Limits of BOTDA Range Extension Techniques

Abstract

Abstract-Brillouin-based temperature and strain sensors have attracted great attention of both the academic and industrial sectors in the past few decades due to their ability to perform distributed measurements. Particularly, Brillouin Optical Time Domain Analysis (BOTDA) systems have been applied in many different scenarios, proving particularly useful in those requiring especially wide coverage ranging extremely long distances, such as in civil structure monitoring, energy transportation or environmental applications. The extension of the measuring range in these sensors has therefore become one of the main areas of research and development around BOTDA. To do so, it is necessary to increase the Signal to Noise Ratio (SNR) of the retrieved signal. So far, several techniques have been applied in order to achieve this goal, such as pre-amplification before detection, pulse coding or Raman amplification. Here, we analyze these techniques in terms of their performance limits and provide guidelines that can assist in finding out which is the best configuration to break current range limitations. Our analysis is based on physical arguments as well as current literature results.Index Terms-Brillouin scattering, distributed optic fiber sensing, distributed Raman scattering, optical fibers, optical pulse coding. I. INTRODUCTIONMONG the different available techniques to perform distributed measurements of strain and temperature, Brillouin Optical Time-Domain Analysis (BOTDA) [1,2] is presently recognized as one of the most consolidated. This fiber sensing technology is widely used in different application domains (civil engineering, pipelines, fire detection, etc.). BOTDA technology finds its root on the non-linear optical effect occurring in optical fibers and called Stimulated Brillouin Scattering (SBS) [3]. SBS is an acousto-optic process that manifests as a narrowband amplification of a probe beam when an intense coherent pump light beam is introduced through the opposite end of a single-mode fiber. The distributed feature is provided to the BOTDA by pulsing the pump wave and analyzing the retrieved probe wave as function of the time-of-flight of the pump pulse within the fiber, as depicted in Fig.1.Range and resolution are two of the most important features of these systems, which are normally limited to 20-30 km with 1-2 meter resolution in standard systems [4]. Attenuation, an intrinsic fiber feature, is the limiting factor in terms of range as it reduces the intensity of the signals within the fiber, therefore decreasing the Signal to Noise Ratio (SNR) as the monitored distance increases. Resolution, which is set by the spatial length of the employed pump pulses, is restricted by the associated issues when the pulse duration is reduced. First, a spectral broadening will manifest on the retrieved wave (leading to a higher uncertainty) and also a shorter interaction length will be obtained among the pump and probe wave, leading to a corresponding SNR decrease. Both factors lead to a non-proportional increase of ...

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