Preface
The first generation of fiber-optic communication systems debuting in 1980 operated at a meager bit rate of 45 Mb/s and required signal regeneration every 10 km or so. However, by 1990 further advances in lightwave technology not only increased the bit rate to 10 Gb/s (by a factor of 200) but also allowed signal regeneration after 80 km or more. The pace of innovation in all fields of lightwave technology only quickened during the 1990s, as evident from the development and commercialization of erbium-doped fiber amplifiers, fiber Bragg gratings, and wavelength-division-multiplexed lightwave systems. By 2001, the capacity of commercial terrestrial systems exceeded 1.6 Tb/s. At the same time, the capacity of transoceanic lightwave systems installed worldwide exploded. A single transpacific system could transmit information at a bit rate of more than 1 Tb/s over a distance of 10,000 km without any signal regeneration. Such a tremendous improvement was possible only because of multiple advances in all areas of lightwave technology. Although commercial development slowed down during the economic downturn that began in 2001, it was showing some signs of recovery by the end of 2004, and lightwave technology itself has continued to grow. The primary objective of this two-volume book is to provide a comprehensive and up-to-date account of all major aspects of lightwave technology. The first volume, subtitled Components and Devices, is devoted to a multitude of silica- and semiconductor-based optical devices. The second volume, subtitled Telecommunication Systems, deals with the design of modern lightwave systems; the acronym LT1 is used to refer to the material in the first volume. The first two introductory chapters cover topics such as modulation formats and multiplexing techniques employed to form an optical bit stream. Chapters 3 through 5 consider the degradation of such an optical signal through loss, dispersion, and nonlinear effects during its transmission through optical fibers and how they affect the system performance. Chapters 6 through 8 focus on the management of the degradation caused by noise, dispersion, and fiber nonlinearity. Chapters 9 and 10 cover the engineering issues related to the design of WDM systems and optical networks. This text is intended to serve both as a textbook and a reference monograph. For this reason, the emphasis is on physical understanding, but engineering aspects are also discussed throughout the text. Each chapter also includes selected problems that can be assigned to students. The book's primary readership is likely to be graduate students, research scientists, and professional engineers working in fields related to lightwave technology. An attempt is made to include as much recent material as possible so that students are exposed to the recent advances in this exciting field. The reference section at the end of each chapter is more extensive than what is common for a typical textbook. The listing of recent research papers should be helpful to researchers using this book as a reference. At the same time, students can benefit from this feature if they are assigned problems requiring reading of the original research papers. This book may be useful in an upper-level graduate course devoted to optical communications. It can also be used in a two-semester course on optoelectronics or lightwave technology. A large number of persons have contributed to this book either directly or indirectly. It is impossible to mention all of them by name. I thank my graduate students and the students who took my course on optical communication systems and helped improve my class notes through their questions and comments. I am grateful to my colleagues at the Institute of Optics for numerous discussions and for providing a cordial and productive atmosphere. I thank, in particular, Renè Essiambre and Qiang Lin for reading several chapters and providing constructive feedback. Last, but not least, I thank my wife Anne and my daughters, Sipra, Caroline, and Claire, for their patience and encouragement. Govind P. Agrawal Rochester, NY |
TABLE OF CONTENTS Chapter 9.1.1 - System Capacity and Spectral Efficiency
9.1.1 System Capacity and Spectral EfficiencyIt is evident from Figure 9.1 that the use of WDM can increase the system capacity because it transmits multiple bit streams over the same fiber simultaneously. When N channels at bit rates B1, B2, ..., and BN are transmitted simultaneously over a fiber of length L, the total bit rate of the WDM link becomes
For equal bit rates, the system capacity is enhanced by a factor of N. The most relevant design parameters for a WDM system are the number N of channels, the bit rate B at which each channel operates, and the frequency spacing Δvch between two neighboring channels. The product NB denotes the system capacity and the product NΔvch represents the total bandwidth occupied by a WDM system. Historically, the WDM technique has been pursued since commercial lightwave systems first became available in 1980 [l]-[5]. In its simplest form, WDM was used to transmit two channels in different transmission windows of an optical fiber. For example, an existing 1.3-µm lightwave system could be upgraded by adding another channel operating near 1.55 µm, resulting in a channel spacing of 250 nm. Considerable attention was directed during the 1980s toward reducing the channel spacing. An experiment in 1985 demonstrated the NBL product of 1.37 (Tb/s)-km by transmitting 10 channels at 2 Gb/s over 68.3 km of standard fiber with a channel spacing of 1.35 nm [3]. However, it was during the 1990s that WDM systems were developed most aggressively [6]-[10]. Commercial WDM systems first appeared around 1995. Initially, their capacity was relatively small (around 40 Gb/s or so) but it exceeded 1.6 Tb/s by 2000. Since then, several laboratory experiments have demonstrated capacities of more than 10 Tb/s, although their transmission distance was limited to below 200 km. Clearly, the advent of WDM has led to a virtual revolution in designing high-capacity lightwave systems. WDM systems are often classified as being coarse or dense, depending on their channel spacing. Although no precise definition exists, channel spacing exceeds 5 nm for coarse WDM but is typically <1 nm for dense WDM systems. It is common to introduce the concept of spectral efficiency for WDM systems as ηs = B/Δvch. Spectral efficiency is relatively low for coarse WDM systems [ηs < 0.1 (b/s)/Hz]. Such systems "waste the bandwidth" in a traditional sense, but are useful for metropolitan-area and local-area networks for which system cost must be kept relatively low. In contrast, long-haul links used for the backbone of an optical network attempt to makeηsas large as possible in order to utilize the bandwidth as efficiently as possible. For a given system bandwidth, the capacity of a WDM link depends on how closely channels can be packed in the wavelength domain. Clearly, channel spacing Δvch should exceed the bit rate B so that the channel spectrum can fit within the allocated bandwidth. The minimum channel spacing is limited by interchannel crosstalk, an issue covered later in this chapter. In practice, channel spacing Δvch often exceeds the bit rate B by a factor of 2 or more. This requirement wastes considerable bandwidth as spectral efficiency is then <0.5 (b/s)/Hz. Many new modulation formats are being explored to bring spectral efficiencies closer to 1 (b/s)/Hz. The channel frequencies (or wavelengths) of WDM systems have been standardized by the International Telecommunication Union (ITU) on a 100-GHz grid in the frequency range of 186 to 196 THz (covering the C and L bands in the wavelength range 1,530-1,612 nm). For this reason, channel spacing for most commercial WDM systems is 100 GHz (0.8 nm at 1,552 nm). This value leads to only 10% spectral efficiency at the bit rate of 10 Gb/s. More recently, ITU has specified WDM channels with a frequency spacing of 25 and 50 GHz. The use of 50-GHz channel spacing in combination with the bit rate of 40 Gb/s has the potential of increasing the spectral efficiency to 80%. |
