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.
Highlights
The use of Brillouin-based distributed fiber sensors has increased considerably in the past few years thanks to their ability to sense strain and temperature in a distributed way over many tens of kilometers with meter-scale resolutions
Of the different features that all these implementations have, the Brillouin frequency shift (BFS) of the fiber is always retrieved by fitting a linear curve to the central spectral region of the Brillouin phase spectrum (BPS). This has allowed the implementation of reliable sensing systems based on the phase response of the stimulated Brillouin scattering (SBS) induced in the sensing fiber [5,6,7,8,9,10,11,12], instead of using the classical SBS gain response typically measured by conventional Brillouin optical time domain analysis (BOTDA) sensors
The results prove that, under conventional scanning conditions and using a large enough number of scanned spectral points, the quadratic fitting of the Brillouin gain spectrum (BGS) confidently provides a better accuracy than using a linear fitting over the BPS
Summary
The use of Brillouin-based distributed fiber sensors has increased considerably in the past few years thanks to their ability to sense strain and temperature in a distributed way over many tens of kilometers with meter-scale resolutions. This has allowed the implementation of reliable sensing systems based on the phase response of the SBS induced in the sensing fiber [5,6,7,8,9,10,11,12], instead of using the classical SBS gain response typically measured by conventional BOTDA sensors It is still unclear under which conditions the linear fitting of the BPS provides better (or worse) accuracy to the BFS determination when compared to the use of the classical quadratic fitting over the BGS. The approach of using a linear fitting of the BPS can result in a better accuracy under very specific conditions, especially when a narrow frequency range has to be scanned around the BFS, enabling for instance very high-resolution or faster measurements
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