Tests of sapphire optical fiber sensors for strain monitoring in high temperature environment

Strain sensors based on optical fiber technology have a number of advantages over more conventional resistive strain gages. Optical fiber sensors are immune to electromagnetic interferences, resistant to corrosion, are safe to be used in explosion or fire hazard areas, and can be embedded inside materials. Commercially available solutions include the Fiber Bragg Grating (FBG) sensors and the Distributed Strain and Temperature Sensors, based on Brillouin scattering; both technologies are using fused silica optical fibers. FBG sensors application is limited to temperature up to 300 °C; solution for high temperature applications (up to 1000°C) have been developed but are not commercially available. Sapphire optical fibers have a very high melting point (2030 °C) and are promising for high temperature monitoring. Sapphire fibers can be embedded inside metals and ceramic materials. However, even if FBG sensors can be written inside sapphire fibers with femtosecond UV lasers, there are no solutions available on the market. This work describes a test to verify the possibility of monitoring strain using a sapphire fiber embedded into a metal specimen, by monitoring the power loss of the light passing through the fiber.