Fibre Cable Types Explained

Multi-mode fibres are described by their core and cladding diameters. Thus, 62.5/125 µm multimode fibre has a core size of 62.5 micrometres (µm) and a cladding diameter of 125 µm. 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. OM4 (defined in TIA-492-AAAD) was finalized in August 2009, and is awaiting final publication by the TIA.

For many years 62.5/125 µm (OM1) and conventional 50/125 µm 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 50/125 µm 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 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 that 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 fiber 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 fibers maintain signal integrity over longer distances, thereby maximizing the bandwidth.