Most fiber-optic sensors rely upon interaction of an external stress, strain, or temperature change on the light contained within the core of the fiber. The minute shifts imposed on the physical parameters of embedded fiber Bragg gratings (FBGs) that alter reflectance or the changes seen in Rayleigh backscatter of light within a standard single-mode fiber due to an external force are critical to sensing the characteristics of the external environment.
But a new distributed fiber-optic sensing capability is being developed by understanding the interaction of light with sound.1 Acoustic ultrasound signals that are stimulated from within the fiber do indeed interact with media outside its physical boundary, making it possible to sense materials and environmental conditions outside the fiber that do not interact with its core—sensing occurs by the fiber “listening” to what cannot be “seen.” The trick is to monitor the decay rates of the ultrasonic waves. Just like the Fresnel laws of optics, the transmission of sound waves towards the outside depends on the matching of mechanical impedance between the silica fiber and the surrounding substance. With better matching, acoustic transmission becomes more effective and the waves in the fiber decay more quickly.
F-SBS distributed sensing
The light/ultrasound interactions that form the basis of sensing outside the fiber are known as forward stimulated Brillouin scattering (F-SBS). The forward-scattering nature of the phenomenon, however, makes spatially distributed sensing rather difficult. By contrast, all known protocols for distributed fiber sensing rely on backscatter, which is readily localized. This challenge was successfully met by two parallel efforts at Bar-Ilan University (Ramat-Gan, Israel) and École Polytechnique Fédérale de Lausanne (EPFL; Lausanne, Switzerland).2, 3 The mapping of liquids outside the cladding boundary, over several kilometers and with tens-of-meters resolution, is now possible.
Progress, however, does not stop there. The coupling between optical and ultrasonic signals along the fiber gives rise to nonlinear wave mixing dynamics that are far richer than standard four-wave mixing driven by the Kerr effect. Kerr and F-SBS nonlinearities both induce and amplify Stokes and anti-Stokes sidebands, with intricate interplay. The nonlinear coefficient associated with F-SBS is on the order of 1 (W*km)-1, which is comparable with the magnitude of the Kerr effect.
Unlike the Kerr effect, however, F-SBS strongly depends on the exact frequency spacing between optical field components. Interactions between light and sound are characterized by sharp resonances, at frequencies of hundreds of megahertz and narrow linewidths that are only a few megahertz wide.
The entire nonlinear mixing dynamics were modeled analytically and observed experimentally by the Bar-Ilan team, who reported a breaking of symmetry that is not encountered when considering the Kerr effect alone (see figure). “We observe a continuous transfer of power from higher-frequency optical waves to lower-frequency ones,” says doctoral student Yosef London. “Consequently, Stokes-wave sidebands are being amplified much more effectively than their anti-Stokes counterparts. Normally, we do not see this.”
The measurement protocol was rather involved. Standard optical time-domain reflectometry was extended to include six spectral orders of nonlinear wave mixing, and the analysis of Rayleigh backscatter separated the contribution of each component. Agreement between analysis and measurement is, however, excellent.
The work is not strictly an exercise in nonlinear optics, though. “Until now, our sensing protocols had assumed that only two optical waves existed in the fiber,” London says. “We know that nonlinear wave mixing actually breaks this assumption. The observation of just two waves, as we did before, is limiting the range, resolution, and precision of F-SBS sensors. With quantitative modeling and measurement of multiple Stokes and anti-Stokes sidebands, as we now obtained, we look to improve performance by an order of magnitude.”
Sensing what is outside the fiber is promising for several potential applications, including leak detection in reservoirs, pipeline integrity, desalination plants, environmental studies in oceans and lakes, and more. The prospects of reaching hundreds of resolution points would help transfer this novel sensing approach from the research laboratory into the field.
1. Y. London et al., APL Photonics, 3, 11, 110804 (Aug. 15, 2018).
2. G. Bashan et al., Nat. Commun., 9, 2991 (2018).
3. D. Chow et al., Nat. Commun., 9, 2990 (2018).