Multimode fiber construction

Multimode fiber (MMF) features a larger core diameter that supports multiple light modes, typically constructed with either step-index or graded-index profiles.Core and Cladding StructureMultimode fiber consists of a central core surrounded by a cladding layer. The core diameter is typically 50–100 µm, much larger than single-mode fiber, which allows multiple light paths (modes) to propagate simultaneously . The cladding, usually 125 µm in diameter, has a lower refractive index than the core, which keeps light confined through total internal reflection . This structure makes MMF more tolerant to connector misalignment and easier to handle compared to single-mode fiber .Step-Index vs Graded-Index ProfilesStep-Index MMF: The refractive index of the core is uniform, creating sharp boundaries with the cladding. Light travels in straight paths, but higher-order modes take longer paths, causing modal dispersion and limiting transmission distance .Graded-Index MMF: The core's refractive index gradually decreases from the center to the edge, which bends light rays and reduces modal dispersion. This design improves bandwidth and allows longer transmission distances compared to step-index fibers .Light Sources and Modal BehaviorMMF typically uses LEDs or VCSELs as light sources. LEDs emit broader beams that excite multiple modes, while VCSELs are used in laser-optimized multimode fibers (LOMMF) for higher-speed applications . The multiple modes in the core can lead to modal dispersion, which limits the maximum distance and bandwidth of MMF links .Multimode Fiber Types (OM1–OM5)Multimode fibers are classified according to ISO/IEC 11801 and TIA standards:OM1: 62.5/125 µm core/cladding, LED source, suitable for 10 Mbit/s to 1 Gbit/s over short distances.OM2: 50/125 µm, LED source, supports higher bandwidth than OM1.OM3: 50/125 µm, laser-optimized for 10 GbE up to 300 m.OM4: 50/125 µm, laser-optimized for 10 GbE up to 400–550 m, supports 40/100 GbE.OM5: 50/125 µm, wideband multimode fiber supporting short-wavelength division multiplexing (SWDM) for multi-wavelength transmission .ApplicationsDue to its larger core and tolerance to alignment, MMF is widely used for short-distance communication, such as within buildings, data centers, and enterprise LANs. It is cost-effective compared to single-mode fiber and suitable for backbone connections, fiber-to-the-desk deployments, and high-power optical applications . In summary, multimode fiber construction combines a large core, cladding, and either step-index or graded-index profiles to support multiple light modes, with specific fiber types optimized for different bandwidths and distances, making MMF ideal for short-range, high-speed optical networks.