Wavelength division multiplexing (WDM) technology relies on the fact that optical fibres can carry many wavelengths of light without interaction between each wavelength. Thus, a single fibre can carry many separate wavelength signals or channels simultaneously. This title covers all key active optical components required for current and next generation communication systems.;Its aim is to help the reader to choose the right device for a given system application, as well as aiding engineers working in optical communication area, (from system to component) to understand the principle and mechanics of each key component they deal with for optical system design. It covers different laser diodes as transmitter and pumping sources, different modulators and different photodetectors, and highlights the operating principles of each component, fabrication technologies and packaging/module technologies.