Continuously micro-tapered long-period gratings: modeling and experimental validation

Micro-tapered long-period gratings (LPGs), based on geometrical corrugations, exhibit position-dependent mode interactions, making typical models inadequate for predicting their optical response. To address this limitation, a modeling framework based on coupled local-mode theory is developed to describe mode coupling in micro-tapered LPGs, explicitly accounting for longitudinal variations of both the modal field distributions and the propagation constants. To the best of our knowledge, this work provides the first experimental validation of a coupled local-mode description applied to LPGs fabricated using the pull-and-heat technique. In particular, the model is validated experimentally through the fabrication of a micro-tapered LPG in a step-index zirconium fluoride optical fiber with a period of Lambda=1.2mm and ten grating periods. Broadband mid-infrared characterization reveals a resonance at the wavelength lambda similar or equal to 3220nm, due to the coupling between the HE11 and HE12 modes, in close agreement with theoretical prediction. These results establish a predictive model for describing longitudinally varying propagation constants and coupling coefficients in continuously micro-tapered LPGs, enabling systematic tailoring of resonance wavelength, spectral bandwidth, and coupling strength, with direct implications for high-sensitivity sensing and filtering applications. The approach is general and can be applied also to microwave structures where typical gratings are obtained via corrugated metallic structures.