Agarose gel filled temperature-insensitive photonic crystal fibers humidity sensor based on the tunable coupling ratio

An Agarose gel filled photonic crystal fiber in-line interferometric sensor for the measurement of relative humidity is proposed and experimentally demonstrated. The sensor is constructed by filling the Agarose gel between the aligned single mode fiber (SMF) and the photonic crystal fiber (PCF). A fiber in-line interferometer is fabricated by splicing the other end of the PCF and another SMF. Due to the tunable refractive index property of the Agarose gel, the mode field diameter of the propagation light is changed with the external relative humidity, which induces the change of coupling ratio between the PCF and the SMF. The relative humidity is measured by interrogating the fringe visibility of the white-light interferogram. The experimental results show that the sensitivity of up to 2.2 dB/RH is achieved. The proposed method possesses high resolution and low temperature cross-sensitivity.