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This paper describes a fiber-optic system which is able to detect ultrasound in structures. The aim of the sensing system is to monitor structures, in particular aircraft structures, by detecting ultrasonic Lamb waves. This type of monitoring technique has recently become a key topic in structural health monitoring. Most common approaches use piezoceramic devices to launch and receive the ultrasound. A new way of fiber-optic detection of Lamb waves is based on fiber Bragg grating sensors. In addition to the well known advantages of fiber-optic sensors, this new interrogation scheme allows the use of Bragg gratings for both high-resolution strain and high-speed ultrasound detection. The focus of the paper is on the ultrasonic part of the system. The theoretical approach and the implementation into a laboratory set-up are elaborated. Experiments have been carried out to calibrate the system and first results on simple structures show the feasibility of the system for sensing ultrasonic Lamb waves.

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We present the design and initial investigation of a fibre optical system which may be used both for intra-cavity and for ring-down measurements of absorption losses. The system consists of a fibre loop containing a length of erbium-doped fibre pumped at 980 nm, with gain adjustment below or above threshold for the two types of operation. The fibre loop is constructed from standard fibre optical components and includes a micro-optical gas cell. The intended application is for measurement of levels of trace gases which possess near-IR absorption lines within the gain bandwidth of the erbium fibre amplifier. We discuss the key issues involved in operation of the system and the level of sensitivity required. Our initial experimental investigations have demonstrated that ring-down times of several microseconds can be obtained, which can be altered through adjustment of the attenuation or gain factor of the loop. Gain control is one of the most important issues and we explain how this may be achieved.