YAG Lasers

Definition: lasers based on YAG (yttrium aluminum garnet) crystals, usually Nd:YAG

The term YAG laser is usually used for solid-state lasers based on neodymium-doped YAG (Nd:YAG, more precisely Nd³⁺:YAG), although there are other rare-earth-doped YAG crystals, e.g. with ytterbium, erbium, thulium or holmium doping (see below). YAG is the acronym for yttrium aluminum garnet (Y₃Al₅O₁₂), a synthetic crystal material which became popular in the form of laser crystals in the 1960s. Yttrium ions in YAG can be replaced with laser-active rare earth ions without strongly affecting the lattice structure, because these ions have a similar size.

YAG is a host medium with favorable properties, particularly for high-power lasers and Q-switched lasers emitting at 1064 nm.

YAG lasers are in many cases bulk lasers made from discrete optical elements. However, there are also monolithic YAG lasers, e.g. microchip lasers and nonplanar ring oscillators.

The most popular alternatives to Nd:YAG among the neodymium-doped gain media are Nd:YVO₄ and Nd:YLF. Nd:YAG lasers nowadays also have to compete with Yb:YAG lasers (see below).

Properties of Nd:YAG

Nd:YAG is a four-level gain medium (except for the 946-nm transition as discussed below), offering substantial laser gain even for moderate excitation levels and pump intensities. The gain bandwidth is relatively small, but this allows for a high gain efficiency and thus low threshold pump power.

Nd:YAG lasers can be diode pumped or lamp pumped. Lamp pumping is possible due to the broadband pump absorption mainly in the 800-nm region and the four-level characteristics.

Figure 1: Energy level structure and common pump and laser transitions of the trivalent neodymium ion in Nd³⁺:YAG.

The most common Nd:YAG emission wavelength is 1064 nm. Starting with that wavelength, outputs at 532, 355 and 266 nm can be generated by frequency doubling, frequency tripling and frequency quadrupling, respectively. Other emission lines are at 946, 1123, 1319, 1338 and 1444 nm. When used at the 946-nm transition, Nd:YAG is a quasi-three-level gain medium, requiring significantly higher pump intensities.

Nd:YAG is usually used in monocrystalline form, fabricated with the Czochralski growth method, but there is also ceramic (polycrystalline) Nd:YAG available in high quality and in large sizes. For both monocrystalline and ceramic Nd:YAG, absorption and scattering losses within the length of a laser crystal are normally negligible, even for relatively long crystals.

Typical neodymium doping concentrations are of the order of 1 at. %. High doping concentrations can be advantageous e.g. because they reduce the pump absorption length, but too high concentrations lead to quenching of the upper-state lifetime e.g. via upconversionhigh-power lasers. Note that the neodymium doping density does not necessarily have to be the same in all parts; there are composite laser crystals with doped and undoped parts, or with parts having different doping densities. processes. Also, the density of dissipated power can become too high in

Table 1: Some properties of Nd:YAG = neodymium-doped yttrium aluminum garnet.

Property	Value
chemical formula	Nd3+:Y3Al5O12
crystal structure	cubic
mass density	4.56 g/cm3
Moh hardness	8–8.5
Young's modulus	280 GPa
tensile strength	200 MPa
melting point	1970 °C
thermal conductivity	10–14 W / (m K)
thermal expansion coefficient	7–8 × 10−6/K
thermal shock resistance parameter	790 W/m
birefringence	none (only thermally induced)
refractive index at 1064 nm	1.82
temperature dependence of refractive index	7–10 × 10−6/K
Nd density for 1 at. % doping	1.36 × 1020 cm−3
fluorescence lifetime	230 μs
absorption cross section at 808 nm	7.7 × 10−20 cm2
emission cross section at 946 nm	5 × 10−20 cm2
emission cross section at 1064 nm	28 × 10−20 cm2
emission cross section at 1319 nm	9.5 × 10−20 cm2
emission cross section at 1338 nm	10 × 10−20 cm2
gain bandwidth	0.6 nm

Other Laser-active Dopants in YAG

In addition to Nd:YAG, there are several YAG gain media with other laser-active dopants:

• Ytterbium – Yb:YAG emits typically at either 1030 nm (strongest line) or 1050 nm (→ ytterbium-doped gain media). It is often used in, e.g., thin-disk lasers.
• Erbium – Pulsed Er:YAG lasers, often lamp-pumped can emit at 2.94 μm and are used in, e.g., dentistry and for skin resurfacing. Er:YAG can also emit at 1645 nm [2] and 1617 nm.
• Thulium – Tm:YAG lasers emit at wavelengths around 2 μm, with wavelength tunability in a range of ∼ 100 nm width.
• Holmium – Ho:YAG emits at still longer wavelengths around 2.1 μm. Q-switched Ho:YAG lasers are used e.g. to pump mid-infrared OPOs. There are also holmium-doped laser crystals with codopants, e.g. Ho:Cr:Tm:YAG.
• Chromium – Cr⁴⁺:YAG lasers emit around 1.35–1.55 μm and are often pumped with Nd:YAG lasers at 1064 nm. Their broad emission bandwidth makes them suitable for generating ultrashort pulses. Note that Cr⁴⁺:YAG is also widely used as a saturable absorber material for Q-switched lasers in the 1-μm region.

Neodymium- or ytterbium-doped YAG lasers in the 1-μm region in conjunction with frequency doublers are often the basis of green lasers, particularly when high powers are required.