סמינר בחומר מעובה: 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 Brillouin Gain Spectrum (BGS) can be measured from which the frequency of maximum gain, the so called Brillouin Frequency Shift (BFS) can be extracted. The Brillouin interaction can be harnessed for the purpose of distributed sensing due to the fact that the BFS is linearly dependent on both the local strain/temperature along the fiber. Classical implementations of BOTDA are slow, limiting them to quasi-static scenarios. 

Three main factors that control the sensing speed are the number of scanning frequencies, the switching speed of the optical frequency scanning mechanism and the number of averages. 

The first factor is practically eliminated by our Double Slope-Assisted BOTDA method which in addition to superb sensing speeds also offers an almost double dynamic range and immunity to pump pulse power variation.

Our novel Ultimate-BOTDA method significantly reduces the influence of the second and third factors as it offers almost instantaneous (~ns instead of ~ milliseconds) frequency transitions and practically removes the measurement polarization dependence, which is the main reason for the need for averaging.

The talk will describe these two dynamic sensing methods and present experimental demonstrations (lab and real world scenarios) of their ability to monitor 10's of meters of optical fiber in a distributed manner (10cm spatial resolution) at ground breaking sampling rates (10's of kHz) with a large dynamic range (1000's of microstrains) which when combined all together can facilitate a world record measurement speed of millions of resolved spatial resolution points per seconds 

Furthermore, a newly discovered physical property of the Brillouin amplification process, we named the Brillouin GDLS (Gain Dependent Line Shift) effect, will be presented and discussed.

מארגן הסמינר: ד"ר ערן סלע