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Until recently, virtually all communication systems have relied on the transmission of information over electrical cables or have made use of radio-frequency and microwave | Fundamentals of Photonics Bahaa E. A. Saleh Malvin Carl Teich Copyright 1991 John Wiley Sons Inc. ISBNs 0-471-83965-5 Hardback 0-471-2-1374-8 Electronic CHAPTER 22 FIBER-OPTIC COMMUNICATIONS 22.1 COMPONENTS OF THE OPTICAL FIBER LINK A. Optical Fibers B. Sources for Optical Transmitters C. Detectors for Optical Receivers D. Fiber-Optic Systems 22.2 MODULATION MULTIPLEXING AND COUPLING A. Modulation B. Multiplexing C. Couplers 22.3 SYSTEM PERFORMANCE A. Digital Communication System B. Analog Communication System 22.4 RECEIVER SENSITIVITY 22.5 COHERENT OPTICAL COMMUNICATIONS A. Heterodyne Detection B. Performance of the Analog Heterodyne Receiver C. Performance of the Digital Heterodyne Receiver D. Coherent Systems AT T undersea fiber-optic communication network of the 1990s 874 Until recently virtually all communication systems have relied on the transmission of information over electrical cables or have made use of radio-frequency and microwave electromagnetic radiation propagating in free space. It would appear that the use of light would have been a more natural choice for communications since unlike electricity and radio waves it did not have to be discovered. The reasons for the delay in the development of this technology are twofold the difficulty of producing a light source that could be rapidly switched on and off and therefore could encode information at a high rate and the fact that light is easily obstructed by opaque objects such as clouds fog smoke and haze. Unlike radio-frequency and microwave radiation light is rarely suitable for free-space communication. Lightwave communications has recently come into its own however and indeed it is now the preferred technology in many applications. It is used for the transmission of voice data telemetry and video in long-distance and local-area networks and is suitable for a great diversity of other applications e.g. cable television . Lightwave technology affords the user enormous transmission capacity distant .