Fibre Optic Sensors - An Introduction

Fibre optic sensors are used all over the world in a range of different environments and applications. They are defined as devices that can measure physical parameters through the effects placed upon the fibre and how light is passed through the fibre.

Fundamentally fibre optic sensors are used most frequently to measure physical properties such as strain, temperature, humidity and pressure although they are also used to monitor velocity and acceleration in some situations. In terms of applications they are frequently used to monitor the health of structures, providing measurements in real time of the forces placed upon a building.

One of these applications is in buildings and also bridges where monitoring is carried out to detect changes in the overall structure. Fibre optic sensors are also frequently used to measure properties in tunnels and dams. In these situations sensors are frequently used to monitor foundations, spatial displacement, subsidence and in some cases post-seismic evaluation after earthquakes.

There are a number distinct advantages to using optic sensors. These include:

• Immunity to EMI (electromagnetic interference) and also RFI (radio frequency interference).

• Safe and stable use in extreme environments where there are high temperatures, explosions and extreme vibration.

• Small in size and high sensitivity.

There are two main types of sensor extrinsic and intrinsic.

• Extrinsic sensors utilise optical fibres as a means of transferring signals. Frequently these fibres enter a light modulator box that has a light beam passing through it.

• Intrinsic sensors differ fundamentally from extrinsic varieties in that the sensing actually takes place within the fibre. In this instance the sensing is a result of the way in which the fibre is affected by direct physical changes. Essentially the strains put upon the fibre change the way in which light passes through the fibre. The change in light behaviour provides a measurement.

Fibre optic sensors are used in a wide range of industries, from building and construction to oil and gas measurements. It is the specific properties of fibre optics that make them so useful within sensing instrumentation.

The use of fibre optic sensors is based upon the stability of light as a way to make measurements and also the physical size and properties of fibre optics more widely.

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