Femtosecond laser micromachining of waveguides intrinsically depends on the bulk glass substrate used for fabrication. Phosphate glasses are important substrates because they can incorporate high concentrations of rare-earth ions, making them an ideal host material for fabricating compact high gain waveguide lasers and amplifiers that operate in the telecommunications window. The research presented in this thesis focuses on how phosphate glasses, as well as specific phosphate glass compositions, will affect the fabrication of waveguiding structures using focused femtosecond laser pulses. In this study, we examine the fundamental relationships between the initial composition of phosphate glasses and the structural changes associated with refractive index modification that result from fs-laser irradiation. Changes to the glass network structure have been studied using scanning confocal micro-Raman and fluorescence microscopy. Systematic changes in the Raman spectrum and the excited photoluminescence indicate atomic level changes to the phosphate network that depend on the femtosecond laser writing conditions and initial phosphate glass composition.

Characteristic changes in the spectral positions of the (POP)_sym and (PO2)_sym phosphate network vibrations have been identified. Shifts in these Raman modes to higher and lower wavenumbers indicate a decrease and/or increase in the P-O network bond length, resulting in an overall expansion and/or contraction of the phosphate network that depends on the amount and the rate of femtosecond laser energy deposition, as well as on the initial phosphate glass structure. In addition to investigations of changes to the Raman peaks, the use of fluorescence spectroscopy of fs-laser modified phosphate glass has been studied. Under conditions where no usable waveguides could be produced by direct fs-laser writing techniques, a photoluminescence at 620 nm was observed inside the modified area. This peak has been attributed to the presence of fs-laser induced POHC (Phosphorous Oxygen Hole Center) defects, indicative of phosphate network depolymerization as a result of laser irradiation. The overall intensity, and thus concentration of induced POHC defect fluorescence depends on the initial glass composition in the same manner as the changes to the Raman spectrum. Such a relationship indicates that the mechanisms of glass network expansion caused by the absorption fs-laser pulses typically result in broken phosphate bonds and a lower density with the exception of a very specific type of phosphate glass composition where network changes were not observed.

Through this research, which was done in collaboration with Prof. R. K. Brow at MS&T, we have discovered a specific zinc polyphosphate glass composition that has demonstrated ideal microstructure changes to the glass in that it can be used to fabricate subsurface single-pass waveguide amplifiers using fs-laser waveguide writing techniques. The results presented in this study demonstrate that the exact glass composition should be taken into account when fabricating waveguide devices in phosphate glasses, in order to both expand the fs-laser processing conditions and maximize favorable morphological changes for 3-D photonic devices.