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, neodymium, dysprosium, praseodymium, and thulium. They are related to doped fiber amplifiers, which provide light amplification without lasing. Fiber nonlinearities, such as stimulated Raman scattering or four-wave mixing can also provide gain and thus serve as gain media for a fiber laser.
Applications of fiber lasers include material processing, telecommunications, spectroscopy, and medicine. The advantage of the fiber laser is that the light is already coupled into the fiber and can be easily delivered to a movable focusing element. Such a coupling is important for laser cutting or laser welding or laser folding of metals and polymers.
Design and manufacturing of fiber lasers
Unlike most other types of lasers, the laser cavity in fiber lasers 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 active regions several kilometers long, and can provide very high optical gain. They can support kilowatt levels of continuous output power because the fiber's high surface area to volume ratio which allows efficient cooling. The fiber waveguiding properties reduce or remove completely thermal distortion of the optical path thus resulting in typically diffraction-limited high-quality optical beam. Fiber lasers are also compact 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 double-clad fiber. The gain medium forms the core of the fiber, which is surrounded by two layers of cladding. The lasing mode propagates in the core, while a multimode pump beam propagates in the inner cladding layer. The outer cladding keeps this pump light confined. This arrangement allows the core to be pumped with a much higher power beam than could otherwise be made to propagate in it, and allows the conversion of pump light with relatively low brightness into a much higher-brightness signal. As a result, fiber lasers and amplifiers are occasionally referred to as "brightness converters."
There is an important question about the shape of the double-clad fiber; a fiber with circular symmetry seems to be the worst possible design . The design should allow the core to be small enough to support only a few (or even one) modes. It should provide sufficient cladding to confine the core and optical pump section over a relatively short piece of the fiber.
Recent developments in fiber laser technology have led to a rapid and large rise in achieved diffraction-limited beam powers from diode-pumped solid-state lasers. Due to the introduction of large mode area (LMA) fibers as well as continuing advances in high power and high brightness diodes, continuous-wave single-transverse-mode powers from Yb-doped fiber lasers have increased from 100 W in 2001 to >1 kW.
Previously unattainable powers can now be achieved with commercially available off-the-shelf fibers and components. As a result, fiber laser technology is expected to have a profound effect on a broad variety of industrial applications. This white paper describes the technology in greater detail: "KW-power fiber lasers with single transverse mode output".
Fiber disk lasers
Another type of fiber laser is the fiber disk laser. In such, the pump is not confined within the cladding of the fiber (as in the double-clad fiber), but pump light is delivered across the core multiple times because the core is coiled on itself like a rope. This configuration is suitable for power scaling in which many pump sources are used around the periphery of the coil. 
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