energized by optically pumping it with a xenonflashtube. Ruby lasers initially
were used for tattoo and hair removal, but now usually the alexandrite and Nd:
YAG lasers described below are carrying out these functions.- Neodymium:yttrium aluminum garnet (Nd:YAG) lasers use a
neodymium-doped yttrium aluminum garnet (Y 3 Al 5 O 12 ) synthetic crystal,
which is known by the acronym YAG. The Nd:YAG laser can be pumped by a
laser diode or by a lamp. The most commonly used wavelength from Nd:YAG
lasers is 1064 nm. Other emission lines are at 946, 1123, 1319, 1338, 1415, and
1444 nm. In continuous operation versions the output power can be a few
milliwatts for miniature setups to several watts for larger models. Pulsed ver-
sions can generate pulses of a few nanoseconds in duration with microjoule
pulse energies. Applications of these lasers are in the areas of ophthalmology,
oncology, urology, pulmonology, hair removal, prostate surgery, and toenail
infections. - Erbium:YAG (Er:YAG) lasersare erbium-doped YAG crystals that lase at
2940 nm. This wavelength is close to an absorption line of water. Thus, because
water strongly absorbs the light, the laser light interaction with tissue results in
evaporation and ablation, such as the smoothing or resurfacing of skin (for
example, wrinkle removal). Osseous minerals also absorb this wavelength
strongly, so an Er:YAG laser is advantageous for sawing and drilling in bone
and teeth. - Holmium:YAG (Ho:YAG) lasersare holmium-doped YAG crystals that lase
at 2100 nm. Its use in ophthalmology includes correcting mild to moderate cases
of farsightedness and some types of astigmatism. Other applications include
tissue ablation, kidney stone removal, and intraoral soft tissue surgery. - Titanium sapphire lasers(also called Ti:Al 2 O 3 lasers or Ti:sapphire lasers) are
tunable lasers that emit light in the 650–1100 nm range (red to near infrared). As
its name implies, the lasing medium is a sapphire (Al 2 O 3 ) crystal that is doped
with titanium ions. This laser usually is pumped with another laser that emits in
the 514–532-nm region, such as an argon laser. A Ti:sapphire laser can generate
ultra-short pulses, which can be as short as 6 femtoseconds in duration with an
average output power of 0.5–1.5 W. The pulse repetition frequency nominally is
between 70 and 90 MHz. A major application is for multiphoton microscopy,
which is a key noninvasive laboratory tool for studying underlying biological
phenomena. - Alexandrite lasersare tunable in the 700–830-nm range. The average output
power can be up to 100 W. Light in the emission spectrum of these lasers is
absorbed well by melanin and dyes but not by blood. Therefore it is useful for
destroying tissue elements that contain melanin (e.g., moles and hair roots), for
removing tattoos, and for breaking up kidney stones (lithotripsy).
112 4 Fundamentals of Light Sources