A novel fiber-interface directional waveguide coupler was inscribed on the surface of a coreless fiber by femtosecond laser, and was successfully applied to highly sensitive refractive index (RI) measurements. The primary arm was first inscribed to couple light from a lead-in single mode fiber to the fiber interface, then back to a lead-out single mode fiber. A side arm was inscribed parallel and in close proximity to the primary arm. Light propagating in the primary arm could then be efficiently coupled into the side arm when a phase-matching condition was met, which produced a dramatic spectral dip at the coupling wavelength. The proposed device achieved a sensitivity as high as similar to 8249 nm/RIU over an RI range of 1.44-1.45, due to strong evanescent fields excited in fiber-interface waveguides. The proposed in-fiber directional coupler exhibits high mechanical strength, a compact configuration, and excellent RI sensitivity. As such, it has significant potential for practical applications in biochemical sensing. (c) 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement