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An enhanced Goos–Hänchen (GH) shift, which is attributed to the oscillating wave propagating in the polyimide-coated symmetrical metal cladding waveguide, is employed to optically monitor the relative humidity (RH). The interaction of polyimide with the water molecules can change the waveguide structure parameters (thickness and refractive index) and thus alter the magnitude of GH shift. In our experiment, the magnitude of GH shift is measured during the RH variation from 43% to 73% with a step of 5% and the corresponding desiccation process. The proposed optical RH sensor exhibits good linearity and high sensitivity, meanwhile provides short response time and favorable stability over a long period. In addition, since the GH shift signal is position encoded, our sensing mechanism is immune to the intensity fluctuation in the light source. [ABSTRACT FROM PUBLISHER]