Abstract
Distributed optical fiber sensors have gained, in the last decade, considerable attention in structural and environmental monitoring due to a definite number of advantages including the immunity to electromagnetic interferences, the high sensitivity, the small size and the multiplexing and remote interrogation capabilities [1]. In particular, those based on stimulated Brillouin scattering (SBS) allow, through the so-called Brillouin Optical Time Domain Analysis (BOTDA), to measure deformation and temperature profiles by means of a telecommunications grade optical fiber cable for very long distances, up to tens of kilometers [2, 3]. Stimulated Brillouin scattering is a non-linear process occurring in optical fibers at relatively low power levels and gives rise to the conversion of a small fraction of the incident power to backscattered light at a lower frequency [4]. Brillouin scattering arises from acoustic waves guided in the optical fiber. When a pump wave and a frequency down-shifted counter-propagating probe wave are simultaneously injected into the fiber, their interference generates an acoustic wave through electrostriction, and the Bragg diffraction induced by the acoustic wave subsequently scatters the pump wave into the probe wave. Maximum probe gain occurs for a precise value of the pump-probe frequency shift, named Brillouin frequency shift, which depends on the strain and the temperature of the optical fiber [5]. BOTDA is a method that generates Brillouin gain at a specified location along the optical fiber, by pulsing the intensity of the pump wave. Probe amplification is recorded as a function of time, since the instant of launch of the pulsed pump, for a selected numbers of pump-probe frequency shifts. Acquisition time is then converted in fiber locations, by using the group velocity of the optical pump. Standard BOTDA method offers good accuracy and long sensing range.
© 2014 Optical Society of America
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