The characteristics of an interferometric system based on two-wave mixing at 1.06 m 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 undepletedpump approximation is presented. Experimental assessment of the device for both cw and pulse regimes is performed: The sensitivity, the étendue, 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.

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.

A practical coupling device that relies on a dual-core fiber in a loop configuration is presented. Its coupling properties are analyzed in terms of the optical path difference between the cores, which is controlled by the rotation of the fiber about its axis and by a small twist applied to it along the loop. The device actually acts as an anisotropic coupler, and the coupled power can be perfectly controlled from 0-100% by proper adjustment of the loop. A simple implementation of the device was used in the fabrication of a compact single-fiber Michelson interferometer.

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. These schemes require two reflections on the sample surface which strongly limit the sensitivity when the surface is absorbing. The Double Phase Conjugate Mirror (DPCM) is a speckle to plane wavefront converter when one of the two beams creating the mirror is a plane wave [7]. A DPCM was previously used in a heterodyne scheme to produce a wide-field of view interferometer [8] for remote sensing. It has also been used in an homodyne interferometer for ultrasound detection [9].

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