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YAG Lasers

Author: the photonics expert

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

Categories: article belongs to category optical materials optical materials, article belongs to category laser devices and laser physics laser devices and laser physics

DOI: 10.61835/7vp   Cite the article: BibTex plain textHTML

The term YAG laser is usually used for solid-state lasers based on neodymium-doped YAG (Nd:YAG, more precisely Nd3+:YAG). However, 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. Generally, YAG is a host medium with favorable properties, particularly for high-power lasers and Q-switched lasers emitting at 1064 nm.

The most popular alternatives to Nd:YAG among the neodymium-doped laser 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 laser 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:

  • Even a moderate gain requires a high excitation density, which favors detrimental quenching effects.
  • In addition, lasing at 1064 nm, the wavelength with much higher gain, has to be suppressed, for example by using suitable dichroic mirrors for building the laser resonator.

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 (which is particularly relevant in Q-switched lasers). 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.

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.

Nd density for 1 at. % doping1.38 · 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.

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.

Typical Types of Nd:YAG Lasers

Some typical types of Nd3+:YAG lasers, mostly emitting at 1064 nm, are described in the following:

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 laser gain media). It is often used in, e.g., powerful and efficient 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 higher powers are required than with directly green-emitting lasers.

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[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); https://doi.org/10.1063/1.1753928
[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); https://doi.org/10.1364/OL.31.000754
[3]J. W. Kim et al., “Fiber-laser-pumped Er:YAG lasers”, IEEE Sel. Top. Quantum Electron. 15 (2), 361 (2009); https://doi.org/10.1109/JSTQE.2009.2010248
[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); https://doi.org/10.1364/OE.18.007923
[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); https://doi.org/10.1007/s00340-011-4638-5
[6]H. C. Lee et al., “Diode-pumped continuous-wave eye-safe Nd:YAG laser at 1415 nm”, Opt. Lett. 37 (7), 1160 (2012); https://doi.org/10.1364/OL.37.001160

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Questions and Comments from Users


Why do different companies specify different Nd:YAG emission cross-sections? Is that related to different doping concentrations?

The author's answer:

Such differences simply go back to different measurements, not all of which are accurate.

The doping concentration has no influence. In principle, ions could influence each other when they are closely seated for high doping concentrations, but in practice that does not play a role for Nd:YAG.


How can a Nd:YAG laser be pulsed despite continuous pumping?

The author's answer:

That works with Q switching or mode locking, for example.

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