Coherent transmission is now a major research topic within lightwave communication laboratories and could have future long-haul or local loop uses. This tutorial describes the principles of coherent transmission and outlines the differences between present-day direct detection systems. Critical technology Issues are discussed, in particular, the spectral properties of laser sources for coherent transmission, modulators, receiver design, and polarization control. Finally, the status of coherent transmission experiments around the world Is surveyed, and future trends are identified.

© 1987 Optical Society of America

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