Sensitivity Enhancement in Long Period Gratings by Mode Transition in Uncoated Double Cladding Fibers

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This work deals with long period gratings (LPGs) fabricated in double cladding fibers (DCFs) with W-type refractive index profile and analyzes their surrounding refractive index (SRI) sensitivity. The W-type fiber presents an outer cladding with refractive index higher than the inner one, thus some cladding modes can be guided in the ring-shaped outer cladding without interacting with the core mode. However, the transition phenomenon from outer- to inner-cladding of these modes can be induced, through the reduction of the external cladding thickness (i.e. fiber diameter). As a matter of fact, here we demonstrate for the first time the mode transition phenomenon without any additional coating on the fiber. The result is a significant enhancement of the SRI sensitivity, as well as the possibility to tune the SRI range with maximum sensitivity gain. It is illustrated by numerical analysis and experimentally verified by considering LPGs written in pure-silica core with F-doped inner cladding and silica outer cladding fiber, by means of the electric arc discharge technique. The case study is focused on the SRI sensitivity in water-like environment as for biological applications. Proof of concept results have demonstrated that, by proper selection of the outer diameter through chemical etching, the gain in SRI sensitivity reaches from 40 to 60 as compared with unetched fiber, with maximum sensitivity around 420 nm/RIU with water as surrounding material.