Semiconductor optical amplifiers (SOA's) are essentially laser diodes, without end mirrors, which have fiber attached to both ends. They amplify any optical signal that comes from either fiber and transmit an amplified version of the signal out the second fiber. An SOA can be constructed in a small package, and they work for 1310 nm or 1550 nm systems. In addition, SOA's transmit bidirectionally, making the reduced size of the device an advantage over regenerators or EDFA's. Drawbacks to SOA's include high-coupling losses, polarization dependence, and a higher noise figure. Figure 7.1 illustrates the basics of a semiconductor optical amplifier.