Recent developments in interrogating fibre Bragg-grating sensors for composite Smart structures

Abstract

Fibre optic Bragg grating sensors are attracting considerable interest for a number of sensing applications because of their intrinsic and wavelength-encoded operation. There is great interest in the multiplexed sensing of smart structures and materials, particularly for real-time evaluation of physical measurands (eg. temperature, strain) at critical monitoring points. The primary emphasis is on dynamic measurement of strain. The requirement is to monitor a large number of in-fibre grating sensors, either continuously in real time, or by polling them with an acquisition time of less than 1 msec. Without such capability, it will not prove possible to interrogate the vibrational modes of structures. In this paper, the various interrogation methods are briefly reviewed and are compared with our new method of using an acousto-optic tunable filter. 1. Introduction:
The in-fibre grating is a highly-attractive sensor. In particular, it is extremely compact, and the cylindrical sensing fibre also serves as the optical communications medium (Ref.1). The advantages of this form of sensor for strain sensing are:-
1. The sensor can be formed in a continuous length of fibre, which also acts as the telemetry link.
2. Silica is an excellent mechanical material, with a low-creep characteristic and excellent immunity to corrosion.
3. The sensor is small, lightweight and hence readily incorporated in composite materials.
4. The cylindrical geometry of the sensor/optical link gives a uniform mechanical structure with no discontinuities to provide stress concentration points.
5. The sensor is immune to electromagnetic interference.
6. Multiple gratings can be incorporated in, and interrogated via, a single length of fibre.
The main technical problems are in reading the wavelength of the gratings, particularly when there is the constraint of requiring instrumentation suitable for space applications. There is also a potential problem of undesirable temperature sensitivity, when required for use as a strain sensor. The latter problem (which is, of course, also seen with resistive strain gauges) is preferably resolved by arranging the interrogation system to perform simultaneous measurement of temperature and strain, or by arranging the thermal response to be cancelled. In our recent work, we have made significant progress in both these directions. Before discussing our new advances, however, we will first summarise the present state of the technology for interrogating Bragg gratings.

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