Non-uniform nano-coated long-period fiber gratings for sensing applications

This work numerically and experimentally investigates the spectral modifications due to non-uniform azimuthally symmetric high refractive index (HRI) nano-coatings deposited on long-period fiber gratings (LPFGs). First, a uniform overlay was deposited on a LPFG by using electrostatic self-assembling technique, which guarantees a fine control on the coating thickness. Successively, UV (λ=193nm) laser micromachining was applied to locally and selectively remove the coating with high spatial resolution and preserving the azimuthal symmetry of the structure. As the overlay removal was performed starting from the middle of the grating, strong modifications of the LPFG transmission spectrum occurred. Phase-shift phenomenon and selective fringes generation in correspondence of all the attenuation bands can be observed. The two-effects occurring are ruled by the longitudinal length of the uncoated region and the overlay features (thickness and optical properties). The wavelength selectivity, combined with the effects of the HRI coatings on the cladding modes distribution, can be successfully applied for the development of advanced and high performances sensing devices.