50µm fibre optimised for 10GbE applications


Fibre development

Increasing bandwidth requirements have prompted the development of a novel 50µm-diameter multimode fibre designed for transmission of 10 Gigabit Ethernet by 850nm vertical-cavity surface-emitting laser.

By Meghan Fuller, Lightwave

Cable Design Technologies (CDT) and Draka Comteq subsidiary Chromatic Technologies have developed a novel 50µm-diameter multimode fibre (MMF) specifically designed to work with vertical-cavity surface-emitting lasers (VCSELs) in the 850nm window for 10 Gigabit Ethernet (10GbE) applications. Moreover, both Corning and Tyco Electronics have confirmed that they will soon unveil similar fibre lines. So do these product releases signal a new trend away from 62.5µm MMF for certain applications?

Herb Congdon, director of fibre systems marketing and technical support at Tyco Electronics, says that the use of the new "laser-optimised" 50µm MMF is not yet a trend. Rather, the new fibre is one of the most promising technologies for applications such as SANs and LANs, where more bandwidth is now required.

"People are moving to richer, multimedia content," explains Subodh Sharma, commercial development manager of premises networks at Corning. "PowerPoint files are typically 2 to 3Mbits each, and people are sending them off to 10 or 20 others at a time. Plus, people are using the Internet more often, using streaming video and audio. Considering a LAN application, that's a lot of bandwidth going through at one time."

In 1999, the Institute of Electrical and Electronics Engineers formed a study group to examine the use of 10GbE for bandwidth-intensive applications. Known as IEEE 802.3ae, the standard should be finalised this summer.

Ethernet has always been the predominant protocol used in LAN environments. In the early 1980s, Ethernet supported 10Mbit/s, but by the early 1990s Ethernet had jumped to 100Mbit/s, also called Fast Ethernet. These protocols ran over copper or fibre and used a light emitting diode (LED) light source in the latter applications.

In 1998, a new protocol emerged: Gigabit Ethernet, a 10-fold increase over Fast Ethernet. However, the protocol necessitated a switch from LEDs to lasers, since LEDs cannot modulate fast enough to support data rates higher than 622Mbit/s.

Now, 10GbE is on the horizon. But, before such speeds could be realised, a new fibre type had to be developed - one optimised for higher bandwidth and longer distances. While the new laser-optimised 50µm MMF and standard 50µm MMF have the same physical strength and handling properties, they feature a different index of refraction.

When fibre manufacturers designed standard 50µm MMF, they placed the peak of the bandwidth curve at 980nm. They centred the curve so that it covered both the 850nm and 1300nm windows, because all the electronics used either 850nm or 1200nm wavelengths for their optical sources.

When designing laser-optimised 50µm MMF, manufacturers changed the index of refraction profile of the fibre; they shifted the bell curve so that the peak fell at 850nm instead of 980nm. "Now, you get the most bandwidth at 850nm," explains Congdon, "which enables you to support 10GbE or 10Gbit/s data rates with the most fibre."

The new 50µm MMF will also support all of the legacy applications found in the LAN today as well as 10 Gigabit Fibre Channel.

VCSEL laser-optimised fibre
The new laser-optimised fibre uses VCSEL transceivers as its light source; they are less costly than the alternatives, including long-wavelength lasers over single-mode fibre (SMF) and Fabry-Perot lasers over MMF in the 1300nm window.

The use of VCSELs in the 850nm window enables the fibre to extend to longer distances. The IEEE 802.3 proposal specifies a reach of 300m or roughly 900ft, a length that meets most applications with the LAN environment. Both standard 50µm and 62.5µm MMF will also support 10GbE, but they cannot support the necessary distances.

One attractive feature of the new fibre is that it supports the lowest levels of Ethernet all the way through 10GbE. "End users may not necessarily be running at 10-gig now," says Sharma. "They may be running at Fast Ethernet speeds - at 100Mbit/s - but they can put this fibre in and still operate at Fast Ethernet speeds using LEDs. This fibre can be installed as an upgrade path so you don't have to re-cable in the future."

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