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A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The term laser originated as an acronym for light amplification by stimulated emission of radiation.
In most cases, the gain medium is a fiber doped with rare earth ions such as erbium (Er 3+), neodymium (Nd 3+), ytterbium (Yb 3+), thulium (Tm 3+), or praseodymium (Pr 3+), and one or several fiber-coupled laser diodes are used for pumping. Therefore, most fiber lasers are diode-pumped lasers.here are some lasers which have a semiconductor optical amplifier (SOA) as the gain medium in a resonator made of fibers. Even though the actual laser process does not occur in a fiber, such fibers are sometimes called fiber lasers. They typically emit relatively small optical powers of a few milliwatts or even less.