The invention relates to methods of writing a light guiding structure in a bulk glass substrate. The bulk glass substrate is preferably made from a soft silica-based material having an annealing point less than about 1380 DEG K. Pulsed laser beam is focused within the substrate while the focus is translated relative to the substrate along a scan path at a scan speed effective to induce an increase in the refractive index of the material along the scan path. Substantially no laser induced physical damage of the material is incurred along the scan path. Various optical devices can be made using this method.
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