In this paper, we demonstrate the use of single-mode Cobalt-doped fibers for active distributed temperature sensing with optical heating. A high-power EDFA at 1550nm is used for optically heating the fiber, while a Brillouin Optical FrequencyDomain Analysis (BOFDA) set-up operating at 850nm is employed to monitor the corresponding temperature changes. Owing to the use of a dual wavelength scheme, longer sensing distances can be reached even with relatively high doping concentrations. As a proof of concept, we demonstrate the possibility to distinguish two 1-m fiber sections, one immersed into water and another one surrounded by air.
Brillouin fiber sensors permit to perform distributed temperature and strain measurements along an optical fiber through the changes of the Brillouin Frequency Shift (BFS). In conventional Brillouin schemes operating in the time-domain, the BFS is detected by analyzing the interaction between a pulsed pump wave, and a counter-propagating probe wave. The duration of the pump pulse determines the spatial resolution, which however is limited to about 1m due to phonon lifetime. Alternatively, Brillouin optical frequency-domain analysis (BOFDA) and Brillouin optical correlation-domain analysis (BOCDA) configurations provide cm-scale, or even mm-scale distributed sensing capabilities. In particular, BOFDA sensors make use of a continuous wave (CW) pump wave with superimposed a small-signal modulation. The analysis consists in determining, by use of a vector network analyzer (VNA), the amplitude and phase of the corresponding modulation induced on the probe wave intensity, over a discrete number of modulation frequencies. In BOFDA sensors, spatial resolution can be improved down to the cm-range or mm-range, thanks to the pre-activation of the acoustic wave involved in the scattering process. In this work, we show that mm-scale spatial resolution can be exploited, in a BOFDA configuration, to perform both physical (temperature) and chemical (refractive index) sensing. For the latter, a sidepolished fiber was used in order to make the BFS sensitive to the surrounding refractive index (SRI). A sensitivity of the BFS to the SRI as large as 293 MHz/RIU at n<sub>sm</sub> = 1.40 is demonstrated experimentally and validated numerically.
A new scheme for fast distributed sensing based on Brillouin Optical Time-Domain Analysis (BOTDA) is proposed and demonstrated, based on the use of a frequency swept microwave source for the generation of the probe wave. The entire BOTDA measurement is taken within the duration of the frequency sweep, at a frequency granularity depending on the duration of the sweep, the repetition rate of the pump pulses and the number of averages.
We propose the application of a distributed optical fiber sensor based on stimulated Brillouin scattering, as an integrated system for safety monitoring of railway infrastructures. The strain distribution was measured dynamically along a 60 meters length of rail track, as well as along a 3-m stone arch bridge. The results indicate that distributed sensing technology is able to provide useful information in railway traffic and safety monitoring.
We report an experimental study on a cantilever beam, aimed to verify the feasibility of modal analysis by distributed
Brillouin sensing for structural damage identification. Damage identification was carried out for three defect cases,
analyzing the changes of the natural frequencies and mode shapes of the first two bending modes. Comparison with finite element method (FEM) analysis shows that the damage can be detected and localized, within the limitation dictated by the spatial resolution (30 cm) of our sensor.