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Department of Engineering, ICT and Technologies for Energy and Transport Patent Title Method for measuring dynamic strain along an optical fiber based on the stimulated Brillouin scattering between two optical pulses, and relevant apparatus. Ref. CNR 10047 Assignee(s): CNR CNR Institute: IREA Main Inventor: Romeo Bernini Countries: IT Priority date: 06/11/2009 Abstract The present invention refers to a technique for measuring the dynamic strain along an optical fiber, based on the stimulated Brillouin shif scattering between two optical pulses. The proposed approach is based on the stimulated Brillouin shif scattering ­ available also in standard telecommunication fibers ­ and permits to performs distributed vibration measurements. The present invention is related also to an experimental apparatus which implements the technique and which has been used in order to carried out some experimental tests up to a frequency of 100Hz and with a spatial resolution of 3m along a 30m long fiber. Background Nowadays, distributed fiber optic sensors based on stimulated Brillouin scattering are widely used for distributed strain monitoring. However, the developed sensors require measurement time that range from several seconds to minutes, so they could be used only for a static strain monitoring. Technology The method uses two optical pulses counter­propagating in an optical fiber with a delay T. The relative delay between the two pulses is set in order to obtain a spatial overlap of the two optical pulses in correspondence of the fiber section dynamically strained. In this way, any vibration­induced modulation of the local Brillouin frequency shif, due to a dynamic strain, will be easily measured as an intensity variation of the Stokes pulse peak intensity, as it emerges from the sensing fiber. Advantages and Applications The main advantage of the proposed techniques is that, using a single optical fiber, it’s possible to monitoring the static strain along the fiber and measure the dynamic strain in specific fiber section. This section ­ along the fiber ­ can be changed by simply varying the relative delay between the two optical pulses. The invention can be used in large civil infrastructure monitoring as bridges, highways, railways and pipelines. Development stage The proposed approach has been validated by some experimental laboratory tests up to a frequency of 100Hz and with a spatial resolution of 3m along a 30m long fiber. 75