Accurate Brillouin frequency shift measurements through secondary interaction compensation

Distributed optical fiber sensors based on stimulated Brillouin scattering (SBS) allow temperature and strain measurements at multiple points along a fiber. Conventional interrogation methods based on a pulsed pump fail to reach a cm-scale spatial resolution due to the acoustic response time of the silica fiber. Instead, the Brillouin Optical Frequency Domain Analysis (BOFDA) technique achieves a finer resolution by pre-activating the acoustic wave involved in the scattering process. Unfortunately, BOFDA also suffers from artifacts caused by the “secondary interaction” between the pump and the sidebands of the acoustic wave. These artifacts introduce systematic errors in the estimate of the Brillouin frequency shift. To correct these distortions, earlier proposed methods based on either iterative numerical compensation or high-pass filtering have some drawbacks, such as a long processing time or a degradation of the SNR. Here, we propose a method based on the direct measurement of the secondary interaction, achieved through the injection of a double-sideband-modulated pump with a suppressed carrier. Under these conditions, the SBS interaction between the sidebands of the pump and those of the acoustic wave induces a frequency-doubling modulation in the probe intensity. By acquiring such modulation, one can estimate the secondary interaction signal and subtract it from the original BOFDA signal to mitigate systematic errors. Theoretical and experimental results validate the proposed technique.