Optical-Fiber Sensors: An Overview
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Optical-Fiber Sensors: An Overview D.J. Webb Abstract This article provides an overview of the field of optical-fiber sensing, including a brief introduction to the properties of optical fibers that make them suitable for material characterization and monitoring. Some of the recent developments in the field are described, with an emphasis on Bragg grating sensors, multiplexed systems, and chemical sensing, as well as the new field of microstructured fiber. Keywords: distributed sensors, glass fiber, fiber Bragg gratings, multiplexed sensors, optical-fiber sensors, waveguides.
Introduction: Historical Development Optical measurement techniques have been around for more than a century, but the current renaissance in optical sensing had its birth in the 1970s with the development of low-loss, high-quality, optical-fiber waveguides for the telecommunications industry. Prior to that time, with few exceptions precise optical measurements had been confined to the metrological laboratory, where large optical tables were often needed to maintain components in accurate alignment. By the end of the 1970s, it was increasingly being realized that optical fibers could be used to link the optical source and detection electronics to the measurement region, even permitting the use of the most sensitive interferometric techniques in hostile environments such as wind tunnels and under the sea. By the early 1980s, the increasing effort devoted to developing optical communications technology was reducing the price of components such as lasers, detectors, and couplers, further fueling interest in optical measurement techniques. To a certain extent, the technology was oversold around this time, with many people sharing the view that optical techniques would replace most conventional means of monitoring. Of course, this has yet to happen, and it is unlikely that optics will ever compete on price with the humble strain gauge or thermistor, at least for the foreseeable future. The initial research had focused on the use of optical sensors to monitor physical parameters, such as temperature and pres-
MRS BULLETIN/MAY 2002
sure, but in the 1980s people increasingly realized that optical-fiber sensors also had potential in the fields of chemical and medical sensing. These remain promising areas of research to this day. A major development occurred in 1989 when a simple technique for producing fiber Bragg gratings (FBGs) within optical fibers was invented. Bragg gratings are dealt with in more detail later in this article, but put simply, they are a local modification to the internal structure of the fiber that causes it to reflect light of essentially one wavelength while allowing all other wavelengths to pass. The wavelength reflected varies, depending on the temperature or strain to which the fiber in the region of the grating is subjected; thus by launching light from a large-bandwidth optical source into the fiber and monitoring the reflected wavelength using some sort of spectrometer, the strain or temperature at the grating may be ded
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