This book provides a comprehensive account of fiber-opticcommunication systems. The 3rd edition of this book isused worldwide as a textbook in many universities. This 4thedition incorporates recent advances that have occurred, inparticular two new chapters. One deals with the advanced modulationformats (such as DPSK, QPSK, and QAM) that are increasingly beingused for improving spectral efficiency of WDM lightwave systems.The second chapter focuses on new techniques such asall-optical regeneration that are under development and likely tobe used in future communication systems. All otherchapters are updated, as well.