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Fabrication of the novel multi-D-shape fiber sensor by femtosecond laser machining with the diffractive optical element

A novel class of multi-D-shaped optical fiber suited for refractive index measurement is presented. The multi-D-shaped optical fiber was constructed by forming three D-shaped sections in a multimode optical fiber at localized regions with femtosecond laser pulses by diffractive optical element. Each D-shaped zone covered a sensor volume of 100 µm depth, 250 µm width, and 1 mm length, and the total length for sensing external medium was 3 mm. The mean roughness of the core surface obtained by the AFM images was 214.7 nm, which is relatively smooth. Results of the tensile test indicated that the fibers have sufficient mechanical strength to resist damage from further processing. The multi-D-shaped optical fiber as a high sensitive refractive-index sensor to detect changes in the surrounding refractive index was studied. The results for different concentrations of sucrose solution show that a resolution of 1.27×10−3 RIU is achieved for refractive indices in the range of 1.333 to 1.403, suggesting that the multi-D-shaped fibers are attractive for chemical, biological, and biochemical sensing with aqueous solutions.