Functionalizing optical glasses by femtosecond laser for high temperature sensing: trends, limits and opportunities (Invited Paper)

Ultrashort laser pulses focused inside optical glasses enable localized transformations in volume. For example, isotropic index modifications, so-called Type I, can be used to fabricate complex waveguiding structures, gratings, lenses. Another type of transformation, labeled Type II or nanogratings (NGs), corresponds to the formation of sub-diffraction porous nanostructures controllable by light polarization. NGs are birefringent, making them attractive to the development of 3D geometric phase components, waveplates, optical data storage, or fiber-based sensors, e.g., Fiber Bragg gratings (FBGs). Furthermore, NGs exhibit extraordinary thermal stability, withstanding hundreds of hours at 1000°C, making them particularly useful for FBGs operating in harsh environments. To date, fs-Type II FBGs are principally inscribed in telecom, lightly doped, silica core optical fibers. When operating at high temperatures (>800 °C), NGs progressively relax and erase, causing drifts of the monitored property (e.g., Bragg wavelength), and more drastically a loss of signal. Therefore, there is a need to comprehend and predict what drives NGs erasure over a given thermal treatment, to anticipate potential signal degradation. Following this, novel solutions must be envisioned to go beyond current limitations, ultimately set by the intrinsic nature of the glass substrate, usually SiO2.