End Pumps

End-pumping of lasers and optical amplifiers (also known as longitudinal pumping) is a technique of optically pumping a laser gain medium in which the pumped light is incident in the direction of the laser beam rather than laterally (side-pumping). The geometry of the end pump often allows for a good spatial overlap between the pump source and the laser radiation, which offers a number of advantages。

Advantages of end pumps:

• It is possible to obtain high laser gain in this way, which is also particularly important for optical amplifiers.
• With fewer pumping areas that cannot participate in the laser process, one can also achieve higher power efficiency (typically higher than side pumping).
• Transverse single-mode operation with diffraction-limited beam mass is often achievable if the pump beam is well kept within the volume of the basic transverse mode of the laser resonator, and the gain for the higher-order mode is too low to reach the laser threshold. It is often required that the laser beam radius be slightly lower than the radius of the pump intensity distribution, because otherwise the laser pattern will be affected by the thermal lens optical aberration.

For these reasons, most diode-pumped solid-state lasers, especially those with lower output power, are end-pumped.

Potential drawbacks of end pumps

• Pump light can only be injected from a maximum of two directions, which may limit the total pump power applied.
• In particular, when pumping long laser crystals with a small beam radius, a pump source with high beam quality is required, or a high radiation (brightness) is obtained in combination with a certain level of optical power, so lamp-pumped lasers usually cannot be end-pumped.
• The light intensity and crystal temperature vary in the direction of the beam, especially for unidirectional pumps, but much less for bidirectional pumps.

Due to these limitations, end-pumping generally cannot be used for high-power lasers, especially lamp-pumped lasers. However, there are techniques that can extend the concept of end-pumping to fairly high power; Composite laser crystals, for example, can be used to better distribute absorbed power.