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Femtosecond laser has the characteristics of short pulse width and high peak power and is widely used in laser micromachining，optical communication, medical applications and other fields. Compared with infrared lasers, ultraviolet lasers have shorter wavelength and higher single-photon energy, which makes the processing heat affected area smaller and can perform precise processing on special materials. With the advancement of science and technology, people are demanding higher precision in laser processing, then ultrashort pulse lasers in the ultraviolet spectrum have gradually came into people's sight. At present, there have been many reports on femtosecond solid-state ultraviolet lasers domestic and overseas. However, solid-state lasers are bulky and difficult to dissipate heat. In contrast, the fiber femtosecond laser has a stable and compact structure and is more suitable for industrial processing. However, it is understood that there are very few domestic studies on femtosecond fiber lasers in the ultraviolet band. In recent years, fiber laser technology has been greatly developed. Femtosecond fiber lasers have good beam quality and high laser directivity, besides it can provide high-power, high-repetition output in the near-infrared spectrum. In this experiment, we propose an ultraviolet femtosecond laser system based on β-BaB2O4 crystal. The fundamental frequency laser passes through two BBO crystals to generate second harmonic generation and third harmonic generation. At the same time, we adjust the angle at which each beam of laser passes through each crystal to meet the conditions of phase matching and obtain the maximum conversion efficiency. It’s worth mentioning that the system has a compact structure, low cost, and high conversion efficiency. Besides, it has broad application prospects.
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