R. D. Huber

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.

Photothermal multi-pixel imaging microscope

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 mm2 per hour). Single-pixel detection has geen 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.

Laser ultrasonic inspection of the microstructural state of thin metal foils

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.

High frequency laser-based ultrasound

To obtain micrometer resolution of materials using acoustics requires frequencies around 1 GHz. Attenuation of such frequencies is high, limiting the thickness of the parts that can be characterized. Although acoustic microscopes can operate up to several GHz in frequency, they are used primarily as a surface characterization tool. The use of a pulsed laser for acoustic generation allows generation directly in the part, eliminating the loss of energy associated with coupling the energy from a piezoelectric transducer to the part of interest. The use of pulsed laser acoustic generation in combination with optical detection is investigated for the non‐contact characterization of materials with features that must be characterized to micrometer resolution.

Direct detection of ultrafast thermal transients by use of a chirped, supercontinuum white-light pulse.

An experimental technique is demonstrated that permits direct optical measurement of ultrafast material transients during a single excitation-relaxation cycle. Reflection of a linearly chirped, supercontinuum optical pulse from a gold film with changing surface temperature induced by an ultrafast pump pulse allows the thermal transients to be encoded onto the spectrum of the probe pulse. Calibrating the chirp of the probe pulse and the wavelength sensitivity of the sample permits mapping of the measured transient into the time domain. Measurements are completed over the course of 100 ps with subpicosecond time resolution. Results obtained with this technique are compared with similar measurements obtained with conventional pump-probe correlation techniques.

Optical mapping of the acoustic output of a focused transducer

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.]

A spatio‐temporal approach to acoustical imaging of laser‐generated ultrasound

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...

An application of laser‐based ultrasonic nondestructive evaluation using a fiber‐optics‐based Fabry–Perot interferometer

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.

Detection of ultrafast thermal transients using a chirped, super-continuum white light pulse

Summary form only given. Ultrafast material transients are typically measured with a correlation technique that uses two ultrashort laser pulses and a delay line. The technique presented in this work encodes the temporal information onto the spectral components of a chirped and temporally stretched, super-continuum probe pulse. This preserves the information of the entire material transient onto a single probe pulse. The approach generalizes and improves on the single-shot, narrowband, chirped detection technique used by Z. Jiang and C. Zhang (1998). Other single shot techniques have been recently realized that separate a single probe pulse into a series of time-delayed probe pulses that are spatially separated onto a CCD device. The use of a continuous probe pulse such as the technique presented here offers several advantages including ease of configuring the time record length and temporal resolution while providing a degree of insensitivity to optical misalignment.

Processing of laser-based ultrasound for matched-field imaging

In this paper we describe the results of controlled laser-based ultrasonic experiments for NDE applications, which are aimed at detecting and imaging a well-defined flaw in an aluminum component. A pulsed laser is used to generate ultrasonic signals which are then received by an interferometer so as to synthesize an array for displacement measurements. We demonstrate how these measurements can be coupled to an algorithm to detect, localize, and image the flaw. The technique, called matched-field imaging (MFI), evolves from research in ocean acoustics. It is shown that MFI is a generalization of the well-known (ad hoc) synthetic aperture focusing technique (SAFT). In terms of investigating components for flaws during NDE, the matched-field approach also offers a reasonable technique of imaging the component itself. Because of the low signal-to-noise ratio inherent in laser-based ultrasound, preprocessing is required and accomplished by a correlation canceling technique designed to separate a correlated (re...