Condensed Matter Physics Seminar: Fast, Distributed and High Spatial Resolution Brillouin Sensing

Abstract:

Fiber-optic sensing has already proven itself as an effective way to monitor strain and temperature in Structural Health Monitoring (SHM), as well as in many security applications. The Brillouin effect in optical fibers is now widely utilized for strain/temperature monitoring of quite a few structures such as long gas pipes, undersea electrical cables, tunnels and more. In Brillouin Optical Time Domain Analysis (BOTDA) a pump pulse wave is launched into one end of a sensing fiber, nonlinearly amplifying a counter-propagating CW probe wave launched from the fiber opposite side. By scanning the relative optical frequency between the pump and probe waves, the frequency of maximum gain can be determined, from which the values of the local strain/temperature are extracted. Classical implementations of BOTDA are slow, limiting them to quasi-static scenarios. The two main factors that control the sensing speed are the switching speed of the optical frequency scanning mechanism and the number of averages. Fast BOTDA (F-BOTDA) method, that was developed by our team, practically eliminated the first factor by enabling an almost instantaneous (~ns instead of ~ milliseconds) frequency transitions. In short length fiber sensing (<1Km) removing the measurement polarization dependence is the main reason for averaging. 

We present a polarization-independent fast frequency-scanning BOTDA technique, capable of acquiring the full Brillouin gain spectrum (BGS) at a speed limited only by the fiber length and the employed frequency scanning granularity. This new technique combined with the Fast-BOTDA method helps carrying classical BOTDA technique into the dynamic sensing domain.

Experimental demonstration of our technique showing a polarization-independent measurement along with comparison to classical BOTDA technique is presented. For dynamic demonstration, 500Hz vibrations of a last 2m of a 145m long fiber are captured at a measurement rate of 11300 BGSs per second with a spatial resolution of 1.5m (15ns pump pulse). Improvement of the spatial resolution to 10cm is shown by combing our method with the known Double Pulse Pair BOTDA technique. 

Supervisor: Prof. Moshe Tur

Seminar Organizer: Prof. Yoram Dagan