In an optical fiber amplifier, the one employing Er fiber is called Erbium Doped Fiber Amplifier (EDFA), while the other using Yb fiber is named a Ytterbium Doped Fiber Amplifier (YDFA). The optical fiber amplifiers are generally composed of an pumping LD light source, a wave-mixer such as WDM coupler, optical isolators, a rare earth additive optical fiber, as shown in Fig. below. By removing the optical isolator and then incorporating loop devices, such as an FBG, at the edges of entrance and exit, the optical fiber amplifier becomes a Fabry-Perot fiber laser oscillator. The system of optical fiber amplifier is either of following three: forward-pumped type where the propagating direction of signal and pumping light are the same, backward pumping type where the propagating direction is opposite, and bidirectional pumping type where the pumping light is supplied from both a regular and opposite direction, as shown in Fig. (a),(b), and (c), respectively.

The optical isolator introduced between input edge and gain fiber (see Fig.) plays a role of gain amplification by suppressing a parasitic oscillation (an unintentional or uncontrollable laser oscillation accompanying with a valid oscillation) and feedback of amplified spontaneous emission (ASE). The optical isolator is often also introduced between the optical fiber and output edge for the sake of higher gain.



Fig. Basic configuration of optical fiber amplifier.
(a) Forward-pumped system, (b) backward-pumped system, (c) forward/backward pumped system

Commercially available EDFA amplifies the signal in the wavelength region of 1530~1610 nm. In general, the forward-pumped system superior in noise property uses the pumping light of the wavelength of 980nm, while the backward pumping system superior in output property uses pumping light of the wavelength of 1480 nm. YDFA amplifies the signal in the wavelength region of 1030-1090 nm. The wavelength of pumping light is 915 and 975 nm. In the communication range, a Thulium doped fiber amplifier (TDFA) for 1.45-1.5 and 1.67 µm range amplification and Praseodymium doped fiber amplifier (PDFA) for 1.3 µm range amplification are commercially available besides the  EDFA for C band (1530-1565 nm) and L band (1565-1625 nm).