Noncontact detection of ultrasound with rugged fiber-optic interferometer

An all-fiber-optic probe for noncontact detection of ultrasound in industrial settings is described. The probe consists of a path-matched interferometer built with polarization-maintaining optical fiber. The interferometer is maintained at its linear operating point (quadrature condition) without the need of active feedback or modulation. Ultrasound was monitored on rough metallic surfaces achieving single-pulse subnanometer sensitivity with a 5 MHz bandwidth. Probes with surface clearances of about half a centimeter were investigated. This interferometer provides immunity to ambient disturbances and a very compact package. Measurements obtained in combination with laser-generated ultrasound are also presented.