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Fiber Optics for Load Testing
Published in Eva O.L. Lantsoght, Load Testing of Bridges, 2019
Joan R. Casas, António Barrias, Gerardo Gutiérrez Rodriguez, Sergi Villalba
There are two main Brillouin-based DOFS techniques that have been used in the past few years. First, there is the Brillouin optical time domain reflectometry (BOTDR) technique, which uses the spontaneous Brillouin scattering and was the first to be used for strain and temperature measurements in civil engineering infrastructure. Then, there is the Brillouin optical time domain analysis (BOTDA) technique, which is based on stimulated Brillouin scattering amplifying the usually very weak Brillouin backscattering through the use of counter-propagating lasers (Bao and Chen, 2012;Galindez-Jamioy and López-Higuera, 2012;Leung et al., 2013).
Pervasive Fibre-Optic Sensor Networks in Bridges: A UK Case Study
Published in Nigel Powers, Dan M. Frangopol, Riadh Al-Mahaidi, Colin Caprani, Maintenance, Safety, Risk, Management and Life-Cycle Performance of Bridges, 2018
L.J. Butler, M.Z.E.B. Elshafie, C.R. Middleton
Distributed FOS sensors, on the other hand, have the ability to measure strain changes along the entire length of a fibre optic cable. Based on the optical time domain reflectometry (OTDR) technique, an optical pulse (laser light) is shone down an optical fibre where a photodetector measures the amount of backscattered light in the fibre. Knowing the speed of light, the time information is converted to distance. One particular backscattered light component known as Brillouin scattering, carries information about the temperature and amount of strain in the fibre based on its wavelength shift; this is known as Brillouin Optical Time Domain Reflectometry (BOTDR) (Glišić and Inaudi 2007). The BOTDR cable used as part of this study was a steel-reinforced single-mode low bend loss fibre. Strain values are averaged over a 50 cm length are taken at steps of 5 cm along the entire cable. Unlike FBG sensors, BOTDR based sensing is not suitable for capturing dynamic strain changes as measurement times of 5 minutes or greater are typical depending on the length of cable.
Optical Fiber Sensors for Smart Composite Materials and Structures
Published in Krzysztof Iniewski, Ginu Rajan, Krzysztof Iniewski, Optical Fiber Sensors, 2017
Manjusha Ramakrishnan, Yuliya Semenova, Gerald Farrell, Ginu Rajan
Distributed optical fiber sensors (DOFSs) are capable of providing a continuous measurand profile over the length of the optical fiber and thus are very promising for strain/temperature measurements in large structures such as bridges, buildings, and pipelines [25, 65, 66]. However, given the nature of composite structures, the length is normally limited to 80 m or less with a strain or temperature requirement of at least 1°C or 20 με with a few centimeters resolution. DOFSs are categorized into several types based on the sensing technology and the physical effect underpinning the operating principle: (1) optical time-domain reflectometry (OTDR) and optical frequency-domain reflectometry (OFDR), both based on Rayleigh scattering; (2) Raman optical time-domain reflectometry (ROTDR) and Raman optical frequency-domain reflectometry (ROFDR), both based on Raman scattering; and (3) Brillouin optical time-domain reflectometry (BOTDR) and Brillouin optical frequency-domain reflectometry (BOFDR), both based on Brillouin scattering [67].
Crack monitoring in reinforced concrete beams by distributed optical fiber sensors
Published in Structure and Infrastructure Engineering, 2021
Carlos G. Berrocal, Ignasi Fernandez, Rasmus Rempling
Raman scattering arises from the thermal vibration of the glass molecules in the fiber core as light travels through the fiber and is highly sensitive to temperature variations (Rodriguez, Casas, & Villalba, 2015b). Brillouin scattering is produced by the interaction of backscattered light and acoustic waves generated when changes in the density of the material occur as a result of thermal effects. Brillouin scattering is sensitive to external changes of both mechanical strain and temperature and despite its measuring range can reach lengths of up to more than 300 km (Gyger, Rochat, Chin, Niklès, & Thévenaz, 2014), the spatial resolution that can be achieved is often limited to several centimetres (Güemes, Fernández-López, & Soller, 2010). Rayleigh scattering, on the other hand, refers to the elastic distribution of light in all directions that happens when light interferes with local inhomogeneities in the fiber core that are smaller than the wavelength of the light itself. These inhomogeneities are caused by fluctuations in the density and composition of the fiber core, which makes Rayleigh scattering sensitive to both mechanical strain and temperature changes (Palmieri, 2013). Systems based on Rayleigh scattering are currently limited to a measuring range of up to 2 km, but in exchange they provide an unprecedented spatial resolution that can go down to the the sub-millimetric scale, thereby offering new possibilities for the development of damage detection systems (Rodriguez et al., 2015b).
Optical fibre-based sensors for distributed strain monitoring of asphalt pavements
Published in International Journal of Pavement Engineering, 2018
The distributed optical fibre sensor based on Brillouin scattering is induced by the interaction between light and acoustic phonons propagation in the fibre core. The use of Brillouin scattering for distributed sensing is termed Brillouin optical time domain analysis (BOTDA) by interrogating the frequency shift of the backscattered light in the fibre. The basic approach involves two laser sources, a pump pulse laser source into one end of the fibre and a probe laser source into the other end (Zhang et al.2007). The frequency difference between the two lasers can be equal to the Brillouin frequency shift, and the back Brillouin scattering is simulated (Bao and Chen 2011). External strain or temperature along the fibre will cause Brillouin frequency shift, which is found to be a linear relationship. BOTDA is the sensing interrogator using the linear relationship between the Brillouin frequency shift and strain or temperature to obtain the distributed value of strain or temperature along the fibre (Zhao and Ansari 2013). The functions are expressed as
Distributed fiber optics sensors for civil engineering infrastructure sensing
Published in Journal of Structural Integrity and Maintenance, 2018
Under an electrical field, silica tends to be compressed because the molecules are attracted toward the high optical power area to increase the potential energy, and this causes the change of density in the optical fiber, called electrostriction (Boyd, 2008). That is, electrostriction reconstructs the distribution of medium density. Brillouin scattering is therefore the interaction of the photons of the incident light with the acoustic phonons of the host material (Alasia, 2006; Brillouin, 1922). Similar to the Raman scattering, the photons can gain or lose energy, generating a Stokes/anti-Stokes wave and absorbing or creating acoustic phonons (donor or receiver) (Chandrasekharan, 1965; Mandelberg & Witten, 1962). The acoustic wave, which travels at about 6 km/s in the optical fiber, includes a large range of frequencies by different phonon energies. However, only the frequencies satisfying the Bragg Law condition give the Brillouin scattering (Al Ahbahi, 2005). The vibrational frequencies of acoustic phonons in the host material by scattering are in the order of 11 GHz and depend on density, pressure, and temperature of the material.