1.2 Singlemode vs. Multimode Fiber

Section 1.2: Singlemode vs. Multimode Fiber

To build, test, or design a fiber optic network, a technician must understand the differences between the two main types of optical fiber: singlemode fiber (SMF) and multimode fiber (MMF). While both types operate on the principle of total internal reflection, they differ in core diameter, how light propagates within them, the light sources they require, and the applications they serve.

Fiber Geometry: Core and Cladding Dimensions

All standard optical fibers consist of two primary concentric glass layers: the central core and the surrounding cladding. To protect the glass from moisture and physical damage, a primary buffer coating (typically made of plastic or acrylate, with an outer diameter of 250 µm) is applied during manufacturing.

A key industry standardization is the cladding diameter. For virtually all telecommunications-grade optical fibers—whether singlemode or multimode—the cladding has an outer diameter of exactly 125 µm. Standardizing the cladding size allows manufacturers to build connectors, splicing sleeves, and termination tools with uniform dimensions, reducing installation complexity.

The primary physical difference between fiber types is the diameter of the core:

• Singlemode Fiber (SMF): Features a very small core, typically between 8 and 10 µm in diameter. The industry standard reference size is 9 µm. Singlemode fiber is designated as 9/125 fiber.
• Multimode Fiber (MMF): Features a much larger core, typically 50 µm or 62.5 µm in diameter. These fibers are designated as 50/125 or 62.5/125 fiber.

Modes of Propagation

The term "mode" in fiber optics refers to a stable electromagnetic wave pattern (or path) that light follows as it travels down the core. The core diameter relative to the wavelength of light determines how many modes can exist within the fiber.

Multimode Propagation

With a large core (50 µm or 62.5 µm), a multimode fiber allows hundreds of different light paths (modes) to propagate simultaneously. Some light rays travel straight down the center, while others bounce off the core-cladding boundary at steep angles. Because the bouncing paths cover a greater physical distance than the straight path, the light rays arrive at the end of the fiber at slightly different times. This temporal spreading of the signal is called modal dispersion. It limits the bandwidth and transmission distance of multimode fiber, making it suitable mainly for short-distance applications, such as local area networks (LANs), data centers, and enterprise buildings.

Singlemode Propagation

In singlemode fiber, the core is so small (9 µm) that it is close to the wavelength of the light being transmitted (typically 1310 nm or 1550 nm, which are 1.31 µm and 1.55 µm). Because the core is comparable in size to the wavelength, the fiber acts as a waveguide, allowing only a single electromagnetic mode (the fundamental mode) to propagate. The light travels straight down the center of the core without bouncing. Because there is only one path, there is no modal dispersion. This allows singlemode fiber to transmit signals over tens of kilometers at high speeds without significant signal degradation, making it the standard for telecommunications, cable TV (CATV), and long-haul networks.

Multimode Fiber Classifications and Laser Optimization (OM1 to OM5)

Multimode fibers are classified under international standards (ISO/IEC 11801 and ANSI/TIA-568) into five categories: OM1, OM2, OM3, OM4, and OM5 (where "OM" stands for Optical Multimode).

Legacy Multimode (OM1 and OM2)

• OM1 (62.5/125 µm): Developed in the 1980s, OM1 has a 62.5 µm core. It was designed to work with Light Emitting Diode (LED) light sources. It has a low bandwidth (typically 200 MHz·km at 850 nm) and is limited to 1 Gbps up to 275 meters.
• OM2 (50/125 µm): Also optimized for LED sources, OM2 has a smaller 50 µm core, which reduces the number of propagating modes and slightly increases bandwidth. It supports 1 Gbps up to 550 meters. Because LEDs are slow and have a wide spectral width, OM1 and OM2 are considered legacy fibers and are not installed in new high-speed networks.

Laser-Optimized Multimode (OM3, OM4, and OM5)

Modern high-speed networks use Vertical-Cavity Surface-Emitting Lasers (VCSELs) operating at 850 nm. VCSELs can be modulated at 10 Gbps and faster, but they require 50/125 µm fibers with highly precise graded-index cores to minimize modal delay.

• OM3: Laser-optimized 50/125 µm fiber. It has an effective modal bandwidth (EMB) of 2000 MHz·km at 850 nm and supports 10 Gbps transmission up to 300 meters, or 40/100 Gbps up to 100 meters.
• OM4: High-bandwidth laser-optimized 50/125 µm fiber. It has an EMB of 4700 MHz·km at 850 nm. It supports 10 Gbps up to 550 meters, or 40/100 Gbps up to 150 meters.
• OM5 (Wideband Multimode Fiber - WBMMF): The newest multimode standard. While it has the same bandwidth as OM4 at 850 nm, it is optimized to support Shortwavelength Division Multiplexing (SWDM). This allows OM5 to transmit four separate wavelengths simultaneously in the range of 850 nm to 953 nm over a single fiber, multiplying the cable's data capacity.

Singlemode Fiber Classifications (OS1 and OS2)

Singlemode fibers are classified using the "OS" (Optical Singlemode) designation:

• OS1: Designed primarily for indoor use (tight-buffered cables). OS1 has a higher attenuation, typically around 1.0 dB/km at 1310 nm and 1550 nm, and is limited to distances up to 2 km.
• OS2: Designed for outdoor or loose-tube applications. OS2 is a "low-water-peak" fiber, meaning it is manufactured to eliminate absorption at the 1383 nm water peak. It has an attenuation of 0.4 dB/km at 1310 nm and 0.25 dB/km at 1550 nm, supporting transmission distances up to 40 km or more.

Jacket Color Coding Standards (ANSI/TIA-598-D)

To prevent technicians from misidentifying cables in the field, the ANSI/TIA-598-D standard defines specific color codes for cable jackets:

• Yellow: Singlemode fiber (OS1 and OS2).
• Orange: Legacy multimode fiber (OM1 62.5 µm and OM2 50 µm).
• Aqua: Laser-optimized multimode fiber (OM3 and OM4 50 µm).
• Lime Green: Wideband multimode fiber (OM5 50 µm).

[!IMPORTANT] Never connect singlemode and multimode fibers together. Connecting a 50 µm or 62.5 µm multimode core to a 9 µm singlemode core results in a severe core mismatch. When light travels from the large multimode core into the tiny singlemode core, most of the light spills into the cladding, resulting in a loss of 17 to 20 dB.