Sensing Performance of Fiber-Optic Combs Tuned by Nanometric Films: New Insights and Limits

The deposition of high refractive index (HRI) nanocoatings has become one of the most effective techniques used to underpin light-matter interaction, and hence to optimize sensing performance of fiber optic devices towards single molecule detection. However, it is not always that true. Herein, we report on the general polarization-dependent sensing performance of nanocoated tilted fiber Bragg gratings (TFBGs), with the results demonstrating for the first time that the sensitivity of both P- and S-polarizations can be improved, but also greatly reduced after depositing HRI nanocoating. The investigations were performed for both high- and low-order modes excited in 10° and 4° TFBGs, respectively, coated with nanometer scale metal oxide film using the full-vector complex coupled mode theory (CCMT). The results reveal that the general sensing features significantly stem from the enhanced polarized dispersion properties tuned by the nanocoating through the mode transition. The analysis of evanescent field, dispersion characteristics, and spectral response clearly confirms the improvement and deterioration of both local and general sensing performances. Due to the enhanced polarization difference, the P-polarization usually presents higher sensitivity to bulk/volume RI changes, while the S-polarization could become more sensitive to surface changes, which enables the P-/S-polarization preferable for bulk/surface refractometric sensing applications. This thorough study reveals a new insight on nanocoated fiber optic sensors and turns out to be decisive in the performance optimization, which paves the way to applications in both bulk and surface RI-based sensing.