AbstractNowadays, laser-based systems are widely used in research, industry, medical care and consumer products. The optical modeling and design of such systems is essential in order to find innovative solutions for various applications, and to develop novel applications beyond the current state-of-the-art. This PhD work enhances the modeling and design for two laser-based optical systems: refractive laser beam shapers and lenticular laser engraving systems.
My work on laser beam shaping is motivated by - but not limited to - its application in laser additive manufacturing, such as laser cladding and selective laser melting. Here, high power laser beams are required to provide various predefined irradiance patterns to optimize the laser-material interactions in the melt pool. Refractive beam shapers are selected due to their high optical efficiencies and straightforward co-axial system layouts. For the design of refractive beam shapers, geometric ray mapping technique has been well developed to mainly transform a Gaussian to a flat-top profile in literature. In or- der to generate more flexible profiles, I have developed an enhanced numerical design procedure after investigating the impacts of different sampling strategies and surface reconstruction methods. With this development, both the calculation and simulation ef- forts are reduced due to a clearly reduced number of the required sampling points. In addition, the collimation quality of the output beam is much improved so that the de- sired irradiance pattern can be maintained over a much longer distance, both for the flat-top and other patterns like annular rings.
More often, laser beams are required to have the required irradiance patterns at fo- cus. The state-of-the-art approach is based on Fourier transform lenses, and it has two inherent limitations. First, additional beam expansions might be needed to obtain the desired shaping quality, in case of long focal lengths and/or small beam sizes. Second, the irradiance patterns out of focus are not under control as the phase at focus is not considered. By combining geometrical optics with wave optics, I have developed a new direct design approach using two plano-aspheric lenses to control both irradiance and phase at focus. With the additional phase control, the shaping quality is always ensured without the need for external beam expansions, rendering the system more compact. Be- sides, the generated irradiance patterns can be maintained within a considerably large range, which provides a more tolarant system and improves the performance for certain applications such as deep laser micro-machining. Furthermore, this newly developed method is used to shape laser beams in a 3-Dimensional (3D) volume, so that the irradi- ation patterns at different planes along the optical axis in z direction can be scaled and even to have different aspect ratios in x and y directions, or can change shapes, e.g. from a flat-top to an annular ring.
Finally, I have developed a holistic optical simulation model for lenticular laser en- graving systems. The achieved functionality Changeable Laser Image (CLI) is commonly used as a security feature for identification cards. In this application, laser beams engrave different images through parallel lenticular microlenses at different angles, and different engraved images can be viewed by tilting the card. Usually, the design of such CLI cards strongly depends on experience or is the result of ”trial-and-error”. With the developed optical model, it allows to identify different influencing factors and to optimize the de- sign prior to fabrication. Quantitative evaluation of the system performance in terms of viewing angles and contrast between two neighbouring images is also enabled, with experimental validations. Besides, this developed optical model can also serve as a refer- ence for simulating other applications with light-material interactions, or for evaluating similar lenticular systems offering other visual effects, such as 3D, animation or zoom.
|Date of Award
|16 Sep 2016
|Fabian Duerr (Promotor), Michael Vervaeke (Promotor), Peter Schelkens (Jury), Johan Deconinck (Jury), Erik Stijns (Jury), Hugo Thienpont (Jury), Patrick Guillaume (Jury) & Patrick De Visschere (Jury)