YAG Lasers

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.

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:

• 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. 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	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

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

Property	Value
Nd density for 1 at. % doping	1.38 · 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

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

Property	Value
Yb density for 1 at. % doping	1.38 · 1020 cm−3
fluorescence lifetime	950 μs
absorption cross section at 940 nm	0.75 · 10−20 cm2
emission cross section at 1030 nm	2.2 · 10−20 cm2
absorption cross section at 1030 nm	0.12 · 10−20 cm2
emission cross section at 1050 nm	0.3 · 10−20 cm2
absorption cross section at 1050 nm	0.01 · 10−20 cm2
gain bandwidth	15 nm

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

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., 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), doi: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), doi: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), doi: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), doi: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), doi: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), doi:10.1364/OL.37.001160

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