Materials and Techniques for the Femtosecond Laser Fabrication of Optical Devices in Glass.

摘要

Focused femtosecond laser pulses can be used to permanently modify the refractive index of glasses. If the change in the refractive index of the laser-modified material is positive, the material can be easily used to create optical waveguiding structures. Unfortunately, few materials are known to exhibit this property. In prior research we found a unique zinc phosphate glass composition, 60ZnO ∘ 40P2O5, that exhibits this property and we have used it as a launching point to create more robust glasses as well as optical devices using the femtosecond laser writing technique. In particular, the research presented in this dissertation verifies our claim that the oxygen to phosphorus ratio (O/P) in the glass is responsible for the glass's ability to create high index regions when modified. This O/P ratio of 3.25 has allowed us to create waveguides in zinc phosphate glasses with the inclusion of rare-earth doping, magnesium doping, and aluminum doping. After the creation of these waveguides Raman spectroscopy showed that there were distinct changes to the phosphate glass network such that longer phosphate chains were broken up into smaller chains by the modification process. We were able to show that by mapping the ratio of differently bonded phosphate tetrahedra (denoted by their Qi species) we could precisely map out all laser-modified material.;In a separate study we investigated the use of cylindrical vector beams for femtosecond laser writing. By using cylindrical vector beams, in which the polarization state varies across the beam profile, it is possible to create torus-shaped foci, which possibly yield new waveguide geometries. After creation of these beams with a twisted nematic liquid crystal device we modified torus-shaped regions on and inside glasses. In glasses we attempted to use this torus modified shape to induce optical guiding by producing a low index region around a high index core but were unable to, likely due to the self-focusing of the laser deep subsurface. Nonetheless, Raman spectroscopy verified that all modified regions were contained to the torus shape and that the combination of femtosecond lasers with cylindrical vector beams offers a whole new facet to the waveguide writing process in glasses as well as crystals.;Finally, high-resolution confocal fluorescence spectroscopy was used to identify the kinds of defects that are created during femtosecond laser fabrication of Bragg gratings in silica fibers. Our results show that there is no effect on defect creation due to the presence of fluorine in the cladding of fibers but that germanium plays a significant role in the creation of non-bridging oxygen hole centers. Pre-loading silica fibers with molecular hydrogen prior to femtosecond irradiation leads to passivation of defects in all cases except in the regions containing germanium.