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 9.5.4 - Use of DPSK Format
9.5.4 Use of DPSK FormatSimilar to the case of phase-alternation technique discussed in Section 8.5.2 for suppressing the intrachannel nonlinear effects, one may ask whether pulse phases can be adjusted to reduce the interchannel nonlinear effects. The answer turns out to be yes. The modulation format that has proved quite successful in this respect is the RZ-DPSK format (see Section 2.3.4) in which an optical pulse is present in all bit slots and the information is encoded in the phase of these pulses through DPSK [128]-[132]. Figure 9.25 shows schematically how the field amplitude and optical power vary with time for a DPSK-coded channel in which the phase of pulses is shifted by π whenever a bit transition occurs. It is easy to understand qualitatively why XPM-induced penalties are reduced for systems designed with the RZ-DPSK format. The main reason why interchannel XPM leads to the amplitude and timing jitter is related to the randomness of bit patterns in two neighboring channels. It is easy to see that the XPM will be totally harmless if channel powers were constant in time because all XPM-induced phase shifts will be time-independent, producing no frequency and temporal shifts. However, this is not the case for the RZ-DPSK systems because, even though information is coded through phase shifts, an optical pulse is present in all bit slots. As seen in Figure 9.25, channel powers vary in a periodic fashion under such conditions. Nevertheless, the XPM effects are considerably reduced because all bits experience the same bit patterns in the neighboring channels and undergo nearly identical collision histories, especially if the average channel power does not vary too much along the link. Since all bits are shifted in time by nearly the same amount, little timing jitter is induced by interchannel collisions.
Figure 9.25: Temporal variations of the electric field and optical power for a 10-Gb/s channel coded using the RZ-DPSK format. (After Ref. [129]; ©2002 IEEE.)
where P2(t) = P0U(0,t)2 governs power variations in channel 2 and δ = β2Ωch depends on the channel spacing. If lumped amplifiers are used to compensate for fiber losses, p(z) = exp(-αz) in each fiber section between two amplifiers. By taking the Fourier transform of Eq. (9.5.1), we obtain [129]
where we used L = NALAand assumed that the link is made of NAamplifiers spaced apart by LAsuch that αLA>> 1. Equation (9.5.2) shows that the fiber link acts as a low-pass filter of bandwidth fb = α / (2πβ2Ωch) as far as the transfer of power variations into phase variations is concerned [129]. Using typical parameter values for a WDM system designed with standard fibers, α = 0.2 dB/km, D = 17 ps/(km-nm), and 100-GHz channel spacing, this frequency is found to be around 540 MHz, a value much smaller than typical channel bit rates. Since the spectrum Numerical simulations performed for an eight-channel WDM system confirm this simple analysis [129]. Figure 9.26 compares the calculated eye-opening penalty for the fourth channel in the case of standard RZ-ASK and RZ-DPSK formats, when channels operate at 10 Gb/s and are spaced 100 GHz apart. Each span between two amplifiers consists of 100 km of standard fiber, followed with a DCF of suitable length. Each channel carries chirp-free RZ pulses with 8-mW peak power. Residual dispersion of each channel is compensated at the receiver end to optimize the eye opening, measured in a 20-ps time window. As seen in Figure 9.26, penalty increases rapidly for the standard RZ format but does not exceed even 1 dB at a distance of 3000 km when the DPSK format is employed.
Figure 9.26: Eye-opening penalty (EOP) as a function of link length for one of eight channels in the case of RZ-ASK and RZ-DPSK formats. Eye diagrams at a distance of 3,000 km are also shown for the two formats. (After Ref. [129]; ©2002 IEEE.)
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2(ω) of a periodic pulse train consists of discrete frequency components separated by the bit rate B, only the zero-frequency (dc) component is passed by the low-pass filter. As a result, the XPM-induced phase shift is nearly constant in time even for RZ pulses, and no frequency and temporal shifts occur during interchannel collisions.
