*Additive Manufacturing of Optical Waveguides DOI: http://dx.doi.org/10.5772/intechopen.105349*

"Glassomer" was introduced, which successfully combined the advantages of polymer processing with the excellent material properties of fused silica glass, which was heat treated to form the same optical properties as commercial fused silica glass [48]. Based on the previous two methods, the organic-inorganic hybrid photosensitive resin was prepared, and optical glass was successfully printed by DLP technology [49]. In addition, the use of DLP technology to induce the phase separation of the photosensitive resin provides the possibility for the fabrication of complex structures, and multi-component glass [50]. Lawrence Livermore National Laboratory (LLNL) has also developed a similar organic-inorganic hybrid silica material with sol-gel ink, silica glass printed by DIW technology, and optical glass doped with Ti and Ge [51–53].

## *3.2.1 Silica fiber fabrication based on DLP technology*

The above-mentioned technologies for AM of silica glass lay the foundation for the AM of silica optical fibers. However, the size of silica glasses produced by AM was small, usually only 10 millimeters, which was far from the size of the optical fiber preforms. The main reason may be that the size of the preform is large, impurities are likely to remain in the preform after debinding, and the cooling rate after sintering is too slow, resulting in crystallization or ceramicization, and the overall devitrification of the preform. In response to this problem, Chu. Y et al. optimized the method and successfully used DLP AM technology to manufacture traditional single-mode fiber and multimode fiber in 2019 [54]. Then, the research group continued their work and fabricated multi-component and multicore optical fibers using DLP 3D-printing technology [55]. The process of bismuth and erbium codoped optical fiber (BEDF) manufactured by AM technology is shown in **Figure 6**.

Firstly, silica nanoparticles were dispersed into a UV photosensitive monomer forming a stable photosensitive resin. Secondly, the pre-designed optical fiber preform was printed by DLP printer, and functional materials such as Ge4+, Er3+, and Bi3+ were doped into the core. Ge4+ was used to adjust the refractive index to realize the waveguide structure, but Er3+ and Bi3+ were utilized to achieve the broadband nearinfrared luminescence, presented in **Figure 6a**–**c**. Thirdly, the printed preform was moved to a furnace to remove the organic components and achieves densification. The temperature setting is shown in **Figure 6d**, the mass and size change clearly pointed out that the organics were removed before 600°C and the preform started to densify after 600°C. Finally, the preform was drawn to the fiber at 1855°C. Before drawing, the preform was heated to 810°C and kept for 3 hrs for removing the moisture absorbed by the preform due to the porous structure during storage.

The drawn fiber was characterized by X-ray diffraction (XRD) using the powdered BEDF without coating, and the pattern pointed out that the fiber was amorphous, shown in **Figure 6e** inset. The cross-sectional microscopy images and electron probe micro-analysis (EPMA) of BEDF are shown in **Figure 6f** and **Figure 6g**, respectively. Although the crack and shrinkage were noticed, multicore structures were kept. Elements distribution of BEDFs was as excepted, resulting in a refractive index difference between the core and cladding to form the waveguide structure, shown in **Figure 6g**–**i**. The loss and emission spectra of BEDF are demonstrated in **Figure 6j**–**k**. Typical characteristic peaks of bismuth and erbium were clearly identified.

DLP additive preform manufacturing has received attention from peers. In 2021, Zheng et al. have made progress in the AM of microstructured optical fibers; a ytterbium-doped microstructured optical fiber preform with a diameter of about 12 mm and a length of 20 mm was fabricated. The preform was then drawn to fibers,

#### **Figure 6.**

*(a) Dispersion of silica nanoparticle into ultraviolet curable resin, (b) preform cured by DLP 3D printing with UV light at 385 nm, (c) preforms before and after core filling, the scale bar is 10 mm, (d) temperature setting of the preform debinding process, inset is the remaining ratio of mass and size during the debinding process, (e) temperature change of the fiber drawing process. Insets are the XRD pattern of the drawn fiber and the photo of the drawing tower, (f) fiber cross-sectional images recorded by a microscope with a 50-μm scale bar, (g) cross-sectional view of seven-core BEDF, and EPMA-WDS mappings of different elements from the cross section (scale bar: 10 μm), (h)-(i) three-dimensional refractive index profiles of single- and seven-core BEDF, (j) loss spectrum of the singlecore BEDF, (k) emission spectra of a single-core BEDF excited by the 830-nm and 980-nm lasers [54, 55].*

the core was doped with 0.1 wt% ytterbium oxide, and six air holes were evenly distributed in the cladding [56]. The relevant information is shown in **Table 1**.

However, there are several difficulties in the process of printing silica fiber with DLP technology:


formula suggests that the layer thickness should be slightly lower than the cure depth to ensure a close connection between the layers. In addition, attention should be paid to the balance between printing efficiency and layer thickness exposure time parameter selection while ensuring successful printing.



**Table 1.**

*Additive manufacturing on silica optical fibers (Micro-Stru.: micro-structure, D: outer diameter of preform, L: length of preform, SM: single mode, MM: multimode, d: diameter of optical fiber, dc: diameter of fiber core, MClad: material of fiber cladding, MCore: material of fiber core).*
