This book contains a comprehensive examination of the macroscopic optical and spectral properties of rare earth doped silica glass. This vital resource establishes the relationship among compositions, structures, and properties of rare earth ion doped silica glasses and fibers, providing theoretical guidance for the design and application of active fiber components for laser and amplifier. The book systematically introduces the waveguide structure and design principles of rare earth doped silica fibers, the preparation and characterization methods of these fibers, and the main issues and solutions in their research and development. It also provides an in-depth look at the latest industry development trends and applications of rare earth doped silica fibers. Written by Prof. Dr. Lili Hu and her team, who have been engaged in fundamental research and key technology research of laser glass and laser fibers for more than twenty years, this book is a result of their innovative work in the field of rare earth doped silica glass and fiber research. With its high academic value and enormous social benefits, this book provides valuable references for researchers, institutions, and enterprises involved in specialty fibers research and development. The basis of English translation of this book, originally in Chinese, was facilitated by artificial intelligence. The content was later revised by the author for accuracy.