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In this letter, an optical fiber hydrogen sensor based on a Fabry–Pérot interferometer cavity fabricated by femtosecond laser micromachining near the fiber end and deposited with Pd film is proposed. The relationship between the reflection spectrum and the hydrogen concentration is simulated, and experimentally demonstrated. The shifts of the interferometer output spectrum are 10, 30, 100, and 150 pm respectively, corresponding to the hydrogen concentration of 2%, 4%, 6%, and 8%, respectively. The proposed sensor is compact, easy in fabrication, and has high potential for hydrogen detection.