Laser-Ultrasonics for Industrial Applications

Increased use of advanced materials and more stringent requirements for process and quality control are creating new needs for nondestructive inspection techniques. Ultrasonics is a widely used technique for defect detection in various materials and is being developed, and even in some cases actually applied for microstructural characterization. However, ultrasonics in its present state of implementation in industry suffers several limitations. Probing materials at elevated temperature is made difficult by fluid coupling problems. Inspecting specimens of complex shapes requires sophisticated robotic manipulators to properly orient the transducer. Furthermore, since the technique relies on a piezoelectric resonator to generate and receive ultrasound, it does not have the adequate bandwidth or sensitivity for some applications

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

Ultrasound-modulated optical imaging using a powerful long pulse laser

Peer reviewed: YesNRC publication: Ye

Acousto-optical imaging using a powerful long pulse laser

Peer reviewed: YesNRC publication: Ye

Ultrasound-modulated optical imaging using a confocal Fabry-Perot interferometer and a powerful long pulse laser

Ultrasound-modulated optical imaging combines the good spatial resolution of ultrasonic waves (mm scale) and the spectroscopic properties of light to detect optically absorbing objects inside thick (cm scale) highly scattering media. Light propagating in a scattering medium can interact with an ultrasonic wave thereby being tagged by a frequency shift equal to the ultrasound frequency or its harmonics. In this paper, a confocal Fabry-Perot interferometer(CFPI) is used as a tunable spectral filter to detect selectively the ultrasound-tagged photons. The CFPI allows obtaining high spectral resolution (MHz scale) while maintaining a high light gathering power when compared to other spectroscopic devices of comparable resolution. The contrast between the tagged photons and the untagged photons can be further enhanced by cascading CFPI. Moreover, the fast response of the CFPI allows performing measurements within the speckle decorrelation time typically encountered in biomedical applications. In this paper, the use of a single-frequency laser emitting powerful optical pulses allows illuminating the scattering medium only during the transit time of the probing ultrasonic pulses. Consequently, the acoustic and the optical power are both concentrated in time to enhance the signal-to-noise ratio of the tehcnique while remaining below the biomedical safety limits. The detection of optically absorbing objects (mm size) inside 30- and 60-mm thick scattering media is presented.L\u2019imagerie optique modul\ue9e aux ultrasons combine la bonne r\ue9solution spatiale des ondes ultrasoniques (\ue0 l\u2019\ue9chelle du mm) et les propri\ue9t\ue9s spectroscopiques de la lumi\ue8re pour d\ue9tecter des objets optiquement absorbants \ue0 l\u2019int\ue9rieur de milieux \ue9pais (\ue0 l\u2019\ue9chelle du cm) \ue0 forte diffusion. La propagation de la lumi\ue8re dans un milieu diffusant peut interagir avec une onde ultrasonique, \ue9tant ainsi marqu\ue9e par un d\ue9calage de fr\ue9quence \ue9gal \ue0 la fr\ue9quence des ultrasons ou de leurs harmoniques. Pour le pr\ue9sent travail, on a utilis\ue9 un interf\ue9rom\ue8tre confocal de Fabry-Perot (ICFP) comme filtre spectral r\ue9glable pour d\ue9tecter s\ue9lectivement les photons marqu\ue9s par les ultrasons. L\u2019ICFP permet d\u2019obtenir une r\ue9solution spectrale \ue9lev\ue9e (\ue0 l\u2019\ue9chelle du MHz) tout en maintenant un pouvoir de convergence de la lumi\ue8re \ue9lev\ue9 comparativement \ue0 d\u2019autres dispositifs spectroscopiques ayant une r\ue9solution comparable. Le contraste entre les photons marqu\ue9s et les photons non marqu\ue9s peut \ueatre am\ue9lior\ue9 par des ICFP en cascade. De plus, la r\ue9ponse rapide de l\u2019ICFP permet de r\ue9aliser des mesures pendant le temps de d\ue9corr\ue9lation du speckle observ\ue9 typiquement dans des applications biom\ue9dicales. L\u2019utilisation d\u2019un laser \ue0 fr\ue9quence unique \ue9mettant des impulsions optiques puissantes permet d\u2019illuminer le milieu diffusant uniquement pendant le temps de transit des impulsions ultrasoniques de d\ue9tection. En cons\ue9quence, la puissance optique et la puissance acoustique sont toutes deux concentr\ue9es dans le temps afin d\u2019am\ue9liorer le rapport signal/bruit de la technique, tout en restant sous les limites de s\ue9curit\ue9 pour des applications biom\ue9dicales. On pr\ue9sente la d\ue9tection d\u2019objets optiquement absorbants (taille de l\u2019ordre du mm) \ue0 l\u2019int\ue9rieur de milieux diffusants de 30 et 60 mm d\u2019\ue9paisseur.Peer reviewed: YesNRC publication: Ye

Improvement of sensitivity of acousto-optical imaging using a powerful long pulse laser

Peer reviewed: YesNRC publication: Ye

Laser-Ultrasonics for Industrial Applications

Increased use of advanced materials and more stringent requirements for process and quality control are creating new needs for nondestructive inspection techniques. Ultrasonics is a widely used technique for defect detection in various materials and is being developed, and even in some cases actually applied for microstructural characterization. However, ultrasonics in its present state of implementation in industry suffers several limitations. Probing materials at elevated temperature is made difficult by fluid coupling problems. Inspecting specimens of complex shapes requires sophisticated robotic manipulators to properly orient the transducer. Furthermore, since the technique relies on a piezoelectric resonator to generate and receive ultrasound, it does not have the adequate bandwidth or sensitivity for some applications.</p

Laser-Ultrasonic Monitoring of Metallurgical Transformations in Advanced Ultra-High Strength Steels

Peer reviewed: YesNRC publication: Ye

Heterodyne Detection of Ultrasound from Rough Surfaces Using a Double Phase Conjugate Mirror

Ultrasonic excitation of a solid sample (optically opaque) can be detected by directing a laser beam at one of its surfaces. Surface motion causes a transient phase shift upon the scattered light, which has to be demodulated into an intensity variation prior to its detection by a photodetector. Classical reference beam interferometry (homodyne or heterodyne) is a well-known technique for performing this demodulation. It is characterized by a broad detection bandwidth, but is, following the antenna theorem [1], essentially limited to the detection of one speckle, when used on rough surfaces. In order to circumvent this limitation (i.e., in order to increase the étendue of the interferometer), two different approaches for adapting the signal and reference wavefronts have been considered. The first approach proceeds by creating a reference beam that matched the wavefront of the signal beam. This can be done by using a Fabry-Pérot (FP) [2] which is a self-reference interferometer and means that the reference beam is generated by the signal beam. It can also be done by using two-wave mixing (TWM) in a photorefractive crystal [3,4]. In this case, the reference beam is created by the diffraction of a plane wave pump beam by the hologram written by both pump and signal beams. Alternatively the signal beam wavefront can be adapted to the reference wavefront, which requires, since the reference beam can usually be approximated by a plane wave, the transformation of the speckled beam to a beam with a plane wavefront. Devices using externally pumped [5] or self-pumped phase conjugate mirrors (SPCM) [6] have been reported