Rare-earth doped fibers are optical fibers that have been doped with rare-earth elements such as erbium, neodymium, or ytterbium. These elements are used as active dopants in fiber optics to enhance their ability to amplify light signals, making them crucial components in telecommunications, lasers, and sensing applications. Rare-earth doped fibers exhibit unique optical properties such as high gain coefficients, long lifetimes of excited states, and high efficiency in converting pump light to signal light. They can be used to amplify optical signals at specific wavelengths, allowing for long-distance transmission of data in telecommunication networks. Research in rare-earth doped fibers focuses on developing novel fiber compositions, optimizing doping concentrations, and exploring new fabrication techniques to improve their performance and efficiency. This research area is important for advancing the capabilities of optical communication systems and enabling the development of high-performance fiber optic devices.