Distributed cladding mode fiber-optic sensor

TL;DR

This paper introduces a novel dynamic, random-access fiber-optic sensor that enables distributed measurement of surrounding media with high resolution and accuracy, using stimulated Brillouin gratings without permanent inscribed structures.

Contribution

It presents a new method for dynamic, distributed coupling between core and cladding modes in standard fibers, enabling point measurements at arbitrary locations without permanent gratings.

Findings

Achieved 2 m sensing range with 8 cm resolution.

Successfully distinguished water from ethanol outside the fiber.

Measured local refractive index with an accuracy of 0.0004 to 0.004 units.

Abstract

The analysis of substances outside the cladding has challenged the optical fiber sensors community for decades. A common solution relies on the propagation of light in cladding modes. However, the coupling of light to/from these modes is typically based on permanent gratings in specific locations, which restrict the sensors to point measurements only. In this work, we present dynamic, random-access coupling of light between core and cladding modes of standard fibers, in arbitrarily located short sections. Coupling is based on the stimulation of Brillouin dynamic gratings by two coded pump waves and intermodal scattering of a third optical probe wave. All waves are launched and collected inthe core mode. No permanent gratings are inscribed. Distributed sensing of surrounding media is demonstrated with 2 m range and 8 cm resolution. Measurements distinguish between water and ethanol outside the fiber. The measurement accuracty of the local index outside the cladding is 0.004 - 0.0004 refractive index units.