What is a YAG laser?
Thursday, February 10, 2022
Embrace your imperfections
YAG crystals are man-made garnets. They can be used as gemstones for jewelry, but their main use today is in the laser industry. Interestingly enough, perfect YAG crystals are useless for lasing. It is literally their imperfections that make them great.
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Correction: What are YAG lasers? Plural.
As mentioned earlier, yttrium aluminum garnets (YAG) must contain some impurities to be useful as a lasing material. These impurities, or dopants, come in a wide variety, including erbium (Er), neodymium (Nd), ytterbium (Yb), and many others.
Because so many different dopants can be used, it is a bit ambiguous to speak of a YAG laser in the singular form. That being said, neodymium-doped YAG lasers (Nd:YAG) are the most common and you can usually assume this is what people are referring to when they simply say “a YAG laser”.
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Working mechanism
Nd:YAG is a 4-level gain medium. The most frequently used transition (1064 nm) is between 2 intermediate energy states. Because the lowest of these 2 energy states is usually empty at normal operating temperatures, population inversion can be attained even with low pump power. This allows for low threshold pump power and high gain efficiency.
To better understand population inversion, I suggest reading the well-written Wikipedia article. For the numbers specific to Nd:YAG, refer to the RP-Photonics entry.
Hazards
Nd:YAG lasers (the most widespread of YAG lasers) emit light at 1064 nm. Though light at this wavelength can damage the eye, it is invisible and hence does not trigger the blink reflex. It is therefore particularly important to respect all safety guidelines when manipulating them.
532, 355 and 266 nm wavelengths can also be attained using frequency doubling, tripling and quadrupling. Again, particular care must be taken when handling these lasers. They are commercially available in powers sufficient to be considered Class IV.
Applications
Medicine
Nonetheless, if handled with care, Nd:YAG lasers can be used in medicine to destroy tumors, for cosmetic surgeries like hair and tattoo removal, as well as to correct many eye problems. One such example is the treatment of posterior capsular opacification (PCO), which is a condition that impairs vision and frequently occurs after cataract surgery.
Manufacturing
YAG lasers can output high powers in the kilowatt range. They are available in continuous wave (CW) and pulsed modes. The fact that they can be used with flexible optical fibre enables easier beam delivery. Their relatively small size also makes them portable, enhancing their practicality.
They are used to cut, weld, mark and engrave metals, plastics, semiconductors and transparent materials like glass. Nd:YAG lasers are also used to strengthen materials by laser peening.
Future outlook
YAG lasers, particularly the most common Nd:YAG laser, have a bright future. They are a well-established player in the laser market with a large install base and many useful applications.
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