You can identify single-mode and multimode fiber by examining jacket color, printed labels, core diameter, connector type, or using specialized test equipment.

Visual and Label Indicators

• Jacket Color: Single-mode fibers are typically yellow, while multimode fibers are usually orange (OM1/OM2), aqua (OM3/OM4), or lime green (OM5), though color may vary by manufacturer .
• Printed Labels: Check the cable jacket for markings. Labels like OS1 or OS2 indicate single-mode, while OM1, OM2, OM3, OM4, or OM5 indicate multimode. Core/cladding dimensions may also be printed, e.g., 9/125 µm for single-mode and 50/125 µm or 62.5/125 µm for multimode .
• Connector Color: Blue connectors often correspond to single-mode, while beige or black connectors are commonly used for multimode, though this is not universal .

Core Diameter

• Single-Mode Fiber (SMF): Core diameter is typically 8–10 µm, allowing only one light path, which minimizes dispersion and supports long-distance, high-bandwidth transmission .
• Multimode Fiber (MMF): Core diameter is larger, usually 50 µm or 62.5 µm, supporting multiple light paths. This makes it suitable for shorter distances but more prone to modal dispersion .

Testing Methods

• Optical Time-Domain Reflectometer (OTDR): Measures fiber length, identifies faults, and can distinguish single-mode from multimode based on signal behavior .
• Visual Fault Locator (VFL): A low-power laser or LED can show how light propagates through the fiber. Single-mode fibers transmit a narrow, focused beam, while multimode fibers spread the light more broadly .
• Microscope Inspection: Viewing the fiber end under a microscope can reveal core size differences, but this requires caution and proper safety equipment to avoid eye damage .

Practical Considerations

• Light Source: Single-mode fibers typically use lasers, while multimode fibers often use LEDs for short distances .
• Distance and Bandwidth: Single-mode is ideal for long-haul telecommunications and high-bandwidth applications, while multimode is suited for LANs, data centers, and shorter runs . By combining these methods—visual inspection, label checking, core measurement, and testing—you can confidently determine whether a fiber is single-mode or multimode, ensuring proper network performance and compatibility.