Frequency shift estimation technique near the hotspot in BOTDA sensor.

Abstract

When studying the spatial resolution and Brillouin frequency shift (BFS) uncertainty in Brillouin optical time domain analysis (BOTDA) system with a hotspot, people usually focuses on the point with best performance in the hotspot and neglect the huge error around it. The error is caused by the Brillouin gain spectrum (BGS) contributed by both the hotspot and the ambient segment, which is distorted from standard Lorentzian curve. Due to the distortion of BGS, the estimated BFS near the hotspot is shifted from the actual value and results in BFS error along the fiber. The distorted BGS can be double-peak or single-peak curve that is dependent on both the length of the hotspot that contributes to BGS and the BFS difference between the hotspot and the ambient segment. A fitting technique considering the combined contributions of hotspot and ambient segment is proposed to recover the distorted BGS near the hotspot and evaluate BFS. It is demonstrated that BFS error around hotspot is greatly reduced compare to the conventional Lorentzian fitting and dual Lorentzian fitting schemes.