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The concept of distributed sensors was first introduced in the 1980s with the development of the optical time-domain reflectometer (OTDR) and its application to the sensor field. OTDR refers to the use of a single optical fiber as a linear continuous sensor, providing multiple measurement points distributed spatially in a continuous sequence over the entire fiber length as depicted in Fig. 18.1. A single optical fiber cable can, de facto, replace thousands of traditional single-point sensors such as thermocouples or strain gauges, saving complex installation, calibration, and maintenance costs. As a result, temperature and/or strain information is made available through profiles as a function of distance in one shot, providing simultaneous temperature/strain information of thousands of locations with a single instrument. More recently the measurement of acoustic and vibrational signals has also been performed with these distributed architectures. The geometry of optical fibers makes them ideal candidates for long-range distributed sensors. Furthermore, the wide range of commercially available fiber cable embodiment offers suitable solutions for various monitoring applications, even in the case of extreme environmental conditions (robust design for rough conditions, resistant to corrosion, compatible with temperature range from 0 K to 970 K, intrinsically safe in hazardous environments, etc.). Standard fiber optic cables offer decades of guaranteed lifetime even in demanding environments such as for subsea applications. Distributed sensing techniques are based on the recording and the analysis of the scattered light at every location along the fiber. Light scattering is associated with inhomogeneities in the propagation medium. Different types of inhomogeneities give rise to different scattering phenomena, ranging from density fluctuation generating Rayleigh scattering (both coherent and incoherent) to thermally excited acoustic waves (or phonons) and molecular vibrations causing Brillouin and Raman scattering. The details of the scattering processes discussed here have been fully covered in Chapter 6.
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