Search

A model‐reference approach is developed to solve the signal enhancement problem of a laser ultrasonics application for nondestructive evaluation. In this problem a sophisticated laser thermoelastic propagation model is used to synthesize the surface displacement of the specimen under test. Once synthesized, this model response is used as the reference signal in an optimal (minimum error variance) signal enhancement scheme. Both fixed and adaptive processors are considered in this application where it is shown that a significant improvement in signal levels can be achieved over the usual methods to enhance noisy data acquired from a Michelson interferometric measurement system and increase its overall sensitivity.

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.

A laser-based ultrasonic technique suitable for characterization of the microstructural state of metal foils is presented. The technique relies on the measurement of the intrinsic attenuation of laser-generated longitudinal waves at frequencies reaching 1 GHz resulting from ultrasonic interaction with the sample microstructure. In order to facilitate accurate measurement of the attenuation, a theoretical model-based signal analysis approach is used. The signal analysis approach isolates aspects of the measured attenuation that depend strictly on the microstructure from geometrical effects. Experimental results obtained in commercially cold worked tungsten foils show excellent agreement with theoretical predictions. Furthermore, the experimental results show that the longitudinal wave attenuation at gigahertz frequencies is strongly influenced by the dislocation content of the foils and may find potential application in the characterization of the microstructure of micron thick metal foils.

This report describes the science and engineering performed to provide state-of-the-art acoustic capabilities for nondestructively characterizing mesoscale (millimeter-sized) objects--allowing micrometer resolution over the objects entire volume. Materials and structures used in mesoscale objects necessitate the use of (1) GHz acoustic frequencies and (2) non-contacting laser generation and detection of acoustic waves. This effort demonstrated that acoustic methods at gigahertz frequencies have the necessary penetration depth and spatial resolution to effectively detect density discontinuities, gaps, and delaminations. A prototype laser-based ultrasonic system was designed and built. The system uses a micro-chip laser for excitation of broadband ultrasonic waves with frequency components reaching 1.0 GHz, and a path-stabilized Michelson interferometer for detection. The proof-of-concept for mesoscale characterization is demonstrated by imaging a micro-fabricated etched pattern in a 70 {micro}m thick silicon wafer.

Photothermal microscopy is a useful nondestructive tool for the identification of fluence-limiting defects in optical coatings. Traditional photothermal microscopes are single-pixel detection devices. Samples are scanned under the microscope to generate a defect map. For high-resolution images, scan times can be quite long (1 mm 2 per hour). Single-pixel detection has been used traditionally because of the ease in separating the laser-induced topographical change due to defect absorption from the defect surface topography. This is accomplished by using standard chopper and lock-in amplifier techniques to remove the DC signal. Multi-pixel photothermal microscopy is now possible by utilizing an optical lock-in technique. This eliminates the lock-in amplifier and enables the use of a CCD camera with an optical lock in for each pixel. With this technique, the data acquisition speed can be increased by orders of magnitude depending on laser power, beam size, and pixel density. Keywords: Photothermal microscopy, absorption, nondestructive testing, multilayer coating 1 INTRODUCTION Fluence-limiting defects in optical coatings can be differentiated from benign defects with photothermal microscopy.

Ultrasonic nondestructive evaluation is an extremely powerful tool for characterizing materials and detecting defects. A majority of the ultrasonic nondestructive evaluation is performed with piezoelectric transducers that generate and detect high frequency acoustic energy. The liquid needed to couple the high frequency acoustic energy from the piezoelectric transducers restricts the applicability of ultrasonics. For example, traditional ultrasonics cannot evaluate parts at elevated temperatures or components that would be damaged by contact with a fluid. They are developing a technology that remotely generates and detects the ultrasonic pulses with lasers and consequently there is no requirement for liquids. Thus the research in laser-based ultrasound allows them to solve inspection problems with ultrasonics that could not be done before. This technology has wide application in many Lawrence Livermore National Laboratory programs, especially when remote and/or non-contact sensing is necessary.

The use of laser-based ultrasonics in the testing of materials and structures offers various advantages over more traditional ultrasonic methods, but is often less sensitive when applied to real materials. Although high energy laser pulses can generate large ultrasonic displacements, nondestructive evaluation requires that the ablation regime be avoided, thus limiting the amount of optical energy which may be used. For this reason, signal processing of laser generated ultrasonic waveforms detected using laser interferometers may be required to extract the desired information from a nondestructive laser ultrasonic test. A model-based signal processing technique offers a way to enhance the signal-to-noise ratios significantly for ultrasonic waveforms obtained using laser-based systems with the generation of the ultrasound occurring in the nondestructive thermoelastic regime.

A Michelson interferometer is used to map the ultrasonic displacement of the lens at the end of a delay rod of a 50‐MHz immersion transducer. The purpose of mapping the displacement is to provide a source function to a model that predicts the ultrasonic propagation in, and interaction with, various materials. The output of the Michelson interferometer can be calibrated, and then used to determine the displacement of the transducer lens surface moving at ultrasonic frequencies. Using the interferometer, the displacement of the transducer lens is measured at discrete points along its surface. This displacement map then provides the ultrasound propagation model with the actual source function. Direct comparison between a model with a simulated source function and experimentally obtained data is presented. [Work performed under auspices of the U.S. Department of Energy by the Lawrence Livermore National Laboratory under Contract No. W‐7405‐ENG‐48.]

Fiber optics lend increased flexibility to laser‐based ultrasonic nondestructive evaluation (NDE). In this work, fiber‐optic cables are used to transmit light from a laser to the detection site, and then from the detection site to a Fabry–Perot interferometer. The use of fibers allows both the detection laser and interferometer to be placed at a considerable distance from the object under test. A direct line‐of‐sight of the object from the main equipment is not required, since the fibers may be fed through walls and around obstacles. In addition, by containing the laser light in the fibers, the chance of accidental exposure to powerful laser beams that may otherwise be transmitted through air is decreased. Laser‐based ultrasonics is generally less sensitive to traditional contact ultrasonics, and in addition, some light is lost in the coupling of laser light energy into optical fibers, further decreasing the sensitivity; thus the need for signal processing of the received signals is of great importance. In this work, the waveforms obtained using the Fabry–Perot interferometer and the corresponding signal processing performed on the data to enhance the resulting image for NDE are discussed.

In this paper an application of spatio‐temporal array signal‐processing techniques applied to broadband ultrasonic data gathered from a pulsed laser system is discussed. Using a laser source to heat a material specimen under test for flaws, a spatio‐temporal processor capable of estimating the displacement field of the specimen is applied. The peak surface displacement is displayed as an image showing the initial source (displacement field) propagating throughout the material as well as any flaws (scatterers) that may be present within the specimen. Clearly, this method of imaging enables a unique methodology for nondestructive evaluation (NDE). Here, a pulsed laser generates an acoustic (ultrasonic) wave by heating the material and causing thermoelastic expansion. The resulting ultrasonic wave propagates throughout the material and is receied by an array of interferometers created synthetically. Assuming a spherically propagating wave field, the processor creates an image of the field by estimating the p...

Laser generation and detection of ultrasonic signals is a technology that expands the applications for ultrasonic nondestructive evaluation. It is a remote, noncontacting method of generating and detecting acoustic energy in materials. Laser‐based ultrasonics has been shown to have less sensitivity than traditional ultrasonic testing because the amount of optical energy that can be introduced to a material to generate ultrasonic waves nondestructively is limited by the ablation threshold of the material. In addition, detection sensitivity is limited by the surface conditions of the material as well as the feasible power of the laser used in the interferometer. To increase the signal‐to‐noise ratio of laser‐based acoustic detection, beamforming techniques were applied to modeled and measured data. A thermoacoustic wave propagation model of laser generation and detection was used to simulate data from a bistatic array. The corresponding measured bistatic data set was beamformed in the same manner as the sim...