Ultrafast varifocal lenses in laser material processing and particle tracking velocimetry

Abstract

Narrow depth-of-field (DOF) comes as a price when tight lateral focusing is required for applications such as high precision manufacturing and three-dimensional (3D) velocimetry. The thesis addresses the challenges in these fields by using an acoustically-driven ultrafast varifocal lens. The first part of this thesis presents the improvement in efficiency of material processing by using an ultrafast varifocal lens. High-throughput laser materials processing demands precise control of the laser beam position to achieve optimal efficiency, but existing methods can be both time-consuming and cost-prohibitive. Here, we demonstrate a new high-throughput material processing technique based on rapidly scanning the laser focal point along the optical axis using the ultrafast variable focal length lens. Our results show that this scanning method enables higher processing rate over a range of defocus distances, and that the e↵ect becomes more significant as the laser energy is increased. This method holds great potential for improving material processing efficiency in traditional systems, and also opens the door to applying laser processing to pieces with uneven topography that have traditionally been difficult to process.

The second part of the thesis presents a novel idea for high-speed 3D imaging system by using the ultrafast varifocal lens. This 3D imaging system provides a means of high-speed 3D particle tracking velocimetry. This contribution is critical because the ability to understand and visualize complex flow structures in micro-fluidic and biological systems relies heavily on the resolving power of 3D particle velocimetry techniques. The simple technique in this thesis is capable of acquiring volumetric particle information with the potential for microsecond time resolution. By utilizing a fast varifocal lens in a modified wide-field microscope, we capture both volumetric and planar information with microsecond time resolution. As a proof of concept, this technique is demonstrated by tracking particle motions in the complex, 3D flow in a high Reynolds number laminar flow at a branching arrow-shaped junction.