Fibre Optic Cable
Introduction
In the 1950's more research and development into the
transmission of visible images through optical
fibres led to some success in the medical world
where it was being used in remote illumination and
viewing instruments. In 1966 Charles Kao and George
Hockham proposed the transmission of information
over glass fibre and realised that to make it a
practical proposition, much lower losses in the
cables were essential.
This was the driving force behind the developments
to improve the optical losses in fibre manufacturing
and today optical losses are significantly lower
than the original target set by Charles Kao and
George Hockham.
The advantages of using fibre
optics
Because of the Low loss, high bandwidth properties
of fibre cables they can be used over greater
distances than copper cables. In data networks this
can be as much as 2km without the use of repeaters.
Their light weight and small size also make them
ideal for applications where running copper cables
would be impractical and, by using multiplexors, one
fibre could replace hundreds of copper cables. This
is pretty impressive for a tiny glass filament, but
the real benefit in the data industry is its
immunity to Electro Magnetic Interference (EMI), and
the fact that glass is not an electrical conductor.
Because fibre is non-conductive it can be used where
electrical isolation is needed, for instance,
between buildings where copper cables would require
cross bonding to eliminate differences in earth
potentials. Fibres also pose no threat in dangerous
environments such as chemical plants where a spark
could trigger an explosion. Last but not least is
the security aspect, it is very, very difficult to
tap into a fibre cable to read the data signals.
Fibre construction
There are many different types of fibre cable, but
for the purposes of this explanation we will deal
with one of the most common types, 62.5/125 micron
loose tube. The numbers represent the diameters of
the fibre core and cladding, these are measured in
microns which are millionths of a metre.
Loose tube fibre cable can be indoor or outdoor, or
both, the outdoor cables usually have the tube
filled with gel to act as a moisture barrier to the
ingress of water. The number of cores in one cable
can be anywhere from 4 to 144.
Over the years a variety of core sizes have been
produced but these days there are three main sizes
that are used in data communications, these are
50/125, 62.5/125 and 8.3/125. The 50/125 and
62.5/125 micron multi-mode cables are the most
widely used in data networks, although recently the
62.5 has become the more popular choice. This is
rather unfortunate because the 50/125 has been found
to be the better option for Gigabit Ethernet
applications.
The 8.3/125 micron is a single mode cable which
until now hasn't been widely used in data networking
due to the high cost of single mode hardware. Things
are beginning to change because the length limits
for Gigabit Ethernet over 62.5/125 fibre has been
reduced to around 220m and now using 8.3/125 may be
the only choice for some campus size networks.
Hopefully, this shift to single mode may start to
bring the costs down.
What's the difference between single-mode and
multi-mode?
With copper cables larger size means less resistance
and therefore more current, but with fibre the
opposite is true. To explain this we first need to
understand how the light propagates within the fibre
core.
Light propagation
Light travels along a fibre cable by a process
called 'Total Internal Reflection' (TIR), this is
made possible by using two types of glass which have
different refractive indexes. The inner core has a
high refractive index and the outer cladding has a
low index. This is the same principle as the
reflection you see when you look into a pond. The
water in the pond has a higher refractive index than
the air and if you look at it from a shallow angle
you will see a reflection of the surrounding area,
however, if you look straight down at the water you
can see the bottom of the pond.
At some specific angle between these two view points
the light stops reflecting off the surface of the
water and passes through the air/water interface
allowing you to see the bottom of the pond. In
multi-mode fibres, as the name suggests, there are
multiple modes of propagation for the rays of light.
These range from low order modes, which take the
most direct route straight down the middle, to high
order modes, which take the longest route as they
bounce from one side to the other all the way down
the fibre.
This has the effect of scattering the signal because
the rays from one pulse of light arrive at the far
end at different times; this is known as Intermodal
Dispersion (sometimes referred to as Differential
Mode Delay, DMD). To ease the problem, graded index
fibres were developed. Unlike the examples above
which have a definite barrier between core and
cladding, these have a high refractive index at the
centre which gradually reduces to a low refractive
index at the circumference. This slows down the
lower order modes allowing the rays to arrive at the
far end closer together, thereby reducing intermodal
dispersion and improving the shape of the signal.
So what about the
single-mode fibre?
Well, what's the best way to get rid of Intermodal
Dispersion?, easy, only allow one mode of
propagation. So a smaller core size means higher
bandwidth and greater distances. Simple as that ! :)
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