Optical CDMA Networks

Principles, Analysis and Applications

Hooshang Ghafouri-Shiraz, M. Massoud Karbassian

• Publisher: Wiley-IEEE Press
• Year: 2012
• ISBN:
  9781119942009
• Edition: 1
• Topic: Scienze fisiche
• Subtopic: Onde e meccanica ondulatoria

This book focuses heavily on the principles, analysis and applications of code-division multiple-access (CDMA) techniques in optical communication systems and networks.

In this book, the authors intimately discuss modern optical networks and their applications in current and emerging communication technologies, evaluating the quality, speed and number of supported services. In particular, principles and fundamentals of optical CDMA techniques from beginner to advanced levels are heavily covered. Furthermore, the authors concentrate on methods and techniques of various encoding and decoding schemes and their structures, as well as analysis of optical CDMA systems with various transceiver models including advanced multi-level incoherent and coherent modulations with the architecture of access/aggregation networks in mind. Moreover, authors examine intriguing topics of optical CDMA networking, compatibility with IP networks, and implementation of optical multi-rate multi-service CDMA networks.

Key features:

• Expanded coverage of optical CDMA networks, starts from principles and fundamentals
• Comprehensive mathematical modelling and analysis from signal to system levels
• Addresses the applications of modern optical networking in the current and emerging communication technologies
• Greater focus on advanced optical multi-level incoherent and coherent modulations, spreading codes, and transceiver designs
• Detailed hardware specifications, system-level block diagrams, and network nodes' functionalities

This book appeals to researchers, practicing engineers, and advanced students. It is a practical resource for readers with an interest in optical communications and networks.

Chapter 1

Introduction to Optical Communications

1.1 Evolution of Lightwave Technology

The invention of the solid state ruby laser (acronym for light amplification by stimulated emission of radiation) in May 1960 [1] and the He-Ne gas laser [2] in December 1960 has led to some wide-ranging and very significant scientific and technological progress. This so-called ‘discovery of the century’, followed by the first use of semiconductor lasers [3–5] in communications, heralded the start of optical communications. The laser provided a powerful coherent light source together with the possibility of modulation at high frequency, and this opened up a new portion of the electromagnetic spectrum with frequencies many times higher than those commonly available in radio communication systems. In addition the narrow beam divergence of the laser made enhanced free-space optical transmission a practical possibility.

Since optical frequencies are of the order of 100 THz, and information capacity increases directly with frequency bandwidth, the laser potentially offers a few order of magnitude increase in available bandwidth compared with microwave systems. Thus, by using only a small portion of the available frequency spectrum, a single laser could, in principle, carry millions of telephone conversations or TV channels.

With the potential of such wideband transmission capabilities in mind, a number of experiments [6] using atmospheric optical channels were carried out in the early 1960s. These experiments showed the feasibility of modulating a coherent optical carrier wave at very high frequencies. However, the high cost of development for all the necessary components, and the limitations imposed on the atmospheric channel by rain, fog, snow and dust make such high-speed systems economically unattractive. However, numerous developments of free-space optical channel systems operating at baseband frequencies were in progress for earth-to-space communications [7, 8].

It soon became apparent that some form of optical waveguide was required. By 1963, bundles of several hundred glass fibres were already being used for small-scale illumination, but these early fibres had very high attenuations and so their use as a transmission medium for optical communications was not considerable. Optical fibres can provide a much more reliable and versatile optical channel than the atmosphere. Initially, the extremely large losses of more than 1000 dB/km observed in even the best optical fibres made them appear impractical. In fact, to compete with existing coaxial cable transmission lines, the glass fibre attenuation had to be reduced to less than 20 dB/km. It was in 1966 that C.K. Kao (2009 Nobel Laureate) and G.A. Hockman [9] speculated that these high losses were as a result of impurities in the fibre material, and that the losses could be reduced to the point where optical waveguides would be a viable transmission medium. This was realized in 1970 when Kapron, Keck and Maurer [10] of the Corning Glass Works fabricated a fibre having 20 dB/km attenuation. At this attenuation, repeater spacing for optical fibre links become comparable to those of copper systems, thereby making lightwave technology an engineering reality. A whole new era of optical fibre communications was thus launched.

The ensuing development of optical fibre transmission systems grew from the combination of semiconductor technology, which provided the necessary light sources and photodetectors, and optical waveguide technology upon which the optical fibre is based. The result was a transmission link that had certain inherent advantages over conventional copper systems in telecommunications applications. For example, optical fibres have lower transmission losses and wider bandwidths as compared to copper wires.

This means that, with optical fibre cable systems, more data can be sent through one fibre, over longer distances, thereby decreasing the number of channels and reducing the number of repeaters needed over these distances. In addition, the low weight and the small hair-sized dimensions of fibres offer a distinct advantage over heavy, bulky wire cables in crowded underground city ducts. The low weight and small size are also of importance in aircraft where small lightweight cables are advantageous, and in tactical military applications where large amounts of cable must be unreeled and retrieved rapidly.

An especially important feature of optical fibres relates to their dielectric nature. This provides optical waveguides with immunity to electromagnetic interference, such as inductive pick-up from signal-carrying wires and from lightning, and freedom from electromagnetic pulse effects, the latter being of particular interest for military applications. Furthermore, ground loops are no longer an issue, fibre-to-fibre crosstalk is very low and a high degree of data security is afforded since the optical signal is well confined within the waveguide. Of additional importance is the advantage that silica is the principal material of which optical fibres are made. This material is abundant and inexpensive since the main source of silica is sand.

The recognition of optical fibre advantages in the early 1970s created a flurry of activity in all areas related to optical fibre transmission systems. This development led to the first laboratory demonstration of optical communication with glass fibre in the early 1970s. Such a progress resulted in significant technological advances in optical sources, fibres, photodetectors and fibre cable connectors. Since then, research on material for optical fibre transmission has made dramatic progress. Fibre loss was reduced from 1000 dB/km to 20 dB/km in 1970 and to 4 dB/km in 1973. By using longer-wavelength transmission the optical fibre losses were further reduced to 2 dB/km in 1974, 0.5 dB/km in 1976 and 0.2 dB/km by 1979.

Also, a study of the spectral response of glass fibres showed the presence of low-loss transmissions at 850 nm, 1300 nm and 1550 nm as shown in Figure 1.1. Although the early optical links used the 850 nm window, the longer wavelength windows exhibit lower losses, typically 0.5 dB/km at 1300 nm and 0.22 dB/km at 1550 nm. As a result, most modern links use 1300–1550 nm wavelength light sources.

Figure 1.1 Optical signal attenuation against wavelength

New types of fibre materials have also been investigated [11–14] for use in the 3 μm–5 μm wavelength bands. It was found that fluoride glasses have extremely low transmission losses at mid-infrared wavelengths (i.e. 0.2 μm < λ < 8 μm) with the lowest loss being around 2.25 μm. The material that has been concentrated on is a heavy-metal fluoride glass which uses ZrF₄ as the major component. Although this glass potentially offers intrinsic minimum losses of 1.01–0.001 dB/km, fabricating long lengths of these fibres is difficult. Firstly, ultra-pure materials must be used to reach this low level. Secondly, fluoride glass is prone to devitrification. Fibre-making techniques have to take this into account to avoid the formation of microcrystallites, which have a drastic effect on scattering losses.

1.2 Laser Technologies

The prospects for lower fibre losses at longer wavelengths led to intensive research on lasers and photodetectors. The advent of the semiconductor laser in 1962 meant that a fast light source was available. The material used was gallium arsenide (GaAs) which emits light at a wavelength of 870 nm. With the discovery of the 850 nm window, the wavelength of emission was reduced by doping the GaAs with aluminium (Al). Later modifications included different laser structures to increase device efficiency and lifetime. As the longer wavelength windows exhibit lower losses, various materials—in particular InGaAsP/InP—were also investigated to produce devices for operation at 1300 nm and 1550 nm. These efforts have also been successful. Commercial systems that used 1300 and 1550 nm technologies appeared in early and mid 1980s, respectively. Semiconductor sources are now available which emit at any one of the above wavelengths, with modulation speeds of several Gbits/s being routinely achieved.

The field of semiconductor lasers which has already reached a considerable level of development has recently undergone more changes. Initially, semiconductor laser wavelengths were at the infrared end of the spectrum. However, by shortening the wavelengths it becomes possible to concentrate the light in a smaller area thus increasing the energy density and producing a light source that is suited to optical data processing devices such as optical disk a...