NEW GENERATION OF OPTICAL ULTRASONIC SENSORS ADAPTED TO INDUSTRIAL CONSTRAINTS

摘要

In the field of non destructive testing the ultrasonic techniques are widely used for thickness measurements, flaw detection or material characterization. Ultrasonic vibrations are usually generated and detected with piezoelectric transducers, coupled to the tested part by a direct contact or through a water bath or a water jet. This contact, as well as the presence of a coupling liquid, have a negative impact for their use in a lot of industrial applications, particularly for on line control, for the characterization of parts at high temperature or for the inspection of advanced materials. This explains the interest of optical techniques for the detection of ultrasound which, despite the fact that they are less sensitive than piezoelectric sensor, have the great advantage of being non contact and of having a large frequency bandwidth. Optical systems developed until now are based on the coherent detection of phase modulation, imprinted on the beam by the vibrating surface, that uses the exact superposition of the wave issued from the target with a plane reference wave. These devices thus only operate with a plane wave signal beam. This requires either to polish the tested surface (what is generally not possible in industrial environment) or to filter the beam (what leads to a strong loss in light power, and subsequently a strong reduction of the detection performances). As a consequence, although largely used in laboratories, laser ultrasonic systems see their spread in industry greatly slowed down by these constraints. The use of dynamic holographic materials as, for example, photorefractive crystals, relaxes these constraints and allows to realize an interferometer with a large field of view and thus with a large light gathering power. Different systems were proposed and two of them, which have led to industrial developments, will be presented below. They are the photorefractive beam combiner, that is an homodyne detection setup in which a reference wavefront adapted to the signal beam is reconstructed holographically, and the adaptive photodetector, based on an effect close to the photorefractive effect, the non steady state photoelectromotive force, in which the phase modulation is electronically detected directly on the photorefractive crystal.