HIGH-RESOLUTION LONG-RANGE DISTRIBUTED BRILLOUIN ANALYSIS USING DUAL-LAYER PHASE AND AMPLITUDE CODING
Stimulated Brillouin Scattering (SBS) is a non-linear effect which can couple between two optical waves counter propagating through a waveguide. Effective coupling occurs when the difference between the two optical frequencies matches a particular value known as the Brillouin frequency shift. The Brillouin shift is dependent on both temperature and strain, therefore mapping of the local Brillouin gain spectrum along standard fibers has been used in distributed sensing of both quantities for 25 years. Much effort is being dedicated to increasing the range and the number of addressed points of high-resolution SBS analysis, driven by structural health monitoring applications in the civil engineering, construction, aerospace and transportation sectors.
The most common type of Brillouin sensor is based on Brillouin optical time domain analysis, B-OTDA. Commercial B-OTDA instruments can provide a measurement sensitivity of 1 °C or 20 µe, over a range of 50 km and with a spatial resolution of 2-3 m. The resolution limit in B-OTDA stems from the lifetime of hyper-sonic acoustic waves in silica, which is of the order of 5 ns and corresponds to a spatial resolution no better than 1 meter. In order to improve resolution, a different method named Brillouin optical correlation domain analysis, B-OCDA, was proposed during the late 90's. In B-OCDA the buildup of the acoustic wave takes place only at discrete and narrow correlation peaks along the fiber, and pulse duration and resolution are effectively decoupled. The technique was expanded by our group to the phase coding of both optical waves, in order to introduce correlation peaks in the fiber. Using phase-coded B-OCDA, our group and collaborators showed a distributed measurement of a 1600 meter long fiber with a spatial resolution of 2cm.
In this research we present an improved Brillouin analysis scheme reaching a distance of up to 8.8km with a 2 cm resolution corresponding to 440,000 resolution points. This protocol employs both phase and amplitude coding techniques to localize the SBS interactions and improve the measurement signal-to-noise ratio. Furthermore, high-resolution Brillouin analysis protocols were implemented in the study of composite material beams, demonstrating their potential in real-world applications.
* Research was carried out towards the M.Sc. Degree in Electrical Engineering at Bar-Ilan University, with the supervision of Prof. Avi Zadok