Abstract
The distributed sensor is proven to be a powerful tool for civil structural and material process monitoring. Brillouin scattering in fiber can be used as point sensors or distributed sensors for measurement of temperature, strain, birefringence and vibration over centimeters (Brillouin grating length) for point sensor or the pulse length for the distributed sensor. Simultaneous strain and temperature measurement with a spatial resolution of 20 cm is demonstrated in a Panda fiber using Brillouin grating technique with the temperature accuracy and strain accuracy of 0.4 °C and 9 μɛ. This technique can also be used for distributed birefringence measurement. For Brillouin optical time domain analysis (BOTDA), we have developed a new technique to measure differential Brillouin gain instead of Brillouin gain itself. This technique allows high precision temperature and strain measurement over long sensing length with sub-meter spatial resolution: 50-cm spatial resolution for 50-km length, using return-to-zero coded optical pulses of BOTDA with the temperature resolution of 0.7 °C, which is equivalent to strain accuracy of 12 μɛ. For over 50-km sensing length, we proposed and demonstrated frequency-division-multiplexing (FDM) and time-division-multiplexing (TDM) based BOTDA technique for 75-km and 100-km sensing length without inline amplification within the sensing length. The spatial resolution of 2 m (100 km) and Brillouin frequency shift accuracy of 1.5 MHz have been obtained for TDM based BOTDA and 1-m resolution (75 km) with Brillouin frequency shift accuracy of 1 MHz using FDM based BOTDA. The civil structural health monitoring with BOTDA technique has been demonstrated.
Highlights
For over two decades, distributed optical fiber sensors based on Brillouin scattering have gained much interest for their potential capabilities of monitoring temperature [1] and strain [2] in civil and structural engineering, environmental monitoring, and geotechnical engineering
Stimulated Brillouin scattering (SBS) enhances the Brillouin scattering in a Brillouin optical time domain analysis (BOTDA) [3,4,5,6,7,8] with intense signal and better spatial resolution comparing with a spontaneous scattering based Brillouin optical time domain reflectometry (BOTDR) [9,10,11]
Xiaoyi BAO et al.: Recent Progress in Optical Fiber Sensors Based on Brillouin Scattering at University of Ottawa sensor, the pulsed light is used as a pump and has a higher frequency over continuous-wave (CW) light, so that the CW light (Stokes wave) experiences gain through SBS process; on the contrary, in the case of Brillouin loss, the CW light used as a pump experiences loss after propagating in the sensing fiber and the pulsed light gains energy whose frequency is lower than that of the CW light
Summary
For over two decades, distributed optical fiber sensors based on Brillouin scattering have gained much interest for their potential capabilities of monitoring temperature [1] and strain [2] in civil and structural engineering, environmental monitoring, and geotechnical engineering. For a short-range fiber, with low pump depletion in Brillouin gain and small excess amplification on the probe pulse in the Brillouin loss, two techniques give the similar results. In a long-ranged BOTDA system via Brillouin gain, the maximum input power of CW light is severely limited by the pump depletion of pulsed pump light due to SBS threshold, which decreases Brillouin signal and induces measurement errors at the far end of the fiber, and shortens the measureable sensing length. In a Brillouin-loss-based system, the excess amplification on the probe pulse induced by CW pump can distort Brillouin spectrum and produce measurement errors at the far end of the sensing fiber. As the CW is detected in DC (direct current) mode with high contrast due to the low pump power and low DC offset, the broadband detection maintains the high spatial resolution
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