**3.1 Polymer optical fiber fabricated by additive manufacturing technology**

Polymer optical fibers (POFs) are ideal materials for short-range communications and are increasingly used due to their low cost and high elastic strain limit. POF is usually fabricated by extrusion or drilling method, until Cook et al. reported that POFs were fabricated using FDM 3D-printing technology in 2015 [39]. The commercially available 3D-printing filament consisting of a propriety polystyrene mixture containing styrene-butadiene-copolymer and polystyrene was used as the raw materials. An optical fiber geometry consisting of a solid core surrounded by six air holes was chosen as an example and shaped to preform by FDM 3D printer, as shown in **Figure 5a**. The preform was then annealed and drawn to fibers. The lightguiding images of fiber end faces at 630 nm and 515 nm, white-light output projected onto a screen, and the relative schematic setup are demonstrated in **Figure 5b**. The

loss of the as-made POF was ~1.5 dB/cm, ~0.75 dB/cm, and ~ 1.51 dB/cm at 632 nm, 1064 nm, and 1550 nm, respectively.

Compared with the traditional POF manufacturing technology, the optical fiber preform manufactured by AM technology greatly simplifies the manufacturing process and saves a lot of time and labor costs. In addition, due to the integrated manufacturing, the waste of materials caused by drilling and other methods in the traditional manufacturing process is avoided. Despite the great advantages of using AM technology to manufacture preforms, the loss of manufacturing optical fibers is still high. These losses mainly come from the scattering caused by the gap between layers during the printing process, which can be reduced by an additional annealing process or optimized fiber drawing process [39].

After the first demonstration of POF by AM technology, various types of POFs fabricated by AM technology were reported, such as terahertz (THz) fiber, photonic bandgap fibers, anti-resonant fibers, Bragg fibers, step index fiber with two materials, and magnetically doped fiber with multi-materials by FMD, DLP, STL, and Polyjet methods [40–46].
