The Evolution of Quality-Switched Lasers
November 2012 | Volume 11 | Issue 11 | Original Article | 1296 | Copyright © 2012
Quality-switched (QS) lasers and their applications have evolved greatly since the ruby laser's effect on tattoo ink was first reported in the
1960s. The 1983 description of selective photothermolysis explained the efficacy of QS lasers for the treatment of cutaneous pigmented
lesions and tattoos and cemented their status as the gold standard for these targets. Within the past decade, the uses for QS lasers have
expanded dramatically, including nonablative rejuvenation and the treatment of onychomycosis. Additional applications and refined techniques
and technologies promise to maintain the stature of QS lasers as an integral part of the laser surgeon's arsenal.
J Drugs Dermatol. 2012;11(11):1296-1299.
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Quality-switching was first demonstrated in 1962 by R. W. Hellwarth.1,2 The quality factor, or Q, of a resonant laser cavity measures its internal energy loss. The higher the Q, the lower the energy loss. Hellwarth's laser employed rotating mirrors within the laser cavity. When the mirror was parallel to the long axis of the laser, it did not reflect light to the output mirror and the Q was low. However, when the mirror rotated perpendicular to the long axis of the cavity, the Q was momentarily high and an intense, short pulse of light was generated.3 Qualityswitching can also be accomplished by blocking one mirror in a laser cavity, exciting the lasing medium, then abruptly unblocking the mirror to release stored energy in a short, much higher peak power pulse than would otherwise be attainable. Pulses emitted are typically in the nanosecond (ns) domain, or one billionth of a second (10-9 s). For perspective regarding the brevity of this pulse, in approximately one nanosecond, light travels only 1 foot.
The ruby laser was first introduced for dermatologic use in the 1960s, but it was not until several years later that it incorporated quality-switching, which improved its selectivity for small targets like melanosomes and tattoo pigment.4-6 The active medium, a ruby (aluminum oxide) crystal, was doped with chromium crystals, emitted 694-nm light within the red portion of the visible spectrum.7 The device was powered by a helical flash lamp that surrounded the centrally placed crystal.7 The 694-nm light exhibits a high affinity for melanin, and the quality-switched (QS) ruby laser emits pulses of 25 to 40 ns in duration.8 Goldman et al first used the QS ruby laser on human skin and reported their experience with tattoo removal in 1965.9
The neodymium-doped yttrium aluminum garnet (Nd:YAG) laser was developed by Joseph E. Geusic of Bell Laboratories and introduced to the scientific literature in 1964.10 Neodymium ions were placed in the host crystal YAG and powered by a xenon arc lamp, which used a curved reflective surface to concentrate light energy on a centrally placed Nd:YAG crystal rod.11 The Nd:YAG laser emits light in the near-infrared range with a 1,064- nm wavelength. The 1,064-nm laser light is absorbed by the skin's 3 major chromophores (water, melanin, and hemoglobin); however, it has less affinity for melanin and hemoglobin than shorter-wavelength visible light lasers. This limited absorption and deeper skin penetration allows for safe Nd:YAG laser use in darker-skinned patients. In the QS mode, the Nd:YAG laser can be used for dermal pigmented lesions.12 Once the 1,064- nm light is passed through a potassium titanyl phosphate (KTP) crystal in the laser cavity, it is frequency doubled (wavelength halved) to emit 532-nm green light. This wavelength is highly absorbed by both hemoglobin and melanin.
In addition to the ruby laser and the Nd:YAG laser, solid-state lasers utilizing the alexandrite crystal can be QS. Alexandrite was first mined from the Ural Mountains in Russia and has been used by jewelers for more than a century, but it was not until the 1990s that it was used for medical applications. A stimulated, or "excited," alexandrite crystal produces photons of 755-nm light in the near-infrared spectrum, and with quality-switching, the pulse width can range from 50 to 100 ns.13 The 755-nm wavelength affords deeper cutaneous penetration than shorter-wavelength visible light and is well absorbed by melanin.
The theory of selective photothermolysis dictates that the pulse duration of laser energy be less than the thermal relaxation time of the target, which is itself defined by the size and shape of the