What is modal dispersion?

Optical fibre comes in two general forms, single-mode and multi-mode. Single-mode fibre (small core) supports just a single propagation pathway through the fibre, whereas multi-mode fibre (large core) allows for various different pathways for light to travel along the fibre. In the latter case the ‘modes’ or ‘light patterns’ which travel a longer pathlength arrive later at the end of the fibre than the shorter pathlength ‘modes.’ When there are a large number of modes excited in the fibre this results in pulses of light spreading along the fibre. This intrinsically limits the pulse rate and hence the data rate possible when using multi-mode fibre. On the other hand, in a single-mode fibre light that travels along its length does not experience the same pulse spreading or so-called modal dispersion.

 

This pulse spreading means that a series of pulses in a multi-mode fibre will eventually begin to overlap. This severely limits the transmission reach possible as the pulse rate increases. Conversely, for a given fibre link length it limits the data rates possible. Note that the multi-mode fibre commonly used in fibre networks is OM1 (with >2000 modes at a wavelength of 850 nm) and OM2,3,4 (>680 modes at a wavelength of 850 nm). Multi-mode fibre typically has a core diameter of 50 or 62.5 microns (i.e. easy to align) compared to a much smaller ~8 microns (difficult to align) for single-mode fibre.  For example, in the case of an OM1 fibre link that is 1 km in length the maximum data rate is well below 1G. In order to support 1G the fibre link would need to be reduced in length to around 250 m.

The limit on the transmission reach imposed by the data rate in multi-mode fibre is clearly an issue. It can however be overcome by recabling and simply using single-mode fibre. As mentioned in a single-mode fibre only one mode propagates. Therefore, all the light from a pulse injected into the fibre travels at virtually the same speed because it is all contained within the one mode, resulting in negligible pulse spreading. Therefore, the issue of pulse broadening is avoided. For these reasons many companies have started to recable existing legacy multi-mode fibre links, switching them with single-mode fibre.

This begs the question, what if just a single-mode were to be excited in a multi-mode fibre, would that allow for single-mode fibre performance to be realized on existing multi-mode fibre without necessitating the removal and recabling of the entire fibre network which is often costly and results in service interruptions. The answer is yes, but this has only recently become feasible with the advent of multiplexers capable of selectively exciting the various modes within an optical fibre. Several solutions have been proposed with Modular Photonics’ offering the market-leading OMPlex series devices which excite just the fundamental mode in OM1-4 fibres, allowing for single-mode performance on old multi-mode fibre networks. As the graph below shows this means that the trade-off between data rate and fibre length is circumvented – allowing for 1G, 10G and even 40G on fibre lengths up to 2 km.