A novel class of multiphase D-shaped optical fiber suitable for refractive index measurements is presented. The multiphase D-shaped optical fiber was constructed by forming six D-sections in a multimode optical fiber at localized regions with femtosecond laser pulses. Each D-shaped zone covered a sensor volume of 100 μm depth, 245 μm width, and 1 mm length. Each bottom plane of a D-shaped zone forms an angle of 60 degree with the bottom plane of an adjacent D-shaped zone. The multiphase D-shaped optical fiber as a highly sensitive refractive-index sensor to detect changes in the surrounding refractive index was studied. Results show that a refractive index resolu- tion of 2.82 × 10 -4 RIU was achieved for sucrose solutions with refractive indices in the range of 1.333 to 1.403, suggesting that the multiphase D-shaped fibers are attractive for chemical, biological, and biochemical sensing.
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