INTRODUCTION
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.
APPLICATIONS
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
