Introduced in December 1996, the 1550-nm semiconductor optical amplifier A 1901 SOA, from Alcatel Optronics (Nozay, France), was specially designed to provide telecommunication manufacturers with a key component for the growing market of all-optical switching and routing systems. Its performance specifications, such as polarization insensitivity, low gain ripple, and fast switching time, are such that it can be used as either a high-speed optical gate or as a wavelength converter. The A 1901 SOA is based on molecular-beam epitaxial growth of a gallium indium arsenide phosphide active region; its low tensile bulk separate confinement heterostructure leads to a high internal gain (>30 dB) with an extremely low polarization dependence (typically 0.5 dB). The Nd:YAG laser-welded package is highly reliable, and advanced fiber-pigtailing ensures efficient fiber-to-fiber gain (>25 dB). Low internal capacitance gives the fiber module fast (<1 ns) switching times. It has potential in future large-scale optoelectronic integration for realizing large switching matrices and offers an interesting component for wavelength conversion (u¥to 40 Gbit/s) by exploiting optical nonlinearities in the SOA active layer.