**1. Introduction**

Optical waveguides have achieved great success in information transmission in the past decades, mainly due to the ultralow loss, large capacity, high power, and excellent mechanical robustness. Optical fiber as one of the most useful optical waveguides plays an essential role in telecommunications and forms today's Internet backbone. In this chapter, optical fiber is briefly presented in Introduction part, then some AM technologies are focused, and finally, the fabrications of optical fibers based on AM technology are introduced including the fabrication process and perspective.

Optical fiber is a flexible, transparent fiber made of glass or plastic that acts as a light-transmitting tool. Optical fiber usually consists of a core surrounded by a transparent cladding and a coating in order, shown in **Figure 1a**. The refractive index of the core is higher than cladding, creating the waveguide structure to transmit light by total internal reflection (TIR) as demonstrated in **Figure 1b**. Charles K. Kao firstly promoted that the loss of optical fiber could be reduced by removed impurities and applied as the communication medium when he worked at ITTT Standard Telephones and Cables in 1966. This pioneering work made him earn the Nobel Prize in Physics in 2009 [1–3]. However, it was impossible to fabricate ultrapure silica as Kao mentioned due to the technical limitation at that time. Fortunately, the first low-loss (20 dB/km

**Figure 1.** *(a) Diagram of typical optical fiber; (b) TIR in optical fiber.*

at 632.8 nm) silica fiber was achieved by Robert D. Maurer from Corning in 1970 [4], and the modified chemical vapor deposition (MCVD) technology was invented by J. B. MacChesney from Bell Labs in 1974 [5]. Then, the optical fibers have developed rapidly and formed today's internet backbone. In 1999, Kao, Maurer, and MacChesney received the Charles Stark Draper Prize because of making the communication revolution possible [6].

Nowadays, there are some specialty optical fibers except the optical fiber for information transmission. The most representative ones are active fiber and microstructure optical fiber. For the active fiber, rare earth (RE) ions or metal ions are doped into optical fibers, generating luminescence under excitation, such as ytterbium (Yb) [7], erbium (Er) [8], thulium (Tm) [9], holmium (Ho) [10], and bismuth (Bi) [11]. Specific functions can also be achieved by codoping of two or more ions, for example, an ultrabroadband emission covering O-L telecommunication band was obtained from Bi/Er codoped optical fiber under 830-nm pumping, shown in **Figure 2a** [12, 13]. For the microstructure optical fiber, it usually consisted of one or more materials arranged periodically along the fiber length, realizing the refractive index modulation. The principles of light transmission are photonic bandgap effect and anti-resonance effect besides the TIR mentioned above. Microstructure optical fiber has many unique and novel physical properties, such as controllable nonlinearity, endless single-mode behavior, adjustable singular dispersion, low bending loss, and large mode field. **Figure 2b–e** shows the structures of typical PCFs [14–16].

The fabrication of optical fiber usually consists of two steps of preform manufacturing and fiber drawing, shown in **Figure 3**. The fiber drawing process is usually operated on a fiber drawing tower. The silica preform is heated to around 2000°C and becomes soft, then a thin bare fiber can be pulled out and cooled to solid, and finally, the bare fiber is coated and rolled into a coil, demonstrated in **Figure 3c**. For preform fabrication, chemical vapor deposition (CVD) is usually used for regular optical fiber, and microstructure optical fiber preform is manufactured by the stacking method. CVD utilizes SiCl4 and GeCl4 to oxidize into SiO2 and GeO2 at high temperature, which are deposited layer by layer onto the inner side of the quartz tube and sintered to form an optical fiber preform. According to the different deposition ways and heating source, CVD technologies can be divided into plasma chemical vapor deposition (PCVD) [17], outside vapor deposition (OVD) [18], vapor axial deposition (VAD) [19], and also MCVD [5] mentioned above. **Figure 3a** shows the schematic view of the four preform fabrication processes. Stack-and-draw method is usually used for PCF fabrication; glass capillaries

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

#### **Figure 2.**

*(a) Emission spectra of Bi/Er codoped optical fibers [12]; (b)-(e) structures of large mode area fibers, hollow core fiber, Bragg fiber, and anti-resonance fiber.*

and rods of a specific size are stacked according to the designed structure as shown in **Figure 3b** and then drawn into PCF [20].

However, as the Internet has evolved into the ubiquitous Internet of things (IoT), the role of optical fibers is expanding from passive telecommunications transmission medium to lasers, sensors, devices, and beyond. This is creating the demand for

#### **Figure 3.**

*Optical fiber fabrication process, (a) preform fabrication-based CVD technology, (b) preform fabrication-based stack technology, (c) fiber drawing process.*

increasingly sophisticated optical fibers. Unfortunately, the traditional fabrication technologies, for example, the CVD mentioned above, have limited capability in both material and structure flexibility for diverse and custom-designed functionalities. AM technology provides a solution to this problem.
