With a rise of electric vehicles and a vigorous development of new energy industry, an urgent need for high power circuit and battery technologies has been put forward. As an important conductive material, a high-effective and stable welding of copper has become has become a focus of attention. Since copper has a high thermal conductivity and the near-infrared laser absorption rate of copper is relatively low at room temperature and will suddenly rise near the melting point, the laser welding process can be easily affected by the surface quality of workpieces. Copper laser welding is still a very challenging issue.
The defects in copper laser welding can be addressed by using long pulse green light (pulse width greater than 100 μs). In the green band, the laser absorption rate of copper can reach 40%, and the focused green light has a smaller focal spot and a higher power density, thus a high-quality welding spot can be easily accessed. Therefore, using long pulse green laser as a welding light source can significantly improve the productivity and quality of copper wielding.
Currently, high energy laser of visible spectrum is mainly based on a short or ultra-short pulse laser apparatus, such as a Q-switched laser apparatus. For a quasi-continuous high energy solid-state laser system, the conversion efficiency is relatively low due to the beam quality and the peak power density of the fundamental frequency light. A conventional long pulse green light laser apparatus with intracavity frequency doubling improves the fundamental frequency light power density in the nonlinear crystal by inserting a lens into the resonator. However, this method requires to restrict the assembly position of the nonlinear crystal to a quite small range, and the nonlinear crystal can be easily damaged.