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Fringe generation with non-uniformly coated long-period fiber gratings.

In this work, the spectral characteristics of non-uniform symmetrically ring shaped coatings deposited on long-period fiber gratings (LPFGs) have been theoretically and experimentally investigated. To optimize the structure performances, the device was designed with a simulation tool based on vectorial analysis of modes in a multilayer cylindrical waveguide and coupled mode theory. Electrostatic self-assembling technique was selected to deposit with fine control uniform azimuthally symmetric coatings on the cladding of the LPFG. UV laser micromachining operating at 193nm was used to selectively remove the coating with high spatial resolution and with azimuthal symmetry. By locally and selectively removing portions of the overlay surrounding the LPFG from the middle of the grating, strong modifications of its spectral characteristics were observed. Phase-shift effects and multiple interference fringes have been observed for all the attenuation bands, strongly depending on the length of the uncoated region and the overlay features (thickness and optical properties). This provides a valid technological platform for the development of advanced photonic devices for sensing and telecommunication applications.