Fiber optic sensors are fiber-based devices that use optical fibers to detect certain quantities such as mechanical strain or temperature, concentrations of chemical species, acceleration, rotations, pressure, vibrations and displacements.
These sensors are mainly used in remote sensing applications. Most of the fiber optic sensors are multiplexed along the length of a fiber by using light wavelength shift for each sensor or by determining the time delay as light passes along the fiber.
Fiber optic sensor systems consist of three photoelectric sensing modes such as retroreflective, through-beam and diffuse reflective modes as these systems are operated based on photoelectric sensing technology. Individual and bifurcated sensors are two fiber optic sensing systems that are operated in these modes.
A bifurcated fiber-optic assembly used for both diffuse reflective and retroreflective sensing combines the emitter and the receiver cable assemblies to achieve detection. In the fiber optic through-beam mode, the individual sensing systems sense the desired quantity when the light beam that extends from the emitter to the receiver is interrupted.
Fiber optic sensors are resistant to electromagnetic interference, and they do not conduct electricity. Hence they can be used for applications that involve highly inflammable material or high voltage electricity.
Working Principle of Fiber Optic Sensors
Fiber optic sensors work based on the principle that light from a laser or any superluminescent source is transmitted via an optical fiber, experiences changes in its parameters either in the optical fiber or fiber Bragg gratings and reaches a detector which measures these changes.
A typical fiber optic sensor system consists of a fiber-optic cable connected to a remote sensor or an amplifier. The fiber optic cable consists of a glass or plastic core surrounded by a layer made of cladding material.
The difference in densities between the core and the layer enables the cables to act based on the total internal reflection principle, which states that the light striking a boundary between two components will be totally reflected without any loss in light energy. The reflected light is then transmitted to a sensor/detector that converts the light energy into an electrical signal.
Benefits of Fiber Optic Sensors
Fiber optic sensors are small and light weight. Resistant to high temperature and explosive environments, they possess electrically insulating material which also make them suitable for use in applications subject to high voltages and there are no risks of electrical sparks.
In addition to this fiber optic sensors are very resistant to electromagnetic and radio frequency interference. They are highly sensitive, have excellent range and resolution and multiplexing capabilities.
Applications
Fiber optic sensors are used in a number of different applications. In mechanical properties testing, fiber optical sensors are used to measure mechanical strain. They can also be used to measure acceleration, velocity, pressure, temperature and displacement.
In heritage structures, fiber optic sensors can be used to evaluate post-seismic damage, analyze cracks, monitor restoration and monitor displacement. Similarly in dams they can detect and monitor leakages, foundation defects and measure spatial displacement.
The video below shows ABB's adoption of fibre optic sensors in high voltage applications.

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