End pumping (also sometimes called longitudinal pumping) is a technique of optically pumping a laser gain medium, where the pump light is injected along the laser beam.
If the pump beam stays well within the volume of the fundamental transverse mode of the laser resonator (at least within the laser crystal), transverse single-mode operation with diffraction-limited beam quality is often possible, because higher-order modes then have too low gain to reach the laser threshold. (A laser beam radius somewhat below the radius of the pump intensity distribution is often required, since otherwise the laser mode is affected by the aberrations of the thermal lens, particularly for high pump power levels and intensity distributions which deviate strongly from a flat-top shape.) In addition to the high beam quality, end pumping also makes it possible to achieve a high power efficiency (usually higher than achieved with side pumping). For these reasons, most diode-pumped solid-state lasers, particularly those with lower output powers, are end-pumped.
Disadvantages of end-pumped laser designs are that pump light can be injected only from at most two directions, that the optical intensity and crystal temperature vary along the beam direction, and that this approach leads to constraints on the beam quality of the pump source. Therefore, end pumping often cannot be used for high-power lasers, and in particular not for lamp-pumped lasers. There are techniques, however, to extend the end pumping concept to fairly high powers; for example, multi-segmented rods (→ composite laser crystals) can be used for better distributing the absorbed power.
See also: optical pumping, lasers, solid-state lasers, side pumping, gain media, beam quality, Spotlight article 2007-04-01
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