Fiber Optic Sensors
In the past two decades, the fiber optic sensor has developed from the experimental stage to practical applications. For instance, distributed fiber optic sensors have been installed in dams and bridges to monitor the performance of these facilities. With the rapid advent of optical networks, the cost of fiber optic sensors has substantially dropped because of the commercially viable key components in fiber optic communications such as light sources and photodetectors. We anticipate that fiber optic sensors will become a widespread application in sensing technology.
This text covers a wide range of current research in fiber optic sensors, although it is by no means complete. Each of the 10 chapters is written by an authority in the field. Chapter 1 gives an overview of fiber optic sensors that includes the basic concepts, historical development, and some of the classic applications. This overview provides essential documentation to facilitate the objectives of later chapters.
Chapter 2 deals with fiber optic sensors based on Fabry–Perot interferometers. The major merits of this type of sensor include high sensitivity, compact size, and no need for fiber couplers. Its high sensitivity and multiplexing capability make this type of fiber optic sensor particularly suitable for smart structure monitoring applications.
Chapter 3 introduces a polarimetric fiber optic sensor. With polarization, a guided lightwave of a particular fiber can be changed through external perturbation, which can be used for fiber sensing. Thus, by using a polarization-maintaining fiber, polarization affecting the fiber can be exploited for sensing applications. One of the major features of this type of sensor is that it offers an excellent trade-off between sensitivity and robustness.
Chapter 4 reviews fiber-grating-based fiber optic sensors. Fiber grating technology (Bragg and long-period gratings) is one of the most important achievements in recent optic history. It provides a powerful new component in a variety of applications including dispersion compensations and spectral gain control (used in optics communications). In terms of fiber optic sensor applications, in-fiber gratings not only have a very high sensitivity but also provide distributed sensing capability due to the easy implementation of wavelength division multiplexing.
Chapter 5 introduces distributed fiber optic sensors. One of the unique features of fiber optic sensors is the distributed sensing capability, which means that multiple points can be sensed simultaneously by a single fiber. This capability not only reduces the cost but also makes the sensor very compact. Thus, many important applications such as structure fatigue monitoring (e.g., monitoring the performances of dams and bridges) can be implemented in an effective way. Both continuous and quasi-distributed sensors are discussed. The continuous type of distributed sensor is based on the intrinsic effect existing in optic fibers (such as Rayleigh scattering). The most widely used type is optical time domain reflectrometry (OTDR), which has become an indispensable tool for checking the connections of optics networks.
Chapter 6 discusses fiber specklegram sensors. Fiber specklegram is formed by the interference between different modes propagated in the multimode optics fibers. Since this interference is common-mode interference, it not only has a very high sensitivity for certain environmental perturbations (such as bending) but also has less sensitivity to certain environmental factors (such as temperature fluctuations). Thus, this is a very unique type of fiber optic sensor.
Chapter 7 introduces interrogation techniques for fiber optic sensors. This chapter emphasizes the physical effects in optic fibers when the fiber is subjected to external perturbations.
Chapter 8 focuses on fiber gyroscope sensors. First, the basic concepts are introduced. The fiber gyroscope sensor is based on the interference between two light beams propagated in opposite directions in a fiber loop. Since a large number of turns can be used, a very high sensitivity can be realized. Second, more practical issues related to fiber optic gyroscopes such as modulation and winding techniques are reviewed. It is believed that fiber optic gyroscopes will be used more and more in many guiding applications (such as flight by light) due to the consistent reductions in their cost.
Chapter 9 introduces fiber optic hydrophone systems. This chapter focuses on key issues such as interferometer configurations, interrogation=demodulation schemes, multiplexing architecture, polarization fading mitigation, and system integration. Some new developments include fiber optic amplifiers, wavelength division multiplexing components, optical isolators, and circulators.
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