In a laser, atoms, molecules or ions are raised to energetic states, for example, by the passage of a discharge current or illumination with a powerful arc lamp or another laser. These excited states can be stimulated by photons to emit additional photons which are identical in wavelength, direction and phase to the original photons. In this way, numbers of such photons may be amplified enormously to produce a parallel beam of visible, infrared or ultraviolet radiation. The principal medical lasers are the carbon dioxide laser, emitting in the far infrared, the argon laser with a blue-green beam, the Nd YAG laser with a near infrared beam, and the dye laser which may be “tuned” over a wide range of the visible and infrared spectrum. All laser beams have the property that their wavefronts are virtually parallel planes, so that they may be focused by a lens into the smallest possible spot. The applications of a particular laser depend upon the absorption and scattering properties of soft tissue at the different wavelengths available.
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