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1、Unit 19 Optical Fiber Communication,Passage A The General System Passage B Advantages of Optical Fiber Communication Passage C Fiber-optics Communication,Passage A The General System Communication may be broadly defined as the transfer of information from one point to another. When the information i
2、s 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 modulate
3、d carrier is then transmitted to the required destination where it is received and the original information signal is obtained by demodulation.,Sophisticated techniques have been developed for this process by using electromagnetic carrier waves operating at radio frequencies as well as microwave and
4、 millimeter wave frequencies. However, “communication” may also be achieved by using an electromagnetic carrier which is selected from the optical range of frequencies.,An optical fiber communication system is similar in basic concept to any type of communication system. A block schematic of a gener
5、al communication system is shown in Figure 1.1(a), 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 m
6、edium, 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 convert
7、s 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 d
8、own 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 suffers loss, and is subject to de
9、gradations due to communication by random signals and noise, as well as possible distortions imposed by mechanisms within the medium itself.,Therefore, in any communication system there is a maximum permitted distance between the transmitter and the receiver beyond which the system effectively cease
10、s to give intelligible communication. For long-haul applications these factors necessitate the installation of 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 communications sys
11、tem shown in Figure1.1(a) may be considered in slightly greater detail, as given in Figure1.1(b). In this case the information source provides an electrical signal to a transmitter comprising an electrical stage which drives an optical source to give modulation of the lightwave carrier. The optical
12、source which provides the electrical-optical conversion may be either a semiconductor laser or light emitting diode (LED). The transmission medium consists of an optical fiber cable and the receiver consists of an optical detector which drives a further electrical stage and hence provides demodulati
13、on of the optical carrier. Photodiodes (p-n, p-i-n or avalanche) and, in some instances, phototransistors and photoconductors are utilized for the detection of the optical signal and the optical-electrical conversion. Thus there is a requirement for electrical interfacing at either end of the optica
14、l link and at present the signal processing is usually performed electrically.,Figure 19.1 The General Communication System and The Optical Fiber Communication System,The optical carrier may be modulated using either an analog or digital information signal. In the system shown in Figure1.1(b) analog
15、 modulation involves the variation of the light emitted from the optical source in a continuous manner. With digital modulation, however, discrete changes in the light intensity are obtained (i.e. on-off pulses). Although often simpler to implement, analog modulation with an optical fiber communicat
16、ion system is less efficient, requiring a far higher signal to noise ratio at the receiver than digital modulation. 1 Also, the linearity needed for analog modulation is not always provided by semiconductor optical sources, especially at high modulation frequencies. For these reasons, analog optical fiber communication links are generally limited to shorter distances and lower bandwidths than digital links.,Figure 1.2 shows a block schematic of a typical digital optical fiber link. Initially
