4. Optical amplifiers

4.1 Fiber-doped amplifiers

The first generation of commercially available erbium-doped fiber amplifiers (EDFAs) (pumped at 1,480 nm, single wavelength) has evolved, firstly towards lower-noise EDFAs (pumped at 980 nm), and secondly towards wavelength-division multiplexing (WDM) applications. In the latter case, EDFAs are generally combined with optical filters known as gain equalizers, which reduce in-band optical gain ripples, notable for low input optical power levels. Increasing the number of channels at the EDFA's input requires increasing the saturation optical power at the EDFA's output, for equivalent optical gain. This is why EDFAs with increasingly high output power are appearing on the market (they use either several gain stages, the first being pumped at 980 nm to obtain...