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The impact of Raman scattering and modul ation instability is studied in Brillouin time-domain analysis systems. It turns out to be very detrimental for long-range sensing as a result of the extended interaction length combined to the high pump peak pulse power. The conditions under which these effects limit the sensing range are determined and the modeling is very well confirmed by experimental results. Keywords: Fibre optics, optical fibre sensor, stimulated Brillouin scattering, distributed fibre sensor, non-linear optics, stimulated Raman scattering, modulation instability. 1. INTRODUCTION The recent raising interest in Brillouin fiber sensors for application in civil engineering, in oil & gas industry, in perimeter security and in intrusion detection has stimulated the research efforts to extend the detection range. In time-domain distributed Brillouin sensors (BOTDA) pulses are used to interrogate locally the interaction along the fiber. In order to achieve better performances in terms of range and spatial resolution, pump pulse and signal powers must be raised. These powers cannot be made arbitrarily large and must be kept below the observation threshold of any other noise-fed nonlinear effects. In these systems the most critical nonlinear effects are those in which a signal wave is amplified through a forward interaction, in other words when the background noise is amplified while co-propagating with the pump pulse. In such a situation the interaction length may cover many tens of kilometers and even a weak nonlinear amplification much less efficient than stimulated Brillouin scattering (SBS) may result in a large amplified spontaneous signal that eventually depletes the pump. The id entified nonlinear effects are the modulation instability (MI), which occurs only in fibers presenting an anomalous dispersion at the pump wavelength, and the Raman scattering (RS). A spectral self-broadening of the pump due to MI was observed in sensors based on spontaneous Brillouin scattering |
