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What is a Fiber-Optic Sensor?

Mal Baxter
Mal Baxter

A fiber-optic sensor is a technology that uses light to conduct analyses of physical properties in remote sensing applications. Intrinsic sensors use the optical fiber itself as the sensing element, while extrinsic sensors use fiber-optic cables to transmit light information from a remote sensor to electronic processors. Light can be used to detect changes in many physical and chemical material properties to analyze factors such as temperature, pressure, or vibration. These sensors work in various industrial and scientific contexts, such as downhole measurement of oil wells or as hydrophones for sonar and seismic applications. Lightweight fiber-optics possess many inherent advantages over preceding technologies, including small, lightweight sizes, electromagnetic resistance, durability, and instantaneous and accurate transmission of information.

Optical fibers conduct light from sources such as lasers or light-emitting diodes (LED) through cylindrical dielectric waveguides. These fibers allow light to reflect in a wave form with minimal loss over even great distances. The fiber is made of a dielectric core surrounded by a cladding layer and coated by a jacket; these fibers may be bundled together into thicker cables. A fiber-optic sensor provides rugged performance in extreme conditions that prohibit direct observation; these can include dangerous and remote areas such as inside engines or explosive and corrosive environments.

Woman holding a book
Woman holding a book

Intrinsic sensors might measure the flow of material through gaps of up to one meter in the light path. This material makes certain changes to the quality of the light, which can then reveal vital information for analysis. Variations in optical path length allow measurements of intensity, polarization, phase, and other wavelength characteristics of light. The fiber-optic sensor propagates the modulated environmental effects using light sources and detectors. Additional readings such as strain, temperature, and viscosity are possible.

Optical and electronic sensors can also use optical fibers to carry information to electronic processors. These extrinsic-type sensors might be specifically designed for certain harsh conditions: for example, reading temperatures inside jet engines and transformers, where heat or electromagnetic fields prohibit other methods of measurement. An extrinsic fiber-optic sensor typically uses a multimode fiber that can allow multiple wavelengths, or light beams, for more complex information transmission. Information from an electronic sensor is converted through an optical transmitter and carried over optical lines to its destination base.

The simplest type of fiber-optic sensor is known as the intensive type; it measures intensity modulation. Spectral sensors measure light modulated by an environmental effect and are used to capture information relating to light properties such as radiation, fluorescence, and absorption. Interferometric sensors function as solid state fiber-optic gyros in aerospace, navigation, and mining. The medical field utilizes fiber-optic sensors to obtain blood gas and dosage information; these passive sensors are low-cost, mass produced, and appear to present no ill effects for patients. Fiber-optic smart structures are embedded into manufactured materials and large-scale structures like bridges and dams; applications continue to be developed for existing and new technologies.

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