Polymeric Microwire-Enabled Fiber Bragg Grating for Simultaneous Humidity and Temperature Sensing

Humidity monitoring is of great significance in various applications ranging from food safety, agricultural and industrial fields, to environmental monitoring. Here, we propose and fabricate an ultracompact fiber-optic sensor comprised of fiber Bragg grating (FBG) and polymeric microwire on the fiber end face for simultaneous sensing of humidity and temperature. By only a one-step fiber drawing process, photocurable polymeric microwire can be conveniently fabricated on the fiber end face, showing excellent advantages including optical multimode interference in microwire, direct interaction between optical mode and surrounding environment, and thermal and humidity effects of polymeric adhesive. In the potential applications, polymeric microwire demonstrates a high relative humidity sensitivity of 360.8 pm/%RH and temperature sensitivity of −120 pm/°C. Line-by-line FBG, inscribed around polymeric microwire, demonstrates temperature sensitivity of 10 pm/°C, and has no response on the surrounding humidity perturbation because of its location in the fiber core. Thus, humidity monitoring could be extracted from microwire-based humidity measurement through excluding the accompanying temperature effect via FBG element, finally realizing simultaneous sensing of humidity and temperature.