What is the difference between Single-Mode and Multimode Fibre?

In order to understand the difference between Single-Mode and Multimode Fibre, you first have to have an appreciation of how light signals are propagated down a length of optical fibre.

The passing of optical signals down a length of optical fibre is based on the principles of refraction, which is something that occurs when light waves travel from one medium to another, and those two mediums have different refractive indices. A light wave’s wavelength will either increase or decrease, while its frequency will remain constant. This will cause the light wave to change direction. When light meets a boundary between two transparent materials that have different refractive indexes (like two different types of glass), if the angle of incidence is narrow enough, the light is reflected.

Suppose you want to shine a torch beam down a long, straight hallway. Light travels in straight lines, so you can see the beam of light illuminate the hallway from end to end., so it is no problem. If the hallway has a in it then the light cannot turn the corner, unless you angle a mirror at the bottom of the hallway to reflect the beam around the corner.

This is exactly what happens in an optical fibre, as the light in a fibre optic cable travels through the core by constantly being reflected by the outer cladding. This principle is known as Total Internal Reflection (TIR).

With multimode fibre, multiple rays of light enter the core of the fibre, each with a slightly different reflection angle that creates different modes of light using the principle of Total Internal Reflection. Multimode fibre is only ideal for short distance communications due to the effects of modal dispersion with some rays of light arriving by a shorter path than others due to the different angles by which the rays of light are introduced into the fibre. These rays, or modes of light arriving at slightly different time intervals has the effect of distorting the overall signal.

Multi-mode optical fibre tends to be used for short distances up to around 600 metres, so is suitable for connections within buildings or campus type areas. Typical multimode data rates range from 10Mbps to 10Gbps.

Single-mode fibre is designed with a much smaller diameter core then multimode fibre and therefore only carries a single ray of light (single mode). Speeds in access of 40Gbps are possible.

Fibre optic patch cables are used to link different equipment components in a network, typically switch to switch connections, where fast efficient communication is required. These patch cords can be terminated with a number of different connectors depending of the equipment requirements.

LC (Lucent Connector)  -  Small form factor connector which has a 1.25mm ferrule which is ideal for installations where space is at a premium. This connector was first developed by Lucent Technologies for Telecommunications applications. This connector is commonly seen with SFP and XFP transceivers.

 SC – (Subscriber Connector)  -  They are one of the most common types of connector and were found on most of the first generation of Gigabit Interface Converters (GBICs). SC connectors were invented by NTT, a Japanese manufacturer.

ST – (Straight Tip)  -  These connectors are mainly used in data communications installations and have a very simple bayonet locking system that is spring loaded. ST connectors were designed by AT&T.

FC – (Ferrule Connector)  -  These connectors are becoming less common and have a screw type connection. They are often replaced by LC and SC connectors these days.

Fibre patch cords are sometimes called fibre jumpers and are basically a length of fibre optic cable terminated with a suitable connector for the required installation. As previously described, fibre patch cables are commonly fitted with ST, SC, LC or MTRJ connectors. Fibre patch cables come as either single-mode or multimode and and the outer jackets are normally colour coded yellow for single-mode and orange for multimode.

Many manufacturers supply Fibre patch cords suitable for multimode or single-mode operation and will normally offer these patch cords with various combinations of connector types, and in a range of lengths.

This article on fibre patch cords was written by David Christie, MD at NSTUK Ltd,  Website http://www.ipexpress.co.uk .