Alain Blouin

Detection of ultrasonic motion of a scattering surface by two‐wave mixing in a photorefractive GaAs crystal

The performance of an interferometric system based on two‐wave mixing at 1.06 μm in undoped GaAs crystal, for the remote detection of transient motion of a scattering surface, is described. In this system, the wave scattered by the surface is mixed inside the photorefractive crystal with a pump wave directly derived from the laser to provide the reference wave of the interferometer. The system shows several features appropriate to industrial applications, although its sensitivity is less than passive interferometric systems of the confocal Fabry–Perot type presently in use, except in the low frequency range (below 1 MHz).

Detection of ultrasonic motion of a scattering surface by photorefractive InP:Fe under an applied dc field

The characteristics of an interferometric system based on two-wave mixing at 1.06 µm in photorefractive InP:Fe under an applied field for the detection of ultrasonic motion of a scattering surface are described. A theoretical analysis of possible configurations for the detection of small phase modulation in the undepleted-pump approximation is presented. Experimental assessment of the device for both cw and pulse regimes is performed: The sensitivity, the etendue, the response time, and the behavior under ambient vibrations or moving inspected samples are provided. This adaptive device presents many features appropriate for industrial inspection and compares advantageously with the passive confocal Fabry–Perot device that is now widely used.

All-optical measurement of in-plane and out-of-plane Young's modulus and Poisson's ratio in silicon wafers by means of vibration modes

An all-optical technique was developed to determine the in-plane and out-of-plane Youngs modulus, E, and Poissons ratio, σ, in silicon wafers through excitation and detection of vibration modes. The technique is remote, non-destructive and works on-line, making it an attractive inspection tool for use in semiconductor foundries. Vibration modes were generated by a pulse from a frequency-doubled Q-switched Nd:YAG laser and detected by a commercial heterodyne Michelson interferometer. Because of their importance as component substrates in the electronic and MEMS industries, both the 100 and 111 silicon wafers were investigated. It was found that the resonance frequencies of the first pure torsional and of the first hybrid (torsional and flexural) modes varied continuously and symmetrically with respect to a 90° rotation of the wafer around the centre, while the resonance frequencies of the first and second pure flexural modes remained constant. On the basis of this experimental observation, we could correlate the resonance frequencies of the two former and two latter modes with the in-plane and out-of-plane mechanical parameters E and σ, respectively. A novel inverse method for circular anisotropic plates was then derived to extract the in-plane and out-of-plane parameters from the resonance frequency measurements. The results obtained by this approach were in good agreement with the standard values calculated by the theory of elasticity.

Non-contact photoacoustic tomography and ultrasonography for tissue imaging

The detection of ultrasound in photoacoustic tomography (PAT) and ultrasonography (US) usually relies on ultrasonic transducers in contact with the biological tissue. This is a major drawback for important potential applications such as surgery and small animal imaging. Here we report the use of remote optical detection, as used in industrial laser-ultrasonics, to detect ultrasound in biological tissues. This strategy enables non-contact implementation of PAT and US without exceeding laser exposure safety limits. The method uses suitably shaped laser pulses and a confocal Fabry-Perot interferometer in differential configuration to reach quantum-limited sensitivity. Endogenous and exogenous inclusions exhibiting optical and acoustic contrasts were detected ex vivo in chicken breast and calf brain specimens. Inclusions down to 0.5 mm in size were detected at depths well exceeding 1 cm. The method could significantly expand the scope of applications of PAT and US in biomedical imaging.

Non-contact biomedical photoacoustic and ultrasound imaging

The detection of ultrasound in photoacoustic tomography (PAT) usually relies on ultrasonic transducers in contact with the biological tissue through a coupling medium. This is a major drawback for important potential applications such as surgery. Here we report the use of a remote optical method, derived from industrial laser-ultrasonics, to detect ultrasound in tissues. This approach enables non-contact PAT (NCPAT) without exceeding laser exposure safety limits. The sensitivity of the method is based on the use of suitably shaped detection laser pulses and a confocal Fabry-Perot interferometer in differential configuration. Reliable image reconstruction is obtained by measuring remotely the surface profile of the tissue with an optical coherence tomography system. The proposed method also allows non-contact ultrasound imaging (US) by applying a second reconstruction algorithm to the data acquired for NCPAT. Endogenous and exogenous inclusions exhibiting optical and acoustic contrasts were detected ex vivo in chicken breast and calf brain specimens. Inclusions down to 0.3 mm in size were detected at depths exceeding 1 cm. The method could expand the scope of photoacoustic and US to in-vivo biomedical applications where contact is impractical.

Performance of laser-ultrasonic F-SAFT imaging

The resolution and signal-to-noise ratio of laser-ultrasonics to detect small and buried defects can be greatly enhanced by using the synthetic aperture focusing technique (SAFT). Originally developed in the time domain, SAFT can also be implemented in the frequency domain (F-SAFT) using the angular spectrum approach for a significant reduction in processing time. In this paper, an F-SAFT based data processing method especially adapted to laser-ultrasonic data is presented. This method allows for further significant improvements towards laser-ultrasonic imaging of small defects. It includes temporal deconvolution of the waveform data, control for an optimal aperture and frequency bandwidth as well as spatial interpolation of the subsurface images. All the above operations are well adapted to the frequency domain calculations and embedded in the F-SAFT data processing. Also, the aperture control and spatial interpolation allow a reduction of sampling requirements to further decrease both inspection and processing times. The above improvements are illustrated using laser-ultrasonic data taken from an aluminum sample with flat-bottom holes.

Improved resolution and signal-to-noise ratio in laser-ultrasonics by SAFT processing.

Laser-ultrasonics is an emerging nondestructive technique using lasers for the generation and detection of ultrasound which presents numerous advantages for industrial inspection. In this paper, the problem of detection by laser-ultrasonics of small defects within a material is addressed. Experimental results obtained with laser-ultrasonics are processed using the Synthetic Aperture Focusing Technique (SAFT), yielding improved flaw detectability and spatial resolution. Experiments have been performed on an aluminum sample with a contoured back surface and two flat-bottom holes. Practical interest of coupling SAFT to laser-ultrasonics is also discussed.

Adhesive bond testing of carbon–epoxy composites by laser shockwave

Adhesive bonding, particularly of composite laminates, presents many practical advantages when compared with other joining methods but its use is limited, since there is presently no non-destructive inspection technique to ensure the quality of the bond. We are developing a technique based on the propagation of high amplitude ultrasonic waves to evaluate the adhesive bond strength at high strain rate. Compression waves are generated by a short and powerful laser pulse under water confinement and are converted after reflection on the assembly back surface into tensile waves. The resulting tensile forces normal to the interfaces can cause a delamination inside the laminates or a disbond. The adhesion strength is probed by increasing the laser pulse energy until disbond. A good bond is unaffected by a certain level of stress whereas a weaker one is damaged. The method is shown completely non-invasive throughout the whole composite assembly. The sample back surface velocity is measured by an optical interferometer and used to estimate stress history inside the sample. The depth and size of the disbonds are revealed by a post-test inspection by the well established laser-ultrasonic technique. Experimental results confirmed by numerical simulations show that the proposed method is able to differentiate weak bonds from strong bonds and to estimate quantitatively the bond strength.

Ultrasound-modulated optical imaging using a powerful long pulse laser

Ultrasound-modulated optical imaging (or tomography) is an emerging biodiagnostic technique which provides the optical spectroscopic signature and the localization of an absorbing object embedded in a strongly scattering medium. We propose to improve the sensitivity of the technique by using a pulsed single-frequency laser to raise the optical peak power applied to the scattering medium and thereby collect more ultrasonically tagged photons. Moreover, when the detection of tagged photons is done with a photorefractive interferometer, the high optical peak power reduces the response time of the photorefractive crystal below the speckle field decorrelation time. Results obtained with a GaAs photorefractive interferometer are presented for 30- and 60-mm thick scattering media.

Ultrasound-modulated optical imaging using a high-power pulsed laser and a double-pass confocal Fabry–Perot interferometer

We report the use of short ultrasonic bursts and high-peak-power laser pulses to detect absorbing objects in thick scattering media (SMs). The detection of ultrasound-tagged photons is performed with a double-pass confocal Fabry-Perot interferometer. Photons shifted by the fundamental and harmonic frequencies of the ultrasonic bursts were observed. Absorbing objects were detected in 30- and 60-mm-thick SMs including a sample of biological tissue.