3. Optical Amplifiers

The principles, design, and operation of erbium-doped and Raman amplifiers, two of the most important classes used in modern lightwave communication, are described. Developed over two decades, erbium-doped fiber devices act as lumped optical gain elements in terrestrial, submarine, and access networks, underpinning nearly all commercial data traffic today. Raman amplifiers have allowed significant reach and capacity increases and, unlike erbium-doped devices, are not confined to a specific lightwave band. In contrast to alternatives such as parametric and semiconductor amplification technology, erbium-doped and Raman amplifiers have been commoditized and can be readily designed and constructed from a mature set of components that includes specialty fibers, semiconductor pumps, dedicated filters, and passive elements. The design of both types of amplifiers is described, along with the most important engineering rules that allow for optimal device construction. Mitigation of noise and distortion mechanisms is detailed for both types of amplifiers when operating with commercial fiber plants.