A single-mode optical cable consists of a small-diameter core (≈9 µm) surrounded by cladding (≈125 µm), designed to guide a single light mode with minimal signal loss.

Core

The core is the central part of the fiber where light travels. In single-mode fibers, the core diameter is typically 8–10 micrometers, with 9 µm being the most common standard . This small core allows only one transverse mode of light to propagate, reducing modal dispersion and enabling long-distance transmission with high bandwidth . The core is made of high-purity glass to minimize attenuation and maintain signal integrity over distances up to 100 kilometers or more .

Cladding

Surrounding the core is the cladding, usually 125 micrometers in diameter, made of glass with a slightly lower refractive index than the core . The cladding ensures total internal reflection, keeping the light confined within the core and preventing signal loss. This reflective layer is critical for maintaining the single-mode propagation and high transmission efficiency.

Coating and Jacket

Outside the cladding, single-mode fibers are typically coated with a protective polymer layer to prevent physical damage and microbending, which can cause signal attenuation . The outer jacket provides mechanical protection and environmental resistance, allowing the fiber to be deployed in various conditions, including indoor, outdoor, and underground installations.

Optical Properties

Single-mode fibers are designed to operate at telecommunication wavelengths, commonly 1310 nm and 1550 nm, with low attenuation (0.3–0.4 dB/km) and high bandwidth . The small core ensures that light travels in a single path, minimizing reflections and dispersion, which is essential for long-haul and high-speed networks .

Summary

The typical single-mode fiber structure can be summarized as:

• Core: 8–10 µm, high-purity glass, single light mode
• Cladding: 125 µm, lower refractive index, total internal reflection
• Coating: Protective polymer layer
• Jacket: Mechanical and environmental protection This structure allows single-mode fibers to support long-distance, high-bandwidth communication, making them the backbone of modern telecommunications, data centers, and FTTH networks .