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.
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.
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 .
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.
|mass density||4.56 g/cm3|
|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||1.38 · 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|
|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|
Typical Types of Nd:YAG Lasers
Some typical types of Nd3+:YAG lasers, mostly emitting at 1064 nm, are described in the following:
- Lamp-pumped lasers can be made with long cylindrical Nd:YAG laser rods. As the four-level laser transition does not cause any reabsorption by non-excited Nd ions, such lasers can be operated with a very low fractional excitation of the laser-active ions.
- Diode-pumped lasers are usually made with relatively small laser crystals, i.e., with dimensions of only a few millimeters. Exceptions are certain high-power slab lasers and side-pumped rod lasers.
- 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, often optimized for single frequency operation with small emission linewidth.
- Many YAG lasers are Q-switched, generating nanosecond light pulses.
- For mode locking, Nd:YAG lasers are less suitable, as the limited gain bandwidth does not allow for very short pulses.
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  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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See also: vanadate lasers, YLF lasers, laser crystals, neodymium-doped laser gain media, chromium-doped laser gain media, ytterbium-doped laser gain media, rare-earth-doped laser gain media, ceramic laser gain media, solid-state lasers, lamp-pumped lasers, diode-pumped lasers, nonplanar ring oscillators, The Photonics Spotlight 2006-09-16
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