FTTH Optical Fibre Overview

What is Optical Fibre ?
Optical fibre is effectively a "Light Pipe" carrying pulses of light generated by lasers or other optical sources to a receiving sensor (detector). Fibre light transmission can be achieved over considerable distances supporting high speed, high bandwidth applications unsustainable by today's copper based networks. Conceived in the 1960's, optical fibre has been highly developed and standardised to form a reliable, proven backbone of today's modern telecommunication transmission systems.

Fibre Basics

Fibre is manufactured from high purity silica glass-like rods drawn into fine hair-like strands and covered with a thin protective plastic coating. Fibres are subsequently packaged in various cable configurations before installation in the external and/or internal networks. Whilst there are many different fibre types, this document concentrates on fibre for FTTH applications.

Fibre is essentially made up of a core, cladding and outer coating. The light pulses are launched into the core region. A surrounding cladding layer keeps the light traveling down the core and prevents light from leaking out. An outer coating, most commonly made of a protective polymer layer, is applied during the drawing process.

The fibre core can be designed in varying geometrical sizes. These impact how the light pulse travels thus producing differing optical performance.

Fibres are connected together either by fusion splicing (ends welded together) or by butt jointing using a mechanical splice or precision optical connector (end polished).

A number of key parameters determine how effective the light pulses transmit down the fibre. The two main fibre performance parameters are attenuation and dispersion.

Attenuation is the reduction of light power over distance. Even with the highly pure materials used to manufacture the fibre core/cladding, light power is lost over distance by scattering and absorption within the fibre. Fibre attenuation limits the distance light pulses can travel while still being detectable. Attenuation is expressed in decibels per kilometer (dB/km) at a given wavelength or range of wavelengths.

Dispersion is inversely related to bandwidth, which is the information carrying capacity of a fibre. This can be broadly described as the amount of distortion or spreading of a pulse during transmission. If pulses spread out too far, the detection unit at the other end of the fibre is not able to distinguish one pulse from another, causing loss of information. Chromatic dispersion occurs in all fibres and is caused by the various light colours (components of a light pulse) traveling at slightly differing speeds along the fibre.

There are many other parameters, which affect fibre transmission performance. Further information can be found in IEC 60793 series of specifications.

Types of Fibre

The two main fibre types used for optical transmission are described as single-mode and multimode.

Single-mode fibres

Single-mode fibre comprises a small core size (<10um) which supports one mode (ray) of light.

Single-mode fibre provides the lowest optical attenuation loss and highest bandwidth transmission carrying capacity of all the fibre types. Single-mode fibre incurs higher equipment cost compared with multimode fibre systems. Most of the world’s fibre systems are based on this type of fibre. For FTTH single-mode applications, the reference to the ITU-T G.652 recommendations should adequately cover most user’s needs.

More recently, a newer type of single mode fibre was introduced to the market that has decreased optical losses at reduced fibre bends. This fibre is standardized as ITU-T G.657 and is available from several fibre suppliers. This type of fibre is most beneficial when optical fibre needs to be installed in environments where cables need to be installed in tighter bends like in-home wiring,…

Graded Index Multimode Fibres

Multimode fibres comprise a larger core size (50 or 62.5 micron) which supports many modes (different light paths through the core). Depending on the particular launch characteristics, the input pulse power is divided over all or part of these modes.

The different propagation speed of individual modes (modal dispersion) can be minimised by adequate fibre design. Multimode fibre can operate with less costly light sources and connectors, but incurs a higher fibre cost than single-mode fibre. Multimode fibres are used effectively for short distance transmission networks e.g. campus and in-building applications. The ISO/IEC11801 specification gives the data rate and reach of multimode fibre grades referred to as OM1, OM2 and OM3.