Micro-Deformed Looped Fiber Taper With High Sensitivity to Refractive Index

We report a highly sensitive refractive index sensor based on a looped micro-deformed tapered fiber. The micro-deformation is induced by first shaping the taper into a loop, and then, a ball-shaped fiber end bends the taper waist. While the tapered fiber loop is bent, a heat pulse from a CO2-laser is applied for 2 seconds, just enough to avoid the fusion with the balled fiber end. In this way, a small section of the taper's waist is permanently micro-deformed. The micro-deformation has a semicircular shape with a size determined by the diameter of the balled fiber end. The micro-deformation dramatically enhances the sensitivity of the tapered fiber loop to changes in the external refractive index. Refractive index sensitivities of 1052 nm/RIU and 1636 nm/RIU were obtained with micro-deformation diameters of 220 mu m and 570 mu m, in a refractive index range from 1.3 to 1.345, and 1.3 to 1.34, respectively. The high sensitivity of the proposed device in the above mentioned refractive index range is attractive for biological and chemical sensing in water-based solutions.