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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Fibre Optic Sensors - Common Applications and Types

Fibre optic sensors are used within a wide range of different industries. Fundamentally they utilise fibre optics to remotely sense and are frequently used due to their small size and also the fact that no power is needed to operate the sensor. There are two major types of fibre optic sensor, intrinsic and extrinsic; the former uses optical fibre as the actual sensor element whilst the latter use fibres to relay signals from the sensor to a processing unit.

Intrinsic fibre optic sensors utilise the properties of glass and the fibre geometry to measure a number of different quantities. Essentially they measure how light passes through the sensing fibre; then convert this light behaviour into a unit, such as pressure or temperature. They can be used in a variety of different situations; here are just four of the common types of fibre optic sensor used today.

• Strain sensing is a frequent application for this type of sensor as they are capable of measuring mechanical strain directly. This is measured through the ways the geometric properties of the fibre change under strain; this is manifested as a change to the light refractive properties of the fibre.
• Temperature sensing is carried by a practically identical sensor set up. The one difference is that to assess the temperature, the expansion or contraction of the fibre is measured. In this instance, instead of mechanical strain being the force, it is how the fibre is directly affected by changes in temperature that is measured.
• Pressure sensing can be carried out through the same measurement of expansion and contraction. Typically, a fibre will be inserted into a rigid steel tube with a diaphragm at the end. This diaphragm, coupled to the fibre acts as the sensor.
• Sensors are also used to measure humidity. These are slightly different from other sensors as they do not take their measurements from the direct strain placed upon the fibre. Instead, a special coating capable of absorbing and releasing water vapour is used; this coating is attached to the fibre and as it either expands or contracts in correlation to the water vapour it holds, it strains the fibre, changing its refractive properties and subsequently provides a measurement.

These four uses for fibre optic sensors are just some of the most common. Today sensors are used throughout the world in a variety of industries, from biomedical applications and aircraft to petroleum drilling sites.

In a wide variety of industries fibre optic sensors are used as a reliable and agile sensing solution. It is the specific properties of fibre optics and their ability to work remotely without power that makes them popular for a range of industrial and medical applications.

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