 |
 |
 |
Fibre Optics
Click
Here for the Network Cabling Help eBook and Video Tutorials
This
is what one
customer said!
"I
am starting a new job tomorrow putting in structured (cat 5e & 6)
and fibre cable which I have never done before. After watching these videos
and reading the material it has given me a great head start in the job.
Thanks for the information, money well spent."
Regards
Carl W.
|
The installation
and termination of optical fibers 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. |
If you are in the
UK, Optical Technology Training Ltd offer several different courses to
choose from including a City & Guilds qualification.
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 fiber 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.
 |
Fibre
construction
There are many different types
of fiber 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 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 fiber 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.
A banged up computer screen doesn't necessarily mean you need to buy a new computer, with laptop parts service you can actually save your current laptop. Whether you need a laptop memory, system boards, or laptop batteries, the Internet is the place to go.
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.
Site designed and maintained by Ian Patrick