Development and application of the long-range distributed fiber sensor based on brillouin scattering

The limitations of long-range sensing (>20km) based on Brillouin optical time domain analysis (BOTDA) with centimeter spatial resolution, and high strain or temperature resolution include 1) the Modulation instability effect in abnormal dispersion fiber which limits thepulse power as pump or probe wave in the Brillouin gain or loss case; 2) the pump depletion in Brillouin gain case or excess amplification in Brillouin loss case could induce the Brillouin spectrum distortion which limits the CWpower. The coded pulse offers the alternative solution at low pump and probe power to improve the signal-to-noise ratio (SNR). In this chapter, the design of the long-range distributed Brillouin scattering fiber sensor has been discussed, the single pulse and multi-pulses in the coded pulse format have been used for measuring the Brillouin gain and the differential Brillouin gain with different limit on the sensing length and the spatial resolution. For the coded pulses, two most commonly used pulse formats: non-return-tozero (NRZ) and return-to-zero (RZ) are implemented for BOTDA, and it is found that RZ coded pulses offer minimum distortion in the time domain waveform and the Brillouin spectra while NRZ coded pulses introduce spatial broadening which reduces the spatial resolution. For SMF-28 fiber gain saturation occurs at much shorter length (<20km with 20ns coded pulses) due to high Brillouin gain while for 50km LEAF fibre with 20ns coded pulse, no gain saturation is observed due to low Brillouin gain in the fiber with more uniform Brillouin gain across the fiber length. To overcome the gain saturation nonuniform fibers is introduced by reducing effective stimulated Brillouin scattering (SBS) amplification length with different sections of the fiber of different Brillouin peaks, this allows 75km sensing length with 1m spatial resolution and 1°C temperature resolution without in-line amplifiers at single pulse operation. The field application of the distributed Brillouin sensor has been demonstrated for the temperature and strain measurement on concrete structures.