Laser crystals are used with various geometric shapes. One of the common shapes is that of a rod. Most frequently, this means a crystal where the length is substantially longer than the diameter – often several centimeters or even a few tens of centimeters –, and the cross-section is cylindrical. Such long laser rods are required for lamp-pumped lasers; their length is similar to that of the used arc lamps or flash lamps. Much shorter rods (normally with a length of only a few millimeters) are used in diode-pumped lasers.
See the article on laser crystals for more details.
Features of Laser Rods
Rod dimensions can vary substantial between different applications, but should generally be met with good accuracy, since problems with accurate mounting may otherwise result.
Often, relevant parameters are only the length and diameter (for cylindrical rods with flat end surfaces perpendicular to the axis). In addition, one may have slightly tilted outer faces with a specified tilt angle, and ideally indications on the crystals concerning the orientation. Some rods have one or two slightly curved end faces, but flat ends are more common.
Most laser rods consist of a crystalline host medium (e.g. YAG = yttrium aluminum garnet) which is doped with some laser-active rare-earth ion such as Nd3+ (neodymium), Yb3+ (ytterbium) or Er3+ (erbium). However, there are also transition-metal-doped rods, e.g. with Ti3+ doping. Also, the host medium is sometimes not monocrystalline, but polycrystalline with a fine grain structure (→ ceramic gain media) or a glass.
For long rods, the choice of materials is somewhat more limited than for short ones. For example, it is hard to make large YVO4 (vanadate) crystals, which are therefore normally available only with quite small sizes. Also, long Ti:sapphire rods can be made but are not common, since one could hardly apply the required high pumping intensity with that format.
An important parameter is the doping concentration, i.e., the concentration of laser-active ions. Higher doping can improve the pump absorption efficiency and/or allow the use of a shorter or thinner rod, but may also favor parasitic processes such as energy transfers which degrade the power conversion efficiency.
Unfortunately, specifications for the doping concentration are not always precisely met. (They could be checked with absorption spectroscopy.) Another potential problem is that the doping may not be perfectly homogeneous (particularly for rods with high doping concentrations). That can lead to variations not only of the laser gain, but also of the refractive index, which leads to beam distortions. A high optical homogeneity is usually desirable, particularly when high laser beam quality is required.
Although in most cases nominally homogeneous doping is used, there are laser rods with an intentionally introduced doping gradient, and composite laser crystals obtained by bonding of different materials, e.g. Nd-doped and undoped YAG.
While usually one uses only one type of laser-active ions and no other optically relevant dopant, there are also co-doped rods, typically for exploiting some useful energy transfer processes. Some examples:
- Cr:Nd:YAG ceramic rods are used for solar-pumped lasers.
- Chromium–thulium–holmium-doped YAG (CTH:YAG) rods are used for 2.1-μm holmium lasers.
The laser-active dopants often absorb visible light in certain wavelength ranges, and that causes a color appearance, which is well visible at least for large (thick) rods. For example, neodymium-doped rods appear pink, and erbium-doped rods can have a similar color.
Surface Finish and Coatings
High-quality surface finish (with precise orientation, high flatness, good scratch/dig specifications) is generally required for the two end faces, through which the laser beam is usually sent. The optical quality of the outer surface (mantle) is usually much less critical, although it often serves for the entry of pump light (for side-pumped lasers). That surface is also used for cooling.
The end faces are usually equipped with anti-reflection coatings for the relevant laser wavelength(s) in order to minimize power losses and interference effects. In some cases, reflecting coatings are applied to laser rods.
The quality of the interface polish and the coatings can have a substantial impact on the threshold intensity (or fluence) for laser-induced damage. That damage threshold is relevant for pulsed lasers, particularly those with high peak power.
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