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Professional Fibre Optic & Network Cabling Services

Portsmouth Communications

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Multimode Fibre Optics

Multimode fibre optic cable (OM1,OM2,OM3,OM4) has a large diameter core that allows multiple modes of light to propagate. Because of this, the number of light reflections created as the light passes through the core increases, creating the ability for more data to pass through at a given time. Because of the high dispersion and attenuation rate with this type of fibre the quality of the signal is reduced over long distances. This application is typically used for short distance, data and audio/video applications in LANs.

Multimode fibre is usually 50/125 and 62.5/125 in construction. This means that the core to cladding diameter ratio is 50 microns to 125 microns and 62.5 microns to 125 microns.

Step-Index Multimode Fibre

Due to its large core, some of the light rays that make up the digital pulse may travel a direct route, whereas others zigzag as they bounce off the cladding. These alternate paths cause the different groups of light rays, referred to as modes, to arrive separately at the receiving point. The pulse, an aggregate of different modes, begins to spread out, losing its well-defined shape. The need to leave spacing between pulses to prevent overlapping limits the amount of information that can be sent. This type of fibre is best suited for transmission over short distances.

Graded-Index Multimode Fibre

Contains a core in which the refractive index diminishes gradually from the centre axis out toward the cladding. The higher refractive index at the centre makes the light rays moving down the axis advance more slowly than those near the cladding. Due to the graded index, light in the core curves helically rather than zigzag off the cladding, reducing its travel distance. The shortened path and the higher speed allow light at the periphery to arrive at a receiver at about the same time as the slow but straight rays in the core axis. The result: digital pulse suffers less dispersion. This type of fibre is best suited for local-area networks.