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Highlights 1. An optical fiber-based humidity sensor with ultrafast response is proposed by depositing GO onto a tilted fiber grating (TFG) with large tilted angle. 2. The experimental results show sensitivities of 18.5 pm/%RH and 0.02 dB/%RH with highly linear coefficient in the dynamic range of 30%-80%RH. 3. The sensor is successfully applied to monitor human breathing cycles with different breathing frequencies. 4. The sensor also shows the properties of easy fabrication, low hysteresis, ultrafast response, and high repeatability and reliability.