Nd:YAG laser
A solid-state laser whose gain medium is yttrium aluminium garnet doped with about 1 % neodymium, emitting at 1064 nm. Its 230 µs upper-state lifetime stores energy for Q-switched pulses of millijoules to joules in 5–20 ns, frequency-doubled to 532 nm when green light is needed.
The Nd:YAG laser uses neodymium ions (Nd³⁺) substituted for yttrium in a crystal of yttrium aluminium garnet, Y₃Al₅O₁₂, as its gain medium. Its main output is at 1064 nm in the near infrared, with weaker transitions at 946 nm and 1319 nm. A dopant level of about 1 atomic percent, 1.4 × 10²⁰ ions per cm³, is typical. Since its demonstration at Bell Laboratories in 1964 it has been the most widely used solid-state laser: continuous versions produce milliwatts to kilowatts, and Q-switched versions deliver pulses of millijoules to joules lasting 5–20 ns, often converted to 532, 355 or 266 nm by harmonic generation.
Four-level operation
Nd:YAG is a four-level system. Pump light raises Nd³⁺ ions into absorption bands, the strongest near 808 nm, from which they relax within nanoseconds to the metastable ⁴F₃/₂ level. The laser transition at 1064 nm runs to the ⁴I₁₁/₂ level, about 0.25 eV above the ground state, which is almost empty at room temperature and drains quickly by phonon emission. A population inversion therefore forms with only a small fraction of ions excited, giving low thresholds. The ⁴F₃/₂ lifetime is about 230 µs at 1 % doping, long enough to accumulate inversion between pulses.
Optical pumping was originally done with krypton arc lamps or xenon flashlamps, whose broad spectra overlap the narrow absorption lines poorly. Diode lasers at 808 nm, matched to the strongest line, replaced lamps in most new designs, raising electrical-to-optical efficiency and reducing heat load. The quantum defect sets a floor on the heat deposited: pumping at 808 nm and lasing at 1064 nm leaves
of the absorbed pump energy as heat. Pumping directly into the upper level at 885 nm reduces this to 17 %.
Pulsed operation and harmonics
The long storage time suits Q-switching: with the cavity held at high loss by an acousto-optic or electro-optic switch, or a Cr:YAG saturable absorber in passive designs, pumping builds inversion for up to a few hundred microseconds, and opening the switch releases it in a single pulse. A 100 mJ pulse of 10 ns duration has a peak power of 10 MW. The stimulated-emission cross section is quoted between about 2.8 and 6.5 × 10⁻¹⁹ cm² depending on how the line structure is treated, giving a saturation fluence of 0.3–0.7 J/cm² for amplifiers.
High peak power makes nonlinear conversion efficient. Second-harmonic generation in KTP or LBO produces 532 nm green, and sum-frequency and further doubling give 355 nm and 266 nm. Mode-locked Nd:YAG and Nd:YVO₄ lasers produce picosecond pulses; the gain bandwidth, about 0.5 nm, is too narrow for much shorter pulses.
Thermal effects
Heat deposited in the rod produces a radial temperature gradient, and with it a positive thermal lens whose focal power scales with pump power, together with stress-induced birefringence that depolarizes the beam. These effects limit the average power obtainable in good beam quality from a single rod. Slab, thin-disk and end-pumped geometries mitigate them, and YAG's thermal conductivity of roughly 10–14 W/(m·K), high for an oxide crystal, helps.
Where it is used
Nd:YAG lasers mark, weld, cut and drill metals in laser machining; remove tissue and treat the eye (the Nd:YAG capsulotomy after cataract surgery); pump titanium-sapphire and dye lasers with their 532 nm harmonic; provide the pulses for lidar, range-finding and laser-induced breakdown spectroscopy; and drive Raman and fluorescence spectrometers. In continuous high-power industrial cutting, ytterbium fiber lasers at 1.03–1.08 µm have displaced much of this market with higher efficiency and better beam quality, while Q-switched Nd:YAG remains common where high pulse energy from a compact source is needed.
Pitfalls
1064 nm light is invisible yet focused by the eye onto the retina, and scattered pulsed light at these energies is hazardous; eyewear must be rated for the wavelength and the optical density required. Coatings and optics must be rated for the pulsed laser-induced damage threshold at the pulse length in use, usually quoted in J/cm² for 10 ns pulses. Thermal lensing changes with pump level, so a cavity aligned at low power can become unstable at full power.
Common questions
Is Nd:YAG the same as Nd:YVO₄ or Nd:glass?
No. The same Nd³⁺ ion in other hosts gives nearby wavelengths and different properties: Nd:YVO₄ lases at 1064 nm with a larger cross section and shorter lifetime, suiting high-repetition-rate Q-switching, and Nd:glass lases near 1054–1062 nm with a broad line and the ability to be made in large pieces, used for the highest-energy pulsed lasers.
Why is the green of a 532 nm laser made from 1064 nm?
Few materials lase efficiently in the green. Doubling a 1064 nm Nd laser in a nonlinear crystal is an efficient route, and it is how many green laser pointers work, usually with Nd:YVO₄ as the gain medium.
References: J. E. Geusic, H. M. Marcos and L. G. Van Uitert, Laser oscillations in Nd-doped yttrium aluminum, yttrium gallium and gadolinium garnets, Appl. Phys. Lett. 4, 182 (1964); W. Koechner, Solid-State Laser Engineering 6th ed. (Springer, 2006); A. E. Siegman, Lasers (University Science Books, 1986).