Additive manufacturing (AM) holds great development potential in several applications. In particular, the fabrication of metal parts has been rapidly developing as an industry recently. In such applications, a powder feeder with low material loss is required to ensure effective material usage. Thus, the manufacturing process using laser-induced forward transfer (LIFT) is attractive as a candidate for advances in powder feeder technology. We show that stainless steel particle with diameters of 26 to 53 μm can be transferred via the particle LIFT process; the transfer of particles started at a peak fluence of ∼0.05 J cm − 2 (26- and 36-μm diameters) and ∼0.15 J cm − 2 (53-μm diameter), and the particles were fully transferred (i.e., the probability of transfer reaches unity) at a peak fluence of 0.2 (26 μm), 0.4 (39 μm), and 0.5 J cm − 2 (53 μm). The initial velocity, v0, at which the particle leaves the substrate increased proportionally with the increase in peak fluence. When the point of irradiation of the laser beam was displaced from the contact point of the particle on the substrate, the particle was transferred with a transfer angle (i.e., not vertically). These results demonstrate the possibility of transferring metal particles via the LIFT process. To use this technology for AM, the laser power should be adjusted to control the transfer angle of the particle.
Laser processing of transparent materials, particularly deep hole machining, has been extensively studied from the viewpoint of its industrial use. This time, we tried to improve the processing depth by laser ablation processing for back surface of fused silica glass using a picosecond laser with a wavelength of 532 nm. The numerical aperture of lens was 0.4, and the laser was focused on the back surface of the glass. In addition, the input laser was divided into double pulses and compared with the processing results of a single pulse. As a result, it was shown that the processing depth of back surface ablation is higher than that of surface ablation. It was also found that by using a double-pulse that sets the first laser to a laser output below the fluence threshold and the second laser to the laser over the threshold, a deeper processing depth can be obtained. At the same time, when the shapes of the processing traces were observed, the processing traces were not a simple concave-shaped hole, but a donutshaped processing traces with a raised center. In double-pulse laser processing where the first laser is set above the threshold, we find that the processing traces show traces that are close to single-pulse traces.
The process of removing transparent material such as glass by means of laser ablation is widely performed in the industry. In many cases, the ultrashort pulse laser is used as the laser source, but the laser wavelength is diverse. The ablation mechanism of the transparent material generally is considered nonlinear absorption, but it is known that the absorption coefficient depends on the wavelength. However, there are few examples that discuss the ablation mechanism for multiwavelengths. We performed laser ablation of glass with a 15-ps pulse width and 1064, 532, and 355 nm wavelengths and constructed a unified understanding of the principle by numerical calculation using combined rate equation and beam propagation method. In addition, we measured the three-dimensional profile of the processing traces using a microscope and investigated the dependence of ablation volume to the wavelength. As a result, we acquired the results of ablation efficiency, laser fluence threshold, and inverse absorption coefficient of fused silica glass using various wavelengths in a single set-up. We were able to explain the ablation diameter and depth by numerical calculation considering the multiphoton ionization process and avalanche ionization process. In addition, the calculated laser efficiency indicated that the wavelength suitable for ablation on glass was 355 nm.
Recent years have seen an increase in the demand of laser processing of transparent materials because of the numerous applications, such as the formation of through-silicon vias, glass scribing, the creation of optical wave guides, and so on. Furthermore, laser processing is expected to be used for fabricating photonic devices and circuits. Although significant research efforts have focused on laser processing of transparent materials, many unexplained mechanisms remain to be elucidated. In particular, mechanisms that remain unclear include plasma absorption and the process whereby traces expand when using double pulses. In 2017, to improve the laser-processing speed and efficiency, we proposed a method for cutting transparent materials called the “double-pulse explosion drilling method,” which uses two laser pulses of differing wavelengths to create internal modifications in a material. In the present study, we use the double-pulse method to drill through transparent materials and investigate how the second pulse affects laser-beam absorption and the generation of processing traces. We used a picosecond laser with pulses at 532 and 1064 nm for the first and second pulse, respectively. The target material was fused silica glass. The results clarify how the use of double pulses improves the processing efficiency. This presentation gives the experimental results and discusses the processing mechanisms at work in the double-pulse method.
We show the effects of double pulse processing with a pico-second laser for internal modification of transparent materials. In recent years, the need for laser processing of transparent materials has been growing. Many applications for such laser processing exist, such as the formation of through-silicon via, scribing glass and creating optical wave guides. Creation of photonic devices and circuits are also expected. A large amount of applied research on the processing of transparent materials has been conducted, but many of the processing mechanisms remain unexplained. In general, fluence thresholds are related to free-electron density caused by multi-photon ionization and avalanche ionization. Additionally, the internal fluence threshold of transparent materials is lower than surface fluence thresholds. We applied the double pulse method to the internal modification of transparent materials, and investigated how the second pulse affects the generation of microcracks. We used a pico-second laser with a 532-nm wavelength for the first pulse and a 1064-nm wavelength for the second pulse. The target was fused silica glass. In this paper, we show our experimental results and discuss the processing mechanism of the double pulse method. We call it the double pulse explosion drilling method. Moreover, we discover a processing method that can be applied to the high-rate drilling and scribing of transparent materials.
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