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A novel refractive index-based sensor implemented within a silicon photonic integrated circuit (PIC) is reported. The design is based on a double-directional coupler (DC) integrated with a racetrack-type resonator (RR) to enhance the optical response to changes in the near-surface refractive index via the optical Vernier effect. Although this approach can give rise to an extremely large ‘envelope’ free spectral range (FSR<i>_Vernier</i>), we restrict the design geometry to ensure this is within the traditional silicon PIC operating wavelength range of 1400–1700 nm. As a result, the exemplar double DC-assisted RR (DCARR) device demonstrated here, with FSR<i>_Vernier</i> = 246 nm, has a spectral sensitivity <i>S_Vernier</i> = 5 × 10⁴ nm/RIU.