Polarization dependency in metal oxide coated tilted FBG refractometers

Tilted fiber Bragg gratings (TFBGs) present a refractive index modulation blazed by a few degrees in the plane perpendicular to the optical fiber axis. This results in two kinds of couplings: self-backward coupling of the core mode and numerous backward couplings between the core mode and the cladding modes. TFBG transmitted amplitude spectra are therefore characterized by several tens of cladding mode resonances that possess their own sensitivity to the surrounding refractive index (SRI). TFBGs naturally allow the realization of refractometers accurate to 10-4 RIU (refractive index unit) in the SRI range between 1.33 and 1.45 (refractive index of silica). In this work, we demonstrate that a dense thin film of zinc oxide, a high refractive index material (n=1.9), deposited by RF sputtering in two steps, on the TFBGs tightly modifies their transmitted amplitude spectra. While the wavelength difference between orthogonally polarized modes in nude TFBGs remains within a few picometers, the association of a nanoscale metal oxide coating increases this difference up to 500 picometers. This results in two main assets: the sensitivity is extended to SRI values above 1.45 while the strong polarization dependency makes the demodulation process easier.