Tapered long-period fiber gratings working in inverted mode through all-fiber ring shaped illumination

In this work we present the possibility to obtain long-period fiber gratings (LPFGs) working in inverted mode through simple and cost effective all-fiber ring shaped illumination. Permanently and locally bent single-mode optical fibers are used to provide a ring shaped illumination with core mode depletion depending on the bend features. When this kind of illumination interacts with a tapered LPFG, light power transfer from input cladding modes towards the core mode occurs depending on the grating characteristics. The final result is a transmission spectrum with zero transmission in the whole spectral range except for the optical wavelengths corresponding to mode coupling. Two important aspects of the proposed structure can be envisaged. The first one relies on the possibility to additionally obtain mode coupling involving asymmetric cladding modes by modifying the illumination symmetry acting on the bend shape. The second one is the possibility to tune the visibility of the peaks corresponding to the coupled cladding modes by changing the surrounding medium on the straight fiber located after the bent region without involving the LPFG section. Here, both aspects have been experimentally demonstrated and their effectiveness proved in sensing applications involving refractive index measurements. The proposed approach thus opens up new sensing architectures as well as new fluidic filters for communication applications.