Enhancement of the Dynamic Range in Slope-Assisted Coherent Brillouin Optical Time-Domain Analysis Sensors

We present two techniques that provide an extension of the dynamic range of coherent Brillouin optical time-domain analysis (BOTDA) sensors for dynamic measurements. These types of BOTDA sensors rely on self-heterodyne detection of a phase-modulated probe wave, and the dynamic range for fast measurements is limited to the linear region of the radio-frequency (RF) phase-shift spectrum measured. The first method for range extension that we introduce is based on launching pump pulses containing multiple frequency components. This makes the Brillouin spectra generated by each component overlap, providing a wider linear region of the detected RF phase-shift spectrum and allowing measurement of larger Brillouin frequency shift variations. The second method relies on shortening the length of the pump pulses, which leads to the broadening of the detected RF spectra. The theoretical fundamentals of both range enhancing techniques are presented. Moreover, we experimentally demonstrate that they provide a threefold to fourfold enhancement in the dynamic range. Finally, the factors limiting their performance are determined: For the multi-frequency pump pulse technique, it is the worsening of Kerr nonlinear effects due to the simultaneous propagation of multiple spectral components in the fiber, and for the pulse-shortening method, it is the signal-to-noise ratio (SNR) penalty linked to the reduction of the magnitude of the Brillouin interaction.

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