The installation
and termination of optical fibres used to be regarded as somewhat of a
'Black Art' but with standardization and easier terminating techniques
this is no longer true. A basic knowledge of the subject, together with
a quick lesson and some practice can get you started in fibre optics, but
to really understand the subject and gain full in-depth knowledge will
require some formal training.
There are also hundreds of books on fibre optics and a search on the Barnes and Noble web site will find nearly 600 titles. Without reviewing them all it is difficult to know what to recommend, but two of the best sellers in this category seem to follow on quite nicely from this page without getting too involved with mathematics. The two books are the Introduction to Fibre-Optics by John Crisp and Understanding Fiber Optics, Third Edition by Jeff Hecht.
Right, lets get on with the lesson
First a bit history
In 1870, John Tyndall demonstrated
that light follows the curve of a stream of water pouring from a container,
it was this simple principle that led to the study and development of applications
for this phenomenon. John Logie Baird patented a method of transmitting
light in a glass rod for use in an early colour TV, but the optical losses
inherent in the materials at the time made it impractical to use. 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, as they
began using them in remote illumination and viewing instruments. In 1966
Charles Kao and George Hockham proposed the transmission of information
over glass fibre, and they also 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 out by Charles Kao and George Hockham.
The advantages of using fibre optics
Because of the Low loss, high bandwidth properties of fibre cable 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 benefits
in the data industry are 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.
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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 which stops 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 only 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,
this was 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 225m, 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.
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