Optical amplifier
An optical amplifier is a device that amplifies an optical signal directly, without the need to first convert it to an electrical signal. An optical amplifier may be thought of as a laser without an optical cavity, or one in which feedback from the cavity is suppressed. Optical amplifiers are important in optical communication and laser physics. They are used as optical repeaters in the long distance fiberoptic cables which carry much of the world's telecommunication links. There are several different physical mechanisms that can be used to amplify a light signal, which correspond to the major types of optical amplifiers. In doped fiber amplifiers and bulk lasers, stimulated emission in the amplifier's gain medium causes amplification of incoming light. In semiconductor optical amplifiers (SOAs), electron-hole recombination occurs. In Raman amplifiers, Raman scattering of incoming light with phonons in the lattice of the gain medium produces photons coherent with the incoming photons. Parametric amplifiers use parametric amplification. The principle of optical amplification was invented by Gordon Gould on November 13, 1957. He filed patent No. 804,539 on April 6, 1959 titled "Light Amplifiers Employing Collisions to Produce Population Inversions" (subsequently amended as a continuation in part and finally issued as No. 4,746,201A on May 4, 1988). The patent covered “the amplification of light by the stimulated emission of photons from ions, atoms or molecules in gaseous, liquid or solid state.” In total, Gould obtained 48 patents related to the optical amplifier that covered 80% of the lasers on the market at the time of issuance. Gould co-founded an optical telecommunications equipment firm, Optelecom Inc., that helped start Ciena Corp with his former head of Light Optics Research, David Huber and Kevin Kimberlin. Huber and Steve Alexander of Ciena invented the dual-stage optical amplifier (US Patent 5,159,601) that was a key to the first dense wave division multiplexing (DWDM) system, that they released in June 1996.
