Fiber Optics Sensors
Fiber optic technology started in 1970’s, for long distance telecommunications, and it has experienced an exponential growth during the last four decades. Sensing applications are a small spin-off from this technology, taking profit of developments done f
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Department of Aerospace Materials and Processes, Polytechnic University of Madrid, Madrid, Spain † Aerospace Engineering Research Group, Pontificia Bolivariana University, Medell´ın, Colombia
Introduction
Fiber optic technology started in 1970’s, for long distance telecommunications, and it has experienced an exponential growth during the last four decades. Sensing applications are a small spin-off from this technology, taking profit of developments done for optoelectronic components and concepts. Optical fibers are cylindrical dielectric waveguides for the propagation of the light, made out from high purity, low loss optical materials, usually silica (Optical fibers made from polymers and other transparent materials are also commercially available). The refractive index of the core (about 1.46 for silica) is slightly higher than surrounding material or cladding, due to the presence of dopants. Light is confined at the core, because optical rays arriving to the interface core/cladding with an angle higher than the total reflectance angle, as defined by the Snell law, will follow total reflections and will remain confined at the core. Only when the fiber is locally bent with a sharp radius, the light may escape. The OF is externally protected from mechanical scratches with a polymeric coating (the ‘buffer’), and frequently several optical fibers are bundle together, and assembled with high strength mechanical fibers, such as Kevlar, to make a robust product known as optical cable, that can withstand rough industrial manipulation. Glass optical fibers have an external diameter of 125 µm (human hair diameter range from 50 to 100 µm), and classify in two main groups: monomode and multimode optical fibers. Monomode OF, of small core (about 10 µm , depending on the intended optical wavelength). The electromagnetic waves traveling by the core must satisfy the Maxwell equations; the cylindrical contour conditions only allow for a discrete number of solutions, V, dependent on the core diameter, and the wavelength. When V is less than 2.4, only one mode (2 orthogonal W. Ostachowicz, J. Güemes (Eds.), New Trends in Structural Health Monitoring, CISM International Centre for Mechanical Sciences, DOI 10.1007/978-3-7091-1390-5_5, © CISM, Udine 2013
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A. Güemes and J. Sierra-Pérez Core n2 c m
=kn1senc
n1
kn1
Cladding
Figure 1. Schematic representation of an optical fiber.
polarizations) propagates through the optical fiber, it is called monomode. Monomode OF offer some advantages for optical communication, such as: – Optical attenuation is smaller for monomode OF, due to the lower difference of refractive index between core and cladding, requiring a lower concentration of dopants. – Multimode OF will suffer group dispersion due to different modes traveling at different speed. The multimode optical fibers have as only advantage a larger core diameter (30 to 100 µm). The larger core makes simpler the alignment with the optical sources and connections. They are preferred when used only as guides for light, as in many
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