Femtosecond laser-inscribed fiber-optic sensor for simultaneous temperature and relative humidity measurements

In this paper, we propose a fiber optic sensor for simultaneous temperature and relative humidity measurements based on femtosecond laser processing of an asymmetric single mode-multimode-single mode interferometer, in which half of the core and cladding of a single-mode fiber are removed by a femtosecond laser to form a microcavity, with the remaining grooved portion of the single-mode fiber and the external environment acting as an interferometric arm that is transmitted to the full core portion for coupling. Since the single-mode fiber itself is insensitive to ambient humidity and fragile and prone to breakage after femtosecond laser processing, the microcavity is coated with polyimide to restore the robustness of the structure, which also significantly improves the temperature and humidity sensitivity of the structure. Experimental results show that the temperature sensitivity is −819.7 pm/°C in the temperature range of 40–90 °C and the humidity sensitivity is 503.1 pm/%RH (relative humidity) in the range of 40–75%RH. The proposed sensor is attractive for simultaneous temperature and relative humidity measurements due to its simple construction and easy of fabrication.