What are the transmission distances for single-mode and multimode optical cables?

Single-mode fiber: The central glass core is relatively thin (the core diameter is typically 9 μm; in some countries, 8.3 μm or 10 μm refers to this type as well, with an outer diameter of 125 μm), hence the designation 9/125. It can only transmit a single mode of light. Due to its very low intermodal dispersion, it supports transmission distances of several hundred kilometers and is suitable for long-distance communication. Telecommunications carriers generally choose single-mode fiber for long-distance transmission.

Fiber capable of transmitting multiple modes of light at a given operating wavelength. It is classified into step-index and graded-index types based on the distribution of its refractive index. Standard multimode fiber has a numerical aperture of 0.2 ± 0.02, with a core diameter of 50 μm and an outer diameter of 125 μm. Its transmission parameters include bandwidth and loss. Since multimode fiber supports hundreds of transmission modes, and each mode has different propagation constants and group velocities, the fiber exhibits narrow bandwidth, high dispersion, and high loss. It is therefore suitable only for short- to medium-distance, low-capacity fiber optic communication systems.

Multimode fiber allows light of different modes to propagate through a single fiber. Because multimode fiber has a larger core diameter, it allows for the use of relatively inexpensive couplers and connectors. The core diameter of multimode fiber ranges from 50 μm to 100 μm.

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There are essentially two types of multimode fiber: graded-index and stepped-index. For graded-index fiber, the refractive index of the core is lowest at the outer periphery and gradually increases toward the center, thereby reducing modal dispersion. In contrast, for stepped-index fiber, the refractive index remains essentially constant throughout the core and drops abruptly only at the cladding interface. Stepped-index fiber generally has a lower bandwidth than graded-index fiber. In network applications, the most popular multimode fiber is 62.5/125, where 62.5/125 indicates that the fiber core diameter is 62.5 μm and the cladding diameter is 125 μm. Other common types include 50/125 and 100/140.