The word laser is an acronym standing for "Light Amplification by Stimulated Emission of Radiation." A laser beam is created from a substance known as an active medium, which when stimulated by light or electricity produces photons of a specific wavelength. Lasers are characteristically monochromatic, unidirectional, coherent, and emitted from a stimulated active medium. Monochromacity means that a laser beam is made of a single wavelength of light, and all dental lasers are found in the visible or infrared portion of the electromagnetic spectrum. Once a laser beam is produced it will travel in one direction (unidirectional), though the divergence of the beam varies by type of laser and the associated transmission hardware. Coherence is the property that not only is a laser a single wavelength but all the peaks and valleys of each wave travel in unison.
The active media in dentistry can be solid state, gas, or semiconductor. Solid state lasers are a crystal matrix host doped with the light emitting, excitable atoms; such as erbium laced yttrium, aluminium, and garnet (Er:YAG). CO2 is a popular laser where the active medium is sealed in an air tight chamber. Diode lasers have a semiconductor that when stimulated with electricity, laser light is emitted.
Stimulated emission is a phenomenon that occurs within the active medium. For example, in solid state Er:YAG lasers the erbium is stimulated by light from a flashlamp with a process known as optical pumping. As an erbium atom absorbs a photon, its electrons are elevated to higher energy level. When the electrons return to a lower energy state, two identical photons are emitted and these photons can further stimulate more atoms in a chain reaction, resulting in amplification of the light produced. Mirrors surrounding the active medium called a resonator further increase this light energy. One of the mirrors called the output coupler is less than one hundred percent reflective. Light leaks from the output coupler, and these are the photons that form the laser beam. Once the beam is created it is carried to the target tissue by various types of beam transfer hardware. Mirrors in articulated arms and optical fibers are common examples of this hardware.
The laser beam is collimated and exits in the handpiece as a column of light. The beam will diverge at various rates depending on the device, the handpiece, and the tip used. Once the beam reaches the tissue it will have a specific spot size depending on the distance from the handpiece and its specific divergence. The energy density refers to the actual amount of energy reaching the tissue within the spot size. This energy density varies considerably depending on the parameters of energy, divergence, and distance.