WGM resonator on optical fibre tips for in-situ temperature and salinity monitoring

In-situ monitoring of salinity and temperature variations is essential for environmental pollution surveillance, marine hydrology and other dynamic liquid analysis scenarios. Conventional methods such as using thermometers and electrochemical approaches often face challenges such as environmental interference and relatively bulky device structures, limiting their applicability in dynamic detection scenarios. Optical fibre sensing strategies feature high sensitivity and miniaturisation; however, their interface tunability is constrained by the intrinsic material uniformity of optical fibres. Herein, a whispering-gallery-mode (WGM) optical microcavity platform based on a seven-core optical fibre was developed via two-photon polymerisation to enable in situ, high-sensitivity detection of temperature (-22.31 pm/K from 292 to 338 K) and salinity (249.37 pm/% over a broad range) within a single miniature device. High-quality polystyrene (PS) microsphere cavities fabricated on the fibre tips considerably enhanced light-matter interactions while maintaining excellent anti-interference capability and system stability. Under complex aqueous conditions, the optimised microcavity retained highquality factors in the order of 103, with long-term stability. We believe that this technology holds great promise for the development of next-generation miniaturised, highly integrated and multi-physical parameter optical sensing platforms, providing a robust and reliable solution for medical diagnosis and other applications requiring precise and dynamic detection capabilities.