In this study, a variable focusing lens with a driving mechanism was developed. The surface curvature of the lens is formed via the interaction of elastic film tension and liquid pressure generated by compression of a bellows, which enables the focal length to be changed. This paper discusses the optical characteristics of the lens system. The focal length range is 250 – 750 mm with beam diameters of 0.3 mm – 0.9 mm, respectively, when a YAG laser is irradiated. In the study, the lens system was applied to laser beam welding of mild-steel sheet with a thickness of 0.5 mm for 3,000 shots of laser irradiation at a peak power of 4.7 kW and a duration of 10 ms.
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