Optomechanical time-domain reflectometry

Abstract

Optical fibres constitute an exceptional sensing platform. However, standard fibres present an inherent sensing challenge: they confine light to an inner core. Consequently, distributed fibre sensors are restricted to the measurement of conditions that prevail within the core. This work presents distributed analysis of media outside unmodified, standard fibre. Measurements are based on stimulated scattering by guided acoustic modes, which allow us to listen where we cannot look. The protocol overcomes a major difficulty: guided acoustic waves induce forward scattering, which cannot be mapped using time-of-flight. The solution relies on mapping the Rayleigh backscatter contributions of two optical tones, which are coupled by the acoustic wave. Analysis is demonstrated over 3 km of fibre with 100 m resolution. Measurements distinguish between air, ethanol and water outside the cladding, and between air and water outside polyimide-coated fibres. The results establish a new sensor configuration: optomechanical time-domain reflectometry, with several potential applications.

Invited Article: Distributed analysis of nonlinear wave mixing in fiber due to forward Brillouin scattering and Kerr effects

“…However, the effective stimulation of guided acoustic waves requires the use of a narrowband, coherent light source. To work around this difficulty, experiments were repeated over 512 choices of the central optical frequency  , between wavelengths of 1558 nm and 1560 nm, and traces were averaged with respect to  [36]. The ensemble average over many central optical frequencies helps reduce noise due to coherent Rayleigh back-scatter, and provides a mapping of local power levels along the fiber under test, similar to that of incoherent OTDR.…”

“…4 The observations contribute a more complete description of nonlinear wave mixing in fiber, for a more general case where the Kerr effect is not the exclusive underlying mechanism. On top of the interest to basic research, the results are also relevant to emerging new concepts of distributed fiber-optic sensing that are based on F-SBS [36][37]. These measurement schemes allow for the analysis of media outside the fiber, where light cannot reach [36][37].…”

“…The mapping of F-SBS using Rayleigh back-scatter is at the basis of a new concept for distributed fiber-optic sensing, which we refer to as opto-mechanical time-domain reflectometry [36]. The protocol is based on Eq.…”

“…However, as discussed in Section 2, the analysis of dualtone traces is only valid when no other spectral field components exist in the fiber. The nonlinear wave mixing generation of spectral sidebands currently restricts the input power levels and the measurement signal-to-noise ratio, range and resolution [36].…”

Recent Progress in Fast Distributed Brillouin Optical Fiber Sensing

“…The optical fiber is easy to access to the all-optical network in adverse working conditions thanks to its lightweight and compact structure while maintaining high resolution, high sensitivity, and a wide measurement range. Then, physical parameters outside of the fiber can also be obtained by measuring the acoustic impedance along the fiber, which has been demonstrated in recent reports [2,3]. Because of the above merits, a variety of optical fiber sensors are developed for modern industry.…”