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A novel relative humidity (RH) sensor based on a mismatching fused Mach–Zehnder interferometric (MFMZI) fiber structure which can be operated without coating moisture sensitive material, has been proposed and experimentally demonstrated. A segment of SMF with around 1200 μm in length is sliced between lead-in and lead-out SMFs to construct the U-shaped cavity as sensing part. The optimal structure mismatching and sensing section result in definite improvement of the RH sensitivity. The maximum sensitivities of the optimal MFMZI fiber sensor can reach 0.1667 nm/%RH and 0.62 dB/%RH with the humidity range beyond ∼87.5%RH being acquired by employing the U-shaped cavity of 1222 μm in length. With the help of the beam propagation method, the light transmission characteristics of the various mismatching parameters within the humidity sensor are also analyzed. In addition, different from other mismatching fused fiber sensors, the proposed HR sensor has such distinguished features as compact size, ease of fabrication, which has potential applications in a variety of production application.

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