A novel type of high-sensitivity Surface Plasmon Resonance (SPR)-based refractive index sensor is demonstrated, with a Kretschmann prism-coupling setup enclosed in an optical cavity and interrogated by a telecom wavelength laser source. Two thin layers of Au and SiO 2 are deposited on the prism (SPR chip). Analogously to the sensors commonly referred as “Intensity modulated SPR sensors”, in the presented setup refractive index variations of the sample induce a shift of the SPR central wavelength, which in turn leads to a variation of the SPR chip reflectivity. Since the SPR chip is one of the cavity mirrors, the corresponding loss can be sensitively determined by measuring the internal photon lifetime by a cavity ring-down technique [1]. The interrogating laser is periodically shut down and the subsequent exponential decay of the on-resonance transmitted intensity is recorded to retrieve the ring-down time. In …