Data Cabling and Networking Specialists

Fibre

Multi-mode optical fibre (multimode fibre or MM fibre or fibre) is a type of optical fibre mostly used for communication over shorter distances, such as within a building or on a campus. Typical multimode links have data rates of 10 Mbit/s to 10 Gbit/s over link lengths of up to 600 meters more than sufficient for the majority of premises applications.

The equipment used for communications over multi-mode optical fibre is less expensive than that for single-mode optical fibre. Typical transmission speeds/distances limits are 100 Mbit/s up to 2 km (100BASE-FX), 1 Gbit/s for distances up to 500 - 600 meters (1000BASE-SX), and 10 Gbit/s for distances up to 300 meters (10GBASE-SR).

Because of its high capacity and reliability, multi-mode optical fibre generally is used for backbone applications in buildings. An increasing number of users are taking the benefits of fibre closer to the user by running fibre to the desktop or to the zone. Standards-compliant architectures such as Centralized Cabling and Fibre to the Telecom Enclosure offer users the ability to leverage the distance capabilities of fibre by centralizing electronics in telecommunications rooms, rather than having active electronics on each floor.

Multi-mode fibres are described by their core and cladding diameters. Thus, 62.5/125 m multimode fibre has a core size of 62.5 micrometres (m) and a cladding diameter of 125 m. In addition, multi-mode fibres are described using a system of classification determined by the ISO 11801 standard  OM1, OM2, and OM3  which is based on the bandwidth of the multi-mode fibre.

For many years 62.5/125 m (OM1) and conventional 50/125 m multi-mode fibre (OM2) were widely deployed in premises applications. These fibres easily support applications ranging from Ethernet (10 Mbit/s) to Gigabit Ethernet (1 Gbit/s) and, because of their relatively large core size, were ideal for use with LED transmitters. Newer deployments often use laser-optimized 50/125m multi-mode fibre (OM3). Fibres that meet this designation provide sufficient bandwidth to support 10 Gigabit Ethernet up to 300 meters. Optical fibre manufacturers have greatly refined their manufacturing process since that standard was issued and cables can be made that support 10 GbE up to 550 meters. Laser optimized multi-mode fibre (LOMMF) is designed for use with 850 nm VCSELs.

The migration to LOMMF/OM3 has occurred as users upgrade to higher speed networks. LEDs have a maximum modulation rate of 622 Mbit/s because they can not be turned on/off fast enough to support higher bandwidth applications. VCSELs are capable of modulation over 10 Gbit/s and are used in many high speed networks.

VCSEL power profiles, along with variations in fibre uniformity, can cause modal dispersion which is measured by differential modal delay (DMD). Modal dispersion is an effect caused by the different speeds of the individual modes in a light pulse. The net effect causes the light pulse to separate or spread over distance, making it difficult for receivers to identify the individual 1's and 0's. The greater the length, the greater the modal dispersion. To combat modal dispersion, LOMMF is manufactured in a way that eliminates variations in the fibre which could affect the speed that a light pulse can travel. The refractive index profile is enhanced for VCSEL transmission and to prevent the pulse spreading. As a result the fibres maintain signal integrity over longer distances, thereby maximizing bandwidth.