Carbon-steel tube surface mounted FBG sensors under high-temperature environment, part I: Polyimide coated and femtosecond laser written

Fiber Bragg Grating (FBG) sensors need to be mounted at the outer surface of any metallic test piece to monitor its structural integrity through measuring physical quantities, such as strain, pressure, vibration, and temperature. High-temperature epoxies or ceramic epoxies are used to mount the said sensors to operate under high-temperature environment conditions. The unsteadiness of the outer surface of the metallic piece and non-uniformity of the applied epoxy over the sensing head, affect its operation and measurement accuracy in the long run. Here, in the first part of a twopart article, we have investigated the reliable functioning and spectrum evolution of the FBG sensors, mounted on a carbonsteel tube, for temperature up to 500° C. Polyimide coated femtosecond laser written FBG sensors are surface mounted on the said tube using a high-temperature adhesive. There is no peak splitting, and chirp phenomenon experienced by the sensors at the end of two thermal cycles. The second part of the two-part article focuses on the mounting and operation of gold-coated FBG sensors in the high-temperature operating environment.