Luo Xiaoying
Beijing Kaiplin Optoelectronics Technology Co., Ltd
introduction
Laser beam merging is a process in which multiple elements of laser light are coupled into a single beam. Based on the phase, light intensity, polarization and spectrum characteristics of semiconductor lasers, it uses the refraction, reflection and diffraction effects of optical components to change or not change the oscillation characteristics of the laser unit to increase the output power, increase the laser brightness and improve the beam quality. Incoherent beam combining is currently the main way to achieve high-power semiconductor laser output, which can be distinguished by the spectral width of the combined light source and the wavelength spacing of the beam combining unit [1].
Development of bundle technology
1. Single-wavelength combined beam: space photosynthetic beam, polarization combined beam
Beam shaping using laser units with the same or similar lasing wavelengths, spatial beam binding, polarization beam binding, and fiber beam binding, is the basis for multiple laser beam bindings of different wavelengths [1]. Polarization beam combining uses the linear polarization characteristics of semiconductor lasers to pass two beams perpendicular to each other in the direction of vibration through the polarization beam mirror, so as to realize the output of beam coincident, the power is nearly doubled, and the beam quality remains unchanged, Figure 1 is the schematic diagram of Keplin space photosynthetic beam and polarization beam.
Fig.1 Schematic diagram of the photosynthetic beam and polarization beam combining in Keplin space
In addition, due to the large optical anisotropy of GaN materials, it is easier to achieve high polarization in the fabrication of blue light chips. At the same time, the electronic band structure of GaN materials also plays an important role in its high degree of polarization [2]. Therefore, compared with the infrared semiconductor laser chip ~92% polarization, the polarization degree of blue light can reach 99% or even higher, and the polarization beam combining efficiency is higher.
The single-tube beam combination light source is based on a laser single tube, and its slow-axis beam quality is relatively good, without beam shaping, and after the fast and slow axis are collimated, the coupling is directly realized through the dense spatial arrangement in the fast-axis direction and the overall polarization beam. Due to the large spacing of the laser cells and the low effect of thermal channeling, a single blue laser tube can operate at a power of 5 watts [3]. Dozens of single-tube bundles are used to realize the power of a single-wavelength laser output from 100~200 μm core diameter fibers from tens of watts to 300 watts, which has the advantages of high brightness, low cost and good reliability. NUBURU in the United States, FBH in Germany, and United Win Laser, Ruike Laser and Caplin in China have all reported the realization of single-wavelength laser modules with output power greater than 100 W from 100 μm fiber. Among them, Caplin launched a 105 μ m@250 W Blu-ray module, which is the leading multi-tube single-wavelength beam combination product reported in the commercial field.
Fig.2 Dense spatial arrangement technology of Keplin blue laser
2. Fiber optic bunching technology
Fiber combiner is an optical fiber device prepared on the basis of Taper Fused Fiber Bundle (TFB). It is a bundle of optical fibers that is stripped of the coating layer, then arranged together in a certain way, heated at a high temperature to melt it, and at the same time stretches the fiber bundle in the opposite direction, and the heated area of the fiber melts into a molten cone bundle [4]. After cutting from the cone waist, the cone output is spliced with an output fiber. Fiber bunching technology allows multiple individual fiber bundles to be combined into a single large-diameter bundle to achieve higher optical power transmission. Figure 3 is a schematic diagram of the Kaplin blue laser fiber technology.
Fig.3 Schematic diagram of Caplin blue laser fiber technology
In 2023, Kaplin will be the first in China to launch and deliver a 2 kW, 600 μm core diameter, NA0.22 blue laser system equipped with a blue 105 μ m@250 W module to customers.
3. Wavelength combining: coarse wavelength combining, and dense wavelength combining
Directphotonics in Germany uses dense wavelength beam combining technology to achieve 5 laser beam combining with wavelength spacing of 4 nm. At present, the company has launched infrared fiber-coupled semiconductor laser source products with a power of 500~2 kW, a beam quality of 5 mm·mrad, and a core diameter of 100 μm, which are used in metal cutting. The same technology can be applied to blue semiconductor lasers. The research team of Professor Tang Xiahui of Huazhong University of Science and Technology has done relevant research work. The five laser beams with a wavelength interval of 10 nm were combined, with a beam combining efficiency of 26.54% and a beam quality of 3.75 mm·mrad.
4. Spectral beam combining
The spectral beam combining technology can simultaneously realize the simultaneous combination of multiple laser beams with wavelength spacing as low as 0.1 nm, which further increases the number of beam binding units, multiple lasers of different wavelengths are incident on the dispersive elements at different angles, and coincide at the dispersion elements, and then diffracted output in the same direction under the dispersion of the elements, and each beam overlaps each other in the near and far fields, obtaining a combined laser output with the power of the sum of the unit beams and the beam quality consistent with the cell beam [1]. In order to achieve narrowly spaced spectral beam binding, a diffraction grating with strong dispersion is usually used as the beam binding.
The surface grating combined with the feedback of the external cavity mirror does not need to independently control the spectrum of the laser unit, which significantly reduces the difficulty and cost, and is also one of the best solutions to achieve high-power and high-brightness semiconductor laser output. It is the main way to achieve high-performance spectral beam combining.
In 2022, Teradiode, a U.S.-based company that uses wavelength beam combination (WBC) technology, reported a 400 W, 50 μm fiber output and a 1 kW, 100 μm fiber output blue light product. At the 2023 International Conference, it demonstrated 1 kW, 50 μm, NA0.1 fiber output and 1.8 kW, 100 μm, NA0.1 fiber output products, which are currently the highest brightness blue light products in the world, directly increasing the brightness of high-power semiconductor lasers by 2 orders of magnitude, pointing out a new direction for the development of high-power and high-brightness blue semiconductor lasers, which can be directly applied to thick plate metal cutting, remote laser welding, etc.
Welding characteristics of blue laser in the field of non-ferrous metals
The blue laser welding system uses a Kaplin blue laser as a light source, and is equipped with a collimated focusing welding head, an optical fiber-blue light composite welding head and a worktable, which can realize a variety of welding methods such as continuous and pulsed. The system excels in lap and butt welding, and is particularly suitable for the welding of highly reflective materials such as copper, gold, stainless steel, and alloys.
In the welding process, He (or Ar) gas was used as the protective atmosphere, and the welding process under different flow rates, welding speeds, powers, and defocusing amounts was verified. The blue laser can not only do multi-layer stacking welding of copper foil with a thickness of 20 μm, but also realize copper butt welding with different thicknesses such as 0.3 mm and 0.6 mm. During the welding process, the surface of the weld is stable, no spatter and the surface is smooth, and no porosity is found inside the weld under the 40X microscope, as shown in Figure 4.
Fig.4 Results of 0.3/0.6 mmT2Cu welding with 500/1 kW blue light
In addition to the overlapping and butt welding of thin sheet copper, the blue laser has excellent welding performance in the new energy electrode hairpin, electronic chip beryllium bronze pin welding, etc. For thick plate welding, considering that blue light cannot achieve deep penetration welding and the light spot is large, we explored the composite application scheme of blue light and red light. Red light can achieve high power with a small core diameter to increase penetration depth, while blue light takes advantage of the high absorption rate of copper to quickly melt the material while enhancing the absorption of red light. In addition, the large spot of blue light can also expand the melt pool and delay the solidification of the melt pool, so as to achieve low spatter, low porosity and high-quality welding to a certain extent.
Blue lasers also have certain advantages in welding stainless steel. Due to the high absorption rate of blue light in stainless steel, blue laser can effectively convert energy into heat, quickly melt the surface of stainless steel, and achieve high-quality welding. At the same time, the blue laser has a higher energy density and a smaller heat-affected zone, which helps to reduce thermal distortion and oxidation of the welding area, thereby improving the welding quality. Fig. 5(a) shows the 3 kW infrared laser melting the stainless steel surface, and Fig. 5(b) shows the 500 W blue laser melting the stainless steel surface. Figure 5(c) shows the effect of 1 kW blue light welding 1 mm stainless steel.
Fig.5 Results of 304SUS for Keplin Blu-ray processing
For the welding of aluminum and copper-aluminum composite materials, blue laser also has certain applicability. The absorption rate of blue light is relatively low, but with the right power density and spot shape, the blue laser can also achieve effective welding of aluminum materials. In addition, a composite application combining blue and red lasers can be considered to improve the welding effect of aluminum materials to overcome the limitation of low blue light absorption rate of aluminum materials [5]. In addition, blue light also provides a new possibility for the composite welding of copper and aluminum, due to the high temperature of infrared laser during welding, resulting in a large number of brittle intermetallic compounds when copper and aluminum are composite, and the composite application of blue light and red light laser can also play a role in the welding of copper and aluminum composites to improve the welding quality and efficiency. Fig. 6 (a) shows the blue light 500 W welding T2 copper with 1060 aluminum, and Fig. 6 (b) shows the effect of blue light soldering 1060 aluminum.
Fig.6 Keplin Blu-ray 500 W welding T2CU+1060AL and 1060AL
Blue laser and its composite light source with infrared laser are widely used in the field of non-ferrous metal welding and additive manufacturing, which significantly improves the energy conversion efficiency and the stability of the manufacturing process.
summary
As a new direction in the field of semiconductor lasers, compared with 1080 nm near-infrared wavelength lasers, the absorption rate of non-ferrous metal materials such as copper, gold, and aluminum has been increased by several times to dozens of times, and the absorption rate of metals such as titanium, nickel, and iron has also been improved to a certain extent.
High-power blue lasers will lead the revolution in the field of laser manufacturing, and further improving the brightness of blue lasers and reducing costs are the future development trends. In the field of additive manufacturing, cladding and welding of non-ferrous metals, it will be given an increasingly wide range of application scenarios.
In the stage of low brightness and high cost of blue light, the composite light source of blue laser and near-infrared laser can significantly improve the energy conversion efficiency of the existing light source and the stability of the manufacturing process under the premise of controllable cost.
It is of great significance to develop spectral beam combining technology, solve its engineering problems, combine high-brightness laser unit technology, realize kilowatt-level high-brightness blue semiconductor laser source, and explore new beam combining technology. As the power and brightness of lasers increase, blue lasers will gain important applications in defense and industry, both as direct and indirect light sources.
About the Author:
Luo Xiaoying, product manager of Beijing Kaiplin Optoelectronics Technology Co., Ltd., has been engaged in technology development and product management, mainly engaged in product development and manufacturing in core areas such as semiconductor lasers.
From: Photoelectric Hui
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