Distributed Brillouin optical fiber temperature and strain sensing at a high temperature up to 1000 °C by using an annealed gold-coated fiber.

In this study, the distributed temperature and strain sensing with an annealed single mode gold-coated optical fiber over a wide temperature range up to 1000 °C is demonstrated by using the differential pulse pair (DPP) Brillouin optical time domain analysis (BOTDA). Owing to the protection provided by the gold coating, the fiber can withstand high temperature environments and maintain a high strength, which enables the gold-coated fiber acting as a repeatable high-temperature sensor. After annealing twice to remove the internal stress, the temperature coefficient of the gold-coated fiber is stable and consistent with a nonlinear function. Owing to the residual stress accumulated during the cooling process of coating and the low yield strength of gold, a pre-pulling test is essential to measure the strain of a gold-coated fiber. An equal axial force model is used to recalculate the strain distribution induced by the large temperature difference within the furnace. The high-temperature strain coefficient of an annealed gold-coated fiber decreases with temperature, i.e. from ~0.046 MHz/με at 100 °C to ~0.022 MHz/με at 1000 °C, mainly due to the increase in Young's modulus of silica with temperature. To the best of our knowledge, this is the first time that an annealed gold-coated fiber has been applied for distributed high-temperature strain sensing, which demonstrates the potential applications for strain monitoring in complex, high-temperature devices such as jet engines or turbines.