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YAG Lasers

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

German: YAG-Laser

Categories: lasers, optical materials

How to cite the article; suggest additional literature

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

Nd3+: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.

energy level structure of the trivalent neodymium ion in Nd:YAG

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, 1415 and 1444 nm. When used at the 946-nm transition, Nd:YAG is a quasi-three-level gain medium, requiring significantly higher pump intensities. All other transitions are four-level transitions. Some of these, such as the one at 1123 nm, are very weak, so that efficient laser operation on these wavelengths is difficult to obtain:

However, with careful optimization, even on these weak transitions one can obtain substantial output powers [4].

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 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.

PropertyValue
chemical formulaY3Al5O12
crystal structurecubic
mass density4.56 g/cm3
Moh hardness8–8.5
Young's modulus280 GPa
tensile strength200 MPa
melting point1970 °C
thermal conductivity10–14 W / (m K)
thermal expansion coefficient7–8 · 10−6/K
thermal shock resistance parameter790 W/m
birefringencenone (only thermally induced)
refractive index at 1064 nm1.82
temperature dependence of refractive index7–10 · 10−6/K

Table 1: Some properties of YAG = yttrium aluminum garnet, which are similar for Nd- or Yb-doped YAG.

PropertyValue
Nd density for 1 at. % doping1.36 · 1020 cm−3
fluorescence lifetime230 μs
absorption cross section at 808 nm7.7 · 10−20 cm2
emission cross section at 946 nm5 · 10−20 cm2
emission cross section at 1064 nm28 · 10−20 cm2
emission cross section at 1319 nm9.5 · 10−20 cm2
emission cross section at 1338 nm10 · 10−20 cm2
gain bandwidth0.6 nm

Table 2: Some properties of Nd:YAG = neodymium-doped yttrium aluminum garnet.

PropertyValue
Yb density for 1 at. % doping1.38 · 1020 cm−3
fluorescence lifetime950 μs
absorption cross section at 940 nm0.75 · 10−20 cm2
emission cross section at 1030 nm2.2 · 10−20 cm2
absorption cross section at 1030 nm0.12 · 10−20 cm2
emission cross section at 1050 nm0.3 · 10−20 cm2
absorption cross section at 1050 nm0.01 · 10−20 cm2
gain bandwidth15 nm

Table 3: Some properties of Yb:YAG = ytterbium-doped yttrium aluminum garnet.

Other Laser-active Dopants in YAG

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

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 higher powers are required than with directly green-emitting lasers.

Bibliography

[1]J. E. Geusic et al., “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)
[3]J. W. Kim et al., “Fiber-laser-pumped Er:YAG lasers”, IEEE Sel. Top. Quantum Electron. 15 (2), 361 (2009)
[4]Li Chaoyang et al., “106.5 W high beam quality diode-side-pumped Nd:YAG laser at 1123 nm”, Opt. Express 18 (8), 7923 (2010)
[5]X. Délen et al., “34 W continuous wave Nd:YAG single crystal fiber laser emitting at 946 nm”, Appl. Phys. B 104 (1), 1 (2011)
[6]H. C. Lee et al., “Diode-pumped continuous-wave eye-safe Nd:YAG laser at 1415 nm”, Opt. Lett. 37 (7), 1160 (2012)

(Suggest additional literature!)

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
and other articles in the categories lasers, optical materials

In the RP Photonics Buyer's Guide, 67 suppliers for YAG lasers are listed.


Dr. R. Paschotta

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) and software could become very valuable for your business!

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