Ultra-compact Vernier humidity sensor with high sensitivity and millisecond response based on photocurable hydrogel and a graft-type self-growing method

A novel, to the best of our knowledge, ultra-compact Vernier humidity sensor is proposed based on photocurable hydrogel and a graft-type self-growing method. Inspired by grafting techniques in botany, the optical cavity is captured and coupled into the structure with the self-growth of the waveguide based on beam excitation. The sensor integrates the previously required multiple optical paths of the Vernier structure into a tiny probe on the end face of an optical fiber. Experiments demonstrate that the sensor possesses ultrahigh sensitivity of 5.12 nm/%RH in the high RH range of 11.2%–98%. The tiny cone diameter endows the sensor with an ultrafast time response of 23 ms and 221 ms. Moreover, the specially designed sensor can exhibit mechanical properties, enabling the distinction between static and dynamic moisture as an actuator similar to insect antennae. The sensor provides a new route for the next generation of research on complex chimeric probe devices and is of great significance for the preparation of micro-composite probes and the single-point measurement of humidity.