Fiber optic amplifiers operate on the same principle as a laser except that there is no external optical cavity as there is for a laser. The active lasing medium is a host ion, which in the case of the OFAs is a RE ion like Er3+ added to the host glass or crystal structure. Specifically, erbium ions in concentrations of about 1000 ppm-weight % are added to the core of an SM fiber to make an EDFA. A source of pump energy is provided by a diode laser. An energy-level diagram for Er3+ in silica is shown in Fig. 10.1. The pump laser light from either a 980- or 1480- nm source laser excites the Er ions to an excited state where they make a rapid, nonradiative decay to a long-lived metastable state, 4I13/2. From there they undergo spontaneous or stimulated emission to the lower-energy 4I15/2 state. The incoming signal, with wavelengths spanning the range from 1500 to greater than 1600 nm, will be amplified by stimulated emission. An example of how an EDFA can be used as an in-line amplifier is shown simply in Fig. 10.2. In this case, the incoming signal light at, say, 1550 nm is pumped with either a forward or backward diode laser. Isolators located before and after the EDFA prevent the pump signal from being transmitted along with the amplified signal.