What Is A High Power Blue Semiconductor Lasers?

The Brightness and High Power Blue Semiconductor Lasers are constantly improving to new limits, which will also lead to more and wider applications. In addition to efficient metal material processing, blue semiconductor lasers expect cross-sector applications, in particular the mechanical engineering sector will enable laser material processing with blue light underwater. For manufacturing, this is of course a huge advantage. In addition, the lighting industry can also use high-quality lighting technology based on blue semiconductor lasers.

1. Limitations of high-power lasers at near-infrared wavelengths

Over the past few decades, high-power CW lasers have become a common tool 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 laser technology appeared before 2000, when a high-power 10.6µm wavelength carbon dioxide (CO2) laser and a near-infrared 1064nm wavelength semiconductor-pumped Nd:YAG solid-state laser were developed. However, due to its wavelength, carbon dioxide lasers are difficult to transmit through optical fibers, which poses 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 solutions for high-brightness, high-power lasers that could be delivered through optical fibers. Today, fiber lasers have replaced CO2 lasers in the vast majority of applications and have been effectively used in many industrial processing applications. Especially in recent years, it has become the main force of industrial lasers, such as laser welding and cutting, which has higher speed, efficiency and reliability than carbon dioxide lasers.

However, these CW high-power fiber lasers typically operate at near-infrared (NIR) wavelengths, within 1 µm, which is fine for many applications. For example, it is suitable for the processing of steel with an absorption rate of more than 50%, but it is limited because some metals reflect 90% or more of near-infrared laser radiation incident on their surfaces. Especially welding yellow metals such as copper and gold with near-infrared lasers, due to the low absorption rate, this means that a lot of laser power is required to start the welding process. There are generally two laser welding processes: conduction mode welding (where the material is simply 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 results in a highly absorbed laser beam due to the multiple interactions the laser beam has with the metal and metal vapor as it travels through the material. However, actuating the keyhole with a near-infrared laser requires considerable incident laser intensity, especially if the material being welded is highly reflective. And once the keyhole is formed, the absorption rate will rise sharply, and the high metal vapor pressure generated by the high-power near-infrared laser in the molten pool will cause spatter and porosity, so the laser power or welding speed needs to be carefully controlled to prevent excessive Spatter is ejected from the weld. Metal vapors and "bubbles" in the process gas may also become trapped as the molten pool solidifies, creating porosity in the weld joint. Such porosity weakens the weld strength and increases the joint resistivity, resulting in a lower quality welded joint. Therefore, NIR lasers are very challenging to process materials such as copper with <5% absorption at 1 µm. In order to process these high-reflectivity materials better, methods such as increasing the laser absorption rate of the material by generating plasma on the processed material have been adopted. However, because these methods limit material processing to deep penetration processes, conduction mode welding cannot be used for thin materials, and there are inherent risks of sputtering and controlled energy deposition. Therefore, existing 1 µm laser systems have their limitations when processing highly reflective materials such as non-ferrous metals, as well as in underwater applications.

In order to develop these near-infrared laser-controlled applications, people must conduct research on new laser light sources. In addition, in order to reduce greenhouse gases, new energy vehicles are replacing gasoline engines and internal combustion engines with electric engines. The large amount of copper used in the construction of electric motors, especially power batteries, has created a huge demand for reliable copper processing solutions, while in other renewable energy systems such as wind turbines, there is an equally wide range of applications.

2. The birth of high power blue laser

The development of industrial laser technology has always been developed along the roadmap of production technology and new social requirements. In the past 60 years, from digital economy and society, to sustainable energy, to healthy life, laser technology has made great contributions to solving important tasks in the future of mankind. Today, laser technology is an integral part of many core areas of our economy, from production technology to automotive engineering, medical technology, measurement and environmental technology, and information and communication technology. As metal processing technology continues to advance and user requirements continue to increase, lasers require innovations in terms of cost and energy efficiency as well as laser system performance. The market demand for efficient processing of highly reflective metals has stimulated the development of blue high-power laser technology, which will surely open the door to new technologies in metal processing.

For non-ferrous metals, their absorption of light energy increases as the wavelength of light decreases. For example, the light absorption of copper at wavelengths below 500nm will increase by at least 50% compared with infrared light, so short light wavelengths are more suitable for copper processing. The problem is that developing short-wavelength, high-power lasers for these industrial applications is difficult; few high-power options are available, and even those that do exist are expensive and inefficient. For example, there are some solid-state laser sources on the market based on frequency doubling that can be used in this wavelength range, producing laser light at wavelengths of 515nm and 532nm (green spectrum). However, these laser sources rely on their nonlinear optical crystals to convert the pump laser energy to the energy of the target wavelength. The conversion process results in high power loss, and the laser requires complex cooling systems and complex optical setups.

In order to meet this challenge, people put their attention on blue semiconductor lasers. One is because Blu-ray has its specific properties. High-reflectivity metal materials have a high absorption rate of blue light, which means that blue light has a huge advantage in metal processing of highly reflective materials (such as copper, etc.). As shown in Figure 1, the absorption of blue light by copper is more than 13× (13 times) higher than that of infrared light. In addition, the absorption rate does not change much when copper is melted. Once the blue laser starts welding, the same energy density will keep the welding going. Blu-ray laser welding is inherently well-controlled and less-defective, and the result is fast and high-quality brazed welds. At the same time, blue light is less absorbed in seawater, so it has a longer transmission distance, which makes it possible to develop the field of underwater laser material processing. In addition, blue light is relatively easy to convert to white light, so floodlights and other lighting applications can be implemented very compactly using blue lasers. The second is that semiconductor lasers based on gallium nitride materials can directly generate laser light with a wavelength of 450nm without further frequency doubling, so they have higher energy conversion efficiency.

The laser with a wavelength of 450nm is expected to increase the processing efficiency of copper materials by nearly 20 times compared with the wavelength of 1µm. Compared with traditional near-infrared laser welding processes, high-power blue lasers have quantitative and qualitative advantages. Quantitative advantages: increased welding speeds and a wider process window translate directly into faster productivity and minimized production downtime. Qualitative advantages: greater process latitude, spatter-free and porosity-free high-quality welds, as well as higher mechanical strength and lower electrical resistivity. The consistency of welding quality can greatly improve the production yield. In addition, the blue laser can also conduct heat conduction welding mode, which is not possible with the near-infrared laser.

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