1. MILLENNIUM CABLING SYSTEM Optical Fibre

2. Essential Elements of a Communication Link
Optical - Electronic
Interface
Communication Link
Equipment 1
Equipment 2
Could be a
LASER or an LED.
Could be a PIN
or an APD.
A fibre optics link is composed of a number of basic components.
a) Optical Electronic Interface - the transmitter
b) Optical Fibre Cable
d) Optical Electronic Interface - the receiver
This is the simplest form of link and it provides a point to point communication, that is, a given transmitter is coupled to a designated receiver, with communication in one direction only. Return communication must be provided by a separate link.
Cable connectors and couplers are analogous to their electrical counterparts. The design of these components has to satisfy not only the required optical performance, but also mechanical and environmental specifications so that the installation and operation will be no more difficult to achieve than with copper cables.
Over the recent years equipment designed for the connection and termination of optical fibre has become increasingly less skill sensitive and increasingly more reliable.
FIBRE IS NO LONGER A DARK ART

3. BASIC OPTICAL THEORY
THE REFRACTIVE INDEX of a medium ( sometimes written as n)
is defined as the ratio of:
The speed of light in a vacuum
The speed of light in a medium
For silica glass (which fibre is made of )this is
300,000 Kilometres /sec
200,000 Kilometres/sec
The path of a ray of light in different materials is influenced by the fact that light travels more slowly in an optically dense medium than it does in a less dense one. A measure of this effect is the refractive index.
When a ray of light passes through the boundary between media of different refractive indices a small portion of the light is reflected back into the originating medium. By decreasing the angle of attack it is possible to reach a state when the angle of incidence and the angle of reflection are equal.
At this point all the light is reflected back into the originating medium, this is called Total Internal Reflection and theoretically 100% efficient. In practice the efficiency can reach 99.9% compared for example with 85% for a silvered mirror.
This phenomena is the basis for the transmission of light in optical fibres.
Giving a Refractive Index of 1.5

4. TOTAL INTERNAL REFLECTION
To get Total Internal Reflection of light in a Fibre, there are two conditions that have to occur.
The core and cladding must be Optically different
The light must enter the fibre at some angle greater than the critical angle
Air
Water
Refracted Ray
Incident Ray
Refracted ray
Reflected Ray
Example 1
Example 2
Example 3
AIR
WATER
Example 1.
The incident ray has hit the interface at an angle less than the Critical Angle, and is refracted. ( This is like the spoon in a glass of water).
Example 2.
The incident ray has hit the interface at the Critical Angle and therefore cannot escape, and is bent along the line of the interface
Example 3.
The incident ray has hit the interface at an angle GREATER than the Critical Angle and is therefore REFLECTED back into the medium.

5. PROPAGATION OF LIGHT INTO A FIBRE.
Cladding 
Core
Axis A
Angle
Cone of
Acceptance
Angle A is the Critical Angle any light aimed at the fibre within this angle will
be reflected and will not enter the fibre.
The N.A.( Numerical Aperture of the fibre ) = SIN only rays within angle 
will pass into the fibre.
The higher the NA value, the easier it is to get light into the fibre .
In the diagram above, the axis is the middle of the core. Angle A is the Critical Angle, Angle  is any angle greater than the Critical Angle.
The Cone of Acceptance (or Numerical Aperture) is all those angles which are greater than the Critical Angle, ( rotated through 360 degrees).
Any light introduced into the Cone of Acceptance will enter the fibre and will propagate down the core down the fibre.
The Critical Angle changes with the diameter of the core, for Singlemode Fibre is normally around 83 degrees and 62.5µm Multimode fibre it is normally 74 degrees.

6. WHAT IS OPTICAL FIBRE? Cladding Rays of light Core
All optical fibres have a core and a cladding. The core has a higher refractive index than the cladding, and this property constrains the light to remain in the core.
The amount of light emerging from the end of the fibre is always less than that entering due to losses caused by scattering and absorption in the core and by imperfect reflection at the optical face. This loss is dependent on the length of the fibre and produces an exponential decay, which means that repeated equal increments of length always cause the same proportional decrease in power.
Light is transmitted along a fibre by a multitude of different paths ranging from one which is parallel to the axis to those propagating close to the core and cladding. Each path at a different angle is called a transmission mode.

7. DISPERSION - BANDWIDTH
Fibre cross section index section pulse propagation in Fibre Pulse
Refractive Input Light Output
Type : step-index multimode
125 um n
A out r
100 um
Type : graded index multimode r
125 um n
A out
50um
Type : singlemode fibre
125 um n
A out r
10um
Multimode Step Index fibres exhibit a distinct change in refractive index between the core and the cladding. It is clear that the distances travelled by various modes and hence the time taken are not equal. Consequentially a short pulse of light launched into the fibre will have various transmission delays and will arrive at the exit dispersed over an extended period of time.
This limits the maximum data rate since the rapid train of pulses will merge into one another and may not be distinguishable. There will also be a reduction in amplitude caused by the pulse spreading.
Multimode Graded index fibre is constructed with a refractive index that is continuously graded from a maximum at the centre to a minimum at the interface between the core and the cladding. The previously stepped path is replaced by a smooth curve, and since the same phenomenon occurs on the opposite side of the axis, light is transmitted down the fibre following a smooth oscillating path. From this it can be seen that more of the modes arrive at the end of the fibre at the same time and that the dispersion of the pulse and its consequent reduction in amplitude are reduced.
In Singlemode fibre the core is so narrow that it constrains the pulse of light to a single mode thereby dramatically reducing the dispersion of the signal and its attenuation.

8. COMPARED TO COPPER CABLE
Fibre has a much larger bandwidth
Fibre has a much lower attenuation
Fibre is unaffected by electromagnetic interference
Fibre does not radiate electromagnetic energy
Fibre is small and light
Fibre has a much larger bandwidth
so more information can be sent down the fibre
Fibre has a much lower attenuation
so the information can travel further
Fibre is unaffected by electromagnetic interference
so it can be used safely near electrical machines and sources of interference
Fibre does not radiate electromagnetic energy
so it is totally secure and it will not interfere with other cables and equipment
Fibre is small and light
so it is easy to install and takes up little space in the cable trays and ducts

9. DIFFERENT TYPES OF FIBRE
The terms “62.5/125” means a core of 62.5 microns in diameter and a cladding of 125 microns in diameter.

10. TYPICAL FIBRE CONSTRUCTION

11. TIGHT BUFFERED OPTICAL FIBRE
Nine-hundred micron tight buffered fibre is the smallest fibre suitable for direct termination with optical connectors. For this reason it is the preferred type of fibre for use indoors, where it is common to fit connectors directly to the end of the fibres in the patch panels and remote outlets.
It is also used for “pigtails” where an external fibre can be fitted with a connector by splicing on a previously terminated piece of fibre with a connector fitted.
- Easy strip up to 150mm
- very robust
- easy to directly terminate with a connector
- individual colours to aid fibre identification

12. FIBRE TYPES Why are there different fibre types? The smaller the core,
the lower the attenuation
the higher the bandwidth
But
more difficult to terminate and measure

13. FIBRE TYPES
so, small core fibre such as 9/125um are usually used in long distance telecomm applications
50/125um and 62.5/125um fibres are usually used in short haul datacomms applications, typically up to 2Km

14. OPTICAL FIBRE PERFORMANCE
Wavelength of operation
Attenuation
Bandwidth

15. ELECTROMAGNETIC SPECTRUM
Wavelength in Meters
Frequency in Hz
Increases

16. WAVELENGTH WINDOWS Wavelength These are the windows of operation
1st Window
850nm
2nd Window
1300nm
3rd Window
1550nm
These are the windows of operation
1st window 850 nm
2nd window 1300nm
3rd window 1550nm
these are all in the infrared spectrum

17. OPTICAL FIBRE SPECIFICATION
Fibre Bandwidth Attenuation Dispersion Refractive
Type MHz - km dB/km ps/nm - km Index
850nm nm nm nm nm nm
50/
62.5/
1300nm nm
Singlemode
Note the refractive index is required for OTDR set up.
Note bandwidth is not stated for singlemode but is roughly equivalent to 30GHz. km.

18. OPTICAL FIBRE SPECIFICATION
BBR Optical Fibre meets or exceeds the specifications for -:
ISO 11801
EIA/TIA 568A
FDDI
Gigabit Ethernet
Token Ring
Higher performance is available to special order eg 800 MHz-km fibre for IBM ESCON trunk links
Singlemode fibre is standard telecommunications grade

19. OPTICAL FIBRE SPECIFICATION
BBR Optical fibre cables will still be within specification after the rigours of installation if :-
The cable is installed according to instructions and bend radius, tension, crush and torsional load limits are not exceeded.
The cable is used within its environmental constraints and temperature ranges
Note:
The fibre is proof tested to 1%, (100 Kpsi) to ensure long life. Many other fibres are only tested to 0.5%.

20. OPTICAL FIBRE TESTING OTDR - Optical Time Domain Reflectometry
Filename : 03-B TDR
Test : 2PT FIBER ATTENUATION
Measurement Results
Curser (C) : m
Marker (M) : m
Length diff : m
2 Pnt. Loss : dB
Loss/Length : dB/km
0.000
-0.00
-3.00
-6.00
-9.00
-12.00
-15.00
-18.00 dB
OTDR trace showing a faulty fibre (caused by an over tight nylon tie?)
The OTDR works by sending a pulse of laser light down the fibre. When this pulse of light energy encounters physical changes in the fibre itself or changes in the transmission characteristics caused by connectors, splices, bends or breaks, some of the light is reflected back towards the source.
An important factor in the calibration of an OTDR is knowing the Refractive Index of the fibre that is being tested. The RI of a fibre governs the speed at which the light pulse travels down the fibre and is required to be set on the instrument to ensure that the time domain is accurate for the type of fibre being tested.
The OTDR is a very useful instrument, which can point out problems in the system, their location and enable the installer to rectify these faults quickly.

21. OPTICAL FIBRE TESTING OTDR - Optical Time Domain Reflectometry
Filename : 03-B TDR
Test : 2PT FIBER ATTENUATION
-0.00 dB
-3.00
-6.00
-9.00
-12.00
-15.00
-18.00
0.000
The above slide shows the use of a tail lead.
This lead is fitted in order to measure the loss across the remote connector.
Measurement Results
Cursor (C) : m
Marker (M) : m
Length diff : m
2 Pnt. Loss : dB
Loss/Length : dB/km

22. Power Meter and Light Source
OPTICAL FIBRE TESTING
Power Meter and Light Source
The light source and power meter give absolute dB loss for each fibre but give much less information than the OTDR.
It is often a more appropriate test for very short lengths however.

23. OPTICAL FIBRE CONNECTORS
The most popular connector in datacomms is the “ST” connector
But the “SC” is specified in ISO 11801
The SC can be used for both Multimode and Singlemode, but the ST is nearly always used for Multimode.

24. OPTICAL FIBRE CONNECTORS
Two optical connectors are matched together using a device referred to by different names, ie “uniter”, “adaptor”, or “coupler”
The “uniter” or “adaptor” is usually bulkhead mounted in a patch panel.
SC UNITER ST UNITER

25. OPTICAL FIBRE CONNECTORS
The connectors are fitted to the fibre using :-
“Pot and Polish”
“Crimp and Cleave”
“Hot Melt”
“Anaerobic” (cold cure)
The connectors may be fitted on site directly onto the ends of the fibre, or they may be factory fitted onto the “pigtails” which are then spliced onto the main fibre on site.
BBR only recommend “Pot & Polish”, “Hot Melt” or the Anaerobic connectors for the 15 year warranty.

26. OPTICAL FIBRE SPLICING
Two methods, fusion splicing and mechanical splicing.
The two fibres to be spliced are cleaved to give an accurate 90 degree end faces and then brought together between two electrodes. An electric arc struck between the two electrodes melts the fibre ends together. Typical splice loss is 0.2dB.

27. OPTICAL FIBRE SPLICING
Mechanical Splicing
The mechanical splice is essentially a precision made glass or elastomer tube.
The optical performance is nearly as good as a fusion splicer. The start up tooling costs for mechanical splicing is very low , but ongoing cost per splice is quite high.
Anybody doing more than 1000 splices per year would be better off with a fusion splicer.

28. OTHER OPTICAL COMPONENTS
Other components used are :-
in-line joints
distribution joints
transition joints
rack mounted patch panels
wall mounted patch panels