Editorial Feature

New Uses for Multimode Optical Fibers

Over the past decade, single mode optical fibre has become firmly established as the preferred medium for terrestrial communications. Without the vast data capacity of these types of fibre (practical limit currently estimated to be 100Gbits/s), the current revolution in communications, covering home entertainment, banking, shopping and even online medical diagnosis, would be impossible. Fibre optic technology is on the verge of changing the way we do many things in our everyday life.

Single Mode Fibres

Single mode fibres were developed from an older multimode fibre technology. This older technology suffered from limitations in terms of capacity, now overcome by the use of single mode fibre. But suddenly short haul multimode fibre systems are finding a new lease of life in wide areas of industry and technology. The national measurement system is being developed to meet the new needs that this brings.

Multimode Fibre

As with single mode fibre, the greatest impact of multimode fibre is in communications. The ease of connection of multimode fibre can give this technology a significant cost advantage over single mode systems. Whereas single mode fibre links span many tens of kilometres, multimode links are used within or between buildings connecting computers within a single organisation. There is also a growing trend to install all-optical control systems within vehicles, especially within aircraft, in which the weight advantage of fibre over copper, coupled with its freedom from EM interference, makes the fibre option very attractive.

Use of Short Fibre Lengths

The use of short lengths of fibre presents a major measurement challenge. Traditional measurement techniques related to multimode fibres over long lengths of fibre with controlled launch condition and precise fibre lay. This does not match the conditions under which short lengths of fibre are use in real systems. This makes prediction of fibre loss bandwidth, splice loss, power and component specification difficult. To achieve this, NPL (National Physical Laboratory), in association with the University of Leeds, is developing new techniques for measuring the transmission parameter of short lengths of multimode fibres (figure 2). It is also developing the theoretical background that will provide an understanding of the effects of handling polarisation, cabling and a range of other external influences on the function of the fibre as a communications medium.

Figure 1. Fibre measurement facility at NPL.

Figure 2. The effect of launch conditions on the transmitted near-field of a 2m multimode fibre.

Applications of Multimode Fibres

Multimode fibre also has many uses outside the communications field. Fibre-based sensors are being used in medicine to help diagnose disease, and industry they are used to monitor flow in fluids to determine temperature distribution. Fibre bundles are used as laser delivery systems, as light source for illumination of products and as endoscopes. The latter have a long history in medicine and are increasingly being used for remote viewing in hostile environments, such as nuclear reactors or pipe systems. All of these applications pose their own measurement challenges. For example, NPL scientists have been involved in determining the effect such fibre links have on the spectral content of light from remote sensors.

Summary

The fibre optics industry is a multibillion dollar business which is growing at almost 20% each year. In this industry, in which new technology dominates the headlines, the old is finding it still has a bright future.

Primary author: Dr Jerry Benson

Source: Materials World, Vol. 5 no. 9  pg. 412 September 1997.

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