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Non-contact ultrasound applications are raising more and more interests. However, very few applications have been reported, for now in medical applications. In this work, compact airborne piezotransducers have been designed, built and characterized for a novel medical application: contactless corneal imaging. The emission transducer is cylindrical with a 5.5 mm diameter and the reception transducer is a 2D array of 4x16 elements. A specific emission-reception electronic is also designed for these transducers to drive their high electric impedances that are larger than kiloOhms. The measured parameters - directivity, sensitivity and frequency bandwidth – of both transducers respond well to the needs of corneal imaging application.

The speed of the surface Rayleigh wave, which is related to the viscoelastic properties of the medium, can be measured by non-invasive and non-contact methods. This paper presents a new method for non-contact ultrasound surface wave elastography using airborne ultrasound surface motion camera (AUSMC). To demonstrate and validate the approach the experiments were carried out on different gelatin phantoms of different concentrations (3%, 5%, 8%) and ×12 scaled eye phantom. By studying the dispersion curve of the surface wave speed, shear modulus can be extracted. The study presents the preliminary results that support the capability of the AUSMC system to estimate without contact the shear modulus of semi-infinite media or thin layer over semi-infinite media.

In security applications, several methods are used for detection of hidden objects. Metal detectors and palpation are the most common methods. Non-contact acoustic waves imaging of elastic heterogeneities in superficial body tissues could be an interesting alternative.

Focusing and beam steering is achieved by using a time-reversal process and a single transducer coupled to a solid cavity that is immersed in water. This low-cost technique makes it possible to focus acoustic energy anywhere on a 3D domain with a spatio-temporal resolution comparable to that of multiple transducers array. A short pulse is emitted from a transducer stuck at the surface of the solid cavity. The multiple-scattered field is measured in front of the solid cavity using a hydrophone needle at a reference point. This signal is then time reversed and remitted by the transducer. Around the reference point, one can observe a spatio-temporal recompression. The sidelobe level as well as the focal width no longer depend on the transducer aperture but on the dimensions of the solid cavity and the multiple paths covered by the acoustic waves in the solid. Moreover, it is shown how the experimental impulse responses on the front face of the cavity can be used to control the emitting ultrasonic field. This “synthetic time-reversal” technique is shown to be as powerful as a real time-reversal process.

Generation of ultrasonic displacements under the thermoelastic effect has the advantage of being contactless and nondestructive. Unfortunately, the directivity and energy of the sound field of such a source are limited. In order to change these parameters the authors use a moving thermoelastic source by moving the laser beam on the solid surface. Moving the laser beam allows control of the ultrasonic beam directivity and an increase in the energy of the sound field. For this purpose, a large impulse of dye laser deflected by an acoustooptic cell working under the Bragg mode is used. Experimental results, in an aluminum sample give good agreement with the theoretical prediction. >

Heterodyne interferometers using two‐wave mixing in photorefractive cubic crystals for ultrasound detection on rough surfaces are demonstrated. The speckled scattered beam from a rough surface sample interferes with a planar coherent pump beam inside a photorefractive crystal. A third frequency‐shifted beam is used to read the grating. The diffracted readout beam and the transmitted signal beam are wavefront matched, resulting in an optimal heterodyne interference signal. The signal to noise ratio for the two commonly used crystallographic configurations with cubic crystals, G∥〈110〉∥ and G∥〈001〉, where G is the grating wave vector, are investigated. Very good sensitivity is demonstrated for the detection of small amplitude ultrasonic surface displacements.

The generation of acoustical waves in solids by pulsed laser irradiation has the advantage of being contactless and nondestructive in the case of the thermoelastic mode. Unfortunately, acoustical characteristics of such a source are limited. To modify these parameters, a phased array of thermooptical sources was made by moving the laser beam, and control of beam directivity was obtained. Achievement of such an array is accomplished with an acoustooptic deflector synchronized with a large pulse dye laser. The influence of the velocity and geometry of the moving source is studied, and comparison with a motionless source is made. >

The advantages of ultrasonic remote detection using a heterodyne photorefractive interferometer are demonstrated. The speckle nature of the light reflected by a rough surface (or after propagation through a multimode fiber) does not degrade the interferometer sensitivity. Thanks to the process of wave mixing in photorefractive crystals, this interferometer has a large etendue. Furthermore, the heterodyne detection provides an absolute measurement of the ultrasonic amplitudes. We investigate the influence of an applied ac electric field. Two photorefractive crystals, undoped BSO and BSO:Cr, which have different optical characteristics are compared. Finally, examples of application to NDE are demonstrated.

The time reversal process is used to detect surface and sub-surface flaws and leads to a self focusing process of Rayleigh waves. The surface waves are generated with an array of linear transducers coupled to a specimen surface with a Plexiglas wedge. The advantages of such a technique are demonstrated with the detection of surface and sub-surface holes of subwavelength dimensions. In a pulse-echo measurement, the signal to noise ratio is exponentially increased when the self focusing repetitive process is used Measurements are calibrated with an optical interferometer and results are compared with a surface focusing model to point out the sensitivity of such a detection method.

The D.O.R.T. method is an appropriate method to detect defects in complex and noisy solid samples. This method applies to any type of transducer array and linear waves. It is an efficient mean to separate the echo of a defect from the microstructure contribution or from another defect. The detection of defects in titanium alloy containing grain structure are shown. Results illustrating the detection at 5 MHz of flat bottom holes (FBH) with different diameters located at 140 mm depth in a titanium alloy sample are shown. Then we show that this method applies to imaging with Lamb waves. The detection of 0.6 and 0.8 mm holes in a 1 mm thick duralumin plate is presented.