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ADAPT is a Human Resources Community Initiative supported by the European Union through the European Social Fund (ESF)
Introduction to Optical Fibre
Systems
ADAPT is a Human Resources Community Initiative supported by the European Union through the European Social Fund (ESF)
Overview
Advantages of fibre
Optical fibre in Local Area Networks
Principles of Optical Fibre
Attenuation in Optical Fibres
Types of Optical Fibre
ADAPT is a Human Resources Community Initiative supported by the European Union through the European Social Fund (ESF)
Source: Master 2_1
Wavelength: Light can be characterised in terms of its wavelengthAnalogous to the frequency of a radio signal
The wavelength of light is expressed in microns or nanometers
The visible light spectrum ranges from ultraviolet to infra-red
Optical fibre systems operate in three IR windows around 800 nm, 1310 nm and 1550 nm
200 400 600 800 1000 1200 1400 1600 1800
Spectrum of light (wavelength in nanometers)
Fibre operating windows
Visible light
Wavelength and Spectra
ADAPT is a Human Resources Community Initiative supported by the European Union through the European Social Fund (ESF)
Advantages of FibreLow attenuation, large bandwidth allowing long distance (>100 km) athigh bit rates ( > 1Gb/s)
Small physical size
Low physical mass, low material cost
Cables can be made non-conducting, thus eliminating electromagneticinterference, shock hazards and providing electrical isolation
Negligible crosstalk between fibre channels in the same cable
High security, since tapping is very difficult
Upgrade potential to higher bit rates is excellent
ADAPT is a Human Resources Community Initiative supported by the European Union through the European Social Fund (ESF)
Disadvantages of FibreJointing fibre can be more difficult and expensive
Bare fibre is not as mechanically robust as copper wire
Fibres are not directly suited to multi-access use, alters nature of networks
Higher minimum bend radius by comparison with copper
Minimum bend radius for fibre cable is typically 10-20 times the cable outer
diameter (4-8 for copper)
ADAPT is a Human Resources Community Initiative supported by the European Union through the European Social Fund (ESF)
Application AreasThe most common current applications are:
Public network trunk telecommunications links
Local area and Metropolitan area networks
Short range data transmission
Video transmission
The most promising emerging applications are:
Fibre to the home (FTTH)
Very high speed LANs (1 Gb/s +)
Unrepeatered ultra-long trunk undersea links
Optical interconnects
ADAPT is a Human Resources Community Initiative supported by the European Union through the European Social Fund (ESF)
Applications for Fibre in Buildings
Campus Backbone
Building Backbone
Horizontal Cabling
Most fibre is used in campus and building backbonesHorizontal cabling is mainly copper at present but may become fibre
ADAPT is a Human Resources Community Initiative supported by the European Union through the European Social Fund (ESF)
Physical Cabling StructuresPhysically cabling is always much more complex than a simple point-to-point link
Cabling is installed in a variety of ways or topologies
Typical example is a building backbone
Each fibre link on its own is a simple point- to-point link
But from a network perspective the system may be configured as a ring or star network
Patch panels allow the collection of fibre links to be configured in different ways Building backbone
cabling
ADAPT is a Human Resources Community Initiative supported by the European Union through the European Social Fund (ESF)
Optical Fibre Principles
ADAPT is a Human Resources Community Initiative supported by the European Union through the European Social Fund (ESF)
Optical Fibre Construction
Cladding
Core
Fibre consists of a core surrounded by a cladding.
Most common material is silica glass
The core has a refractive index greater than that of the cladding
Core diameters range from 7 microns to 1 mm
Basic Fibre
ADAPT is a Human Resources Community Initiative supported by the European Union through the European Social Fund (ESF)
Propagation in a fibreBecause of the differences between the glass in the core and the cladding light bends or reflects as it reaches the core-cladding boundary
Some light rays can therefore be "trapped" in the core
Light rays trapped in the core can travel or "propagate" down the fibre even where the fibre bends.
Cladding
Core(i)
(ii)
A
B
Fibre Axis
Example: Light ray (i) propagates because at A it is reflected back into the core. This is repeated at point B and so on. Light ray (ii) is not trapped in the core and is lost in the cladding.
ADAPT is a Human Resources Community Initiative supported by the European Union through the European Social Fund (ESF)
Attenuation in Optical Fibres
ADAPT is a Human Resources Community Initiative supported by the European Union through the European Social Fund (ESF)
Decibels and AttenuationBasic decibel power equation relates two absolute powers P1 and P2:
Power ratio in dB = 10 Log [P1/P2]10
In a fibre or other component with an input power Pin and an output power Pout the loss is given by:
Loss in dB = 10 Log [Pout/Pin]10
By convention the attenuation in a fibre or other optical component is specified as a positive figure, so that the above formula becomes:
Attenuation in dB = -10 Log [Pout/Pin]10
ADAPT is a Human Resources Community Initiative supported by the European Union through the European Social Fund (ESF)
Absolute power in DecibelsIt is very useful to be able to specify in dB an absolute power in watts or mW.
To do this the power P2 in the dB formula is fixed at some agreed reference value, so the dB value always relates to this reference power level.
Allows for the easy calculation of power at any point in a system
Where the reference power is 1 mW the power in an optical signal with a power level P is given in dBm as:
Power in dBm = 10 Log [P/1mW]10
Where the reference power is 1 W the power in an optical signal with a power level P is given in dB as:
Power in dB = 10 Log [P/1W]10
ADAPT is a Human Resources Community Initiative supported by the European Union through the European Social Fund (ESF)
Watts to dBm Conversion TablePower (watts) Power (dBm)
1 W +30 dBm100 mW +20 dBm10 mW +10 dBm5 mW +7 dBm2 mW +3 dBm1 mW 0 dBm
500 mW -3 dBm200 mW - 7 dBm100 mW -10 dBm50 mW -13 dBm10 mW -20 dBm5 mW -23 dBm1 mW -30 dBm
500 nW -33 dBm100 nW -40 dBm
ADAPT is a Human Resources Community Initiative supported by the European Union through the European Social Fund (ESF)
Types of AttenuationAbsorption Loss:
Caused by the fibre itself or by impurities in the
fibre, such as water and metals.
Scattering Loss:
Intrinsic loss mechanism caused by the interaction
of photons with the glass itself.
Bending loss:
Loss induced by physical stress on the fibre.
ADAPT is a Human Resources Community Initiative supported by the European Union through the European Social Fund (ESF)
Transmission Windows
Wavelength in nanometers
600 700 800 900 1000 1100 1200 1300 1400 1500 1600
LossdB/Km
0.1
1
10
1st windowcirca 850 nm
2nd windowcirca 1330 nm
3rd windowcirca 1550 nm
ScatteringLoss only
TotalLoss
Three low loss transmission windows exist circa 850, 1330, 1550 nmEarliest systems worked at 850 nm, latest systems at 1550.
ADAPT is a Human Resources Community Initiative supported by the European Union through the European Social Fund (ESF)
Bending Loss in FibresAt a bend the propagation conditions alter and light rays which would propagate in a straight fibre are lost in the cladding.
Macrobending, for example due to tight bends
Microbending, due to microscopic fibre deformation, commonly caused by poor cable design
Macrobending is commonly caused by poor installation or
handling
Microbending is commonly caused by poor cable design
ADAPT is a Human Resources Community Initiative supported by the European Union through the European Social Fund (ESF)
Types of Optical Fibre
ADAPT is a Human Resources Community Initiative supported by the European Union through the European Social Fund (ESF)
Types of Optical Fibre
Three distinct types of optical fibre have developed
Concern here is to examine propagation in the different fibres
The three fibre types are:
Step index fibre
Graded index fibre
Singlemode fibre (also called monomode fibre)
Multimode fibres
ADAPT is a Human Resources Community Initiative supported by the European Union through the European Social Fund (ESF)
Dispersion in an Optical FibreFibre type influences so-called "Dispersion"
The higher the dispersion the lower the fibre bandwidth
Lower fibre bandwidths mean less information capacity
Modal Dispersion:
Reduced by using graded index fibreEliminated by using singlemode fibre
Material Dispersion:
Reduced by using Laser rather than LED sourcesReduced by operating close to 1320 nm
ADAPT is a Human Resources Community Initiative supported by the European Union through the European Social Fund (ESF)
Propagation in different Fibre TypesStep index fibre
Graded index fibre
Singlemode fibre
Very small core size
ADAPT is a Human Resources Community Initiative supported by the European Union through the European Social Fund (ESF)
SummaryOptical fibre systems utilise infared light in the range 700 nm to 1600 nm
Fibre has a number of significant advantages
Most fibre systems are point-to-point, containing fibre, connectors and splices
Fibre in normally used as a backbone in Local Area Networks