Laser is an acronym for “light amplification by stimulated emission of radiation.”
In 1917, Albert Einstein described the theoretical basis of lasers in his paper Zur Quantentheorie der Strahlung.1 The first functional laser was constructed in 1957 by Theodore Maiman, a physicist at Hughes Research Laboratories in Malibu, California.
A laser is a resonant cavity flanked by two mirrors and filled with an active medium which can be gas, liquid, or solid. One of the mirrors is 100% reflective and the other is partially reflective (slightly leaky). A laser also has a pump or external energy source. A laser is pumped by passing current through the active medium or by using a flash lamp. When a laser is pumped, energy is absorbed by the atoms of the active medium, raising electrons to higher energy levels. The high-energy electrons then spontaneously decay to their lower energy “ground state,” emitting a photon in a random direction. This process is called spontaneous emission. Most of these spontaneously emitted photons are absorbed and decay; however, the photons emitted in the direction of the long axis of the resonant cavity are retained as they bounce between the two mirrors of the laser. When these photons encounter an atom in the excited state, they stimulate an excited electron to decay to its ground state and emit another photon of the same wavelength in the same direction. This process is called stimulated emission. When more than half of the atoms in the active medium reach the high-energy state, population inversion occurs. This is a necessary condition for a laser to start working. As light is amplified in the active medium through the process of stimulated emission, the partially reflective mirror begins emanating light which is uniform in wavelength, direction, phase, and polarization. This creates the familiar laser beam (Figure 3-1).
Components of a laser include an active medium flanked by two mirrors and a pumping mechanism which delivers energy to the active medium.
Properties of Laser Light
Laser light is monochromatic, unidirectional, and uniform in phase and polarization.
Laser beam spreads over distances, and can be focused with lenses.
Once laser light exits the main resonance chamber, it has to be delivered to tissue via one of two major delivery mechanisms, optical fiber or a waveguide. Light in the visible spectrum easily travels through an optical fiber and can be delivered directly to target tissues using a handpiece without significant energy loss. Even the near-infrared light of the Nd:YAG laser (1.06 µm) can be delivered through a fiber, however, infrared light of the CO2 laser (10.6 µm) cannot be delivered via optical fiber, a major shortcoming of this highly popular laser. CO...