*3.2.2 Silica fiber fabrication based on DIW technology*

At present, silica fiber preforms can also be made by DIW AM technology using inks mixed with silica and organics besides the DLP method. Erbium-doped optical fiber (EDF) was reported by the University of Southampton using DIW method [57]. Hydrophobic fumed silica and erbium chloride were doped in organics, such as tetraethylene glycol dimethyl ether and polydimethylsiloxane, to form the ink. Then, the ink was printed layer by layer with a printing speed of 40 mm/s at room temperature, and each layer was fixed at 500 μm. The printed rod was debinded and consolidated to glass. Finally, the glass rod was inserted into a fluorinated tube and drawn to EDF, and the fluorinated tube was used to provide the refractive index contrast, resulting in the waveguide structure. The typical absorption peaks of erbium were clearly observed at 980 nm and 1535 nm with the absorption of 62.98 dB/m and 151.49 dB/m, respectively. In addition, the overall absorption was still notable mainly due to the residual OH, PDMS, and impurities from the starting materials.

### *3.2.3 Silica fiber fabrication based on SLS technology*

The use of SLS technology to print silica optical fibers fully utilizes the advantages of SLS technology in producing complex geometric shapes and multi-component printing. Optical fiber preforms with various structures and multi-components are formed by selectively scanning powder layers layer by layer with a CO2 laser beam [58]. Microstructured fibers and anti-resonant fibers were printed by SLS technology. In order to evaluate the performance of the printed silica, a solid preform with 12 mm diameter was printed and inserted into a glass tube and drawn to fiber. A refractive index difference of 4x10−4 was achieved between the printed preform and glass tube. The attenuation of the optical fiber was measured in the range of 600–1150 nm, and the lowest attenuation was 23 dB/m around 800 nm. The high attenuation was mainly induced by the purity of the starting silica powder and parameters setting during the 3D-printing process.

After a series of SLS manufacturing technology optimization, photonic crystal fiber, anti-resonance fiber, and multicore Ge-doped optical fiber preforms were successfully fabricated by the same research group. The multicore Ge-doped optical fiber realizes a waveguide structure with a step index of refraction, and the lowest loss after the drawing reduced to 8.32 dB/m around 800 nm. Experiments have proved that the transmission loss can be effectively reduced by optimizing the powder characteristics and printing parameters [59].
