Mode Splitting in ITO-Nanocoated Tilted Fiber Bragg Gratings for Vector Twist Measurement

We proposed and demonstrated a method to improve the birefringence of tilted fiber Bragg gratings (TFBGs) via high-refractive-index indium tin oxide (ITO) coatings. Firstly, we find the mode splitting phenomenon in an ITO film-coated TFBG. Numerical results show that the ITO coating can modulate the wavelength separations between two adjacent p- and s-polarized resonances. A cyclic dependency between the resonance separation and the thickness of the ITO film has been discovered. With proper film thickness selection, two groups of well-separated orthogonally polarized resonances can be achieved. Then we investigated the twist angle sensing capability of such a sensor with an optimal film thickness of 270 nm. The experimental results show that the intensity of the split resonances present reverse responses to applied torsion, which can be used for vector twist measurement. By monitoring the peak-to-peak differential intensity of these two orthogonally polarized resonances, the proposed sensor shows a twist sensitivity of ∼0.27 dB/deg and a linear response of 99.8% for the torsion angle range from -45° to 45°, together with the capability of orientation recognization. We believe that this configuration with improved birefringence opens research directions for vector physical parameter measurements such as vibration, bending, and pressure.