Effect of Dopant Diffusion on the Long-Term Stability of Fabry–Pérot Optical Fiber Sensors

Abstract

We present a research to determine the long-term stability of intrinsic Fabry–Perot (F–P) optical fiber sensors in high-temperature environments. In-fiber sensors were created in 125 μ m diameter single mode fiber (Corning SMF28 Ultra) and a 125 μm diameter PCF ESM-12B pure SiO2 fiber spliced to a SMF28 fiber with a low reflectivity Cr layer at the interface. The outcome is a low finesse optical cavity either formed by a short length of Ge doped SMF fiber, or a short length of pure and undoped SiO2 core PCF fiber. We demonstrate the manufacturing technique required for these intrinsic F–P sensors as well as the stability of their optical characteristics at temperatures up to the range of 850 °C to 1050 °C. We report on the effect of annealing on stability after exposing sensors to temperatures of 1000 °C above nominal working temperatures. In the temperature range above 900 °C we observe increasing levels of nonreproducible drift characteristics. Stability is demonstrated up to 1000 °C. After extended exposure of sensors to high temperatures we observe deviations from the initial smooth second-order response of phase versus temperature, which has been attributed to a change in core diameter in the fiber leading to the sensor at the distal end due to Ge diffusion at the high temperatures. The down lead is exposed to over a length of 17 cm. The dopant diffusion of an SMF28 ultra fiber has been studied using Energy Dispersive X-rays analysis (SEM/EDX), to measure the radial distribution of Ge concentration before and after being heated for a period of 100 days.