“Laser” is an acronym that stands for light amplification by stimulated emission of radiation. This concept was first described by Albert Einstein in 1917. The first successful laser, which was developed in 1954, used ammonia gas as the medium and emitted microwaves (MASER). In 1960, the first laser to emit visible light utilized a ruby medium and had applications in retinal surgery. Over the past 40 years there has been an explosion in modern medical laser technology. Lasers have gone from being mere adjuncts to treatment, to viable alternatives, and in certain cases, the standard of therapy.

All lasers are comprised of three components; the lasing medium, excitation source, and two mirrors (one fully reflective and one partially reflective). Laser light is unique because it is monochromatic (all one wavelength), coherent (all in phase), and collimated (all one direction), and has high power density. This is in contradistinction to ambient light, which is polychromatic and incoherent. Lasers are usually named by their lasing medium, which can be gas, liquid, or solid. Laser light can be delivered as an unadulterated beam, or via fiberoptic cables. The benefit of fiberoptic cables is that they are small, can be easily modified, and manipulated into hard-to-reach cavities. However, laser light passing through fiberoptic cables loses some of its coherence and collimation, due to total internal reflection within the cable.

As laser light comes into contact with a target tissue, four interactions are possible: transmission, reflection, scatter, and absorption. Absorption of laser energy in human tissue occurs with three main chromophores: hemoglobin, water, and melanin. Laser-tissue interaction is based on the concept of selective photothermolysis. The skin and other tissues contain many heterogeneous pigments and structures, each of which most effectively absorbs laser energy at certain wavelengths. This absorbed energy is then converted to heat and thermal damage to target tissues. For example, the highest absorption for blood is at 420nm, whereas water has several absorption peaks ranging from 950nm to 10um. Thus, depending on the target tissue, a specific laser can be chosen to selectively affect that tissue while minimizing damage to the surrounding tissue. Lasers can be used to vaporize, coagulate, or incise. They generally cause less inflammation than steel, which can lead to less scarring and wound contracture, and possibly less pain. However, wound healing is often slower than with scalpel incisions, which can contribute to a higher incidence of delayed postoperative complications such as bleeding and wound breakdown.

Different lasing media produce different wavelengths of light ranging from the infrared to the ultraviolet (Table 39-1). Various
lasers, their wavelengths, major chromophores, and common uses are listed in Table 39-1.