What is step-index multimode fiber

Step-index multimode fiber is an optical fiber with a large core and a uniform refractive index, allowing multiple light modes to propagate via total internal reflection.

Core Structure and Refractive Index

Step-index multimode fiber has a core with a uniform refractive index surrounded by a cladding with a lower refractive index, creating a sharp "step" at the core-cladding interface . The core diameter is relatively large, typically 50 µm or more, which permits multiple light rays, or modes, to travel simultaneously through the fiber . The cladding acts as a mirror, reflecting light back into the core to maintain total internal reflection .

Light Propagation

In this fiber, light travels in a zigzag path along the core, reflecting off the core-cladding boundary . Because multiple modes follow different paths, they arrive at the fiber end at slightly different times, causing modal dispersion. This temporal spreading of light pulses can lead to inter-symbol interference (ISI), limiting the fiber's bandwidth and making it suitable primarily for short-distance, low-speed communication .

Advantages

Step-index multimode fibers are robust, cost-effective, and easy to manufacture . The large core diameter simplifies alignment with optical transmitters and receivers, making them ideal for local area networks (LANs), data centers, and campus networks .

Limitations

The main drawback is high modal dispersion, which reduces the information-carrying capacity and limits the fiber's use to short-reach applications. Unlike graded-index multimode fibers, step-index fibers do not compensate for different path lengths of light modes, resulting in lower bandwidth .

Summary

Step-index multimode fiber is defined by its uniform core refractive index, large core diameter, and multiple light propagation modes, making it suitable for short-distance, cost-sensitive optical communication, while its modal dispersion limits high-speed or long-distance applications .