Laser pumping

Laser pumping is the act of energy transfer from an external source into the laser gain medium. The energy is absorbed in the medium, producing excited states in its atoms. When the number of particles in one excited state exceeds the number of particles in the ground state or a less-excited state, population inversion is achieved. In this condition, the mechanism of stimulated emission can take place and the medium can act as a laser or an optical amplifier. The pump power must be higher than the lasing threshold of the laser.

The pump energy is usually provided in the form of light or electric current, but more exotic sources have been used, such as chemical or nuclear reactions.

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Optical pumping

Main article: Optical pumping

Flash lamps are the oldest energy source for lasers. They are used for lower energies in both solid-state and dye lasers. They produce a broad spectrum of light, causing most of the energy to be wasted as heat in the gain medium. Flash lamps also tend to have a short lifetime.

In the most common configuration, the gain medium is in the form of a rod located at one focus of a mirrored cavity of elliptical cross-section perpendicular to the rod's axis. The lamp is a cylinder located at the other focus of the ellipse. Often the mirror's coating is chosen to transmit shorter wavelengths to minimize thermal lensing. In other cases an absorber for these wavelengths is used. The larger the ellipse the smaller the aberrations, giving higher intensity in the center of the rod. The closer the ellipse is to a circle, the more symmetric the pumping is, which improves beam quality. Typically, the lamp is surrounded by a cylindrical jacket with a dielectric coating that reflects unsuitable wavelengths of light back into the lamp. This light is absorbed and some of it is re-emitted at suitable wavelengths by means of fluorescence. The jacket also serves to protect the rod in the event of a violent lamp failure, and may provide a flow path for coolant. The rod and the lamp are relatively long to minimize the effect of losses at the end faces and to provide a sufficient length of gain medium. Flat mirrors are also often used at the ends of the pump cavity to reduce loss. Cylindrical laser rods support whispering gallery modes due to total internal reflection between the rod and the cooling water, which is not true for other rod cross-sections. Inexpensive rods have unpolished outer diameters, while expensive rods can have a cylindrical lens on one side to focus the pump light into the rod. An unpolished rod lowers the intensity at the center of the rod worsening the beam profile. A lamp jacket or rod without an antireflection coating also leads to losses.

Variations on this design use more complex mirrors composed of overlapping elliptical shapes, to allow multiple flashlamps to pump a single rod. This allows greater power, but is less efficient because not all of the light is correctly imaged into the rod, leading to increased thermal losses. This approach may allow more symmetric pumping, increasing beam quality, however.

Another configuration uses a rod and a flashlamp in a cavity made of a diffuse reflecting material such as spectralon. This doesn't couple the light as well into the lasing medium, since the light makes many reflections before reaching the rod. The increased number of reflections is compensated for by the diffuse medium's higher reflectivity: 99% compared to 97% for a gold mirror.[1] This approach is more compatible with unpolished rods or multiple lamps.

Laser host materials are chosen to have a low absorption, only the doping absorbs. Therefore any light at frequencies not absorbed by the doping will go back into the lamp reheating the plasma. Quartz tubes can operate at 900°C and the cathode needs to be hot, so cooling with water seems to be cool down the lamp to far^{[citation needed]}. Used lamps have deposed their cathode material on the glass and are therefore inefficient. Arc lamps can be made any size and power. If the lamp is thick enough the light in the lamp is thermal equilibrium with the gas and optimal brightness is achieved. For pulsed lasers the lamp voltage may be turned off for up to 10 ms after each laser's output pulse before to much ions recombine. Generally cathodes do suffer due to sputtering due to high voltage (peaks) and there is a distinction between arc lamp and flash lamp.

A laser of a suitable type can be used to pump another laser. The pump laser's narrow spectrum gives it much more efficient energy transfer than flash lamps. Diode lasers pump diode pumped solid state lasers.

Microwaves or radiofrequency EM radiation can be used to excite gas lasers.

The sun has been used to pump lasers. See: Applied Physics Letters, Volume 49, Issue 7, pp. 369–370 (August 18, 1986); and Appl. Phys. Lett. 90, 261120 (June 25, 2007).

Electrical pumping

Electric glow discharge is common in gas lasers. For example, in the helium-neon laser the electrons from the discharge collide with the helium atoms, exciting them. The excited helium atoms then collide with neon atoms, transferring energy. This allows an inverse population of neon atoms to build up.

Electric current is typically used to pump semiconductor lasers.

Electron beams pump free electron lasers and some excimer lasers.

Other types

Chemical reaction is used as a power source in chemical lasers. This allows for very high output powers difficult to reach by other means.

Nuclear fission is used in exotic nuclear pumped lasers (NPL), directly employing the energy of the fast neutrons released in a nuclear reactor. [2] [3]

The United States military tested an X-ray laser pumped by a nuclear weapon in the 1980s, but the results of the test were inconclusive and it has not been repeated.^{[citation needed]}

See also

• Laser construction