Basics of Lasers in Dermatology

This process leads to the fundamental construction of a laser: a population of atoms in a resonant cavity flanked by reflectors that are exposed to some sort of excitation mechanism (known as the pump) with an output mechanism for the laser beam to exit. In practice, the material used to supply the atoms (known as the gain medium) varies and also determines the wavelength and properties of the laser beam due to differences in the discrete energy states of orbiting electrons. Whatever the gain medium being used, the important properties of a laser resulting from these principles is that the beam is monochromatic (consisting of a single wavelength or a very narrow band), coherent (the light is emitted in the same phase and direction), collimated (a narrow beam diameter with limited divergence), and intense (high power per unit area). Consider the differences between a laser pointer and a flashlight; from across the room, the laser pointer output is a small spot of light on the wall whereas the flashlight has long dispersed to a weak, broad swath of light.

Types of Lasers

Different gain media have been used to create a variety of lasers with different properties. In general, lasers fall into 1 of 4 categories: gas discharge, diode, dye, and solid-state lasers.⁹

Although a gas discharge laser theoretically is the simplest laser, whereby a gas is excited by an electric discharge and the excited particles of gas create the laser beam, there are practical considerations such as excessive heat production, which may necessitate the use of cooling coils or some other method for heat dispersion. The excimer laser is a specific type of gas discharge laser in which a noble gas is mixed with halogen and high-current pulses are used to generate excited dimers, hence the term excimer. The excited dimers consisting of 1 halogen molecule and 1 noble gas molecule are only linked in the excited state, thus allowing for more stability in the excited state and enabling a higher proportion of molecules to be in that state at any given time, which increases population inversion and thus helps to maximize the output energy.

Diode lasers employ the use of diodes, or semiconductors that allow current to flow in one direction but not the other (theoretically with infinite resistance in one direction and no resistance in the other direction), thus creating a downstream method to achieve a high-power laser output; however, despite its theoretical efficiency, the use of diode lasers has been limited due to practical considerations of the divergence and quality of the output.

Dye lasers consist of a liquid solution of organic dye in a solvent that is pumped by an optical source. While gas discharge lasers involve excitation of a gas, there is a clear corollary with dye lasers with liquid taking the place of the gas; however, this modality has certain limitations, including the use of toxic materials that degrade naturally; the need to switch cuvettes when changing gain media, which serve as the lasing medium; and a relatively low-power output. One benefit of the dye laser, as alluded to above, is the operator’s capability to switch out cuvettes containing different dyes, thus using one machine to generate widely varying laser beams.

Solid-state lasers are most often used in dermatology. These devices utilize a conducting medium (eg, garnet, sapphire, ruby) doped with trivalent rare-earth ions or transition metal ions (eg, neodymium, ytterbium, erbium, titanium, chromium). This process is a relatively reliable and flexible methodology for generating stable lasers, thus explaining its widespread use. Additionally, these solid-state lasers are particularly amenable to modifications (eg, Q-switching).

Considerations for Lasers

Quality switching (known as Q-switching) is a method used to generate a shorter burst of a higher-power laser output.¹⁰ The longer the electrons have to become excited within a resonant cavity, the higher the number that may end up in an excited state, thus allowing for a higher ultimate energy output to a certain point. The quality of a medium, in general, refers to the ability of light exiting a medium to return. Within a cavity, the ability of light to go back and forth through the lasing medium is critical in achieving stimulated emission and thus laser beam output; however, in a low-quality medium, population inversion can still occur to allow a greater proportion of electrons to reside in a higher energy state. There are multiple mechanisms to switch the quality of a medium, but the ultimate result always is for the quality to be switched to high so that the light beams can immediately start achieving stimulated emission of a “primed” population of high-energy, population-inverted electrons, resulting in a much higher output power.