Investigation on the thermal properties of fs laser inscribed flouride fibre Bragg gratings

<p dir="ltr">In recent years, heavy metal fluoride glasses have gained increasing interest in their applications in fiber optics due to their extended transparency window in the midinfrared range and substantially lower attenuation loss in comparison with industry standard silica fiber. ZBLAN glass is the most chemically stable composition for this family of glasses where ZBLAN-based fibers have been used for the development of midinfrared light sources for various applications. One of the most important developments for mid-infrared light sources is the achievement of all-fiber laser cavities, where the usage of FBGs in fiber mirrors serves as a critical step for achieving this objective. By using the femtosecond laser direct inscription technique, line-by-line gratings were inscribed into passive ZBLAN fibers where a wide parameter scan was conducted and a wide inscription window for different repetition rates ranging from 1 - 150 kHz was identified. The effect of changing the thermal profile of the femtosecond laser pulse during inscription and the resultant complex refractive index modification was explained systemically in the results chapter. Short 5 mm test gratings were then inscribed and it was found that 1 kHz and 50 kHz repetition rates produce the strongest FBG in terms of reflectivity. Subsequent annealing study has revealed an exponential decrease in reflectivity after 30 minutes of annealing for 1 kHz grating while the 50 kHz grating experienced a much less linear reduction in reflectivity. The ion migration study revealed only the migration of the fluoride ions during the inscription procedure. Direct refractive index measurement techniques such as qradri-wave lateral shearing interferometry and micro-reflectivity were used to reveal the exact refractive index change behavior pre and post-annealing. It was found that the large reduction in reflectivity for 1 kHz grating was a result of stress accumulated within the transition region in between the grating planes being annealed away after annealing, which contributes to a sharp reduction in the refractive index contrast between the grating planes and transition region. For 50 kHz gratings, the inscription procedure was found to be in the heat accumulation regime where most of the stress was annealed away during the inscription, and therefore this grating exhibits significantly better thermal stability with only slow ion migration happening during the annealing process. To the best of our knowledge, this thesis presents the first experimental study explaining the complex refractive index modification and associated thermal behavior of the femtosecond inscribed FBGs within fluoride glasses. The results of this thesis pave the way for precise engineering of the FBG properties within ZBLAN fibers to develop all-fiber laser cavities for different application purposes and resulted in a major publication in APL Photonics in 2024 and another paper is in the process of writing.</p>