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 $220~\mu \text{m}$ and $570~\mu \text{m}$ , 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.

中文翻译：

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具有高折射率敏感度的微变形环形光纤锥度

我们报告了一种基于环形微变形锥形光纤的高灵敏度折射率传感器。微变形是先将锥体成型为环，然后球状纤维末端弯曲锥腰部来引起微变形。当锥形光纤环弯曲时，来自 CO ₂激光器的热脉冲施加 2 秒，刚好足以避免与成球的光纤末端融合。通过这种方式，锥形腰部的一小部分永久微变形。微变形具有半圆形形状，其尺寸由成球纤维末端的直径决定。微变形极大地增强了锥形光纤环对外部折射率变化的敏感性。获得了 1052 nm/RIU 和 1636 nm/RIU 的折射率灵敏度，微变形直径为 $220~\mu \text{m}$ 和 $570~\mu \text{m}$ ，分别在 1.3 至 1.345 和 1.3 至 1.34 的折射率范围内。所提出的装置在上述折射率范围内的高灵敏度对于水基溶液中的生物和化学传感很有吸引力。