What is Multi-Mode Fiber

Core Definition

Multi-mode fiber (MMF) is an optical fiber that allows multiple light propagation modes (or paths) to travel simultaneously. Due to its relatively large core diameter, light can propagate through the fiber at various angles and paths (modes). This is akin to a wide highway where multiple vehicles can travel side by side.

The term "mode" refers to the different electromagnetic field distribution patterns of light within the fiber. In simple terms, you can think of it as the various paths light takes as it travels through the fiber.

Key Characteristics of Multi-Mode Fiber

1. Larger Core Diameter

The most distinctive feature of multi-mode fiber is its thicker core. Standard core diameters are typically 50μm or 62.5μm (often referred to as OM1’s 62.5/125μm). In contrast, single-mode fiber has a core diameter of only 8-10μm. The larger core allows multiple modes of light to be coupled into the fiber.

2. Operating Wavelength

Multi-mode fiber typically operates in shorter wavelength bands, primarily at 850nm and 1310nm.

3. Light Source

Multi-mode fiber uses relatively inexpensive light sources, such as light-emitting diodes (LEDs) or Vertical-Cavity Surface-Emitting Lasers (VCSELs). These sources produce less focused light with a wider divergence angle, but the larger core size allows effective coupling of light into the fiber. VCSEL lasers are widely used in modern multi-mode fibers (e.g., OM3/OM4) to support higher data rates.

4. Transmission Characteristics: Short Distance, Lower Bandwidth

• Core Limitation: Modal dispersion. Since different light modes travel along different paths with varying lengths, they arrive at the fiber’s endpoint at different times. A perfect light pulse entering the fiber becomes broadened and distorted due to this "path difference," causing pulses to overlap.

• This phenomenon, known as modal dispersion, significantly limits the bandwidth and transmission distance of multi-mode fiber. The longer the distance, the more pronounced the pulse broadening, eventually making the signal unrecognizable.

• As a result, multi-mode fiber is primarily used for short-distance communication.

5. Cost

• The cost of multi-mode fiber itself is slightly higher than single-mode fiber due to the larger core requiring more material.

• However, its biggest advantage is the significantly lower cost of associated optical equipment (transceivers) compared to the laser-based modules used in single-mode systems, resulting in a lower overall system cost.

  
  

Why Does Modal Dispersion Occur? A Simple Analogy         

Imagine a race:

• You have a group of people (representing photons) starting at one end of a wide corridor (multi-mode fiber) and running to the other end.

• Some run straight down the center (shortest path).

• Others run along the walls, bouncing back and forth (longer path).

• Some may even zigzag across the corridor (longest path).

• As a result, the group doesn’t arrive at the finish line simultaneously; instead, they spread out over time. The initial "group start" becomes a "scattered arrival."

This is modal dispersion—light pulses broaden over time due to different path lengths.

In contrast, single-mode fiber is like a narrow bridge where everyone must run in a single file along the center, arriving at the endpoint almost simultaneously.

Multi-Mode Fiber Grades (OM1, OM2, OM3, OM4, OM5)

To mitigate modal dispersion, multi-mode fiber has evolved with improved refractive index profiles and materials (e.g., fluorine doping) to enhance performance. The International Organization for Standardization/International Electrotechnical Commission (ISO/IEC) classifies multi-mode fibers into different grades:

OM1

• Traditional 62.5/125μm fiber.

• Typically has an orange jacket.

• Supports only short-distance transmission for 10/100M Ethernet; now largely obsolete.

OM2

• 50/125μm fiber.

• Also has an orange jacket.

• Improved over OM1, supporting 1GbE over longer distances.

OM3

• Laser-optimized 50/125μm fiber.

• Typically has an aqua (water blue) jacket.

• Optimized for VCSEL lasers, significantly reducing modal dispersion.

• Supports 10GbE up to 300 meters, and 40GbE/100GbE up to 100 meters.

OM4

• An enhanced version of OM3 with better performance.

• Also has an aqua jacket (distinguished by labeling).

• Supports 10GbE up to 400 meters, and 40GbE/100GbE up to 150 meters.

• Currently the mainstream choice for data centers.

OM5

• Next-generation wideband multi-mode fiber.

• Has a lime green jacket.

• Designed to support short-wave wavelength division multiplexing (SWDM), allowing multiple wavelengths to be transmitted over a single fiber.

• Primarily aimed at future 400GbE applications.

For high-performance multi-mode fiber solutions, explore ZORA products at www.zoracz.com.

Comparison Table (Multi-Mode vs. Single-Mode Fiber)

Applications

Multi-mode fiber excels in short-distance, high-bandwidth, and cost-effective applications. Typical use cases include:

• Data center interconnects: Connecting servers, switches, and storage devices within or across cabinets.

• Enterprise LANs: Network backbones within buildings or campus networks.

• Audio-video systems: Video transmission in large venues.

• Surveillance systems: Short-distance signal transmission for security cameras.

In summary, multi-mode fiber is the cost-effective champion for short-distance communication, while single-mode fiber dominates long-distance applications. The choice depends on your specific needs: distance, bandwidth, and budget. For cutting-edge multi-mode fiber solutions, visit www.zoracz.com to explore ZORA’s innovative products.