Hu et al. use laser light to ultrasonically shake a metal plate and simultaneously measure the shaking, enabling structural health monitoring for extreme conditions.

Silica glass has a high melting point and is highly resistant to corrosion. Glass optical fibers are thus suitable for use in harsh environments, and fiber based sensors are now employed in oil wells, explosives testing, and structural health monitoring of roads and bridges. Researchers at Virginia Tech have demonstrated an all-fiber multi-parameter sensor bonded to a metal plate. Their sensor is suitable for monitoring temperature, strain, thickness changes, and crack formation in aircraft, power plants, and other metal structures, replacing traditional PZT-based electronic sensors.

The structure is interrogated by a high power laser pulse delivered to a defect in a multimode fiber, which locally heats the metal plate and induces an acoustic vibration. The wave propagation is read out by monitoring the reflection of a CW tunable laser from fiber Bragg gratings (FBGs) inscribed in single-mode fibers located at various points along the plate. Their analysis of the wave propagation allows them to track several acoustic modes of the plate, and to separate temperature and strain-induced effects in the FBG response, removing some of the packaging problems inherent to many FBG multi-parameter sensors.

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