**2. What are optical fibers?**

Optical fibers are dielectric wave-guides, circular in shape that can transport optical information and energy. They have a central core that is surrounded by a concentric cladding that has a slightly lower refractive index than that of the core. Optical fibers are normally made of silica doped with index-modifying dopants such as GeO2 [24–29]. A protective coating of one or two layers of a cushioning material (such as acrylate) is used to reduce cross talk between adjacent fibers and microbending, which occurs when fibers are pressed against rough surfaces. Microbending and cross talk increase the loss of optical energy as the optical beams propagate through the fiber. Fibers are typically incorporated into cables in order to provide environmental protection. Typical cables have a polyethylene sheath that encases the fiber within a strength member such as steel or Kevlar strands. **Figure 2** shows a typical sketch of an optical fiber.

Since the core has a higher index of refraction than the cladding, light will be confined to the core when the condition for total internal reflectance (TIR) is

### **Figure 2.** *Cross-section view of an optical fiber (https://www.newport.com/t/fiber-optic-basics).*

**Figure 3.**

*Close-up view of a fiber preform (https://www.newport.com/t/fiber-optic-basics).*

met. The composition and geometry of the fiber determine the fiber modes or the discrete set of electromagnetic fields, which can propagate in the fiber (**Figure 3**).

Directly connectorized fibers may furthermore provide beam expansion to lower the fiber end facet intensity and reduce the risk of damage at high power levels (**Figure 4**).
