Enhanced Performance of Photo-EMF Detectors Using Asymmetric Interdigital Contacts

摘要

Laser ultrasonics is a promising technique for nondestructive, remote inspection of materials and components for industrial and aerospace applications. In this technique a pulsed laser is used to generate an ultrasonic wave, which probes the part under test. When the ultrasonic wave reaches the surface, the resulting surface displacement is measured with an interferometric laser receiver. One promising receiver uses a reference-beam interferometer with photo-emf detection. In this receiver, a signal beam scattered from the surface under inspection is made to interfere with a reference beam at a nonzero angle at the surface of the photo-emf detector. The resulting optical fringe pattern creates a fixed replica space charge pattern in the detector. When the surface vibrates, the optical fringe pattern at the detector surface vibrates laterally. If the vibrations are at high frequencies, the space charge pattern in the detector is effectively frozen and a current is driven through contacts at the edge of the sample. If the vibrations are at low frequencies, the space charge pattern can track the fringe pattern and no current is collected.