Adaptive Heterodyne Interferometer for Ultrasonic NDE

Abstract

For NDE applications, the remote generation and detection of ultrasound by laser present many advantages over traditional piezo-electric based methods. They provide nonintrusive, point generation and detection with a large frequency bandwidth. For example, it can be used on surfaces of complex geometry and elevated temperature on a production line. Ultrasound generation using absorption of pulse laser energy is well known. Various interferometers for optical detection of ultrasound have been described in the literature [1]. In order for the interferometer to be attractive for NDE applications, the interferometer must also be able to operate, without loss of sensitivity, in an environment where large amplitude low frequency vibrations are generally present. Furthermore, the interferometer must be able to achieve a good sensitivity on rough surfaces. The laser light reflected on a rough surface is characterized by speckles. The random intensity and phase distributions of these speckles require the use of interferometer with large étendue. In addition, the possibility of using an optical fiber in the path of the probe beam without loss in the interferometer sensitivity is highly desirable for applications where access to the specimen is limited. The confocal Fabry-Pérot interferometer [1] has been shown to be well adapted for NDE applications. Unfortunately, since the elimination of the optical side-bands is based on the optical filtering action of the confocal cavity, it is sensitive mostly to high frequencies for a reasonable cavity size, typically above 1MHz for a 1 meter cavity length.