Abstract
According to recent models, non-local effects in dual-probe-sideband Brillouin Optical Time Domain Analysis (BOTDA) systems should be essentially negligible whenever the probe power is below the Stimulated Brillouin Scattering (SBS) threshold. This paper shows that actually there appear non-local effects in this type of systems before the SBS threshold. To explain these effects it is necessary to take into account a full spectral description of the SBS process. The pump pulse experiences a frequency-dependent spectral deformation that affects the readout process differently in the gain and loss configurations. This paper provides a simple analytical model of this phenomenon, which is validated against compelling experimental data, showing good agreement. The main conclusion of our study is that the measurements in gain configuration are more robust to this non-local effect than the loss configuration. Experimental and theoretical results show that, for a total probe wave power of ~1 mW (500 μW on each sideband), there is an up-shifting of ~1 MHz in the Brillouin Frequency Shift (BFS) retrieved from the Brillouin Loss Spectrum, whereas the BFS extracted from the measured Brillouin Gain Spectrum is up-shifted only ~0.6 MHz. These results are of particular interest for manufacturers of long-range BOTDA systems.
Highlights
Brillouin-based distributed temperature and strain sensors are extremely attractive in civil engineering thanks to their unique capabilities to monitor large linear infrastructures [1, 2]
The measurements have been performed over ~50 km of single-mode fiber (SMF) with an essentially homogeneous Brillouin Frequency Shift (BFS) located at 10.865 GHz at the pump wavelength (~1550 nm)
The separation from the BFS frequency leads to an increase in the pulse energy content, which is larger when the modulation frequency is above the BFS
Summary
Brillouin-based distributed temperature and strain sensors are extremely attractive in civil engineering thanks to their unique capabilities to monitor large linear infrastructures [1, 2]. When an intense coherent beam is introduced in the fiber, SBS leads to the generation of two spectral signatures in the counterpropagating direction: a frequency down-shifted gain spectrum (Stokes band) and a frequency up-shifted attenuation spectrum (anti-Stokes band) [6]. The interrogation of this process in a distributed manner requires the use of two different counter-propagating light beams in the fiber. The gain/loss at each point is maximized when the pump-probe frequency separation matches exactly the BFS at that position By scanning for this maximum at all the positions, one can obtain a map of the BFS of the fiber across its whole length
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