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FEATURE DATA CENTRE OPTICS
total bandwidth cannot be higher than either the modal bandwidth or the chromatic bandwidth, and in reality it is always lower than either of the two. It is important to try to keep the two in balance because raising one very high has diminishing benefit if the other is not also raised.’ Wideband multimode fibre is designed to
support wavelengths longer than 850nm because this allowed designers to take advantage of the cables’ naturally occurring reduction in chromatic dispersion as the wavelength lengthens. ‘We get an improvement in the chromatic bandwidth by going to longer wavelengths,’ Kolesar noted. ‘Wideband fibre will also increase the modal bandwidth for these longer
Enterprise customers do not require the sort of infrastructure that the web-scale companies are demanding
on both of the duplex fibres. Te optical transceivers from Avago and Cisco
represent the first commercial applications for SWDM. Both operate over duplex fibre with an LC connector – as opposed to eight fibres with the MPO connector used by conventional 40GBASE- SR4 optics. Tus they provide a simple drop-in upgrade to 40 Gigabit Ethernet for data centre networks previously running at 10G over duplex multimode fibre. Te technology inside the Avago and Cisco modules is proprietary. However, as the joint demonstration from CommScope and Finisar indicates, further optimisation and standardisation of SWDM technology will allow multimode fibre transmission to reach much higher capacities.
Technical challenges Although SWDM is possible using legacy cable plant, the technology is not ideal for multiple wavelengths. Current OM3 and OM4 multimode fibre is optimised for single-wavelength VCSELs operating in a narrow wavelength range around 850nm, and the modal bandwidth of the fibre drops off quickly at wavelengths above this value.
Wideband multimode fibre and compatible transceivers are required to fully utilise the increase in bandwidth without suffering a reduction in reach. ‘Te main challenge that we have with using
short-range wavelength multiplexing over wideband fibre is to strike the correct balance between supporting the legacy applications that operate at 850nm while also providing a better transmission capability for other wavelengths,’ explained Paul Kolesar, engineering fellow in the Enterprise Solutions division of CommScope. Te problem is dispersion. In multimode fibre
there are two primary causes of dispersion. One is caused by the differences in propagation velocity between the many modes in a multimode fibre, called modal dispersion. Te second is caused by the differences in propagation velocity of different wavelengths, or colours, of light, called chromatic dispersion. Teir reciprocal functions are called modal bandwidth and chromatic bandwidth, respectively. Each dispersion mechanism causes the pulses of light that carry information to spread out in time as they propagate. Kolesar commented: ‘In practical terms, the
wavelengths compared to OM4. Overall the net effect will be to provide OM4-like capability over a broader wavelength range of 100nm or more. Tis range is sufficient to support four low-cost wavelengths while also supporting all of the legacy applications of today.’ Input from optical transceiver and systems
manufacturers was also vital, otherwise ‘the transceiver could be so expensive that the total system cost would not be acceptable,’ commented John Kamino senior product manager for multimode fibre at OFS. Developers need to be aware of the capabilities of the fibre and any proposed changes to the system, as this can affect the wavelength grid – a table of all the central frequencies (and corresponding wavelengths) of channels allowed in a given system. Finisar’s Tompson agreed: ‘It does take time
and a number of iterations to agree on what the wavelength grid should be, what channels should be supported to achieve the length distances, and the power consumption that the industry needs.’ He added: ‘It was important for us to support
links on installed fibre. Tere are a lot of 10-gigabit links out there today that go 300m, and so we took more time and spent considerable amount of R&D dollars to create a wavelength grid and a link model that would support WDM multimode up to 300m, even on legacy fibre.’ Product development also has to align with consumer demand and the timing for market
Issue 9 • Autumn 2015 FIBRE SYSTEMS 17
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