Ultrafast autofocusing in laser-based manufacturing
Laser-based manufacturing involves using lasers as a tool in various manufacturing processes, from cutting and shaping materials to welding and heat treating. The high degree of precision and control provided by lasers makes them an extremely valuable tool for manufacturing processes, especially in industries where precision is paramount, such as electronics, automotive, aerospace, and medical devices.
Traditionally, working with non-flat or changing surfaces in laser-based manufacturing was challenging and labor-intensive. Complex focus mapping procedures and ex-situ characterisation often leads to repositioning errors and extended processing times. However, advancements in technology have led to the development of ultra high-speed auto-focusing in laser processing, a significant improvement over these older methods.
Most auto-focusing techniques still require mechanical movement of a motorized stage, which is slower than the lateral speed and can slow down the process of surface detection and re-alignment. This requires feedback, control, and sensing methods to determine the optical focal position.
To address these issues, Dr Du Xiaohan and her colleagues developed a method to track the specific location of a surface and adjust the focus of an optical system simultaneously. This was achieved by employing axial varifocal optics, specifically a TAG lens, which operates at a high speed of 0.1-1 MHz.
The work was carried out when Dr Du Xiaohan was a doctoral student in the group of Professor Craig B. Arnold at Princeton University in the USA and published in Light: Science and Applications. Dr Du is now an assistant professor in the Department of Systems Engineering at the City University of Hong Kong.
A TAG lens, or tuneable acoustic gradient lens, employs sound to dynamically and rapidly alter its focus. It contains a liquid, and a piezoelectric shell generates a sound wave inside the liquid, leading to a refractive index gradient that behaves like an optical lens. The focal length of the lens can be micro-adjusted within microseconds by modulating the frequency of the sound wave, allowing for rapid and precise focusing changes.
The team’s solution involves dynamic z-axis scanning for both detection and movement, without any mechanical axial movement. This significantly speeds up the time between surface detection, focus retrieval, and firing of the fabrication laser pulse.
Dr Du and her colleagues explained the operational principle of their auto-focusing method: “We integrated a single varifocal lens into a dual laser beam setup, consisting of a probing beam and a fabrication beam. The probing beam scans continuously along the z-axis, and the temporal response of its reflection is related to the surface location. Simultaneously, we guide the fabrication beam to the desired position by triggering the fabrication laser at the appropriate time. This approach reduces defocused laser pulses and increases the processing speed when processing non-flat or changing samples.”
The researchers also highlighted the potential of this technique for auto-focusing with a lab-made real-time detection and focusing system, designed to instantaneously follow the surface topography without any mechanical movement in the z direction.