Multimode Fibers: An Option for LANs

Optical fibers are thin strands of silica (glass) that transmit light with minimal loss. At one end, light is transmitted by a laser or light-emitting diode (LED); at the other end, a light-sensitive receiver converts the signal back into a digital transmission.

There are two types of glass optical fiber: single mode and multimode. In premises cabling, multimode fibers are typically used because they can be powered by low-cost optical sources, such as LEDs or vertical-cavity surface-emitting lasers (VCSELs), which operate in the 850 nm transmission window. Multimode fibers support applications from 10 Mb/s to 10 Gb/s over link lengths of up to 550 meters, which is more than sufficient for the majority of premises applications (some multimode fiber will even support 100 Gb/s).

Single-mode fibers are used in long-distance telephony and cable TV applications to support high-bandwidth applications and span transmission distances of hundreds of kilometers. These fibers operate at 1310 nm or 1550 nm transmission windows, which require higher-cost electronics. Occasionally, single-mode fibers are installed in premise backbone cables, or may be installed as “dark” fiber in anticipation of needs.

How Fibers Work

Although optical fibers are slightly smaller in diameter than a human hair, they’re composed of two kinds of glass: the core of the fiber is where light is carried, and the cladding is the glass that surrounds the core. The cladding glass acts as a mirror; as light pulses propagate down an optical fiber, the cladding reflects the light pulses and keeps them contained in the fiber core. Surrounding the cladding glass is an acrylate coating that protects the fiber.

In multimode fibers, many modes of light are transmitted down a fiber simultaneously. Single-mode fiber, as its name implies, allows only one mode of light to travel down the core at a time. Single-mode fibers have a much smaller core diameter.

Multimode Fiber Types

Multimode fibers are described by either their core/cladding diameter, or by using a classification determined by the ISO/IEC 11801 Standard (OM1, OM2, and OM3). The standard specifies that OM3 fibers are capable of 10 Gb/s performance over distances of up to 300 meters.

The predominant type of multimode fiber used in premises applications today is laser-optimized 50/125 mm multimode fiber (OM3). This fiber type provides sufficient bandwidth to support 10 GbE and beyond with cable lengths of up to 550 meters. Laser-optimized multimode fiber (LOMMF) is designed for use with 850 nm VCSELs vs. the LEDs used for lower speeds. Its development extended the distances over which multimode fiber cable can transmit high data rates.

The migration to LOMMF/OM3 has occurred as users upgrade to higher-speed networks. LEDs have a maximum modulation rate of 622 Mbps, so they couldn’t be turned on/off fast enough to support higher-bandwidth applications. VCSELs are a cost-effective transmission source, but to obtain the necessary performance, fiber manufacturers had to modify their multimode fiber designs.

VCSELs power profiles, along with variations in fiber uniformity, can cause differential modal delay (DMD) – an effect that causes the speed of individual light pulses to spread over time, and makes it difficult for transceivers to identify the 1s and 0s. This effect reduces transmission capacity as link lengths increase. To combat DMD, LOMMF is manufactured to eliminate variations in the fiber that could affect the speed that a light pulse can travel. The refractive index profile is enhanced to control VCSEL transmission and prevent the pulse spreading. As a result, the fibers maintain signal integrity over longer distances, thereby maximizing bandwidth.

LOMMF/OM3 is a good choice for all LAN infrastructure applications, and for the data center.

It’s backward compatible with LED signaling technology, so you can use it today to support slower rates, and then upgrade to VCSEL-based 1, 10, and 100 Gb/s transceivers.