Functional coatings on optical fibers enable selective detection of environmental and chemical parameters, but their use is typically limited to point or quasi-distributed sensing due to localized deposition techniques. In this work, we demonstrate a possible transition towards full-length functional coatings on optical fibers using a draw tower process, enabling their potential use in distributed sensor technology. An optical fiber with Pt:WO<inf>3</inf> nanocomposite polymer functional coating is employed as a proof of concept. The results demonstrate the successful application of this functional coating along hundreds of meters of fibers using a draw tower. When integrated into a distributed sensing configuration, the Pt:WO<inf>3</inf> fiber exhibited a clear change in response with varying hydrogen concentrations from 1% to 4% H<inf>2</inf>, with a temperature increase of 2.5 °C at 4 vol.% indicating a promising performance for distributed hydrogen leak detection. This approach opens new opportunities for applying other functional coatings over extended fiber lengths using draw towers, which could be exploited for novel distributed sensing applications

The paper describes the fabrication of two nanocomposite (NC) coated optical fibers, and their use to enhance the capabilities of distributed fiber sensing. The fibers are produced in a conventional optical fiber draw tower. NC coated fibers with reduced graphene oxide were shown to have enhanced mechanical durability at elevated temperatures and have a low humidity sensitivity compared to standard polyimide coated fibers. NC coated fibers with hydrogen-sensitive materials were shown to enable distributed hydrogen leak detection, in a configuration that is scalable to long lengths.

Polymer coating layers are essential for the handling and proper function of optical fibers in their service environment. The analysis and monitoring of the elastic characteristics of coating layers are important for materials research and development, quality assurance, and maintenance. Most measurement protocols are destructive, require specialty samples, and may only be carried out offline. In this work, we monitor the velocities of dilatational acoustic waves in several coating layers of standard fibers, using forward Brillouin scattering processes. The measurements are non-destructive and performed over working fiber. Velocities are measured in three polyimide-based coating layers as functions of temperature up to 220 °C. Thermal changes in velocity are identified with 1% precision. The results suggest that the incorporation of nanoparticles within the polymer coating matrix improves its thermal stability.

The optical fibre coating is essential to ensure high performance and reliability of the optical fibre. Out of all polymer-coated fibres, polyimide coatings provide the highest temperature rating, typically rated for use in optical fibre sensing applications at 300&#x02DA;C (in air), with short excursion to 350&#x02DA;C. In this communication, we assess whether the inclusion of graphene-based nanoparticles, such as graphene and graphene oxide, in a polyimide coating can enhance the durability of optical fibres at high temperatures. Draw tower fabrication of optical fibres with nanocomposite polymer coating is described. Tensile strength tests, performed on aged nanocomposite-coated optical fibres, are used as an indication of their performance at harsh conditions. The results are validated and quantified by distributed temperature and humidity sensing tests performed using these fibres. The results show that this novel class of fibre is more robust to high-temperature ageing and moisture-induced strain than standard polyimide-coated fibres, when used for distributed sensing. The electrical conductivity of the nanocomposite coating is also used in a multi-sensing approach, together with distributed optical fibre sensing, to measure temperature in a reliable way using the same optical fibre. 

Draw tower fabrication of a novel optical fibre with nanocomposite polymer coating is reported. Preliminary results show it is more robust to high-temperature ageing, and moisture-induced strain than standard polymer fibres when used for distributed sensing.