Miniature interferometric humidity sensor based on an off-center polymer cap onto optical fiber facet

In this work, we propose an interferometric humidity sensor based on a polymer micro-cap bonded off-center on the polished end of a standard single mode fiber (SMF). In our sensor, the sensitive part is the micro-cap. In the interferometer fabrication process, we can adjust the misalignment between the core and the micro-cap. It allowed us to control the visibility of the interference pattern. In our device, water molecules absorbed by the polymer microcap causes changes in the height of the microcap. This results in measurable shifts of the interference pattern. The fabricated samples were tested in a calibrated climatic chamber in the range from (similar to)10 to (similar to)95% of relative humidity (RH). The sensitivity and resolution of our device was found to be 148 pm per %RH and 0.04% RH, respectively. The obtained results suggest that our sensors can be useful to monitor relative humidity (RH) in miniature spaces.This work was funded by the Fondo Europeo de Desarrollo Regional (FEDER); the Ministerio de Economia y Competitividad (projects PGC2018-101997-B-I00 and RTI2018-094669-B-C31), and in part by the Gobierno Vasco/Eusko Jaurlaritza IT933-16 and ELKARTEK KK-2018/00078, (mu 4Indust and SMARTRESNAK). Oskar Arrizabalaga acknowledges a PhD fellowship from the Departamento de Educacion, Politica Linguistica y Cultura del Gobierno Vasco/Eusko Jaurlaritza