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FEATURE DATA CENTRE OPTICS


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Taking multimode fibre to new wavelengths


Vendors are developing products that support wavelength multiplexing over multimode fibre, providing an alternative to parallel links. Robert Roe and Pauline Rigby report


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ultimode fibre has been the workhorse of data centre networks for many decades because it provides the lowest cost cabling system for


high-speed connections between servers, switches and storage. Although the optical fibre is more expensive, the larger core size enables the use of cheaper transmitters based on vertical-cavity surface-emitting lasers (VCSELs), which reduces the overall cost of the system. Mega data centre operators like Google and


Facebook are expected to adopt singlemode fibre in the future, taking advantage of singlemode fibre’s ability to support higher capacities over longer distances. Facebook recently stated that its new data centre in Altoona, Iowa, which is built using 40-Gb/s links, will be its last to use multimode fibre. However, there is still a vast installed base of


legacy equipment in data centres which will continue to play an important role in the overall data centre market. Tose operators are looking for cost-effective upgrade paths for their existing networks, as Craig Tompson, director of strategic marketing at Finisar, explains. Tompson said: ‘It is a storyline in our industry


that has not gotten a lot of attention recently because there has been so much attention on the dozen or so companies that are building very large data centres. Te fact of the matter is that the majority of the enterprise customer base does not require the sort of infrastructure that the web-scale companies are demanding.’ According to research firm Gartner, 94 per cent


of all data centre sites built in 2014 were smaller than 250,000 square feet, and this percentage is not expected to change in the next five years. A 300m optical link is sufficient for even the longest distances inside a 250,000 square foot data centre, Tompson contends. He added: ‘We felt compelled to just put a


spotlight back on the rest of the industry that needs continued investment in low-cost


16 FIBRE SYSTEMS Issue 9 • Autumn 2015


multimode fibre. What we are aiming to do is continue to provide that value to the customers so they can continue to buy the lowest cost transceivers and get the lowest cost connectivity for these long distances at these higher data rates.’


Technology roadmap At OFC 2015 in Los Angeles earlier this year Finisar unveiled its technology roadmap for next-generation multimode fibre interfaces. Te roadmap provides a route to higher speeds, both for the installed base of multimode fibre and using new fibre types in the future. Tree key technologies will make this possible: wavelength division multiplexing, higher serial lane rates, and new multimode fibre types that have been optimised around these capabilities. At the show Finisar demonstrated a proprietary


40G ‘SWDM4’ transceiver in a QSFP+ form factor, capable of 300m transmission over duplex OM3 multimode fibre. Te device employs what


The roadmap provides a path to higher speeds for multimode fibre


the company calls shortwave WDM (hence SWDM) with four 10Gb/s wavelengths over a fibre pair, one fibre for each direction. At less than 1.5W power consumption, the transceiver enables a simple upgrade for data centres that have previously deployed duplex multimode fibre at 10Gb/s. In the past few years, VCSELs supporting data


rates of 25Gb/s have reached the market. Combining SWDM with 25Gb/s VCSELs enables 100G transmission over duplex multimode fibre. Finisar also demonstrated a prototype 100Gb/s module capable of 70m transmission over duplex


OM3 multimode fibre and 100m over OM4 fibre, and expects to be able to increase the reach to 300m with further product development. Future development of VCSELs that operate at 50Gb/s will enable even faster generations of both single-lane and multiple-lane optical interfaces. Also at OFC 2015, Finisar’s transceiver


technology was used by CommScope to demonstrate 100 Gigabit Ethernet transmission over duplex multimode fibre. Te demo operated error free without the assistance of forward error correction (FEC) over 100m of CommScope’s LazrSPEED 550 OM4 fibre and over 225m of LazrSPEED 550 WideBand, a new type of fibre that has been optimised for SWDM transmission. Te demonstration, held over a few hours by invitation only on Finisar’s booth, was the first to show the capabilities of wideband multimode fibre (WBMMF). Finisar and CommScope were not the only


companies focusing on enhancements to multimode fibre transmission at OFC this year. Avago Technologies demonstrated its VCSEL- based optical 4-level pulse-amplitude modulation (PAM-4) technology for next-generation optical transceivers. PAM-4 uses four distinct amplitude levels, each representing a combination of two bits (00, 01, 11, 10), effectively doubling the data rate for the same symbol rate. Te vendor also showed a prototype 40G bidirectional (BiDi) QSFP+ transceiver that has since started shipping in volume. Each fibre carries two 20G wavelengths travelling in opposite directions to give 40G of capacity on duplex multimode fibre. On its stand, OFS showcased several new


interfaces for multimode fibre, including a demonstration using a third-party 40G BiDi module to connect 40 Gigabit Ethernet switches over 200m of duplex OM4 fibre. Te optical module from Cisco, announced with the vendor’s Nexus 9000 family of data centre switches, is based on 10G wavelengths at 850nm and 900nm travelling in opposite directions simultaneously


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