficiency increases monotonically as thickness is reduced down to the material optical penetration depth. Moreover, the conversion efficiency rises with the concentration of graphite powder. Principal advantages of this kind of source are the ease of production and miniaturization, the excellent electromagnetic compatibility, wide ultrasonic bandwidth and, consequently, high spatial resolution. The ultrasonic bandwidth can be controlled by varying the laser pulse duration. The possibility of generating ultrasonic signals with high frequency and flat spectral distribution makes the proposed device suitable for biological tissue spectral characterization.

In situ Measurement Technologies in Semiconductor Manufacturing.

In situ measurements in semiconductor manufacturing with lasers are advantageous in that they cause very little disturbance to objects and allow real-time accurate quantification. This paper reviews three in situ measuring methods for gas concentration, particle identification, and wafer temperature based on IR absorption, laser induced breakdown (LIB) and laser ultrasound generation in solids, respectively.

Laser-based ultrasonic NDE using gas-coupled laser acoustic detection

An optical beam-deflection technique has been applied to the ultrasonic inspection of materials. The technique is similar to air-coupled detection, except the airborne wave is detected through beam deflection instead of with an electro-acoustic transducer. It is a noncontact, laser-based, detection technique that does not depend on reflection of the detection laser beam from the surface. It is thus independent of the surface optical properties of the material under test, and is particularly suitable for testing polymer/graphite composites, which have rough surfaces of poor optical reflectivity. The beam is directed parallel to the surface of a solid material under investigation and is incident on a custom-built position-sensitive photodetector. A wave is radiated from the surface into the surrounding gas during reflection of elastic waves in the solid. The transverse acoustic density gradients cause a beam deflection which is clearly observable with the photodetector. Applications of gas-coupled laser acoustic detection (GCLAD) with laser ultrasound generation will be illustrated with (1) waveforms showing numerous, well-resolved echoes, (2) C-scans showing subsurface defects, and (3) surface acoustic waves. The samples tested include several types of polymer/graphite composites. [Work supported by the Army Research Office.]