According to recent models, non-local effects in dual-probesideband 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 longrange BOTDA systems.

Abstract-Brillouin optical time domain analysis (BOTDA) relies typically on the interaction among two counter-propagating waves: 1) a pulsed pump wave and 2) a modulated probe wave. The modulated probe wave has typically two sidebands, located at ±ν B with respect to the pump frequency. Conventional systems detect the time-resolved gain/loss by detecting only the upper/lower wavelength sideband. In this letter, we show that BOTDA can strongly benefit from the use of balanced detection among the two sidebands. In particular, the detected signal can be doubled while the noise only grows by a factor of (2) 1/2 , leading to a (2) 1/2 signal-to noise ratio (SNR) increase. Moreover, any common-mode noise in the probe signal path (e.g., master laser noise, modulator drifts, and so forth) is eliminated, rendering the system more robust. We validate the principle by experimental results that highlight the benefits of the technique in terms of the SNR.Index Terms-Brillouin scattering, distributed optic fiber sensor, balanced detection, temperature sensor.

Received Month X, XXXX; revised Month X, XXXX; accepted Month X, XXXX; posted Month X, XXXX (Doc. ID XXXXX); published Month X, XXXXIn this paper, we present and demonstrate a novel technique for distributed measurements in Brillouin Optical Time-Domain Analysis (BOTDA) based on the use of the nonlinear phase-shift induced by stimulated Brillouin scattering (SBS). Employing a Sagnac Interferometer (SI), the position-resolved Brillouin Phase-shift Spectrum (BPS) along the fiber can be obtained, benefiting from the sensitivity to non-reciprocal phase-shifts of the SI scheme. This proposal simplifies the existing methods to retrieve the BPS distribution along an optical fiber since no phase modulation, no filtering and no high-bandwidth detectors are required. In recent years, Brillouin-based distributed temperature and strain fiber sensors have become strong competitors of conventional multipoint sensing systems thanks to their unique properties, among them, their ability to sense hundreds or thousands of points over a single optical fiber. Brillouin Optical Time Domain Analysis (BOTDA) systems [1], one of the most common techniques, has evolved into a consolidated fiber sensing technology that is widely used for temperature and strain monitoring over an extended distance range (several tens of kilometers) with meter-scale spatial resolution. Also, shorter distances can be monitored with sub-meter spatial resolutions.The underlying physical phenomenon of a BOTDA is the optical effect denominated Stimulated Brillouin Scattering (SBS) [2]. This effect is an acousto-optic process that couples light between two counter-propagating waves by means of an induced acoustic wave. The SBS is a nonreciprocal process since the photons are scattered only in one direction owing to the generated acoustic wave nature. In practical terms, SBS manifests as a counterpropagating narrowband amplification curve (in a spectral region around 0 -νB) and an attenuation one (in a spectral region around 0 + νB) when an intense and coherent pump light beam (0) is fed into one of the ends of the optical fiber. The Brillouin Gain Spectrum (BGS), shows a Lorentzian gain distribution and a Brillouin phase profile, called Brillouin Phase-shift Spectrum (BPS).Conventionally, BOTDA systems measure the Gain and Loss mechanisms of SBS, because they are straightforward to retrieve with direct detection. To provide the distributed characteristic to the BOTDA, the pump wave is pulsed and an amplified/attenuated probe signal is analyzed as a function of the time-of-flight of the pump pulse in the fiber. By fitting the gain/loss profile in every point, one is able to determine the Brillouin Frequency Shift (BFS, νB), from which the variations of temperature or strain can be extracted [3].Some previous works have also paid attention to measuring the distributed BPS [4][5][6][7]. All these systems employ complex phase modulation setups to obtain the probe signal and usually require very high-bandwidth photo-detectors and digitizing elements. Recently, anot...

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Abstract:We evaluate the Brillouin frequency shift (BFS) determination error when utilizing the Brillouin phase spectrum (BPS) instead of the Brillouin gain spectrum (BGS) in BOTDA systems. Systems based on the BPS perform the determination of the BFS through a linear fit around the zero de-phase frequency region. An analytical expression of the error obtained in the BFS determination as a function of the different experimental parameters is provided and experimentally validated. The experimental results show a good agreement with the theoretical predictions as a function of the number of sampling points, signal-to-noise ratio (SNR) and Brillouin spectral linewidth. For an equal SNR and linewidth, the phase response only provides a better BFS estimation than the gain response when the fit is performed over a restricted frequency range around the center of the spectral profile. This may reduce the measurement time of specific BOTDA systems requiring a narrow frequency scanning. When the frequency scan covers most of the Brillouin spectral profile, gain and phase responses give very similar estimations of the BFS and the BPS offers no crucial benefit.