Opto-Mechanics of Single-Mode and Multi-Core Fibers
Optical fibers support guided acoustic modes. These modes are stimulated by optical waves, and induce scattering and modulation of light. These interactions are referred to as guided acoustic waves Brillouin scattering (GAWBS) . Here we describe a new application of GAWBS in fiber sensing, and extend the study of the effect to multi-core fibers (MCFs).
Optical sensors typically rely on absorption, index or scattering. These require spatial overlap between light and the test substance. Standard fibers do not provide such overlap. Hence, chemical sensors rely on photonic crystal fibers, or structural modifications. The transverse profiles of acoustic modes reach the outer cladding boundary. Acoustic oscillations are therefore affected by dissipation to the surrounding medium. We employ GAWBS in sensing of liquids outside unmodified, standard fibers . Acoustic waves are stimulated and monitored from within the core. The mechanical impedance of water and ethanol is measured with 1% accuracy. The method can distinguish between aqueous solutions of different salinity .
MCFs are often designed to exhibit weak coupling among cores. Nevertheless, we show that acoustic modes lead to opto-mechanical inter-core cross-talk in MCFs. Analytic expressions are derived for the magnitude and spectrum of inter-core, cross-phase modulation (XPM) that is induced by GAWBS. The spectrum consists of a series of narrowband resonances. The effect is experimentally observed in a commercially-available, seven-core fiber. Agreement between analysis and measurement is excellent. On resonance, the magnitude of opto-mechanical XPM is comparable with the intra-core Kerr effect.
Last, we employ GAWBS in a new electro-opto-mechanical radio-frequency oscillator. An optical pump stimulates guided acoustic modes, which modulate the phase of a co-propagating optical probe. The probe modulation is detected and fed back to drive the pump modulation. With sufficient feedback, stable, single-mode oscillations at acoustic resonance frequencies are achieved. No electrical filtering is required.
 R. M. Shelby, M. D. Levenson, and P. W. Bayer, "Guided acoustic-wave Brillouin scattering," Phys. Rev. B 31, 5244-5252 (1985).
 Y. Antman, A. Clain, Y. London and A. Zadok, "Optomechanical sensing of liquids outside standard fibers using forward stimulated Brillouin scattering," Optica 3, 510-516 (2016).