4D-printed hydrogel Fabry–Pérot fiber sensors with dual-mode humidity sensing for high-sensitivity detection and touchless morse code recognition

Currently, non-contact gesture interaction primarily relies on visual, infrared, and ultrasonic approaches. In contrast, humidity-sensing-based schemes enable silent human-computer interaction with high privacy and without the need for external excitation. Here, we propose a multistage amplified optical fiber humidity sensor based on a 4D-printed bilayer hydrogel Fabry–Pérot interferometer (FPI). Relative humidity (RH) variations drive directionally controllable differential swelling of the bilayer hydrogel, thereby modulating the FPI cavity length. A push–pull Vernier mechanism further amplifies the optical response, achieving a maximum sensitivity of 37.8 nm/%RH for precise humidity measurement. The humidity variation induced by a finger can be detected through direct intensity demodulation of a single FPI, enabling non-contact Morse code recognition. In addition, the sensor achieves a detection distance of up to 30 mm, with response and recovery times of approximately 0.22 s and 0.42 s, respectively, in a single test. A dual-finger parallel input strategy is further demonstrated to enhance the encoding efficiency. This work provides a compact and highly sensitive optical fiber platform for silent input and non-contact human–machine interaction.