Single-mode optical fibers are primarily made of high-purity silica glass for the core and cladding, often with carefully controlled dopants and protective polymer coatings.

Core and Cladding Materials

The core of a single-mode fiber is typically made from ultrapure fused silica (SiO₂), which provides extremely low optical attenuation and high mechanical strength . The cladding surrounding the core is also silica but is usually doped to have a slightly lower refractive index than the core, enabling total internal reflection to guide light efficiently . Common dopants include:

• Germanium dioxide (GeO₂): Increases the refractive index of the core but can introduce photodarkening under UV exposure .
• Fluorine (F): Often used in the cladding to reduce the refractive index relative to the core, improving light confinement . Some fibers, like Thorlabs' SM300 and SM400, use undoped pure silica cores with fluorine-doped cladding to minimize photodarkening and enhance power handling, especially in the UV region .

Coatings and Protective Layers

Single-mode fibers are coated with polymer layers to provide mechanical strength, flexibility, and environmental protection. These coatings prevent microbending, moisture ingress, and mechanical damage without interfering with optical transmission . Typical coatings include:

• Acrylate coatings: Common for standard fibers, providing flexibility and durability.
• Dual-layer coatings: Some fibers use a soft inner layer for stress relief and a harder outer layer for abrasion resistance .

Specialty Glass Fibers

While fused silica is standard, specialty fibers may use other glass types for specific applications:

• Fluoride fibers: For mid-infrared transmission.
• Chalcogenide fibers: For infrared and sensing applications .

Summary

In essence, single-mode optical fibers rely on high-purity silica glass for both core and cladding, with dopants to control refractive index and polymer coatings for mechanical protection. The precise combination of these materials ensures low attenuation, high bandwidth, and long-distance signal fidelity, making them ideal for telecommunications, sensing, and laser applications .