Denis Drolet

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.

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.

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 etendue 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-Perot (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.

Polarization independent phase demodulation using photorefractive two-wave mixing

We describe both theoretically and experimentally a polarization independent interferometric adaptive photodetector based on photorefractive two-wave mixing. The configuration is based on the simultaneous recording of two independent gratings in a single photorefractive crystal. Applied to the detection of ultrasonic signals, this interferometric photodetector operates with depolarized beams issued from multimode fibers and gives a detection limit close to the ultimate.

Detection of ultrasonic vibrations on rough surfaces through the photorefractive effect

We present and describe two techniques used for optical detection of ultrasonic signals based on the photorefractive effect. These techniques used the wavefront adaptation properties of the photorefractive effect. In the photorefractive beam combiner, a local oscillator matched to the signal wavefront is created, leading to an homodyne detection system having a large etendue. In the double phase conjugate heterodyne detection system the signal beam wavefront is cleaned by a double phase conjugate mirror and transformed in a plane wave that is sent on a classical heterodyne detection system. Both systems are characterized and used to detect ultrasound.

Photorefractive detection of ultrasound

We present and describe different techniques based on the photorefractive effect that are used for the optical detection of ultrasonic signals. These techniques use the wavefront adaptation properties of the photorefractive effect. They are: the photorefractive beam combiner, the double phase conjugate heterodyne detection and the adaptive photodetector based on non steady state photoelectromotive force. Their respective advantages and drawbacks, are overviewed. We insist on the latest development and performances obtained with the photorefractive beam combiner that seems to use the most promising technique for the detection of ultrasonic signals on rough surfaces. We show that a sensor with near optimum sensitivity can be developed with the same photorefractive crystal at different wavelength in the range of 1 micrometer to 1.55 micrometer.

SAFT data processing applied to laser-ultrasonic inspection

By relying on optics for providing the transduction of ultrasound, laser-ultrasonics brings practical solutions to a variety of nondestructive evaluation problems that cannot be solved by using conventional ultrasonic techniques based on piezoelectric transduction [1,2]. Laser-ultrasonics uses two lasers, one with a short pulse for the generation of ultrasound and another one, long pulse or continuous, coupled to an optical interferometer for detection. Laser-ultrasonics allows for testing at a large standoff distance, inspection of moving parts on production lines and inspection in hostile environments, such as the one encountered in the steel industry. The technique features also a large detection bandwidth, which is important for numerous applications, particularly involving material characterization. Another feature of laser-ultrasonics, particularly useful for inspecting parts of complex shapes, is the generation of an acoustic wave propagating normally to the surface, independently of the shape of the part and of the incidence angle of the optical generation beam. This characteristic feature occurs either when the ablation mechanism is used for generation or when light from the generation laser penetrates sufficiently deep below the surface. This last condition occurs usually with many polymer-based materials and on materials with painted surfaces.

Optical Detection of Ultrasound using Two-Wave Mixing in Semiconductor Photorefractive Crystals and Comparison with the Fabry-Perot

Optical detection of ultrasound presents several advantages over conventional ultrasonic techniques; performed without contact and at a distance it can be used to probe parts at elevated temperature, in particular on a production line, and parts of complex shape. However, optical techniques are typically less sensitive than conventional piezoelectric-based techniques1. Hence, efforts have been done to improve the sensitivity of optical techniques while maintaining their useful features such as a large detection bandwidth.

Specifications of an Ultrasonic Receiver Based on Two-Wave Mixing in Photorefractive Gallium Arsenide Implemented in a Laser-Ultrasonic System

Optical techniques for ultrasonic measurements present several advantages over conventional piezoelectric methods. First, they are remote sensing techniques and can be, for example, used for the inspection of materials at elevated temperature or products moving on a production line. Secondly, surfaces of complex shape can be easily probed since these techniques work with scattered light. For specific applications, these advantages compensate the usually lower sensitivity of optical techniques.

Optical Detection of Ultrasound by Two-Wave Mixing in Photorefractive Semiconductor Crystals Under Applied Field

The optical detection of transient surface motion has many practical applications which include, in particular, the vibration monitoring of engineering structures (aircraft, power plants,...) and the detection of ultrasound produced by piezoelectric transducer or by pulse laser excitation. This last application where ultrasound is generated and detected by lasers, presents many advantages over conventional piezoelectric based methods. First, laser-ultrasonics is a remote sensing technique. Consequently it can be used, for example, for inspecting hot materials and products moving on a production line. Second, surfaces of complex shapes can also very easily be probed. For many applications, these advantages compensate the usually lower sensitivity of the laser-based technique compared to piezoelectric transduction.