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Multimode
fibre for IEEE 802.3ae 10 Gbit/s Ethernet standard
Introduction
Increasing demand for transmission capacity has pushed the
development of 10Gbit/s systems. The IEEE 802.3ae 10 Gigabit
Ethernet specification describes the system configuration
for application in Local Area Networks (LAN's) as well as
Wide Area Networks (WAN's).
Where LED driven multimode systems can be realised up to transmission
speeds of 622 Mbit/s the 10 Gbit/s systems make use of cheap
850 nm operating VCSEL (Vertical Cavity Surface Emitting Laser)
diodes. To enable use of this VCSEL source a special version
of the 50/125 μm multimode fibre isdeveloped. Where standard
50 μm core multimode fibres have their bandwidth optimisation
at 1300 nm orin-between 850 and 1300 nm, the special version
has maximum bandwidth at the 850 nm operatingwavelength of
the VCSEL. Fibre parameters are designed to guarantee a system
distance at least 300meter.
Bandwidth requirements
Information carrying capacity of a multimode fibre is generally
determined by its modal bandwidth. Strictly speaking the capabilities
of a system depend on the interaction of its constituent parts,
i.e. laser, fibre and detector. Especially the interaction
of the VCSEL source with a specific launch of its light into
the multimode fibre influences the effective bandwidth of
the system.
In the latest IEC specification for multimode fibres IEC 60793-2-10
a new category A1a.2 is defined.Besides a modal bandwidth
in excess of 1500 MHZ.km in case of overfilled launch, an
"effective" (laser-launch) bandwidth is specified exceeding
2000 MHz.km.
For an easier to handle definition of this "effective" bandwidth
requirement is translated intorequirements of the DMD (Differential
Mode Delay) of the fibre.
Six DMD templates are defined, each consisting of an inner
and an outer mask. The DMD pattern of the fibre has to fulfil
one of the six templates as given in table 1.
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