How to tell if an optical fiber is multimode

Multimode fiber can be identified by its larger core size, typical jacket color (orange, aqua, or lime green), labeling (OM1–OM5), and the type of light source it uses.

Visual Identification

Jacket Color: Multimode fibers usually have an orange jacket for OM1/OM2, aqua for OM3/OM4, or lime green for OM5, while single-mode fibers are typically yellow. Connector Color: Beige or black connectors often indicate multimode fiber, whereas blue connectors are more common for single-mode.

Labeling and Core Size

Cable Markings: Look for printed labels on the fiber jacket. Multimode fibers are labeled as OM1, OM2, OM3, OM4, or OM5, often followed by the core/cladding size, e.g., “50/125” or “62. 5/125” micrometers.

Core Diameter: Multimode fibers have a larger core, typically 50 µm or 62. 5 µm, compared to single-mode fibers which are around 8–10 µm.

Testing Methods

Visual Fault Locator (VFL): A VFL can show how light propagates through the fiber. Multimode fibers allow multiple light paths, which can be observed as a broader light pattern. Optical Time Domain Reflectometer (OTDR): Measures fiber length and loss, helping confirm the fiber type based on modal dispersion characteristics.

Fiber Microscope: Viewing the fiber end under a microscope allows direct observation of the core size, confirming whether it is multimode.

Practical Considerations

Distance and Application: Multimode fiber is ideal for short-distance connections (typically under 500 meters) in LANs, data centers, and building interconnects, using LED or VCSEL light sources. Modal Dispersion: Multiple light paths in multimode fiber can cause modal dispersion, limiting bandwidth over longer distances compared to single-mode fiber. By combining visual cues, labeling, and testing tools, you can reliably determine if an optical fiber is multimode and ensure it is used appropriately for your network application.