Distributed chemical detection outside standard coated fibers using Brillouin optical time domain analysis of cladding modes spectra

Propagation in the cladding modes of standard optical fibers enables the sensing of chemicals outside the fiber boundary, where light in the single core mode cannot reach. Coupling to the cladding modes typically relies on the inscription of permanent gratings, which restricts the operation of the sensors to point measurements only. In addition, most applications rely on bare, uncoated fibers, which are difficult to deploy outside the research laboratory. In this work, we report the spatially distributed analysis of cladding mode spectra in a standard, off-the-shelf coated fiber. The inscription of the gratings, removal of the coating, or other structural modifications are not required. Coupling is based on Brillouin dynamic gratings: Two optical pump fields stimulate an acoustic wave, which couples a probe field to a counter-propagating cladding mode. Spatial mapping is obtained through time-of-flight analysis: pulsed modulation of one pump wave and the monitoring of the output probe power as a function of time. All fields are launched and detected only in the core mode. The coupling spectrum is sensitive to local changes in the refractive index of the coating layer, to the third decimal point. The spatial resolution is one meter. The demonstrated range is a few meters, and is scalable to hundreds of meters. The technique is used to detect and monitor the local immersion of a fiber section in acetone. The results establish a practical method for spatially distributed fiber optic chemical sensors.