End pumping (also sometimes called longitudinal pumping) of lasers and optical amplifiers is a technique of optically pumping a laser gain medium, where the pump light is injected along the laser beam, rather than in a transverse direction (side pumping).
The geometry of end pumping often allows one to achieve a good spatial overlap between pump and laser radiation. That in turn can have multiple benefits:
- It is possible that way to obtain a high laser gain. That is also particularly important for optical amplifiers.
- One can also achieve a high power efficiency (usually higher than with side pumping), since one avoids pumping regions which cannot participate in the laser process.
- 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 optical aberrations of the thermal lens, particularly for high pump power levels and intensity distributions which deviate strongly from a flat-top shape.)
There are also various potential disadvantages of end pumping:
- Pump light can be injected only from at most two directions, and that may limits the applicable total pump power.
- Particularly for pumping a long laser crystal with a small beam radius, one requires a pump source with high beam quality, or in combination with a certain level of optical power a high radiance (brightness). Therefore, lamp-pumped lasers, for example, can usually not be end-pumped.
- The optical intensity and the crystal temperature vary along the beam direction – particularly for unidirectional pumping, but substantially less for bidirectional pumping.
Due to these limitations, 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.
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