Fiber Bragg Gratings are structures, photo-inscribed with femtosecond laser, inside the fiber core and can be used as band pass optical filters (centered around the Bragg wavelength). Bragg wavelengths are easily multiplexed and they give to us the necessary information. Silica-based fiber Bragg gratings are point sensors and can measure temperature up to 1200°C. For higher temperature, sapphire based optical fiber are used, since they can withstand temperature up to 2000°C. However, the sapphire optical fiber is coreless, which leads to an extreme multimodal behavior. Consequently, the measure is less precise and the signal-to-noise ratio is low, compare to a classical silica-based grating. Moreover, each modification of the fiber surface, change the grating spectra.
The thesis objective is the creation of an optical waveguide inside the sapphire fiber, which will leads to less propagation modes inside the fiber, in order to obtain new perspectives for the monitoring in high temperature environments (airplane engines, nuclear reactors, …), which is one of the missions of the DRT/LIST-DIN. To obtain this result, the photo-inscription of a cladding is necessary: the cladding will be ring shape and the internal diameter – i.e. the core – will be few tenths of micrometers. Other techniques are also investigated, such as ion implantation, to create an amorphous sapphire cladding. Then these new structures will be characterized up to 2000°C and under high dynamic pressure (> 10 GPa).