High sensitivity humidity sensor based on gelatin coated side-polished in-fiber directional coupler.

• The in-fiber directional coupler-based relative humidity sensor is proposed and demonstrated for the first time. • The maximum and average RH sensitivities of the sensor reach −18.94 nm/%RH near the 90%RH and −7.005 nm/%RH in the RH range of 70 %–90 %, which is the highest as far as we know. • The proposed sensor has a low temperature crosstalk and a good stability. We demonstrate the first experimental realization of in-fiber directional coupler-based relative humidity (RH) sensor. A segment of single eccentric hole-assistant dual-core fiber (SEHADCF) is directly spliced between two single mode fibers (SMFs) to realize an in-fiber directional coupler (IFDC), and the suspended core of the SEHADCF is exposed to the external environment by using the side-polishing process. Gelatin is covered on the exposed suspended core to form the cladding of the suspended core and acts as a humidity sensitive material. Compared to the previous literatures, we report the highest sensitivity of the RH sensor to our best knowledge. In the RH range of 70 %–90 %, the average and maximum RH sensitivities are −7.005 nm/%RH and −18.94 nm/%RH, respectively. In addition, the temperature response and stability of the IFDC is also experimentally measured. Due to the super-high sensitivity and low temperature crosstalk, the proposed sensor will be expected to be a promising candidate for highly sensitive RH sensing applications in high humidity environment. [ABSTRACT FROM AUTHOR]