1 Limitations of high-power lasers with near-infrared wavelengths Over the past few decades, high-power continuous-wave lasers have become versatile tools in modern manufacturing, covering applications such as welding, cladding, surface treatment, hardening, brazing, cutting, 3D printing, and additive manufacturing. The first development peak of high-power continuous-wave laser technology occurred before 2000, when high-power carbon dioxide (CO₂) lasers with a wavelength of 10.6µm and semiconductor-pumped Nd:YAG solid-state lasers with a near-infrared wavelength of 1064nm were developed. However, CO₂ lasers, due to their wavelength, are difficult to transmit through optical fibers, posing certain difficulties for industrial applications; while solid-state lasers are limited by brightness and power amplification capabilities. After 2000, high-power industrial fiber lasers began to emerge as a solution for high-brightness, high-power lasers that can be transmitted through optical fibers. Today, fiber lasers have replaced CO₂ lasers in most applications and have been effectively used in numerous industrial processing applications. Especially in recent years, they have become the mainstay of industrial lasers, such as in laser welding and cutting, where they offer higher speed, efficiency, and reliability than CO₂ lasers. But these continuous high-power fiber lasers generally operate at near-infrared (NIR) wavelengths, within 1µm, which works for many applications. For example, they are suitable for processing steel with an absorption rate exceeding 50%, but are limited because some metals reflect 90% or more of the incident near-infrared laser radiation on their surfaces. In particular, welding yellow metals such as copper and gold with near-infrared lasers, due to low absorption rates, means that a large amount of laser power is required to initiate the welding process. There are usually two laser welding processes: heat conduction mode welding (where the material is only melted and reflowed) and deep penetration mode welding (where the laser vaporizes the metal and the vapor pressure forms a cavity or keyhole). Deep penetration mode welding leads to high absorption of the laser beam because the laser beam interacts multiple times with the metal and metal vapor as it propagates through the material. However, initiating a keyhole with near-infrared lasers requires a considerable incident laser intensity, especially when the material being welded is highly reflective. Moreover, once the keyhole is formed, the absorption rate increases sharply. The high metal vapor pressure generated in the molten pool by high-power near-infrared lasers can cause spatter and porosity, so laser power or welding speed needs to be carefully controlled to prevent excessive spatter from being ejected from the weld. When the molten pool solidifies, "bubbles" in the metal vapor and process gas may also be trapped, forming pores in the weld joint. Such pores can weaken the weld strength and increase the joint resistivity, resulting in reduced weld joint quality. Therefore, near-infrared lasers are highly challenging for processing materials such as copper, which have an absorption rate of <5% at 1µm. To better process these highly reflective materials, methods such as generating plasma on the processed material to increase the material's absorption rate of the laser have been adopted. However, because these methods limit material processing to deep penetration processes, heat conduction mode welding cannot be used for thin materials, and there are inherent risks such as sputtering and controlling energy deposition. Thus, existing laser systems with a wavelength of 1µm have limitations when processing high-reflectivity materials such as non-ferrous metals and in underwater applications. To develop application fields where these near-infrared lasers are restricted, research on new laser light sources must be conducted. Additionally, to reduce greenhouse gases, new energy vehicles are replacing gasoline engines and internal combustion engines with electric motors. Electric motors, especially in the construction of power batteries, use a lot of copper materials, which creates a huge demand for reliable copper processing solutions. There is also a similarly wide range of application demands in other renewable energy systems such as wind turbines. 2 The birth of high-power blue lasers The development of industrial laser technology has always followed the roadmap of production technology and new social requirements. Over the past 60 years, from the digital economy and society to sustainable energy and healthy living, laser technology has made great contributions to solving important tasks for humanity's future. Today, from production technology to automotive engineering, medical technology, measurement and environmental technology, and information and communication technology, laser technology has become an indispensable part of many core areas of our economy. With the continuous advancement of metal processing technology and the increasing requirements of users, lasers need to innovate in terms of cost, energy efficiency, and laser system performance. The market demand for effectively processing highly reflective metals has stimulated the development of high-power blue laser technology, which is sure to open the door to new metal processing technologies. For non-ferrous metals, their absorption of light energy increases as the light wavelength shortens. For example, copper's absorption of light with wavelengths below 500nm is at least 50% higher than that of infrared light, so shorter wavelengths are more suitable for copper processing. The problem is that developing high-power lasers with short wavelengths for these industrial applications is difficult, with few high-power options available, and even existing options are expensive and inefficient. For example, there are some solid-state laser sources based on frequency doubling available in this wavelength range, generating lasers with wavelengths of 515nm and 532nm (green spectrum). However, these laser sources rely on their nonlinear optical crystals to convert pump laser energy into energy at the target wavelength, a conversion process that leads to high power loss, and the lasers require complex cooling systems and complex optical setups. To address this challenge, attention has been turned to blue semiconductor lasers. Firstly, because blue light has specific properties. Highly reflective metal materials have a high absorption rate of blue light, which means blue light has great advantages in processing high-reflectivity materials (such as copper). As shown in Figure 1, copper's absorption of blue light is more than 13 times higher than that of infrared light. In addition, the absorption rate of copper changes little when it melts. Once blue laser welding starts, the same energy density will allow the welding to continue. Blue laser welding has inherently good control and few defects, resulting in fast and high-quality copper welds. At the same time, blue light is less absorbed in seawater, so it has a longer propagation distance, making it realistic to develop underwater laser material processing fields. Furthermore, blue light is relatively easy to convert into white light, so blue lasers can be used to achieve floodlights and other lighting applications in a very compact manner. Secondly, semiconductor lasers based on gallium nitride (GaN) materials can directly generate laser light with a wavelength of 450nm without further frequency doubling, thus having higher energy conversion efficiency. The processing efficiency of 450nm wavelength lasers on copper materials is expected to be nearly 20 times higher than that of 1µm wavelength. Compared with traditional near-infrared laser welding processes, high-power blue lasers have advantages in both quantity and quality. Quantitative advantages: increased welding speed, expanded process range, which can be directly converted into faster production efficiency, and minimized production downtime. Qualitative advantages: a larger process range, high-quality welds without spatter and porosity, higher mechanical strength, and lower resistivity. The consistency of welding quality can greatly improve production yield (see Figure 2). In addition, blue lasers can also perform thermal conduction welding mode, which is impossible with near-infrared lasers (see Figure 3). 3 The development of high-power blue lasers With the awarding of the 2014 Nobel Prize in Physics and the increasing global awareness of environmental protection, gallium nitride (GaN) light-emitting devices have received widespread attention, especially in the lighting field. By continuously improving the high brightness and high output of blue semiconductor devices, blue semiconductor lasers have entered the era of mass production, but they are mainly used as projector light sources, replacing lamps in projectors, and used together with phosphors that generate green or red light. Because blue semiconductor lasers have a longer lifespan and smaller size compared to light bulbs [1], they have rapidly become popular in lighting and display applications in recent years. However, for laser processing, higher power is required than these blue lasers used for lighting. Due to the many advantages of blue lasers as mentioned above, efforts have been made to develop high-power blue lasers for laser processing. Since a single blue laser semiconductor chip has an output power of only a few watts, increasing its power to a higher power range is very time-consuming and expensive. To achieve the high power required to tap the huge application potential of blue lasers, new technical methods will be needed. So far, the actual power of each blue semiconductor laser chip at a single wavelength is about 5W [2], so beam combining technology that combines the outputs of multiple chips is essential to obtain higher power output. Beam combining methods are divided into coherent and incoherent methods. Among them, the incoherent method is more practical and does not require precise phase control between lasers. Incoherent methods include spatial combining, which spatially combines multiple beams; polarization combining, which combines orthogonally polarized light in a polarization beam splitter; and wavelength combining, which coaxially combines different wavelengths. Each method has its advantages and disadvantages, and each method can also be used in combination. Among them, spatial combining is suitable for combining multiple laser chips of the same wavelength to obtain high-power output [3]. So far, two high-power combining methods have been the most successful, which are briefly introduced below. The first method is to use Laser Bars technology, which systematically generates single laser chips (Single Emitters) on indium gallium nitride (InGaN) wafers. First, multiple individual laser chips are efficiently integrated into a so-called laser bar, and each laser bar can generate at least 50W of blue light. Then, by appropriate electrical connections, cooling, and the use of special optical devices, multiple semiconductor laser bars are mounted and combined into a semiconductor laser stack. The entire semiconductor laser can be composed of one or several semiconductor laser stacks, as shown in Figure 3. Currently, laser bar technology can achieve a blue light power of 2kW [4]. The second method is to use Single Emitter technology. These lasers have a unique "single-chip-based" design feature, with each gallium nitride (GaN) laser single emitter's output collimated separately. If, like the bar technology, a single lens is used to collimate all laser single emitters, the combined beam divergence (BPP) will inevitably increase. However, collimating each laser single emitter with its own dedicated lens can minimize the combined beam divergence, reduce the beam BPP to the lowest, and thus improve the laser brightness. Moreover, as gallium nitride laser single emitters continue to increase their single-tube laser power following their expected development path, this unique "single-chip" design provides the best way to improve the overall laser system power. In addition, the laser single emitter technology has produced the best beam quality currently achievable with 1.5kW output power, which provides a guarantee for galvo-scanned laser remote processing [5]. This scanning system is commonly used in battery, electric vehicle, and consumer electronics manufacturing. Laser output power and dwell time can be adjusted during scanning operations, maximizing productivity by allowing different joint geometries and material thicknesses to be addressed in a single scanning pattern. Table 1 shows the advantages of blue semiconductor lasers compared to near-infrared semiconductor lasers and green solid-state lasers. 1) Blue light has a wide process window, which can handle each stage of battery manufacturing, and can weld thicker and various materials, such as copper, gold, and stainless steel with a thickness of several millimeters. It is ideal for manufacturing prismatic batteries, battery casings, and battery pack and battery integration. 2) Using a 450nm wavelength blue semiconductor light source, copper materials can be melted in thermal conduction mode, allowing precise adjustment of the molten pool geometry of thin copper materials. Stable energy absorption and precise control of the thermal conduction process are particularly important for deep penetration welding of thin copper materials, mainly because it helps prevent cutting or spattering of thin materials due to high pressure. These phenomena are especially likely to occur when welding stacked thin copper foils, which may produce difficult-to-control irregular gaps due to the warping of the stacked foils. When butt welding 34 stacked copper foils with a 580W blue semiconductor laser at a speed of 2m/min, a weld width of >0.8mm can be formed with minimal porosity and low undercut. For fillet welding on the edge of the foil stack, the ends of the foil are successfully melted into a high cross-sectional area and completely attached to the solid foil. In both butt and edge welding, perfect mechanical connections and very good electrical conductivity can be achieved. 3) With 3 copper foils stacked with a thickness of 30μm, the copper foils are scanned with a laser from the top surface at a speed of approximately 10mm/s. Since the fiber output with a core diameter of 100μm is focused at a 1:1 projection ratio, the laser spot diameter on the sample surface is also 100μm, achieving good welding quality and suppressing the impact of heat on debris and the surrounding environment. 4) Printers can produce pure copper using blue semiconductor lasers developed by Osaka University. A 100μm laser focusing spot diameter is achieved on the powder bed, enabling lamination of pure copper with high electrical and thermal conductivity, which was previously difficult to melt with near-infrared lasers. This technology is expected to be applied in industrial fields such as aerospace and electric vehicles. 5) The greater penetration depth has also opened up applications in electric vehicles. Electric vehicle manufacturers are turning to rod winding designs to maximize thermal and electrical efficiency. These three blue laser hairpin welds show consistent quality, which is crucial for improving production efficiency. Blue lasers can produce hairpin welds, which are important for manufacturing high-density and high-strength motors. 6) High power and high brightness also increase the flexibility of the welding process, making it possible to expand the range of processed materials. For example, copper and zinc in brass have significantly different thermal properties, which poses challenges for high-quality welding, but blue industrial lasers can handle it easily, and now brass materials commonly used in home appliance production can be welded. Preliminary studies have shown that blue lasers will effectively solve the problem of welding dissimilar metals. Welding dissimilar metals is a challenge because each material has unique thermal, optical, and mechanical properties. Welding dissimilar metals usually leads to the formation of intermetallic compounds, i.e., regions of different alloys, which impair the mechanical and electrical properties and consistency of the joint. The latest generation of blue semiconductor lasers has a wide range of process parameters, enabling welding of dissimilar materials with minimal defects. Although copper and zinc in brass have significantly different thermal properties, which poses challenges for high-quality welding, it is easy to handle with blue semiconductor lasers. 4 The commercialization of blue lasers first developed abroad With the support of a three-year German government research program EffiLAS (Efficient High-Power Laser Beam Sources), Germany's Laserline developed the first blue kilowatt-level semiconductor laser. In 2018, Laserline continued to launch a 500W 600μm prototype; in 2019, the company first displayed the world's first 1kW 400μm commercial blue semiconductor laser at an exhibition. By early 2020, Laserline had announced the commercialization of a 2kW 600μm blue laser product. NUBURU in the United States is also a major company researching and developing blue lasers, committed to expanding the application scale of blue lasers in manufacturing fields such as consumer electronics, batteries, and electric vehicles. It developed blue semiconductor lasers as early as 2017 and launched a 1500W 100μm ultra-high-brightness blue laser in 2020. Germany's DILAS developed a 1.6W TO-packaged blue single emitter in 2013, which can obtain 100W, 400μm/0.22 NA 450nm laser output. Then in 2014, DILAS used multiple blue single emitters with improved slow-axis beam quality to achieve a fiber with a core diameter of 200μm/0.22 NA through spatial combining and polarization methods, outputting a 100W 450nm fiber-coupled module with a coupling efficiency of 82%, which can be applied in laser medical treatment and laser display. In 2015, DILAS launched a 450nm wavelength blue visible light semiconductor laser system. Shimadzu of Japan announced in 2015 the successful development of a fiber-coupled high-brightness blue direct diode laser "BLUE IMPACT", which uses blue gallium nitride-based semiconductor lasers and is one of the few laser processing light sources that have been commercialized. By February 2019, Shimadzu of Japan announced the development of a blue semiconductor laser with an output power of 1kW in cooperation with Osaka University. In addition, Nichia Corporation of Japan reported a blue laser with a slope efficiency of 1.8W/A and an optical power of about 5W; Sony Corporation reported a blue laser with a slope efficiency of 1.8W/A and an optical power of about 5.2W; Osram Group of Germany also reported a blue laser with a slope efficiency of 1.6W/A and an optical power of 4.5W. Foreign industries have made considerable progress in GaN blue lasers. 5 The development of blue lasers in China follows closely 1) Research progress The development of blue lasers in China is slightly later than abroad, but the scientific research and industrial circles are also stepping up research and development. In recent years, Chinese research institutions and enterprises have followed up one after another and launched a number of blue semiconductor lasers. In 2004, the Institute of Semiconductors, Chinese Academy of Sciences developed China's first GaN blue laser. In September 2020, the Guangdong-Hong Kong-Macau Greater Bay Area Hard Technology Innovation Research Institute (referred to as "Guangdong Hard Science Institute") launched the first independently developed industrial-grade blue semiconductor direct output laser with an output power of 500W; in March 2021, Guangdong Hard Science Institute made further breakthroughs and launched a 1000W blue semiconductor laser. In February 2021, the team of Kang Junyong and Li Jincha from Xiamen University, together with San'an Optoelectronics, made a breakthrough in the joint technology research project. The design and production of ultra-8W high-power InGaN blue lasers have reached the international level. In addition, a number of efficient research institutes such as Huazhong University of Science and Technology are stepping up their research. 2) Commercialization progress Many domestic enterprises have launched blue lasers, such as Raycus, Maxphotonics, and Laserline. Among them, as early as 2019, Raycus launched a 200W 200μm semiconductor laser at the Western Photonics Exhibition in the United States. To meet the needs of domestic and foreign customers for high-cost performance,