![]() ![]() Consequently, nearly two-thirds of the used laser power of a Gaussian beam is potentially wasted during laser micro-structuring process. In addition, the excess energy located at the center of the beam can lead to uncontrolled melting (over melting), which in turn affects the microstructure quality. In particular, the energy per unit of area (fluence) at the tails can be much lower than the ablation threshold leading to undesired heating effects of the surface surrounding the laser spot without any ablation (see Figure 1a). Moreover, previous investigations showed that only 36.8% of the pulse energy is used efficiently at the focal position for a Gaussian beam. ![]() Consequently, the spot area limited by a beam diameter (at 1/e 2 level) includes only 86.5% of the laser beam energy, and the intensity at the boundary is only 13.5% of the peak intensity. Furthermore, the energy distribution of the Gaussian beam gradually decreases from the center to the boundary of the laser spot. This intensity profile preserves its distribution during propagation, and it can be focused to a diffraction-limited spot. However, most commercial lasers provide beams with Gaussian (TEM00) intensity distribution. Since the local intensity at the interference maxima positions is directly related to the intensity distribution in the laser beam profile, the use of a round Gaussian beam leads to inhomogeneous surface textures due to the non-uniform intensity distribution of the input laser beam. During nanosecond-pulsed laser processing of metals, the structuring mechanism is mainly based on recoil vapour pressure and Marangoni convection, that have an effect on the overall picture of melt flow. ĭuring DLIP processing, the material interacts with the laser radiation predominantly at the positions corresponding to the interference maxima, inducing various metallurgical processes such as melting, ablation and recrystallization. Moreover, no chemicals, post-treatments or vacuum conditions are required, making it an eco-friendly, fast and cost-optimized process. Due to the flexibility to achieve highly complex patterns in a one-step process, DLIP is especially interesting for industrial applications. In addition, it has been shown that the number of interfering laser beams, their geometrical arrangement, individual angles, phase and polarization influence the shape of the interference pattern as well as its typical repetitive distance (spatial period). This technique enables a direct fabrication of flexible and perfect periodic surface patterns with a well-defined long-range order based on the interference principle. Nowadays, out of the available LST methods, Direct Laser Interference Patterning (DLIP) has arisen as an innovative and effective tool for high throughput surface micro-structuring. ![]() In recent years, laser surface texturing (LST) has proven to be a suitable tool for producing various surfaces with controllable topography, leading to improved surface properties, such as wettability and self-cleaning, tribology and antifouling properties. Moreover, the presented approach allows the production of microstructures with comparable height and homogeneity compared to the Gaussian intensity distribution with increased throughput of 53%. It is demonstrated that by utilizing top-hat-shaped interference patterns, it is possible to produce on average 44.8% deeper structures with up to 60% higher homogeneity at the same throughput. In particular, the impact of both laser intensity distributions on process throughput as well as fill-factor is investigated by measuring the resulting microstructure height with height error over the structured surface. The interference patterns produced by a standard configuration and the developed setup are measured and compared. In this work, a diffractive fundamental beam-mode shaper (FBS) element is implemented in a four-beam DLIP optical setup to generate a square-shaped top-hat intensity distribution in the interference volume. Uniform periodic microstructure formation over large areas is generally challenging in Direct Laser Interference Patterning (DLIP) due to the Gaussian laser beam intensity distribution inherent to most commercial laser sources. ![]()
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