Silicon-on-insulator photoconductors for non-contact ultrasound vibration measurement using laser speckle

摘要

The motion of a vibrating specular surface can be measured by monitoring the change in speckle pattern of a reflected laser beam. Smaller speckles allow for a more sensitive measurement, but provide a weaker signal when monitored with a correspondingly sized photodiode. The signal of a photoconductor, on the other hand, scales with the ratio of its dimensions, and so can be resized without loss of signal. Here we present a prototype detector made from micron-scale, isolated mesas of intrinsic silicon, fabricated lithographically from a commercial silicon-on-insulator wafer. The prototype has a frequency response extending into the megahertz regime, making it suitable for ultrasound testing applications. Only a single laser beam, with no separate interferometer or optical reference, is required for displacement measurement with laser speckle monitoring, so such a system provides a robust and simple alternative to other optical detection methods. Initial tests have captured ultrasound Lamb wave vibrations and standing waves induced by optical excitation in thin copper and aluminum strips.