**2. Additive manufacturing technology**

AM technology is a kind of rapid prototyping technology, also known as 3D-printing technology. It is a technique of constructing objects by layer-by-layer printing using bondable materials such as metal powder or plastic, based on a 3D digital model. Compared with traditional manufacturing methods, AM technology has outstanding performance in terms of economic cost, time efficiency, and customized design and has been successfully applied to various materials such as metals, polymers, metamaterials, and composite materials. These materials are shaped using different principles such as sintering, melting, extrusion, and laser/ultraviolet light curing, exhibiting different manufacturing accuracies, printing speeds, and resolutions. According to different materials, AM technology can be divided into solid-based AM technology and liquidbased AM technology, shown in **Figure 4**. For the solid-based AM technology, powder and filament are manufactured to objects by laser sintering/melting (selective laser sintering, SLS; selective laser melting, SLM) and nozzle extrusion (fused deposition modeling, FDM). For liquid-based AM technology, ink or photosensitive resin is shaped by gelation (direct ink writing, DIW) and light polymerization (stereolithography, SLA; digital light processing, DLP; and polymer jet, Polyjet). The above technologies have made great contributions to AM technology waveguides, providing new possibilities for the diversity of waveguide structures and functions.
