In order to develop excellent photonic devices, we have to fully understand the physics behind operations of photonic devices. This book thoroughly teaches the fundamental physics currently applied to the development of photonics devices such as energy bands of semiconductors, optical transitions, optical waveguides, and semiconductor junctions. The book also reviews the characteristics of laser diodes, optical filters, and optical functional devices, which have been developed based on the above physics. These photonic devices have been demonstrated in system applications, and several experimental results are described.