Point-measurements of Forward Brillouin Scattering in Optical Fibers

Abstract: Point-analysis of mechanical impedance outside the cladding of a multi-core fiber is demonstrated for the first time, assisted by fiber Bragg gratings. Measurements distinguish between air and water outside a centimeter-long fiber section.

Light propagating in optical fibers may stimulate transverse acoustic wave [1]. The acoustic wavefronts propagate outward from the core towards the cladding boundary, undergo partial reflection and propagate back inward. The acoustic reflection depends on the mechanical impedance of the surroundings. Acoustic oscillations can be monitored through photo-elastic perturbation of optical probe waves [2], and provide estimates of mechanical impedance outside the cladding where light does not reach [3]. The principle was recently used in integrated measurements over tens of meters of fiber [3], and distributed analysis with tens of meters resolution [4,5]. However localized, point-measurements, which are the most ubiquitous form of fiber sensing, are yet to be achieved. Previous reports involved either external transduction or external monitoring of the phenomenon [6,7].

In this work we report opto-mechanical point-sensing outside the cladding, using light that is guided within a commercially-available multi-core fiber. Guided acoustic waves are stimulated by modulated pump light at the inner, on-axis core of the multi-core fiber. The acoustic waves introduce opto-mechanical crosstalk among the constituent cores of such fibers [8]. The photo-elastic perturbations alter the reflectivity spectrum of a fiber Bragg grating that is inscribed in an outer, off-axis core of the same fiber. Although the magnitude of index modulation is on the order of 10^-9 RIU, the measurement protocol is sensitive enough to resolve them. Results identify the spectrum of the stimulated acoustic emission in the few-cm long uncoated region of the fiber. The spectrum is markedly different from that of the rest of the fiber, which is coated with standard coating. Measurements also distinguish between air and water outside the grating. The multiplexing capabilities of fiber Bragg gratings can accommodate quasi-distributed sensing of ambient acoustic impedance over commercially-available fibers.

[1] Shelby, R. M. Physical Review B 31.8 (1985): 5244.

[2] Kang, M. S. Nature Physics 5.4 (2009): 276.

[3] Antman, Y. Optica 3.5 (2016): 510-516.

[4] Bashan, G. Nature communications 9.1 (2018): 2991.

[5] Chow, D. M. Nature communications 9.1 (2018): 2990.

[6] Koehler, J. R. APL Photonics 1.5 (2016): 056101.

[7] Liang, Y. 26th International Conference on Optical Fiber Sensors (2018): WB3.

[8] Diamandi, H. H. Optica 4.3 (2017): 289-297.