A semiconductor Optical Amplifier is essentially like a LaserDiode where the end mirrors have been replaced with anti-reflection coatings. The signalLight is usually sent through a semiconductor single-mode Waveguide with transverse dimensions of a few micrometers and a length of the order of 0.5â€“2â€‰mm. The waveguide Mode has significant overlap with the active region, which is pumped with an electric current. The injection current creates a certain carrier density in the conduction band allowing for optical transitions from the conduction band to the valence band. The gain maximum occurs for photon energies slightly above the bandgap. Semiconductor optical amplifiers are often used in telecom systems in the form of fiber-pigtailed components, operating at signal wavelengths near 1.3 or 1.5â€‰Î¼m, and offering a gain of up to âˆ¼â€‰30â€‰dB.