Communication may be broadly defined as the transfer of information from one point to another. When the information is to be conveyed over any distance a communication system is usually required. Within a communication system the information transfer is frequently achieved by superimposing or modulating the information on to an electromagnetic wave which acts as a carrier for the information signal. This modulated carrier is then transmitted to the required destination where it is received and the original information signal is obtained by demodulation. Sophisticated technique have been developed for this process using electromagnetic carrier waves operating at radio frequencies as well as microwave and millimeter wave frequencies. However, communication may also be achieved using an electromagnetic carrier which is selected from the optical range of frequencies.

1.1 Historical development

       The use or visible optical carrier wave or light for communication has been common for many year. Simple systems such as signal fires, reflecting mirrors and more recently. signaling lamps have provided successful if limited, information transfer. Moreover, as early as 1880 Alexander Graham bell reported the transmission of speech using a light beam. The photophone proposed by Bell just four years after the invention of the telephone modulated sunlight with a diaphragm giving speech transmission over a distance of 200m. However, althoughsome investigation of optical communication continued in the early part of the twentieth century its use was limited to mobile, low capacity communication links. This was due to both the lack of suitable light sources and the problem that light transmission in the atmosphere is restricted to line of sight and is severely affected by disturbances such as rain, snow, fog dust and atmospheric turbulence. Nevertheless tower frequency and hence longer wavelength electromagnetic waves (i.e. radio and microwave) proved suitable carriers for information transfer in the atmosphere, being far less affected by these atmospheric conditions. Depending on their wavelengths these electromagnetic carriers can be transmitted over considerable distances but are limited in the amount of information they can convey by their frequencies (i.e. the information-carrying capacity is directly related to the bandwidth or frequency extent of the modulated carrier which is generally limited to a fixed fraction of the carrier frequency).

       In theory, the greater the carrier frequency, the larger the available transmission bandwidth and thus the information-carrying capacity of the communication system. For this reason radio communication was developed to higher frequencies (i.e. VHF and UHF) leading to the introduction of the even higher frequency microwave and, latterly, millimeter wave transmission. The relative frequencies and wavelengths of these types of electromagnetic wave can be observed from the electromagnetic spectrum shown in Figure 1.1. In this context it also be noted that communication at optical frequencies offers an increase in the potential usable bandwidth by a factor of around 10⁴ over high frequency microwave transmission. An additional benefit of the use of. high carrier frequencies is the general ability of the communication system to communication system to concentrate the available power within the transmitted electromagnetic wave, thus giving an improved system performance .

        A renewed interest in optical communication was stimulated in the early 1960s with the invention of the laser . his device provided a powerful coherent light source together with the possibility of modulation at high frequency. In addition the low beam divergence of the laser made enhanced free space optical transmission a practical possibility. However, the previously mentioned constraints of light transmission in the atmosphere tended to restrict these systems to short distance applications. Nevertheless, despite the problems some modest free space optical communication links have been implemented for applications such as the linking of a television camera to a base vehicle and for data links of a few hundred meters between buildings. There is also some interest in optical communication between satellites in outer space using similar techniques

   Although the use of the laser for free space optical communication proved somewhat limited, the invention of the laser instigated a tremendous research effort into the study of optical components to achieve reliable information transfer using a lightwave carrier. The proposals for optical communication via dielectric waveguide or optical fibers fabricated from glass to avoid degradation of the optical signal by the atmosphere were made almost simultaneously in 1966 by Kao and Hockham and Werts . Such systems were viewed as a replacement for coaxial cable or carrier transmission systems. Initially the optical fibers exhibited very high attenuation (i.e. 1000dB/ km) and were therefore not comparable with the coaxial cables they were to replace (i.e. 5 to 10dB/ km) .

       There were also serious problems involved in jointing the fiber cables in a satisfactory manner to achieve low loss and to enable the process to be performed relatively easily and repeatedly in the field. Nevertheless within the space of ten years optical fiber losses were reduced to below 5 db/ km and suitable low loss jointing techniques were perfected.

        In parallel with the development of the fiber waveguide attention was also focused on the other optical components which would constitute the optical fiber communication system. Since optical frequencies are accompanied by extremely small wavelengths the development of all these optical components essentially required a new technology.Thus semiconductor optical sources (i.e. injection lasers and light emitting diodes) and detectors (i.e. photodiodes and to a certain extent phototransistors) compatible in size with optical fibers were designed and fabricated to enable successful implementation of the optical fiber system. Initially the semiconductor lasers exhibited very short lifetimes or at best a few' hours. but significant advances in the device structure enabled lifetime greater than 1000 hr I and 7000 hr to be obtained by 1973 and 1977 respectively. These devices were originally fabricated from alloys of galllium arsenide (AlGaAs) which emitted in the near infrared between 0.8 and 0.9 m m.

        To obtain both the low loss and low dispersion at the same operating wavelength, new advanced single-mode fiber structures have been realized: namely, dispersion shifted and dispersion flattened fibers. Hence developments in fiber technology have continued rapidly over recent years, encompassing other specialist fiber types such a polarization maintaining fibers, as well as glass materials for even longer wavelength operation in the mid-infrared (2 to 5m m) and far-infrared (8 to 12 m m) regions. In addition, the implementation of associated fiber components (splices, connectors couplers, etc.) and active optoelectronic devices (sources, detectors, amplifiers, etc.) has also moved forward with such speed that optical fiber communication technology would seem to have reached a stage of maturity within its development path . Therefore, high performance, reliable optical fiber communication systems are now widely deployed both within telecommunications networks and many other more localized communication application areas.

        An optical fiber communications system is similar in basic concept to any type of communication system. The function of which is to convey the signal from the information source over the transmission medium to the destination. The communication system therefore consists of a transmitter or modulator linked to the information source, the transmission medium, and a receiver or demodulator at the destination point. In electrical communications the information source provides an electrical signal usually derived from a message signal which is not electrical (e.g. sound), to a transmitter comprising electrical and electronic components which converts the signal into a suitable form for propagation over the transmission medium. This is often achieved by modulating a carrier, which, as mentioned previously, may be an electromagnetic wave. The transmission medium can consist of a pair of wires , a coaxial cable or a radio link through free space down which the signal is transmitted to the receiver, where it is transformed into the original electrical information signal (demodulated) before being passed to the destination. However, it must be noted that in any transmission medium the signal is attenuated. or suffer loss, and is subject to degradations due to contamination by random signals and noise as well as possible distortions imposed by mechanisms within the medium itself. Therefore, in any communications system there is a maximum permitted distance between thc transmitter and the receiver beyond which the system effectively ceases to live intelligible communication. For long haul applications these factors necessitate the installation or repeaters or line amplifiers at intervals. both to remove signal distortion and to increase signal level before transmission is continued down the link.

       For optical fiber communication system shown in