Nd: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 and 1444 nm. When used at the 946-nm transition, Nd:YAG is a quasi-three-level gain medium, requiring significantly higher pump intensities. 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 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 crystalswith doped and undoped parts, or with parts having different doping densities.
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. It is often used in, e.g., 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 high powers are required. [Enc11]
J. E. Geusic et al., “Laser oscillations in Nd-doped yttrium aluminum, yttrium gallium and gadolinium garnets”, Appl. Phys. Lett. 4 (10), 182 (1964)
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)
J. W. Kim et al., “Fiber-laser-pumped Er:YAG lasers”, IEEE Sel. Top. Quantum Electron. 15 (2), 361 (2009)
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)
A directional antenna array usually consisting of one driven one-half wavelength dipole section, one parasitically excited reflector, and one or more parasitically excited directors mounted in a single plane. [Arr11] Z:
A silicon semiconductor device used as a voltage regulator because of its ability to maintain an almost constant voltage with a wide range of currents. Named after Clarence Melvin Zener (1905-1993), American physicist. Read more: http://www.answers.com/topic/zener-diode#ixzz1K34w3alz [Ans11]
Zener Diode Symbol courtesy of Blogspot
Zero Cells A phenomenon common to local market cable television ratings. The Nielsen household meter indicates viewing, but the corresponding diary data shows no record of viewing.
In single-mode fiber, the rate of change of dispersion with respect to wavelength, at the fiber’s zero-dispersion wavelength. [Fib111]
In a single-mode optical fiber, the wavelength at which material dispersion and waveguide dispersion cancel one another. The wavelength of maximum bandwidth in the fiber. Also called zero-dispersion point. [Fib111] Zigbee
A specification for wireless personal area networks (WPANs) operating at 868 MHz, 902-928 MHz, and 2.4 GHz. A WPAN is a personal area network (a network for interconnecting an individual's devices) in which the device connections are wireless. Using ZigBee, devices in a WPAN can communicate at speeds of up to 250 Kbps while physically separated by distances of up to 50 meters in typical circumstances and greater distances in an ideal environment. ZigBee is based on the 802.15 specification approved by the Institute of Electrical and Electronics Engineers Standards Association (IEEE-SA). ZigBee provides for high data throughput in applications where the duty cycle is low. This makes ZigBee ideal for home, business, and industrial automation where control devices and sensors are commonly used. Such devices operate at low power levels, and this, in conjunction with their low duty cycle (typically 0.1 percent or less), translates into long battery life. Applications well suited to ZigBee include heating, ventilation, and air conditioning (HVAC), lighting systems, intrusion detection, fire sensing, and the detection and notification of unusual occurrences. ZigBee is compatible with most topologies including peer-to-peer, star network, and mesh networks, and can handle up to 255 devices in a single WPAN. [Sea05] Zipcord
A two-fiber cable consisting of two single fiber cables having conjoined jackets. A zipcord cable can be easily divided by slitting and pulling the conjoined jackets apart. [Fib111]
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