Ultrasonic Detection System Based on Two-Wave Mixing in Photorefractive Gaas Crystal

Abstract

The generation and the detection of ultrasound at distance by lasers present many advantages over conventional piezo-electric based methods. In particular. it can be used on surfaces of complex geometry and on products at elevated temperature on a production line [1.2]. Detection has been so far usually performed by using interferometers with passive elements. the most useful being the confocal Fabry-Perot interferometer used in a transmission or reflection mode. [1.3-5]. Passive interferometers for surface motion detection can be advantageously replaced by active or adaptive interferometers based on two-wave mixing in photorefractive crystals. In these systems. a signal beam. which acquires phase shift and speckle after reflection on a surface in motion. is mixed in a photorefractive crystal with a pump plane wave to produce a speckle adapted reference wave that propagates in the same direction as the transmitted signal wave and interferes with it. We have recently devised and demonstrated such a scheme. as sketched in fig 1. by using two-wave mixing in a photo refractive BaTi03 crystal [6]. Since in this case. charge separation is essentially caused by diffusion. the transmitted signal wave and the reference wave are apprOximately in phase. thus leading to quadratic detection. Linear detection was obtained by giving two polarizations to the input signal wave and adding a retardation plate at the output. so the reference and transmitted waves were in quadrature [6]. In order to use a photo refractive interferometer for ultrasound detection in most industrial conditions. the grating build-up and erasure time (t) should meet stringent specifications. First. it should be sufficiently long to properly generate the reference beam. which means that the grating should not follow the motion of the interference pattern