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Topic outline

 

  • Time: 20 hours
    Level: Advanced

 
 

Introduction

  • Introduction Resource
  • By using optical fibre, very high data rates (gigabits per second and higher) can be transmitted over long distances (tens of kilometres) without amplifiers or regenerators. Optical fibre has completely...
 

1 Optical fibre communication: introduction

  • 1.1 Uses of optical fibre in communication Resource
  • Using optical fibres, very high data rates (gigabits per second and higher) can be transmitted over long distances (tens of kilometres) without amplifiers or regenerators. As a consequence, optical fibre...
  • 1.2 Wavelength or frequency? Resource
  • In this unit you will find frequent references both to the wavelength of light and the frequency of light. It is important to be able to work with both, and convert from one to the other. In the context...
 

2 Elements of an optical-fibre link

  • 2.1 Light sources and detectors Resource
  • The basic building blocks of an optical-fibre link are the light source, the fibre and the detector (Figure 1).
  • 2.2 Fibre types Resource
  • A strand of glass (or plastic, but the best performance comes from glass) has a core surrounded by a cladding, where the refractive index of the glass in the core is higher than that of the cladding (see...
  • 2.3 Attenuation Resource
  • At approximately what wavelength is the attenuation of optical fibre lowest? What, approximately, is the attenuation at that wavelength? What other wavelengths are used and why?
  • 2.4 Pulse spreading and bandwidth Resource
  • Calculate the maximum signalling rate given by the Nyquist rate for the 1550 nm window, assuming that it runs from 1450 nm to 1610 nm.
  • 2.5 Non-Linearity Resource
  • A linear system can be defined in two ways: (1) one which obeys the principle of superposition, and (2) one possessing the frequency-preservation property.
  • 2.6 Connecting and splicing fibres Resource
  • There are two different types of fibre joint that need to be considered: permanent splices (the equivalent of soldered or crimped connections on copper cables) and demountable connectors.
  • 2.7 Cabling Resource
  • A distinction must be made between the optical fibre – a single strand of glass fibre – and the optical-fibre cable consisting of one or more strands of fibre and various protective coverings.
 

3 Optical components

  • 3.1 Introduction Resource
  • The basic optical-fibre link consisted of the source (laser or LED), the fibre and the detector, as was shown in Figure 1. Improvements in these components can increase the data rate, but the system is...
  • 3.2 Directional couplers Resource
  • A simple yet valuable device is the directional coupler (Figure 19). A directional coupler can be constructed from two single-mode fibres by bringing them into close contact and heating so that the glass...
  • 3.3 Optical amplifiers Resource
  • Figure 22 shows in outline one possible structure for an Erbium-doped fibre amplifier (EDFA).
  • 3.4 Optical switches Resource
  • Optical space switching has been possible for a long time, but has been slow to find widespread application.
  • 3.5 Wavelength multiplexing and demultiplexing Resource
  • Wavelength multiplexers and demultiplexers are needed in order to be able to use wavelength division multiplexing. With just two wavelengths, the multiplexers and demultiplexers can be based on directional...
 

4 Developments and future directions in optical-fibre communications

  • 4.1 Introduction Resource
  • Additional material for this unit, by David Chapman, January 2005
  • 4.2 Fibre in the core network Resource
  • All new trunk transmission – that is, transmission between telephone exchanges – is now over optical fibre. Mostly it uses either PDH or SDH links. Year by year the data rates have increased, so that at...
  • 4.3 Optical networking Resource
  • DWDM improves the utilisation of optical fibre for point-to-point links, but a further step in exploiting the potential of optical fibre comes from optical networking in which routeing or switching is...
  • 4.4 Fibre in the access network Resource
  • In the 1980s there was a belief that it was only a matter of time before fibre would be installed in the access network (from individual private customers to the local telephone exchange, also called ‘the...
  • 4.5 Fibre in LANs Resource
  • Fibre has been slower to be exploited in LANs than in the core transmission network, for similar reasons to the delay in the use of fibre in the access network, but as the data rate demanded of LANs has...
  • 4.6 Conclusion to Section 4 Resource
  • This brief account has introduced a few of the most rapidly developing areas of optical-fibre communications as of January 2004. By the time you are reading it things will certainly have moved on, and...
 

5 Summary

  • 5 Summary Resource
  • The building blocks of a basic optical-fibre communications link are the modulated light source, the fibre and the detector. There are choices to be made between different types of light source and fibre,...
 

References and Acknowledgements

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