This book covers a wide range of technical issues relating to lightwave technologies using high coherence lightwaves. Electromagnetic wave communication started when the first wireless system was invented by Marconi in 1895. However, we had to wait about one hundred years to realize a similar technology in the lightwave frequency region. The invention of lasers in 1960 and two technology innovations in 1970 -low loss silica fiber and semiconductor lasers operating at room temperature - promoted the development of fiber-optic transmission systems. The deployment of high-speed long-haul fiber-optic transmission systems has led to the formation of domestic and international trunk networks. The installed fiber cables in local loop plants provide multimedia communication services including broadband video. However, present lightwave communication systems do not fully utilize the fruitful potential oflightwaves, namely the capacity of extremely high frequency electromagnetic information carrier waves. The frequency oflightwaves used for fiber-optic transmission is about 200 THz 14 (2 x 10 Hz), and the frequency bandwidth of the fiber low loss region is about 13 20 THz (2 x 10 Hz). Recent developments of narrow spectrum width semiconduc­ tor laser and planar optical waveguide devices offer us the possibilities for a new generation of lightwave-based communication systems. This book focuses on system aspects of the new generation lightwave communication technologies such as optical frequency division multiplexing and coherent detection. Chapter 1 overviews lightwave communication system technology.