Laser pumping
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 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.
Smaller ellipses create fewer reflections, (a condition called "close-coupling"), giving higher intensity in the center of the rod.
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.
Longer flashlamps are also more efficient at transferring electrical energy into light, due to higher impedance.
[5] 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 are less efficient because not all of the light is correctly imaged into the rod, leading to increased thermal losses.
[5] Another configuration uses a rod and a flashlamp in a cavity made of a diffuse reflecting material, such as spectralon or powdered barium sulfate.
If the rod has an antireflection coating, or is immersed in a fluid that matches its refractive index, it can dramatically reduce these parasitic reflections.
[8] Pumping with a single lamp tends to focus most of the energy on one side, worsening the beam profile.
A frosted flow tube or diffuse reflector, while leading to lowered transfer efficiency, helps increase this effect, improving the gain.
They produce a broad spectrum of light, causing most of the energy to be wasted as heat in the gain medium.
Low energies give rise to sputter, which can remove material from the cathode and redeposit it on the glass, creating a darkened, mirrored appearance.
With very short pulse durations, care must be taken to ensure that the arc is centered in the lamp, far away from the glass, preventing serious wall ablation.
[12] Dye lasers sometimes use "axial pumping," which consists of a hollow, annular shaped flashlamp, with the outer envelope mirrored to reflect suitable light back to the center.
The dye cell is placed in the middle, providing a more even distribution of pumping light, and more efficient transfer of energy.
This provides better transfer efficiency, eliminating the need for a reflector, but diffraction losses cause a lower gain.
Higher current densities broaden the spectral lines to the point where they begin to blend together, and continuum emission is produced.
Xenon is used extensively because of its good efficiency,[11] although krypton is often used for pumping neodymium doped laser rods.
The output spectrum of an arc lamp is mostly dependent on the gas type, being narrow band spectral lines very similar to a flashlamp operated at low current densities.
The pump laser's narrow spectrum allows it to be closely matched to the absorption lines of the lasing media, giving it much more efficient energy transfer than the broadband emission of flashlamps.
This helps eliminate the standing wave generated by most Fabry–Pérot resonators, leading to a better use of the gain medium's energy.
Gas dynamic lasers are constructed using the supersonic flow of gases, such as carbon dioxide, to excite the molecules past threshold.
Since a good quantity of molecules remain in the upper state, a population inversion is created, which often extends for quite a distance downstream.
Continuous wave outputs as high as 100 kilowatts have been obtained from dynamic carbon dioxide lasers.
[26] Charge-displacement self-channeling can give rise to high energy concentration along a column created and maintained by the ponderomotive expulsion of electrons.
