Listening to a Fiber’s Surroundings with Light

Optical-fiber sensors can sense changes in strain and temperature—and, thus, detect cracks in bridges or abnormal temperatures in gas pipelines.  Now, researchers at the Swiss Federal Institute of Technology (EPFL), Lausanne, Switzerland, are extending the utility of fiber sensors with a method that uses light and sound to determine whether a fiber is in contact with a liquid or solid (Nat. Commun., doi: 10.1038/s41467-018-05410-2).

Looking outside the fiber

When an optical fiber is stretched, or when there’s a change in its temperature or another physical property, the intensity, phase, polarization, or wavelength of the light travelling along it change. Those parameters, however, can be used only to detect physical changes inside the fiber, and can’t sniff out changes in the fiber’s surroundings without the light escaping.  To address that, researchers have turned to an opto-acoustic effect known as forward stimulated Brillouin scattering (FSBS).

FSBS occurs when an intense optical wave propagating in a fiber stimulates transverse acoustic waves. Those acoustic waves bounce off the fiber walls, and their echoes can be detected in the optical signal exiting the fiber. The echoes vary depending on the material the fiber is in contact with, thus providing information about the acoustic impedance of a fiber’s surroundings. 

The problem until now has been that those echoes cannot be spatially resolved.  The authors of the new study, however, have demonstrated a time-based method both to detect changes in the fiber’s surroundings and to determine the location of those changes, achieving a spatial resolution of 15 m in a 730-m length of single-mode fiber. 

Spatial and temporal detection

The EPFL team’s technique measures the longitudinal phase evolution of light that’s perturbed by FSBS transverse acoustic waves. The approach begins by sending two optical pulses along a fiber—the first a long pulse that stimulates a transverse acoustic wave; the second a “reading pulse” in a different wavelength range. The acoustic wave modulates the phase of the reading pulse, generating sidebands on its spectrum.

The researchers analyzed the intensity progression of those sidebands and mapped their longitudinal evolution, mathematically retrieving the changes experienced by the reading pulse and the local response of the FSBS. They could then create FSBS spectra maps of the entire length of the optical fiber.

To test the technique, the team exposed a 30-m section of uncoated optical fiber to water, alcohol and air. The exposed section was clearly identifiable on the FSBS spectra map in each case, because the resonance from the transverse acoustic wave reflection at the fiber boundary was stronger there than in coated fiber.  The acoustic impedances of water and ethanol that the researchers calculated agreed well with standard values.

A new class of fiber sensors?

The EPFL group’s experimental results demonstrate that it’s possible to make acoustic-impedance measurements of a fiber’s surroundings without direct interaction between light and the external material.  In a press release accompanying the work, group leader Luc Thévenaz  said that the technique will “make it possible to detect water leakages, as well as the density and salinity of fluids that come into contact with the fiber,” among other potential applications.

The technique could provide advantages in fiber sensing in terms of reliability and ease of implementation, according to the researchers, who note that further work to reduce noise could help improve the spatial resolution of their technique down to the width of the reading pulse.  They are confident that improved configurations will emerge to take advantage of other sophisticated ways to retrieve distributed information along an optical fiber and that this work is a starting point for a new class of position-resolved sensors.