Reflective High-Sensitivity Humidity Sensor Based on Single Mode-No Core-Seven Core Fiber Structure

A reflective high-sensitive humidity sensor based on a brief no-core fiber (NCF) sandwiched between a single mode fiber (SMF) and a short section of seven-core fiber (SCF) is experimentally demonstrated. The NCF is employed as a coupler to excite and couple the core fundamental mode and the high-order modes in SCF. The reflection spectrum of the multimodal interference is strongly integrated with the surrounding ambient humidity. An enhanced humidity sensitivity of -0.639 dB/%RH over the humidity range of 25-76 %RH is achieved. The sensor is experimentally investigated for two applications: human breathing and soil moisture. The response/recovery time for human breathing is 0.82 s/0.86 s. And a high sensitivity of −0.439 dB/%RH is obtained in the soil moisture range of 36-74.9 %RH. Therefore, the proposed sensor is proved to be an excellent humidity sensor candidate for compact size, high sensitivity, ease of fabrication, low-cost intensity detection and all-fiber configuration.