The laser beam expander is a multi-piece 2x beam expander optimized for additive manufacturing (metal 3D printing) optical path applications. The structure uses a full 304 stainless steel water-cooled structure to improve the long-term stability of the optical structure. The QCS interface and the mirror group are designed as one, which greatly improves the concentricity of the entire optical axis and avoids beam quality problems caused by the optical axis.