Research progress in high-power continuous green lasers

Recently, the rapid development of high-power green lasers has attracted considerable attention from scholars worldwide and has been gradually applied in highly reflective metal processing (such as copper), the 3C electronics industry, medical treatment, underwater laser communications, and military defense. First, this paper introduces the realization of highpower green lasers and reviews research progress achieved globally in high-power continuous green lasers. Compared with solid-state lasers and disk lasers with frequency doubling, fiber lasers have a high electro-optical efficiency, small size, good beam quality, low cost, and good heat dissipation. The single-pass frequency-doubling solution has a simple structure and is easy to adjust and install. The combination of a high-power narrow-linewidth fully polarized fiber laser and a singlepass frequency-doubling scheme is an outstanding ideal solution to achieve high-power and high-brightness miniaturized green laser output. Then, based on the theory of nonlinear effects combined with the second harmonic generation, the formula for efficient frequency doubling is derived. The theoretical and technical study of fiber laser single-pass frequency doubling has been conducted based on the scheme of noncritical phase matching LBO-crystal single-pass frequency doubling with narrow-linewidth linearly polarized fiber laser as the fundamental frequency source, which provides a feasible solution for the localization of high-power green lasers. The main research objectives of this paper are as follows: (1) The research background, application prospects, and development overview of high-power green lasers are introduced. Then, the processing advantages of high-power green lasers are identified and compared with those of near-infrared and blue lasers, and the common technical routes for generating high-power green lasers are compared. (2) Four types of highpower continuous green lasers are introduced: diode-pumped all-solid-state lasers, semiconductor lasers, disk lasers, and narrow-linewidth fiber lasers with frequency doubling. The research progress in narrow-linewidth fully polarized fiber lasers is presented to summarize their current global development status, and the common frequency-doubling crystals and structures and their respective advantageous features are analyzed. The narrow-linewidth linearly polarized fiber laser combined with the LBO-crystal single-pass frequency-doubling solution has the advantages of low cost and simple structures. Thus, this solution is analyzed to provide a feasible solution for generating kilowatt-class high-power green lasers. (3) The key technologies, development trends, and main challenges of the current high-power narrow-linewidth green fiber lasers are presented. The research progress in high-power narrow-linewidth linearly polarized fiber lasers is discussed, and the technologies of narrow-linewidth seed lasers, such as seed source of fiber oscillators and singlefrequency phase modulation, laser power amplification, nonlinear effect suppression, high-efficiency frequency doubling, are investigated in depth. The advantages of fiber oscillator lasers and single-frequency phase modulation seed sources in narrow linewidth and the suppression of nonlinear effects are analyzed. The theoretical study of LBO based on phase matching for an efficient frequency multiplication output is presented according to the Sellmeier equation of crystals. (4) The application of high-power green lasers in the welding processes of metals, such as copper and aluminum alloys, is studied. Finally, an outlook on the development of high-power green laser technology is provided. In the future, more research should be devoted to the study of high-power narrow-linewidth linearly polarized green lasers.