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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 ~1uJ 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-um 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.

Laser welding has great potential as a fast, non-contact joining method for thermoplastic parts. In the laser transmission welding of thermoplastics, light passes through a semi-transparent part to reach the weld interface. There, it is absorbed as heat, which causes melting and subsequent welding. The distribution and quantity of light reaching the interface are important for predicting the quality of a weld, but are experimentally difficult to estimate. A model for simulating the path of this laser light through these light-scattering plastic parts has been developed. The technique uses a Monte-Carlo approach to generate photon paths through the material, accounting for absorption, scattering and reflection between boundaries in the transparent polymer. It was assumed that any light escaping the bottom surface contributed to welding. The photon paths are then scaled according to the input beam profile in order to simulate non-Gaussian beam profiles. A method for determining the 3 independent optical parameters to accurately predict transmission and beam power distribution at the interface was established using experimental data for polycarbonate at 4 different glass fibre concentrations and polyamide-6 reinforced with 20% long glass fibres. Exit beam profiles and transmissions predicted by the simulation were found to be in generally good agreement (R2>0.90) with experimental measurements. The simulations allowed the prediction of transmission and power distributions at other thicknesses as well as information on reflection, energy absorption and power distributions at other thicknesses for these materials. / Thesis (Master, Mechanical and Materials Engineering) -- Queen's University, 2013-09-27 10:41:08.997

In laser transmission welding (LTW), a laser beam passes through the laser-transparent part and is absorbed by carbon black (CB) in the laser-absorbent part. This causes a temperature rise at the interface between the parts which leads to melting, diffusion and ultimately joining of the two components. Weld temperatures increase with laser power at a given scan speed. However at higher temperatures, it has been observed that weld strength of LTW starts to decline due to material thermal degradation. Thermal degradation of materials is a kinetic phenomenon which depends on both temperature and time. Therefore there is no specific temperature for thermal degradation. Thermal gravimetric analysis (TGA) is used to study the thermal degradation of two commonly used thermoplastic materials: polycarbonate (PC) and polyamide 6 (PA6). Each material was studied at two levels of CB. It is shown in this work that increasing the carbon black (CB) level from 0.05 to 0.2wt% has no significant effect on the thermal stability of PA6. However, it is observed that increasing the CB level from 0.05 to 0.2wt% has a noticeable effect on the thermal stability of PC. The TGA data were then used to obtain the kinetic triplets (frequency factor (k_0), activation energy (E), and reaction model (f(α))) of the materials using a non-linear model-fitting method. These kinetic triplets were combined with temperature-time data obtained from a Finite Element Method (FEM) simulation of the LTW process to predict material degradation during LTW. The predicted degradation was then compared with experimental data. It is found that the predicted onset of material degradation is in good agreement with experimentally observed thermal degradation (of both visually observed degradation onset and weld strength decline) for PC and PA6. A semi-empirical model based on the FEM temperature data is also developed in this work as a simpler alternative for obtaining LTW maximum temperature-time profiles for prediction of material thermal degradation during LTW. Comparison of the predicted material conversion using temperature-time profile obtained by FEM and the semi-empirical model shows good agreement. / Thesis (Master, Chemical Engineering) -- Queen's University, 2013-09-27 10:45:24.688

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