| A variable (0.2 to 5 MHz) repetition rate femtosecond laser was applied to delineate the role of thermal diffusion and heat accumulation effects in forming low-loss optical waveguides in borosilicate glass across a broad range of laser exposure conditions. For the first time, a transition from thermal diffusion-dominated transport at 200-kHz repetition rate to strong heat accumulation at 0.5 to 2 MHz was observed to drive significant variations in waveguide morphology, with rapidly increasing waveguide diameter that accurately followed a simple thermal diffusion model over all exposure variables tested. Amongst these strong thermal trends, a common exposure window of 200-mW average power and ∼15-mm/s scan speed was discovered across the range of 200-kHz to 2-MHz repetition rates for minimizing insertion loss despite a 10-fold drop in laser pulse energy. Waveguide morphology and thermal modeling indicate that strong thermal diffusion effects at 200 kHz give way to a weak heat accumulation effect at ∼1-muJ pulse energy for generating low loss waveguides, while stronger heat accumulation effects above 1-MHz repetition rate offered overall superior guiding. The waveguides were shown to be thermally stable up to 800°C, showing promise for high temperature applications. Using a low numerical aperture (0.4) lens, the effect of spherical aberration was reduced, enabling similar low-loss waveguides over an unprecedented 520-mum depth range, opening the door for multi-level, three-dimensional, optical integrated circuits. In contrast to borosilicate glass, waveguides written in pure fused silica under similar conditions showed only little evidence of heat accumulation, yielding morphology similar to waveguides fabricated with low repetition rate (1 kHz) Ti-Sapphire lasers. Despite the absence of heat accumulation in fused silica owing to its large bandgap and high melting point, optimization of the laser wavelength, power, repetition rate, polarization, pulse duration and writing speed resulted in uniform, high-index contrast waveguide structures with low insertion loss. Optimum laser exposure recipes for waveguide formation in borosilicate and fused silica glass were applied to fabricate optical devices such as wavelength-sensitive and insensitive directional couplers for passive optical networks, buried and surface microfluidic and waveguide networks for lab-on-a-chip functionality, and narrowband grating waveguides for sensing. |
