Micro-structured fiber Bragg gratings. Part I: Spectral characteristics

Abstract

In this paper, the authors present a detailed theoretical and numerical analysis of an in-fiber photonic device based on micro-structured fiber Bragg gratings for sensing and telecommunication applications. The investigated structure consists in a fiber Bragg grating with a localized stripping of the cladding layer. The thinning of the cladding layer changes the core propagation features and thus it induces a phase delay on the guided mode, leading to the formation of a defect state inside the original grating band-gap in agreement with the phase-shift grating theory. The behaviour of the defect state is ruled by three main parameters identified in the length and depth of the stripped region and the surrounding refractive index. This special characteristic leads to the possibility to realize a new class of active and passive photonic devices. In particular, here, the theoretical and numerical analysis of the spectral behaviour exhibited by the device and its dependence on the perturbation features have been carried out. Particular emphasis has been focused on the effect of the SRI on the spectral response, providing an exhaustive analysis of the potentiality of the device both for sensing and telecommunication applications. In addition, the investigation of standard uniform and strong fiber Bragg gratings has been carried out. Finally, the case of real devices realized by wet chemical etching in hydrofluoric acid solutions was analysed and a spectral equivalence was found to extend the design rules identified for the ideal case.