Nd:YAG laser

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Overview

Nd:YAG (neodymium-doped yttrium aluminium garnet; Nd:Y₃Al₅O₁₂) is a crystal that is used as a lasing medium for solid-state lasers. The dopant, triply ionized neodymium, typically replaces yttrium in the crystal structure of the yttrium aluminium garnet, since they are of similar size. Generally the crystalline host is doped with around 1% neodymium by weight.^[1]

Laser operation of Nd:YAG was first demonstrated by Geusic et al. at Bell Laboratories in 1964^[2].

Technology

Nd:YAG lasers are optically pumped using a flashlamp or laser diodes. They are one of the most common types of laser, and are used for many different applications.

Nd:YAG lasers typically emit light with a wavelength of 1064 nm, in the infrared.^[3] However, there are also transitions near 940, 1120, 1320, and 1440 nm. Nd:YAG lasers operate in both pulsed and continuous mode. Pulsed Nd:YAG lasers are typically operated in the so called Q-switching mode: An optical switch is inserted in the laser cavity waiting for a maximum population inversion in the neodymium ions before it opens. Then the light wave can run through the cavity, depopulating the excited laser medium at maximum population inversion. In this Q-switched mode output powers of 20 megawatts and pulse durations of less than 10 nanoseconds are achieved.^{[citation needed]}

Nd:YAG absorbs mostly in the bands between 730-760 nm and 790-820 nm.^[3] Krypton flashlamps, with high output at those bands, are therefore more efficient for pumping Nd:YAG lasers than the xenon lamps, which produce more white light and hence more energy therefore goes wasted.^[4]

The amount of the neodymium dopant in the material varies according to its use. For continual wave output, the doping is significantly lower than for pulsed lasers. The lightly doped CW rods can be optically distinguished by being less colored, almost white, while higher-doped rods are pink-purplish.

Other common host materials for neodymium are: YLF (yttrium lithium fluoride, 1047 and 1053 nm), YVO₄ (yttrium orthovanadate, 1064 nm), and glass. A particular host material is chosen in order to obtain a desired combination of optical, mechanical, and thermal properties. Nd:YAG lasers and variants are pumped either by flash lamps, continuous gas discharge lamps, or near-infrared laser diodes (DPSS lasers). Prestabilized laser (PSL) types of Nd:YAG lasers have proved to be particularly useful in providing the main beams for gravitational wave interferometers such as LIGO, VIRGO, GEO600 and TAMA.

Applications

Ophthalmology

Slit lamp photo of Posterior capsular opacification visible few months after implantation of intraocular lens in eye, seen on retroillumination

Frequency-doubled Nd:YAG lasers (wavelength 532 nm) are used in the medical field to correct posterior capsular opacification (after-cataract). These lasers are used for peripheral iridotomy in patients with acute angle closure glaucoma, where it has superseded surgical iridectomy. They are also used in place of argon lasers for pan-retinal photocoagulation in patients with diabetic retinopathy.

Cosmetic medicine

These lasers are also used extensively in the field of cosmetic medicine for laser hair removal and the treatment of minor vascular defects such as spider veins on the face and legs.

Manufacturing

It is used in manufacturing as a means of engraving, etching, or marking a variety of metals and plastics. Nd:YAG lasers are extensively used in manufacturing for cutting and welding steel and super alloys. For automotive applications (cutting and welding steel) the power levels are typically 1-5 kW. Super alloy drilling (for gas turbine parts) typically uses pulsed Nd:YAG lasers (millisecond pulses, not Q-switched). Nd:YAG lasers are also employed to make subsurface markings in transparent materials such as glass or acrylic glass.

Fluid dynamics

Nd:YAG lasers can also be used for flow visualization techniques in fluid dynamics (for example particle image velocimetry or induced fluorescence).^[5]

Dentistry

Nd:YAG lasers are used for soft tissue surgeries in the oral cavity, such as gingivectomy, periodontal sulcular debridement, frenectomy, biopsy, and coagulation of graft donor sites.

Additional frequencies

For many applications, the infrared light is frequency-doubled or -tripled using nonlinear optical materials such as lithium triborate to obtain visible (532 nm, green) or ultraviolet light. A green laser pointer is a frequency doubled Nd:YVO₄ DPSS laser. Nd:YAG can be also made to lase at its non-principal wavelength. The line at 946 nm is typically employed in "blue laser pointer" DPSS lasers, where it is doubled to 473 nm.

References and notes

• Siegman, Anthony E. (1986). Lasers. University Science Books. ISBN 0-935702-11-3. 
• Yariv, Amnon (1989). Quantum Electronics, 3rd Edition, Wiley. ISBN 0-471-60997-8. 
• Koechner, Walter (1988). Solid-State Laser Engineering, 2nd Edition, Springer-Verlag. ISBN 3-540-18747-2. 

1. ↑ Koechner §2.3, pp48–53.
2. ↑ Geusic, J.E., Marcos, H.M, and Van Uitert, L.G.: "Laser oscillations in Nd-doped yttrium aluminum, yttrium gallium and gadolinium garnets". Applied Physics Letters 4 10, 182-184 (1964).
3. ↑ ^{3.0 3.1} Yariv, §10.3, p. 208-211.
4. ↑ Koechner §6.1.1, pp. 251–264.
5. ↑ Palafox, Gilbert N.; Wicker, Ryan B.; and Elkins, Christopher J. (2003). "Rapid in-vitro physiologic flow experimentation using rapid prototyping and particle image velocimetry" (pdf). 2003 Summer Bioengineering Conference: 419. Retrieved on 2007-10-10.

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