An optical fibre interferometer for remote detection of laser generated ultrasonics

A 15 MHz bandwidth active homodyne fibre-optical interferometer for remote detection of laser-induced ultrasonic waves in metallic and non-metallic targets is reported. The fibre interferometer design and associated electronics are described in sufficient detail for it to be easily constructed. The interferometer uses a low-power He-Ne laser and the reflected signal from the natural surface of the target at a distance of 200 mm. Despite its use of an open air path in the sensing arm and an all-fibre design, the interferometer achieved a single-pulse noise equivalent displacement of 0.4 nm and a figure of merit of 0.1 pm .