Multimode optical fibers are not inherently two strands; they can be either single-fiber or dual-fiber depending on the system design.

Fiber Structure

A multimode fiber (MMF) consists of a single core surrounded by cladding and a protective coating. The core is relatively large, typically 50 or 62.5 microns in diameter, allowing multiple light modes to propagate simultaneously, which is ideal for short- to medium-distance communication within buildings or campuses . The cladding ensures total internal reflection, guiding light through the core, while the coating protects the fiber from physical damage .

Single-Fiber vs Dual-Fiber Systems

• Single-fiber systems use one fiber strand for both sending and receiving data by employing different wavelengths of light for each direction. This is useful where space is limited or fiber usage needs to be minimized .
• Dual-fiber systems use two separate fiber strands: one for transmitting data and one for receiving. This is the most common setup for multimode fiber networks because it is simple, reliable, and compatible with standard networking equipment .

Summary

While multimode fibers themselves are single strands of glass, the network configuration may use one or two fibers depending on whether bidirectional communication is implemented on a single fiber or across two fibers. Therefore, multimode fiber is not automatically two strands, but dual-fiber setups are common in practice for full-duplex communication .