16.3.16

You are invited to attend a lecture

By

Or Danon

(M.Sc. student under the supervision of Prof. Moshe Tur)

School of Electrical Engineering, Tel-Aviv University, Tel-Aviv 69978, Israel

Fast and distributed optical fiber strain sensor using novel Brillouin sensing techniques

Fiber-optic based sensors are a promising technology for implementing Structural Health Monitoring (SHM), as well as many other security applications. Sensors based on the Brillouin effect are already used to monitor strain/temperature variations in long gas pipes, submarine cables, tunnels etc. In sensors based on the concept of Brillouin Optical Time Domain Analysis (BOTDA), the Brillouin Gain Spectrum (BGS) of the fiber is interrogated by launching a probe continuous wave (CW) and a pump pulse, with frequency difference around the Brillouin Frequency Shift (BFS). The probe wave gain time dependency is then translated into spatial dependency of the Brillouin gain along the fiber for the specific frequency difference between the probe and the pump waves. In conventional BOTDA, the BGS is interrogated at multiple points so it can be fully reconstructed. This approach, albeit robust, is time consuming as each reading is limited by twice the Time of Flight (TOF) of the pulse in the fiber. A novel technique suggested by our team called Slope Assisted BOTDA (SA-BOTDA) enables to overcome this limitation by interrogating the BGS at only one frequency, on its slope. The BFS is then calculated by knowing the local slope of the BGS.

The SA-BOTDA method however suffers from several drawbacks. First, in distributed sensing, the BFS may vary significantly along the fiber, causing the singular frequency chosen for interrogation to result in zero gain from some areas of the fiber. Second, changes in the pump power, due to various reasons such as polarization fading, coupling losses, depletion etc., cannot be separated from changes in the BFS, as only one parameter is measured.

In this talk we present recent advances which overcome these limitations. The variation of the BFS along the fiber is countered by varying the probe frequency during the travel of the pump pulse, a method called Tailored Probe BOTDA (TP-BOTDA). We demonstrate first experimental results for real time measuring of a vibrating bent cantilever beam, a realistic and challenging scenario applicable for SHM of UAVs, using state of the art measurement system developed within this thesis. We also present experimental demonstrations of techniques which use Double Slope Assisted BOTDA (DSA-BOTDA) to overcome the dependence of the slope on the pump power.

Wednesday, March 16, 2016, at 13:00

Room 011, Kitot Building