Fibre Optics
For many years multimode (62.5/125 fibre optic cabling has been sold into LAN applications as a high bandwidth, future-proof computer network solution. The advent of gigabit Local Area Networks such as ATM, Gigabit Ethernet and Fibre Channel has exposed the distance and bandwidth limitations of 62.5 micron fibre. Users that have brought "data-grade" optical fibre because of its lower price could have a particularly difficult time getting high-speed backbone links to work.

On 25th June 1998, the IEEE approved the Gigabit Ethernet standard, fibre optic section, known as IEEE 802.3z. This will stimulate an explosion in the growth of gigabit backbone links. This growth is inevitable as more ad more users employ 100 Mb/s Fast Ethernet to the desk, giving an aggregate backbone load ten times larger than currently experienced. Anyone installing Fast Ethernet to the desk but leaving 100Mb/s Ethernet or FDDI in the backbone will get no more than 10Mb/s useful throughput at the desk.

Optical Fibre Choices

Optical fibres are comprised of a core and cladding of differing refractive indices. A core of high refractive index is surrounded by a cladding layer of lower refractive index. This difference forms a boundary, which constrains most of the light within the core by the phenomena of total internal reflection. In general there are two types of optical fibre, Singlemode and Multimode.

Singlemode Fibre Optic
This typically has a core diameter of approximately 8um. Above its cut off wavelength, a single mode is transmitted down the fibre. This approach effectively eliminated intermodal dispersion, but with 'bandwidth' is none the less limited by second-order effects such as intramodal dispersion. The combination of huge bandwidth and low attenuation makes singlemode fibre the preferred option for telecommunications systems world-wide. However, singlemode fibres require lasers, producing low numerical aperture light, in order to achieve an effective launch into the fibre. It is the high cost of these devices that has, until now, limited the use of singlemode fibre within LAN's.

Multimode Fibre Optic
Multimode fibres on the other hand, have much larger core diameters, typically 50 or 62.5 um. This effectively permits many modes to be transmitted along different paths down the fibre. Modern graded index multimode fibres have a complex optical core manufactured so that the refractive index varies in a controlled manner, from a high central axis to a lower refractive index material at the outside of the core. Careful design of this profile optimises the transmission characteristics of the fibre.

Plastic Fibre Optic
Plastic fibre has long held the promise of very low cost and easy termination. To date, however, nobody has been able to demonstrate a plastic fibre, at an affordable price, with the distance and bandwidth performance of Category 5 copper cable, let alone any silica glass fibre.

Cost and Performance Trade-off’s
There are three operational wavelengths, long established as the basis for fibre optic data transmission:

850nm - The dominant operating (short) wavelength for most data transmission
1300nm (long wavelength) - Used for higher speed multimode data communications requirements (such as FDDI) and telecoms (with singlemode fibre). 1550nm - Very low attenuation, hence used for telecommunications.

Singlemode fibre optic cabling offers the greatest bandwidth. The additional complication of intermodal dispersion limits multimode bandwidth, being progressively more of an issue with increasing core diameter.

Without the need to manufacture a graded index profile and helped by the economies of scale of the telecommunications market, singlemode fibre is significantly cheaper to manufacture. As far as multimode fibre is concerned, 50/125 is a lower cost solution than 62.5/125.

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