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Highly sensitive and robust refractive index (RI) fiber sensors are proposed, which are based on concatenating two micro air-cavities. The micro air-cavities embedded in the fiber along the core axis are fabricated by combining the techniques of femtosecond laser micromachining and arc fusion splicing. The full-width at half-maximum (FWHM) of an attenuation peak of a sensor in air is 0.048 nm, and the extinction ratio is ~ 15 dB. When the diameter of the cavity is slightly larger than the fiber core diameter, the sensor is highly sensitive to external RI changes. A high sensitivity of 172.4 nm/refractive index unit (RIU) around the RI range of 1.333-1.365 is achieved, which is about six times higher than long-period fiber grating (LPFG)-type and taper-type RI sensors in the same RI range.