Free-space laser communications, also referred to as optical communica­ tions, is a popular subject in today's technological marketplace. A number of conferences on this subject have been organized by professional societies such as SPIE (the International Society of Photo Optical and Instrumenta­ tion Engineering), OSA (Optical Society of America), and IEEE (Instituteof Electrical and Electronics Engineers). The evolving technology of free-space laser communications is emerging as an appealing alternative to RF com­ munications for links between satellites, as well as a promising addition to terrestrial applications such as video or computer linkups between buildings. There is a pressing need for more information on laser communications that is comprehensive enough to provide in-depth knowledge of free-space com­ munications, and that can satisfy the current demands of the research and commercial needs. This book has been designed to provide a comprehensive, unified tutorial to further understanding of the fundamental techniques for laser communi­ cations through the earth's atmosphere. The driving force behind free-space laser communications is the continuous demand for higher bandwidth to deliver high-capacity voice, data, and images to the customer. Free-space propagation distances include ranges that encompass a few millimeters (for example between optical interconnects in a computer using photonics to replace metal interconnects), a fewmeters (such as indoor communications), a fewkilometers (between buildings, campuses, and hospitals), and even up to thousands of kilometers (such as from an aircraft or satellite to the ground).