A fiber laser or fibre laser is a laser in which the active gain medium is an optical fiber doped with rare-earth elements such as erbium, ytterbium, neody- mium, dysprosium, praseodymium, and thulium. Fiber nonlinearities, such as Stimulated Raman Scattering or Four Wave Mixing can also provide gain and thus serve in effect as gain media. Unlike most other types of lasers, the laser cavity in fiber laser is constructed monolithically by fusion splicing the different types of fibers; most notably fiber Bragg gratings replace here conventional dielectric mirrors to provide optical feedback.
To pump fiber lasers, semiconductor laser diodes or other fiber lasers are used almost exclusively. Fiber lasers can have several kilometer-long active regions and provide very high optical gain. They can support kilowatt level of continuous output power because the fiber’s high surface area to volume ratio allows efficient cooling. The fiber waveguiding properties reduce or remove completely thermal distortion of the optical path thus resulting in typically dif- fraction-limited high-quality optical beam. Fiber lasers also feature compact layout compared to rod or gas lasers of comparable power, as the fiber can be bent to small diameters and coiled. Other advantages include high vibrational stability, extended lifetime and maintenance-free turnkey operation.Many high-power fiber lasers are based on doublecladfiber. The gain medium forms the core of thefiber, which is surrounded by two layers of cladding.The lasing mode propagates in the core, whilea multimode pump beam propagates in the innercladding layer. The outer cladding keeps this pumplight confined. This arrangement allows the core tobe pumped with a much higher power beam thancould otherwise be made to propagate in it, andallows the conversion of pump light with relativelylow brightness into a much higher-brightness signal.As a result, fiber lasers and amplifiers are occasionallyreferred to as «brightness converters.»
Applications include: Material processing,telecommunications,spectroscopy, and medicine.