Discriminative Sensing of Acoustic Impedance and Temperature Based on Spatial-Division Multiplexed Forward Brillouin Scattering in Multicore Fiber

A novel method for discriminative sensing of acoustic impedance and temperature based on space-division multiplexing (SDM) and forward Brillouin scattering (FBS) in multicore fiber (MCF) is proposed and experimentally validated. And it is based on different responses of radial acoustic mode (<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</i><sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0,m</sub>) and torsional-radial acoustic mode (<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">TR</i><sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2,m</sub>) to the acoustic impedance and temperature. The linewidth sensitivity coefficients corresponding to a <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</i><sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0,m</sub> mode of the central core and the frequency shift sensitivity coefficients corresponding to the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">TR</i><sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2,m</sub> modes of the central and outer cores in a seven-core fiber were measured. The new phenomena is that the frequency shift-acoustic impedance coefficient of the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">TR</i><sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2,m</sub> mode in the outer core is 6.4 times higher than that in the central core. Therefore, we adopted a combination of the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</i><sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0,m</sub> mode in the central core and the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">TR</i><sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2,m</sub> mode in the outer core, enabling a discriminative measurement of temperature and acoustic impedance. Particularly, the simultaneous use of the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</i><sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0,m</sub> mode in the central core and the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">TR</i><sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2,m</sub> mode in the outer core exhibited a low acoustic impedance measurement error of 0.002 kg·s<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">–1</sup>·mm<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">–2</sup> and a temperature error of 0.1 °C. Moreover, the accuracy of measuring acoustic impedance is 3 times higher than that of large effective area fiber (LEAF). The proposed sensor offers significant advantages, including simplicity of fabrication, large transmission capacity, and high measurement accuracy, making it a promising platform for discriminative acoustic impedance sensing applications.