Metal-Filled Embedded-Core Capillary Fibers as Highly Sensitive Temperature Sensors

We report the fabrication of a metal-filled embedded-core capillary fiber and its use as a temperature sensor. This simplified structure consists of a capillary fiber with germanium-doped elliptical-shaped core placed within the wall of the capillary. In a postprocess stage, the fiber was filled with indium, chosen as the sensing element because of its low melting point and reasonable large thermal expansion coefficient compared to silica. The sensor operation is based on the stress-induced birefringence variations due to metal thermal expansion inside the capillary. The sensor interrogation was performed by wavelength-scanning, and a (8.40 ± 0.06) nm·°C-1sensitivity was measured in the range of 30-45 °C, which is interesting for biological applications. This sensitivity is considerably higher than those usually found in Bragg grating-based sensors and is among the highest ones reported with sophisticated fibers, such as metal-filled and index matching fluid-filled photonic-crystal fibers.