Using pulse with a dark base to achieve high spatial and frequency resolution for the distributed Brillouin sensor

Abstract

A stimulated Brillouin scattering based distributed optical fiber sensor using a probe pulse (<5 ns) of finite extinction ratio (ER) (>20 dB) followed by a dark base of a finite length is proposed to achieve higher spatial and frequency resolution for the first time. The basic mechanism for detecting the small stress or temperature section is to reduce the peak height of the Brillouin spectrum contributed by dc at the stress point so that the Brillouin frequency shift (BFS) of small stress or temperature section can show up at a much lower peak height of the Brillouin spectrum. The finite ER of the positive pulse is used as prepumping of the phonon field to achieve higher contrast for the Brillouin spectrum. The length of dark base is determined by the balanced contribution of the normal base in fiber and the dark base after the pulse to the Brillouin gain spectrum. The theoretical simulation and the experimental results both demonstrate that the proposed novel pulse shape can be used to measure a centimeter stress or temperature section with small uncertainty for the BFS. For two 5 cm stress sections of 15 MHz equivalent strains in BFS with a 30 cm separation, the measured frequency uncertainty is 0.9 MHz.