Automated fabrication of U-shaped silica optical fibers with high reproducibility and sensitivity

An automated bending process was developed for preparing U-shaped optical fibers to minimize variability and enhance the refractive index (RI) response for sensing applications. A silica optical fiber with a core diameter of 600 µm and target bending radius of 1.25 mm was formed into a U-shape by three consecutive bending processes to enhance its stability. A jeweler’s flame burner was employed for localized heating using air instead of oxygen as the oxidizing gas to avoid an excessive temperature rise. The movement of the flame was precisely controlled by a compact linear actuator with a pulse input for micrometer-level adjustments. A CO₂ laser was used to create a smooth end face for the optical fiber and result in a polished finish. Compared with manually fabricated U-shaped probes, the automated process combined with CO₂ laser cutting reduced the coefficient of variation (CV) of optical power from 11.1 % to 3.23 %, representing a 3.4-fold improvement in reproducibility. The RI response at a wavelength of 660 nm was measured to be 115.8 dB per refractive index unit (RIU) within a broad RI range of 1.330–1.430.