Abstract-High spatial ( cm) and spectral ( MHz) resolution Brillouin sensing is realized with enhanced signal to noise ratio using a pre-activated acoustic field and an optical phase control over the interrogating pulse. Pre-activation of the acoustic field preserves the Brillouin natural linewidth and a differential gain technique extends the method to long ranges. Experimentally, fully resolved measurements of the Brillouin frequency shift of a 5 cm spot perturbation at the far end of a 5 km fiber have been performed with a frequency resolution of 3 MHz (2 ), using a 500 ps (5 cm) phase shift pulse.Index Terms-Brillouin echoes, distributed fiber sensor, fiber optics sensors, stimulated Brillouin scattering.

The impact of Raman scattering and modulation 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.

We show that the spectral broadening of the pump pulse through self-phase modulation in a time-domain distributed Brillouin sensor has a considerably detrimental effect in the measurement, especially in the case of long distances and high-resolution pulses. Using 30 ns pump pulses with peak power of 276 mW, self-phase modulation leads to a doubling of the effective gain linewidth after some 20 km, which is equivalent to a contrast loss of 2 dB in the measurement. The impact is higher for shorter pulses (higher resolution). The theoretical modeling is fully confirmed by experimental results. © 2011 Optical Society of America OCIS codes: 290.5900, 190.2640, 060.2370 For at least two decades, Brillouin fiber sensors have attracted great interest in the fiber-sensing community for their temperature and strain-monitoring capability [1,2]. In time-domain-distributed Brillouin sensors, pulses are used to interrogate the local interaction in the fiber. The accuracy on the measurand is scaled by the spectral spreading of the effective gain, which, at its turn, is given by the convolution between the pulse spectrum and the natural Brillouin gain spectrum (BGS). According to standard time-bandwidth relations, the Gaussian pulse is presumably the best candidate for this interrogation when compared to other profiles (rectangular, triangular). However, we show here that this is not the best choice when addressing long ranges, because this pulse shape leads to a significant spectral broadening of the BGS along the fiber. An observed broadening of the BGS was suspected to be caused by self-phase modulation (SPM) in an early work by Lecoeuche et al. [3] and by Izumita et al.[4] in a coherent optical time-domain reflectometer system. SPM leads to small phase chirps during intensity transitions in the pump pulse (leading and trailing edges) that eventually become important in long fibers. The frequency broadening associated with this phase modulation leads to a reduced peak gain and uncertainties in the determination of the Brillouin shift ν B , but it leaves the temporal intensity distribution of the pump pulse unchanged, and hence the spatial resolution is preserved. Although the former [3][4][5] works showed a correct intuition addressing qualitatively the issue, either no theoretical model was given or the model was incomplete [5].We present here a quantitative model of the detrimental impact of SPM supported by a clear experimental demonstration. Two optical pulses with different temporal profiles were judiciously chosen (rectangular and Gaussian) showing the same FWHM and carrying the same energy, to evenly study and compare the SPM impact on their spectrum. Then we clearly experimentally demonstrate the spectral broadening of the BGS due to SPM in a Brillouin distributed sensor in various conditions in terms of pump pulse temporal profiles, power, and width. The results are compared with a theoretical model showing good agreement. SPM is a consequence of the nonlinear Kerr effect in the fiber that results in an intensity-...

Abstract:In this paper we investigate the effect of microstructure irregularities and applied strain on backward Brillouin scattering by comparing two photonic crystal fibers drawn with different parameters in order to minimize diameter and microstructure fluctuations. We fully characterize their Brillouin properties including the gain spectrum and the critical power. Using Brillouin echoes distributed sensing with a high spatial resolution of 30 cm we are able to map the Brillouin frequency shift along the fiber and get an accurate estimation of the microstructure longitudinal fluctuations. Our results reveal a clear-cut difference of longitudinal homogeneity between the two fibers. References and links1. E. P. Ippen, and R. H. Stolen, "Stimulated Brillouin scattering in optical fibers," Appl. Phys. Lett. 21(11), 539-541 (1972). 2. M. Niklès, L. Thévenaz, and P. A. Robert, "Simple distributed fiber sensor based on Brillouin gain spectrum analysis," Opt. Lett. 21(10), 758-760 (1996). 3. L. Thévenaz, "Brillouin distributed time-domain sensing in optical fibers: state of the art and perspectives," Front.Optoelectron. China 3(1), 13-21 (2010). 4. L. Zou, X. Bao, and L. Chen, "Distributed Brillouin temperature sensing in photonic crystal fiber," Smart Mater.Struct. 14(3), S8 (2005

The spectral broadening of the pump pulse through self phase modulation in a time domain distributed Brillouin sensor is demonstrated to have a non-negligible detrimental effect, leading to a doubling of the effective gain linewidth after some 20 km in standard conditions. The theoretical modeling is fully confirmed by experimental results.

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Classical configurations of Brillouin distributed optical fibre sensors show a spatial resolution limited by the phonon lifetime to 1 meter. Novel approaches can substantially overcome this limit through the prior activation of the acoustic wave.