The optical fiber with very high surface-to-volume ratio and a strong waveguide effect provides the fiber based laser source the potential to generate high power laser beam with high quality. In addition to the capacity of generating raw optical power with high beam quality, the fiber laser system has other appealing features, such as supporting robust and compact system designs, allowing ultrashort pulse operation, offering a board wavelength tunability, and providing high gains. Those features stimulate the research on the high power fiber lasers system, and lay the foundation of novel appealing applications, such as remote material processing, aerospace and defense.
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In the past decade, a remarkable increase of the powers produced by fiber lasers with high beam quality has been achieved (see Fig.1). As a result, the high power laser becomes strong counterpart of the solid-state bulk laser, and penetrates rapidly into areas that formerly other lasers were used.
Fig. 1. Progress in output power from diffraction-limited and near-diffraction-limited Yb-doped fibers .
In the early 1960s, the first fiber laser was demonstrated by Snitzer . The doped fiberâ€™s potential for high optical gain was revealed by David Payne and co-researchersâ€™ working on Neodymium- doped fibers in mid 1980s . In 2009, the high power fiber laser, which based on a specifically silica-host ytterbium-doped fiber-based laser (YDFL), obtained 10 kW output in the single-mode (SM) regime.
Although architectures are different, the high-power fiber lasers and amplifiers are mostly archived with rare-earth-doped (RE-doped) double-clad fibers. The double-clad fiber, which was initially demonstrated in 1988, provided the option of cladding pumping, and proved to be one of the key technologies for power scaling. The structure of this double cladding is that the active RE-doped core is surrounded by a much larger â€œinner claddingâ€? (see Fig. 2), and are encircled together by out cladding. The pump beam emitted by fiber-coupled high-power diode bars or other kinds of laser diodes is coupled into the inner cladding, and confined within it by an outer cladding. The confined pump beam will be absorbed into the core while it propagates along the fiber. The laser light is generated in the central core, and the laser light can have very good beam quality â€“ even diffraction limited beam. Thereby, by means of double cladding configurations one realized the conversion from low brightness pump to high brightness single-mode fiber laser output. As the spatial and angular pump acceptance [can be expressed as the product of area and the square of the numerical aperture (NA)] for the inner cladding is significantly improved to the core pump, Such conversion is more effective, and close to 5 orders have been demonstrated experimentally.
Among high power RE-doped fiber lasers,
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