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MATERIAL | Optical Theory |
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A single fibre strand has two areas, a central core and outer
cladding. The central core propagates light along the fibre
by means of Total internal Refraction shown in fig.2. The
majority of losses when light propagates along the core is
due to the molecular water with in the fibre core causing
absorption of the light relative to wavelength as shown in
Fig.1. This graph shows the water absorbption curve of Silica
glass fibre at 850nm, 1310nm and 1550nm. The losses in dB
per km are shown verses wavelengths.
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 Multimode
Transmission Losses
LAN Transmission equipment normally operate at 850, 1300nm.
The dB loses vary depending on wavelength:
850nm = 3.5db/km 1310nm = 0.5db/km
Discontinuities also cause power loss within the fibre core,
at connector points or fusion splices. Return Loss is an issues
which was never considered but with the advent of Gigabit Ethernet
and soon 10Gbps reflection from end faces of connectors will
become a major point of failure
Singlemode Transmission Losses
Long distance Transmission equipment normally operate at 1310
and 1550nm. And soon CWDM or DWDM wavelengths. The dB loses
vary depending on wavelength:
1310nm = 0.5db/km 1550nm = 0.2db/km
Discontinuities also cause power loss within the fibre core,
at connector points or fusion splices. Another type of loss
is Macro Bending loss, normally associated with Single mode
fibre, due to excessively tight bends along the fibre route |
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 Bandwidth
capability at Gigabit speeds
Together with dB loss one needs to be aware of bandwidth carrying
capacity of installed fibre particularly at Gigabit data speeds.
The limitation of bandwidth is a complex issue but general rules
should apply. Always use 50um fibre for multimode applications
and be aware of the fibre route length. For further information
on the effect of high speed applications in fibre or Fibre theory
please ring our sales hot line to discuss training issues and
multi media material. |
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 Fibre
Type vs Bandwidth for Gigabit links
• Multimode, 50/125um = 500
Mhz/km
(Max. Dist. @ 1Gbps = 500mtrs, typically)
• Multimode, 50/125um = 500
Mhz/km
(Max. Dist. @ 2Gbps = 250mtrs, typically)
• Multimode, 62.5/125um =
300 Mhz/km
(Max. 300mtrs)
• Singlemode, 9/125um = Infinite,
(Max. Dist. 120km) |
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