DETECTION OF LEAKY-RAYLEIGH WA YES AT AIR-SOLID INTERFACES BY LASER INTERFEROMETRY Laszlo Adler and Christophe Mattei Adler Consultants, Inc. 1275 Kinnear Road Columbus, OH 43212 Michel de Billy and Gerard Quentin Groupe de Physique des Solides Universites Paris 6 and 7, CNRS U.R.A 17, Tour 23, Place lussieu, Paris cedex OS, France INTRODUCTION Leaky-Rayleigh waves have been studied extensively at liquid-solid interface in the last 30 years [1-4]. Both leaky-Rayleigh and leaky-Lamb waves have found many application in Nondestructive Evaluation, e.g. surface defect characterization, etc [5,6]. Because of the difficulties associated with ultrasonic wave generation and detection in air in the megaHerz region air coupled non destructive evaluation has only recently been explored [7,8]. An optical method recently developed uses heterodyne interferometric probing of refractive index variation [9-10] proportional to the pressure of acoustic waves in the medium, allowed measurements of pulse propagation in air emitted by transducer in the megaHerz region. In this paper, velocities and attenuation measurements of leaky- Rayleigh waves from solids radiated into air is reported. EXPERIMENTAL SET-UP The schematic diagram of the experimental system is shown figure 1. The optical is a Mach-Zender heterodyne interferometer (BMI type SH 130) used to measure the acoustical pressure of the leaky-Rayleigh wave in air. The probe beam shifted in frequency by an acousto-optic Bragg cell (f b=70Mhz) crossed a transparent medium where the index of refraction is modulated by the acoustic wave and is then reflected by a mirror. The phase modulated current measured at the output of the photodiode is given as: l(t)=lo cos[ 2n fb t + v(t)+ <Ds- <Dr]. (1) Review of Progress in Quantitative Nondestructive Evaluation. Vol. 18 Edited by Thompson and Chimenti, Kluwer Academic/Plenum Publishers, 1999 1447