Single-core multimode optical fiber

A single-core multimode optical fiber transmits light through a single, large-diameter core that supports multiple propagation paths (modes) using total internal reflection.

Structure and Core Concept

A single-core multimode fiber consists of a central core surrounded by a cladding layer with a lower refractive index, and an outer protective jacket. The core diameter is typically 50–62.5 micrometers, much larger than single-mode fibers, allowing multiple light rays to propagate simultaneously along different paths or modes . The cladding ensures that light remains confined within the core by total internal reflection, which occurs because the core has a higher refractive index than the cladding .

Light Propagation and Modes

In multimode fibers, light can travel along several distinct paths, including straight-through (axial) and zigzag (higher-order) paths. Each path is called a mode. The multiple modes allow easier coupling of light from sources like LEDs or VCSELs, making multimode fibers suitable for short-distance applications such as LANs and data centers . However, the presence of multiple modes can lead to modal dispersion, where different modes arrive at the receiver at slightly different times, limiting the maximum transmission distance .

Single-Core Advantage

A single-core multimode fiber uses only one core for light transmission, unlike multi-core fibers that have multiple cores. This simplifies network design and reduces cost while still supporting multiple modes for short-range, high-bandwidth communication . The single-core design ensures that all light signals are confined to one channel, which is easier to manage and connect to transceivers.

Practical Considerations

• Launch Conditions: Efficient light coupling requires that the input light primarily enters the core and respects the fiber's numerical aperture (NA) to avoid excessive loss .
• Applications: Single-core multimode fibers are ideal for short-distance networks, such as within buildings or data centers, where high bandwidth is needed but long-distance signal degradation is not a concern .
• Mode Coupling: Over long distances or under disturbances (bending, temperature changes), light may couple between modes, affecting beam quality and signal uniformity .

Summary

The working principle of a single-core multimode optical fiber relies on total internal reflection to guide light through a large core that supports multiple modes. This allows for high-bandwidth, short-distance communication, with easier light coupling compared to single-mode fibers, but with limitations in distance due to modal dispersion .