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1 An Introduction to Optical Communication Systems 1.1 INTRODUCTION Communication is a process in which messages, ideas and information can be exchanged between two individuals. From the early days when languages were developed, the methods people use to communicate have experienced a dramatic evolution. Nowadays, rapid transmission of information over long distances and instant access to various information sources have become conspicuous and important features of our society. The rapidly growing information era has been augmented by a global network of optical fibre [1]. By offering an enormous transmission bandwidth of about 1014 Hz and a low signal attenuation, the low-cost, glass-based single-mode optical fibre (SMF) provides an ideal transmission medium. In order that information can be carried along the SMF, information at the transmitter side is first converted into a stream of coherent photons. Using a specially designed semiconductor junction diode with heavy doping concentration, semiconductor lasers have been used to provide the reliable optical source required in fibre-based lightwave communication. With its miniature size compatible to the SMF, the semiconductor laser diode has played a crucial role in the success of optical fibre communication systems. This chapter has been organised as follows: in section 1.2, the historical progress of optical communication is presented. Before exploring the characteristics of semiconductor lasers, various configurations of optical fibre-based communication systems are discussed in section 1.3. Depending on the type of detection method used, both direct and coherent detection schemes are discussed. Based upon the characteristics of coherent optical communication systems, the performance requirements of semiconductor lasers are presented at the end of the chapter. In particular, the significance of having an optical source that oscillates at a single frequency whilst having a narrow spectral linewidth is reviewed. 1.2 HISTORICAL PROGRESS In the early days of human civilisation, simple optical communication in terms of signal fires and smoke was used. In those days, only limited information could be transferred within line Distributed Feedback Laser Diodes and Optical Tunable Filters H. Ghafouri–Shiraz # 2003 John Wiley & Sons, Ltd ISBN: 0-470-85618-1 2 AN INTRODUCTION TO OPTICAL COMMUNICATION SYSTEMS of sight distances. In addition the transmission quality was strongly restricted by atmospheric disturbances. This form of visual communication was extended and used in the form of flags and signal lamps until the early 1790s, when a French scientist, Claude Chappe [2] suggested a system of semaphore stations. Messages were first translated into a sequence of visual telegraphs. These were then transmitted between tall towers which could be as far as 32 km apart. These towers acted as regenerators or repeaters such that messages could be transmitted over a longer distance. However, this method was slow and costly since messages had to be verified between each tower. With the beginning of a modern understanding of electricity in the 19th century, scientists started to investigate how electricity might be used in long distance communication. The telegraph [3] and telephone [4] were two inventions best representing this early stage of the electrical communication era. During that period of time, optical communication in the atmosphere received less attention and the systems developed were slow and inefficient. The lack of suitable optical sources and transmission media were two factors that hindered the development of optical communication. It was not until the early 1960s when the invention of laser [5] once again stimulated interest in optical communication. A laser source provides a highly directional light source in which photons generated are in phase with one another. By modulating the laser, the coherent, low divergence laser beam enables the development of optical communication. Due to the atmospheric attenuation, however, laser use is restricted to short distance applications. Long distance communication employing laser sources became feasible after a breakthrough was reached in 1966 when Kao and Hockham [6] and Werts [7] discovered the use of glass-based optical waveguides. By trapping light along the central core of the cylindrical waveguide, light confined along the optical fibre could travel a longer distance as compared with atmospheric propagation. Despite the fact that the attenuation of the optical fibre used was so high, with virtually no practical application at that time, this new way of carrying optical signals received worldwide attention. With improvements in manufacturing techniques and intensive research, the attenuation of optical fibre continued to drop. Fibre loss of about 4.2 db kmÿ1 was reported [8] for wavelengths around 1 mm, whilst low-loss fibre jointing techniques also became available. In order to build an optical communication system based on optical fibres, researchers in the 1960s started focusing on the development of other optical components including optical sources and detectors [9–11]. A new family of optical devices based on semiconductor junction diodes was developed. By converting electrical current directly into a stream of coherent photons, semiconductor lasers are considered to be reliable optical laser sources. Based on similar working principles, efficient photodetectors based on the junction diode were developed. By responding to optical power, rather than optical electromagnetic fields, optical signals received are converted back into electrical signals. In this early phase of development, semiconductor lasers used were restricted to pulse operation at a very low temperature. It was not until the 1970s that practical devices operating in continuous wave at room temperature became feasible [12]. The availability of both low-loss optical fibre and reliable semiconductor-based optical devices laid the cornerstone for modern lightwave communication systems. In the late 1970s, lightwave systems were operated at 0.8 mm [13]. Semiconductor lasers and detectors employed in these systems were fabricated using alluminium gallium arsenide alloy AlGaAs [14]. Optical fibres used had a large core of diameter between 50 and 400 mm whilst typical attenuation was about 4 dB kmÿ1. At the receiver side of the system, direct detection was HISTORICAL PROGRESS 3 Figure 1.1 Attenuation of silica-based optical fibre with wavelength (after [44]). used in which optical signals were directly converted to baseband optical signals. The overall system performance was limited by the relatively larger attenuation and inter-modal dispersion of the optical fibre used. In order to reduce the cost associated with the installation and maintainence of electrical repeaters used in the lightwave communication systems, it was clear that the repeater spacing could be improved by extending the operating wavelength to a new region between 1.1 and 1.6 mm where the attenuation of the optical fibre was found to be smaller. Figure 1.1 shows the relation between the attenuation of a typical SMF and optical wavelength. For systems operating at a longer wavelength, semiconductor optical devices were fabricated using quantenary InGaAsP alloy. In order to avoid inter-modal competition associated with high-order oscillation modes inside the optical fibre, optical fibres having a smaller core diameter of about 8 mm were used. In this way, oscillation in an optical fibre was reduced to single mode. For systems operating in such a longer wavelength region, both wavelengths at 1.3 and 1.55 mm have received a lot of attention. For systems operating near 1.3 mm, it was found that the single-mode fibre used had minimum dispersion, and hence maximum bandwidth could be achieved. In the early 1980s, many systems were built using single-mode fibre at around 1.3 mm wavelength. An even lower fibre attenuation of about 0.2 dB kmÿ1 is found at around 1.55 mm. However, the deployment of lightwave systems in the 1.55 mm region was delayed due to the intrinsic fibre dispersion which limits the maximum bit rate the system can support. The problem was later alleviated by adopting dispersion-shifted or dispersion-flattened fibre [15,16]. Alternatively, semiconductor lasers oscillating in single longitudinal modes were developed [17,18]. By limiting the spread of the laser spectrum, this type of laser is widely used in upgrading the 1.3 mm lightwave 4 AN INTRODUCTION TO OPTICAL COMMUNICATION SYSTEMS systems to 1.55 mm wavelengths in which conventional single-mode fibres were used. Since 1988, field trial tests for coherent lightwave communication systems have been carried out [19–21]. In order to improve the bit rate of the present lightwave system whilst utilising available fibre bandwidth in a better way, frequency division multiplexing (FDM) schemes [22] were implemented. Before information is converted into optical signals, electronic multiplexing is often applied in combining the signals. Such a system is normally referred to as coherent optical communication since heterodyne or homodyne detection is used at the receiver end. By mixing the incoming optical signal with an optical local oscillator, coherent detection employs a different technique as compared with the direct detection method. In the 1980s, the development of coherent optical communications was hindered due to poor spectral purity and frequency instability in semiconductor lasers. Due to advances in fabrication techniques, semiconductor lasers nowadays show improved performance. In long-haul optical fibre communication systems, fibre dispersion and intrinsic attenuation are two major obstacles that affect the system performance. In the 1990s, optical fibre communication systems continued to develop in order to tackle these obstacles. To circumvent the fibre dispersion, the non-linear optical soliton able to travel extremely long distances was proven both theoretically [23,24] and experimentally [25,26]. By using optical amplifiers [27,28] as pre-amplifiers, post-amplifiers and optical repeaters, one witnesses the deployment of these wideband amplifiers in optical communication networks. In the coming years, networks employing a densely spaced wavelength division multiplexing (WDM) scheme [29] are expected. As a result, more channels and hence more information will be transmitted over a single optical fibre link. There is no doubt that a new paradigm of communication comprising an optically transparent network is already on the way [30]. 1.3 OPTICAL FIBRE COMMUNICATION SYSTEMS By transferring information in the form of light along an optical fibre, a communication system based on optical fibres starts to grow rapidly. This system, like many other communication systems, consists of many different components. A simple block diagram as shown in Fig. 1.2 represents the various components required in an optical fibre communication system. At the transmitter side, information is encoded, modulated and is then converted into a stream of optical signals. At the receiver side, optical signals received are filtered and demodulated into the original information. For long distance applications, repeaters or regenerators have to be used to compensate the intrinsic attenuation of optical fibre. In order to maximise the amount of information that can be transferred over a single optical fibre link, various multiplexing schemes might also be applied. To ensure successful implementation of optical fibre communication links, careful planning and system consideration is necessary. Apart from the performance characteristics of every component used within the system, it is also necessary to consider interactions and compatibility between various components. Depending on the system requirements, the type of transmission (analogue or digital), required transmission bandwidth, cost and reliability, may vary from one system to another. According to the type of detection method used at the receiver end, it is common to categorise an optical fibre system into either a direct detection or a coherent detection scheme. OPTICAL FIBRE COMMUNICATION SYSTEMS 5 Figure 1.2 Simple block diagram showing various components for optical fibre communication systems. 1.3.1 Intensity Modulation with a Direct Detection Scheme Simply by varying the biasing current injected into a semiconductor laser diode at the transmitter, the so-called intensity modulation with direct detection (IM/DD) scheme was widely adopted. The expression ‘intensity modulation’ derives from the fact that the intensity of the light emitted at the transmitter side is linearly modulated with respect to the input signal for either digital or analogue systems. The expression ‘direct detection’ is used because the optical detector at the receiver end responds to optical power, rather than electromagnetic fields as compared to radio or microwave links. In other words, all optical signals received at the optical detector are demodulated into baseband electrical signals. Due to its simplicity and low cost, the IM/DD transmission scheme has had great success, in particular in point-to-point transmission systems. In order to explore the potential of the optical spectrum, however, coherent detection has to be used. 1.3.2 Coherent Detection Schemes Compared to the IM/DD transmission scheme, coherent optical communication [31–33] is characterised by mixing the incoming optical signal with the local oscillator so that the baseband signal (for homodyne detection) or an intermediate frequency (IF) signal (for heterodyne detection) is generated at the receiver. Since spatial coherence of the carriers and local oscillators is exploited, the expression ‘coherent’ is used to describe such a system configuration. The advantages of coherent detection have long been investigated and were recognised in the 1960s [34], but it was not until the late 1970s that single-mode transmission from an AlGaAs semiconductor laser was demonstrated [35,36]. 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