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"All you'll ever need to know about Network Cabling ......... well almost !"
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 fiber optics, but to really understand the  subject and gain full in-depth knowledge will require some formal training. There are lots of Fibre Optic training companies offering recognised qualifications  and a quick search on the net should find one in your area. There are also hundreds of books on fiber optics and a search on Network Cabling  Help book store will find over 2200 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 Fiber Optic Installer's Field Manual by Bob Chomycz  and Understanding Fiber Optics,  Fifth 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 fiber 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 fiber 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 fiber  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. Fibers 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. 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.

Fiber Optics