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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 short form for yttrium aluminum garnet (Y3Al5O12), a synthetic crystal material which became popular in the form of laser crystals already in the 1960s. Yttrium ions in YAG can be replaced with rare-earth laser-active ions without strongly affecting the lattice structure, because these ions have a similar size.
YAG is a host medium with rather 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 or nonplanar ring oscillators.
The most popular alternatives to Nd:YAG among the neodymium-doped gain media are Nd:YVO4 and Nd:YLF. Besides, Nd:YAG lasers nowadays often 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 large 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.
The most common Nd:YAG emission wavelength is 1064 nm. Starting with that wavelength, outputs at 532 nm, 355 nm and 266 nm can be generated by frequency doubling, frequency tripling and frequency quadrupling, respectively. Other emission lines are at 946 nm, 1320 nm and 1123 nm. When used at the 946-nm transition, Nd:YAG is a quasi-three-level gain medium, requiring significantly high 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% (atm.). 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 upconversion processes. Also, the density of dissipated power can become too high in high-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.
| Property | Value |
|---|---|
| chemical formula | Nd3+:Y3Al5O12 |
| crystal structure | cubic |
| mass density | 4.56 g/cm3 |
| Moh hardness | 8 to 8.5 |
| Young's modulus | 280 GPa |
| tensile strength | 200 MPa |
| melting point | 1970 °C |
| thermal conductivity | 10 to 14 W / (m K) |
| thermal expansion coefficient | 7 to 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 to 10·10-6/K |
| Nd density for 1% atm. 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 1064 nm | 28·10-20 cm2 |
| gain bandwidth | 0.6 nm |
Table 1: Some properties of Nd:YAG = neodymium-doped yttrium aluminum garnet.
Other Laser-active Dopants in YAG
Apart from Nd:YAG, there are several YAG gain media with other laser-active dopants:
- Ytterbium – Yb:YAG emits typically either at 1030 nm (strongest line) or 1050 nm (→ ytterbium-doped gain media). It is often used e.g. in thin-disk lasers.
- Erbium – pulsed Er:YAG lasers, often lamp-pumped can emit at 2.94 μm and are used e.g. in dentistry and for skin resurfacing. Er:YAG can also emit at 1645 nm [2] and 1617 nm, as well as at 550 nm and 561 nm.
- Thulium – Tm:YAG lasers emit around 2 μm wavelength, 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.
Bibliography
| [1] | J. E. Geusic, H. M. Marcos, and L. G. Van Uitert, "Laser oscillations in Nd-doped yttrium aluminum, yttrium gallium and gadolinium garnets", Appl. Phys. Lett. 4 (10), 182 (1964) |
| [2] | D. Y. Shen et al., "Highly efficient in-band pumped Er:YAG laser with 60 W of output at 1645 nm", Opt. Lett. 31 (6), 754 (2006) |
See also: vanadate lasers, YLF lasers, laser crystals, neodymium-doped gain media, chromium-doped gain media, ytterbium-doped gain media, rare-earth-doped gain media, ceramic gain media, solid-state lasers, lamp-pumped lasers, diode-pumped lasers, nonplanar ring oscillators, Spotlight article 2006-09-16
This encyclopedia is authored by Dr. Rüdiger Paschotta, the founder and executive of RP Photonics Consulting GmbH. Contact this distinguished expert in laser technology, nonlinear optics and fiber optics, and find out how his technical consulting services (e.g. product designs, problem solving, independent evaluations, or staff training) could become very valuable for your business!
