adapt is a human resources community initiative supported by the european union through the european...

ADAPT is a Human Resources Community Initiative supported by the European Union through the European Social Fund (ESF)

Introduction to Optical Fibre

Systems


Overview

Advantages of fibre

Optical fibre in Local Area Networks

Principles of Optical Fibre

Attenuation in Optical Fibres

Types of Optical Fibre


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


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


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)


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


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


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


Optical Fibre Principles


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


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.


Attenuation in Optical Fibres


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


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


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


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.


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.


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



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


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


Propagation in different Fibre TypesStep index fibre

Graded index fibre

Singlemode fibre

Very small core size


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

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overview slide

buildings slide

types of optical fibre

different fibre types

dbm conversion table