Optical fibers are mainly applied in telecommunications, which has significantly changed this industry. However, there is also a growing application of optical fibers in sensing applications for measurement. Many components associated with optical fibers are developed to sensing applications. One of the most significant components—fiber optic sensor (also known as optical fiber sensor) is now being widely used in sensing applications.
Fiber optic sensors are fiber-based devices to sense some quantities like temperature, pressure, vibrations, displacement, rotations or concentration of chemical species. The basic instrumentation required for sensor are optical source (often a single-frequency fiber laser), sensing element, optical detector and end-prossesing devices (optical-spectrum analyzer, oscilloscope). A block diagram of fiber optic sensor is showed in the following picture.
Fiber optic sensor offers a wide range of advantages, which makes it being applied in many field successfully. Its advantages are as following.
- Explosion proof: In fiber optic sensor, the primary signal is an optical. Therefore, there is no risk of spark or fiber.
- Immunity to electromagnetic interference: Since the fibers are composed of dielectric such as glass. The fiber optic sensor is immune to radio frequency and electromagnetic.
- Small size, light weight and flexible: This feature extends the applications of fiber optic sensor to many fields, like aircraft.
- High sensitivity: The optical fiber sensors are highly sensitive and have large bandwidth. When multiplexed into arrays of sensors the large bandwidths of optical fibers themselves offer distinct advantages in their ability to transport the resultant data.
- Remote sensing: With the availability of low loss optical fibers, the optical signal can be transmitted up to a long distance transmitted up to a long distance (10-1000m). Thus the remote sensing is possible with the optical fiber.
- Environmental ruggedness and resistant: The optical fiber are manufactured from non-rusting materials such as plastics or glasses, therefore, the fibers have excellent stability when in permanent contact with electrolyte solutions, ionizing radiation etc. Further the fibers can withstand high temperature as high as 350 ca. Special fibers can extend sensor operation beyond 350c to as high as 1200c.
- Compactness: With the availability of solid-state configurations (small size sources and detectors) it is possible to design a compact optical fiber sensor system.
Based on the sensor location, the fiber optic sensors are generally classified into two types, namely intrinsic and extrinsic fiber optic sensor.
Intrinsic fiber optic sensor can provide distributed sensing over very large distances, which is the most useful feature of it. In intrinsic fiber optic sensor, sensing takes place within the fiber itself. Only a simple source and detector are required. This sensor depends on the properties of the optical fiber itself to convert an environmental action into a modulation of the light beam passing through it. The basic concept of the intrinsic fiber optic sensor is showed in the picture below.
Extrinsic fiber optic sensors use an optical fiber cable, normally a multimode one, to transmit modulated light from either a non-fiber optical sensor, or an electronic sensor connected to an optical transmitter. In this sensor, the fiber may be used as information carriers that show the way to a black box. It generates a light signal depending on the information arrived at the black box. The black box may be made of mirrors, gas or any other mechanisms that generates an optical signal. These sensors are used to measure rotation, vibration velocity, displacement, twisting, torque and acceleration. The major benefit of these sensors is their ability to reach places which are otherwise unreachable. The best example of this sensor is the inside temperature measurement of the aircraft jet engine. The following picture shows the basic concept of the extrinsic fiber optic sensor.
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