Confocal laser spectroscopy of glasses modified by ultrashort laser pulses for waveguide fabrication

The work described in this thesis involves the fabrication of waveguiding structures inside glasses using femtosecond (fs) laser pulses and the study of the different atomic scale changes associated with refractive index modification that occur in the fs laser modified glasses. This study helps elucidate the possible processes that occur during fs laser writing of waveguides in glasses.; Waveguide writing inside fused silica and phosphate glass using focused fs laser pulses has been demonstrated. The modification induced inside both glasses is determined to be different. Inside fused silica, the modification involves a single high index region while inside the phosphate glass (IOG-1, Schott Glass Technologies, Inc.), the modification results in a central, low index, non-guiding region bordered by two, high index, waveguiding regions. The waveguides inside both glasses have an index change on the order of 10 −4.; Color center defects have been identified in modified glasses using confocal fluorescence spectroscopy. Modified fused silica exhibits a fluorescence band at 630 nm and at 540 nm, which are attributed to the non-bridging oxygen hole center (NBOHC) and oxygen vacancy defects created by the fs pulses. A fluorescence band at 600 nm is observed in modified phosphate glass, which is assigned to the phosphorus oxygen hole center (POHC) defect. A quantitative analysis of the photobleaching of these defects with exposure to 488 nm light is conducted. Fluorescence imaging of the modified materials is performed to elucidate the location of these defects within the exposed regions in the glass.; Using confocal Raman spectroscopy, atomic scale structural changes in the glass network of modified fused silica are reported and correlated to the changes in the physical properties of the material. The changes in the Raman spectrum of modified fused silica, specifically increases in the 490 cm−1 and 605 cm−1 peaks, indicate that fs pulses induce densification in fused silica, which is mainly responsible for the observed index increase of the modified glass. The Raman changes are similar to changes observed from experiments on modification of silica glass by shock processes or rapid thermal quenching leading to high fictive temperatures. The response of phosphate glass to fs pulses, with a lower index, central region directly induced by the laser beam, is also consistent with this fictive temperature model. These results indicate that, when choosing a glass system in which to write waveguides, a glass with a high refractive index response to increasing fictive temperatures should be chosen.