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

We present a research to determine the long-term stability of intrinsic Fabry-Pérot (F-P) optical fiber sensors in high-temperature environments. In-fiber sensors were created in 125 μm 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.