In-fibre micro-channel: its potential for in-fibre detection

Micro-channels (μ-channels) in microstructure fibres can be regarded as natural in-fibre flow channels. Thus, the advent of microstructure fibres with μ-channels makes it possible to realize in-fibre integrated microfluidic devices, and microstructure fibres with μ-channels provide the possibility of creating new online monitoring systems and define new concepts. Herein, we developed a novel compact in-fibre detection platform, which is combined with a μ-channel in a new-type microstructure fibre, side-hole fibre, for in-fibre detection. The optical component of this proposed in-fibre detection platform is made of a simple cross-axial open-cavity Fabry-Perot interferometer. This miniaturized cross-axial open-cavity Fabry-Perot interferometer is formed by a 45°-angled side-hole fibre, which is fabricated by a simple end-face polishing process. For a 45°-angled fibre, the incident optical axis can be steered based on total internal reflection at the oblique fibre-air interface, and the reflected light will enter the side-hole μ-channel, and the front and rear inner μ-channel walls form the cavity; the natural in-fibre μ-channel functions as a (liquid/gas) flow channel. Experimentally, this proposed in-fibre detection platform was fabricated, and its spectral characteristics were investigated. Its relative humidity characteristics and potential application in breath sensing were calibrated by measuring the evolution of the reflection spectrum. As a whole, the proposed detection platform demonstrates the advantages of simple structure, easy fabrication without additional sensitive materials, and potential application in breath sensing or lab-in-fibre technology.