Analysis of the intrinsic refractometric sensitivity of optical fiber plasmonic sensors

Abstract

Optical fiber refractometers coated by a nano-scale gold layer constitute a miniaturized counterpart to the Kretschmann prism and enable the exploitation of surface Plasmon resonance (SPR) for accurate (bio)chemical sensing purposes. In this paper, we analyze the impact of the gold coating and the optical fiber cladding thickness on the cladding modes distribution. We focus more particularly on the SPR mode and its subsequent refractometric sensitivity. We clearly demonstrate that the optimum gold thickness for SPR generation lies in the range between 30 and 70 nm. We also report that a decrease of the cladding diameter from 125 μm to 80 μm enhances the refractometric sensitivity by ~20 %. Theoretical investigations obtained with a finite-difference complex mode solver in cylindrical coordinates are corroborated by experimental data. We make use of tilted fiber Bragg gratings (TFBGs) photowritten in the core of a standard singlemode silica fiber to individually interrogate the cladding modes. TFBGs couple light to the fiber cladding in reflection and present a comb-like amplitude transmitted spectrum composed of the core mode resonance (so-called Bragg resonance) and several tens of cladding mode resonances. The Bragg resonance provides temperature-insensitivity while each cladding mode resonance finely probes a given range of surrounding refractive index values. TFBGs were coated with gold using a standard sputtering process. Experiments conducted on gold-coated TFBGs immersed in calibrated liquids finally show that their ultimate refractometric sensitivity is of the order of 550 nm/RIU (refractive index unit).