Real-time monitoring of high temperature (>400°C) applications is nowadays a challenge. Temperature is usually monitored by thermocouples, which present disadvantages due to its punctual monitorization, sensibility to electromagnetic interferences and noise in their signal. In this context, Fibre Optic Sensors (FOS) can be solution due to their intrinsic characteristics such as small size, immunity to electromagnetic interferences and to corrosion, light weight, multiplexing and remote sensing capabilities or embedding.

The work presented in here was developed in the framework of the ACHIEF project, where multipoint high temperature sensors to monitor material degradation in two different metallurgical applications operating at high temperatures (>600°C) are required. The FOS technology selected to develop these high-performance temperature sensors were Fiber Bragg Grating (FBG) sensors, because of their high accuracy, sensitivity and multiplexing potential.

Temperature FBG sensors have been developed by inscribing the gratings in metallic and carbon coated optical fibres using an infrared femtosecond (fs) laser to improve the thermal stability and increase the period of the refractive index modulation of the grating. To produce the periodic structure the so-called point by point inscription technique is used. Two different inscription methodologies were developed depending on the optical fibre coating. For the carbon coated fibres the fs laser was directly applied, while, in the case of metallic coated fibres, the metallic coating was previously removed, the grating was inscribed and then, a new metallic coating was applied.

Even though FOS are based on silica and this material is resistant to high temperatures, the standard FBGs (polymeric coated) are fragile and not resistant to being exposed to high temperatures during long periods of time. To improve the fs laser FBGs made, an extra metallic coating based on Ni and Ni alloys was applied. This metallic coating was made based on a combination of sputtering and electroplating deposition techniques, obtaining coating layers thickness between 100 µm to 400 µm.

Finally, these metallic coated FBGs were characterized thermally by long term thermal tests applying static and fatigue test between 200°C to 800°C. And, also by metallographic test to analyse the microstructure. The FBGs manufactured on metallic coated optical fibres and re-coated with Ni or Ni-alloys presented a good resistance and stable signal after being exposed up to 800°C for 50 days.