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A Fabry-Pérot fiber sensor for the simultaneous measurement of refractive index (RI) and temperature based on an in-fiber ellipsoidal cavity is proposed. The sensor head consists of an ellipsoidal air-microcavity, which is formed by splicing together a single-mode fiber and a photonic crystal fiber with a simple arc-discharge technique. The external RI is determined through the analysis of the fast Fourier transform. The sensor is also capable of simultaneous measurement of temperature by tracking the wavelength shift. Easy fabrication, low-cost, and high-contrast make it appropriate for practical application.