Adaptive Hetorodyne Line-Probe Interferometer for Enhanced Directionally-Sensitive Detection of Ultrasound

Optical methods provide a non-contact method of detecting ultrasound at the surface of a test object. Unlike conventional piezoelectric transducers, which require a couplant, optical detection provides an absolute calibration of the ultrasonic displacement amplitude. In addition, they can have a broader bandwidth and a higher spatial resolution of detection than conventional piezoelectric transducers. All these advantages however typically come at the expense of sensitivity. The best extant optical detectors still suffer from a two order of magnitude sensitivity gap with respect to conventional piezoelectric transducers. In this paper, we describe an adaptive heterodyne interferometer receiver using wave mixing in photorefractive bismuth silicate (BSO) crystals which is configured as a line receiver that is directionally most sensitive to ultrasound impinging normal to the line, and is significantly less sensitive to ultrasound impinging in other directions. Such a system is attractive in situations where the ultrasonic scatter from a specific direction is to be selectively pulled out in the presence of scatter from other “noise” sources. The line probe system also provides a way to bridge the sensitivity gap that optical detection thus far has suffered vis-à-vis piezoelectric detection. Results of applications to nondestructive testing of metal surfaces are presented.</p