LASING  MEDIUM (YAG)

      The active lasing ion is Nd³⁺‏ embedded in a host crystal in a manner almost identical to the way in which chromium ions are embedded in an aluminum oxide host in a ruby laser. The most common host crystal is YAG (yttrium aluminum garnet), but other host materials, such as vanadate (YVO₄) or glass, may also be used. The wavelength of the resulting laser beam depends on the host material itself, which modifies the energy levels of the neodymium ion embedded in it. Common host materials and resulting lasing wavelengths are listed in Table 1.1. Although not an extensive list (many other materials exist or are under development), it should give the reader an idea of the types of hosts that can be used and the variations in wavelength (which are minimal and all emit in the near-IR). Of all the materials listed, YAG is the most common material, especially for medium- to high-power units, with vanadate being the favored material for low-power (< 1 W), compact solid-state lasers.

TABLE 1.1. Common Nd³⁺ Hosts and Wavelengths

Nd: YAG (and related materials, such as Nd:YVO₄ and Nd: glass) is a four-level system featuring distinct upper and lower lasing levels. Multiple pump levels allow the material to absorb pump light at a variety of wavelengths in the red and near infrared region of the spectrum. Materials other than neodymium will also lase in an almost identical configuration, including other rare-earth metals, such as holmium and erbium. Ho: YAG lases at 2060 nm and Er: YAG at 2840 nm. None of these lasers is particularly common, although Er: glass is used extensively in fiberoptic communications systems as an amplifier for weak signals at 1549 nm.