A light-in-light-out ultrasonic measurement system

A new designed Michelson heterodyne interferometer is assembled to a Light-In-Light-Out ultrasonic measurement system. This means that a pulsed laser is used in order to generate ultrasound in a sample and a interferometer is used in order to pick-up the ultrasonic displacements at the surface. In this way a non-contact and remote system for material characterization is available. A Nd:YAG-laser is used to generate the ultrasonic waves. The laser pulses are directed to the sample via a scanning system consisting of some movable mirrors and a lens. The key device is the receiver, the heterodyne interferometer. According to theory the light gathering power, the product of effective aperture and solid angle of the field of view, is limited to about lambda (exp 2). Therefore, great attention must be paid in the design and the adjustment of the interferometer. A 100 mW diode pumped cw Nd:YAG-laser is used as light source and a 160 MHz Bragg cell as beam splitter into object and signal beam. In this way a 160 MHz frequency shift between both of these beams is achieved as well. By a careful choice of adjustment components and by expanding the signal beam a very good performance of the system is achieved. The interferometer is compact, rugged and easy to adjust. It could be used to measure ultrasonic displacements from a dark rough surface at a distance of about 700 mm. In the paper, some examples in the field of material characterization are given