Performance of the distributed Brillouin sensor: Benefits and penalties due to pump depletion

Disaster prevention in civil infrastructures requires the use of techniques that allow temperature and strain measurements in real time over lengths of a few meters to tens of kilometres. The distributed Brillouin sensor (DBS) technique has the advantage to combine all these characteristics. The sensing mechanism of the DBS involves the interaction of two counter-propagating lightwaves, the Stokes and the pump, in an optical fibre. Spatial information is obtained through time domain analysis. The sensing data are recorded from the measurement of the pump depletion. We explore the benefits and the drawbacks of this approach and show that there is a power range for which the sensing performances are optima. To achieve that goal, Brillouin fibre generator (BFG) and amplifier (BFA) were studied leading to the derivation of a threshold definition for the BFA, which is the configuration of the DBS. Within that context, numerical and analytical models describing the stimulated Brillouin scattering (SBS) interaction are introduced and validated experimentally. Even if pump depletion is carefully controlled, the Brillouin spectrum shape, and hence the sensor performance, still depend on the sensing parameters such as power, pulse and fibre characteristics. We use a signal processing method grounded in the physics of Brillouin scattering. An analytical approximation, valid for the optimum sensing region, reconstructs the Brillouin spectrum distribution from input sensing parameters and measured data. These data are obtained with a spectrum analysis methodology, based on three original tools: the Rayleigh equivalent criterion, the lengthstress diagram, and the spectrum form factors. This methodology has been successfully used on experimental spectra. The DBS and the signal processing approach were then used to monitor the structural changes in steel pipes and in a composite column, all subjected to heavy loads. The DBS measured the strain distribution of those structures while they were stressed. The DBS provided detailed information on the structure's health at local and global level, associated with deformations, cracks and buckling. This work demonstrates that the DBS is capable of extracting critical information useful to engineers: engineer's experience and judgement in conjunction with appropriate data processing methods make possible to anticipate structural failures. Keywords: Brillouin scattering, optical fibres, SBS threshold, Brillouin generator, Brillouin amplifier, gain saturation, pump depletion, sensor performance, spatial resolution, frequency resolution, signal processing, structural health monitoring, distributed Brillouin sensor, strain measurement