YAG-Laser

The term YAG laser is often 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, such as ytterbium, erbium, thulium, or holmium doped (see below).

YAG is an acronym for yttrium aluminum garnet (Y3Al5O12), a synthetic crystalline material that became popular in the form of laser crystals in the 60s of the 20th century. Yttrium ions in YAG can be replaced by laser-active rare earth ions without strongly affecting the lattice structure, as these ions have similar dimensions. In general, YAG is a matrix medium with good performance, especially for high-power lasers and Q-switched lasers that emit 1064 nm.

Among neodymium-doped laser gain media, the most popular Nd:YAG alternatives are Nd:YVO4 and Nd:YLF. Nd:YAG lasers now also have to compete with Yb:YAG lasers.

Nd:YAG

Nd3+:YAG is a four-level gain medium (in addition to the 946-nm transition discussed below) that provides considerable laser gain even at moderate excitation levels and pump intensities. The relatively small gain bandwidth enables high gain efficiency and low threshold pump power. Lamp pumping is also achievable due to broadband pump absorption and four-level characteristics, mainly in the 800 nm region.

The most common Nd:YAG emission wavelength is 1064 nm. From this wavelength, laser output of 532, 355, and 266 nanometers can be produced by frequency doubling, tripling, and quadruple, respectively. Other emission lines are at 946, 1123, 1319, 1338, 1415 and 1444 nm. When used for the 946 nm transition, Nd:YAG is a quasi-three-level laser gain medium that requires significantly higher pump intensity. All other jumps are four-level jumps. Some of them, such as 1123nm, have very weak wavelengths, making it difficult to get effective laser operation at these wavelengths:

• Even modest gains require high excitation densities, which are prone to harmful quenching effects.
• In addition, the laser at 1064 nm (which has a higher gain) must be suppressed, and a laser resonator can be constructed by using a suitable dichroic mirror.

However, with careful optimization, one can get a lot of output power even at these weak transitions.

Nd:YAG is typically used in monocrystalline form and is fabricated using the straight-pull growth method, but there are also high-quality and large-size ceramic (polycrystalline) Nd:YAG. For single crystals and ceramic Nd:YAG, the absorption and scattering losses over the length of the laser crystal are usually negligible, even for relatively long crystals.

A typical neodymium doping concentration is about 1at. %。 High doping concentrations are often advantageous, for example, because they reduce the length of the pump absorption, and too high concentrations can lead to the quenching of the lifetime of the upper energy state, e.g. by the upconversion process (which is particularly relevant in Q-switched lasers). In high-power lasers, the dissipated power density can become excessive. Note that the neodymium doping density does not necessarily have to be the same for all fractions; Composite laser crystals with doped and undoped parts, or parts with different doping densities.

Attribute	Value
Chemical formula	Y3Al5O12
Crystal structure	Cube
Mass density	4.56g/cm3
Mohs hardness	8–8.5
Young's modulus	280GPa
Tensile strength	200MPa
Melting point	1970°C
Thermal conductivity	10–14W/(mK)
Coefficient of thermal expansion	7–8·10−6/K
Thermal shock resistance parameters	790W/m
Birefringence	None (thermal sensing only)
Refractive index at 1064nm	1.82
Temperature dependence of the refractive index	7–10·10-6/K

Table 1: YAG = some properties of yttrium aluminum garnet, similar to YAG doped with Nd or Yb.

Attribute	Value
The density of Nd is 1at. % of doping	1.38·1020cm−3
Fluorescence lifetime	230μs
Absorption cross section at 808 nm	7.7·10−20cm2
Emission cross section at 946 nm	5·10−20cm2
Emission cross section at 1064 nm	28·10−20cm2
Emission cross section at 1319 nm	9.5·10−20cm2
Emission cross section at 1338 nm	10·10−20cm2
Gain bandwidth	0.6nm

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

Attribute	Value
The Yb density is 1at. % of doping	1.38·1020cm−3
Fluorescence lifetime	950μs
Absorption cross section at 808 nm	0.75·10−20cm2
Emission cross section at 946 nm	2.2·10−20cm2
Emission cross section at 1064 nm	0.12·10−20cm2
Emission cross section at 1319 nm	0.3·10−20cm2
Emission cross section at 1338 nm	0.01·10−20cm2
Gain bandwidth	15nm

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

Typical type of Nd:YAG laser

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

• Lamp-pumped lasers can be made with long cylindrical Nd:YAG laser bars. Since the quaternary laser transition does not cause any reabsorption of non-excited Nd ions, such lasers can operate at very low fractional excitation of laser active ions.
• Diode-pumped lasers typically use relatively small laser crystals, i.e. only a few millimeters in size. The exceptions are certain high-power plate lasers and side-pumped rod lasers.
• YAG lasers are, in many cases, bulk lasers made of discrete optics. However, there are also monolithic YAG lasers, such as microchip lasers and non-planar ring oscillators, which are typically optimized for single-frequency operation with small emission linewidths.
• Many YAG lasers are Q-switched, producing nanosecond pulses of light.
• For mode-locking, Nd:YAG lasers are not well suited because the limited gain bandwidth does not allow for very short pulses.

Other laser active dopants

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

• Ytterb–Yb:YAG is typically emitted at 1030 nm (strongest spectral line) or 1050 nm (ytterbium-doped laser gain medium). It is often used, for example, in powerful and efficient thin-disc lasers.
• Erbium–pulsed Er:YAG lasers, typically lamp-pumped, emit 2.94 μm and can be used in areas such as dentistry and skin resurfacing. Er:YAG can also be emitted at 1645 nm and 1617 nm.
• Thul–Tm:YAG lasers emit at a wavelength of around 2 microns and can be tuned in the ≈100 nanometer width range.
• Holmium–Ho:YAG emits a longer wavelength of about 2.1 μm. Q-switched Ho:YAG lasers are used, for example, to pump mid-infrared OPOs, as well as holmium-doped laser crystals and co-dopants such as Ho:Cr:Tm:YAG.
• Chromium-Cr4+:YAG lasers emit wavelengths around 1.35-1.55 μm and are typically pumped with 1064 nm Nd:YAG lasers. Their wide emission bandwidth makes them suitable for generating ultrashort pulses. Note that Cr4+:YAG is also widely used as a saturable absorber for Q-switched lasers in the 1-μm region.

Neodymium-doped or ytterbium-doped YAG lasers in the 1 μm region are often used in combination with frequency multipliers and are often the basis for green lasers, especially when higher power is required than direct-emitting green lasers.