The word laser is an acronym for l ight a mplification by s timulated e mission of r adiation. In this chapter, brief descriptions of these five terms, within the context of the unique qualities of a laser instrument, are presented as background for a subsequent overview of the uses of lasers in dentistry.

Light
Light is a form of electromagnetic energy that exists as a particle and that travels in waves at a constant velocity. The basic unit of this radiant energy is called a photon.
The waves of photons travel at the speed of light and can be defined by two basic properties: amplitude and wavelength ( Figure 2-1 ). Amplitude is defined as the vertical height of the wave from the zero axis to its peak as it moves around that axis. This correlates with the amount of intensity in the wave: The larger the amplitude, the greater the amount of potential work that could be performed. For a sound wave, amplitude correlates with loudness.

The second property of a wave is wavelength (λ), the horizontal distance between any two corresponding points on the wave. This measurement is important to both how the laser light is delivered to the surgical site and how it reacts with tissue. Wavelength is measured in meters (m). Dental lasers have wavelengths on the order of much smaller units, using terminology of either nanometer (nm), equal to one billionth (10 ⁻⁹ ) of a meter, or micrometer (μm), one millionth (10 ⁻⁶ ) of a meter (replaces the micron [μ] unit, still occasionally seen in laser science).

As waves travel, they rise and fall about the zero axis a certain number of times per second; this is called oscillation. The number of oscillations per unit time is defined as frequency. Frequency is measured in hertz (Hz); 1 Hz equals one oscillation per second. Frequency is inversely proportional to wavelength: The shorter the wavelength, the higher the frequency, and vice versa. Although the hertz as just defined is a basic unit in physics, it also is used more specifically to describe the number of pulses per second of emitted laser energy.

Ordinary light, such as that produced by a table lamp, usually is a warm, white color. “White” as seen by the human eye is really the sum of the many colors of the visible spectrum: red, orange, yellow, green, blue, and violet. The light usually is diffuse—that is, not focused. Laser light is distinguished from ordinary light by two properties. Laser light is monochromatic : It is generated as a beam of a single color, which is invisible if its wavelength is outside of the visible part of the spectrum. In addition, the waves of laser light are coherent , or identical in physical size and shape.dental laser  Thus the amplitude and frequency of all of the waves of photons are identical. This coherence results in the production of a specific form of focused electromagnetic energy.

The beams emitted from laser instruments are collimated (produced with all waves parallel to each other) over a long distance, but once the laser beam enters certain delivery systems such as optical fibers or tips (e.g., in neodymium-doped yttrium-aluminum-garnet [Nd:YAG], erbium, and diode lasers), it diverges at the fiber tip. This monochromatic, coherent beam of light energy can be used to accomplish the treatment objective.

Using a household fixture as an example, a 100-watt (W) lamp will produce a moderate amount of light for a room area, with some heat. On the other hand, 2 W of laser light can be used for precise excision of a fibroma while providing adequate hemostasis at the surgical site, without disturbing the surrounding tissue. The difference between the 100 W of an ordinary light bulb able to light up a room and the 2 W of a laser able to perform a surgical procedure lies in the property of coherence. As an apt analogy, imagine a crew race on a river. The boat that comes in first is the boat in which all of the members of the crew team are working together. At any given moment, they are all at the same stage of the stroke cycle, so that all of their energies are working together to propel the boat. All of the members of the crew team place their oars in the water at the same instant. They all remove their oars from the water at the same instant. They are working together in perfect unison. In similar fashion, all of the light waves in a laser work together in a beam of coherent energy. By contrast, in the boat that comes in last, the crew members may be seen to be at different stages of the stroke cycle. Some have their oars going into the water, and some have their oars coming out of the water; some are at the top of the stroke cycle, and some are at the bottom of the stroke cycle. The team members are not working together as one. The work expended by this disorganized crew, which cannot propel their boat forward with any effective speed, would be analogous to the energy from an ordinary light bulb, which is insufficient for excision of soft tissue.

Radiation

The light waves produced by the laser are a specific form of electromagnetic energy.

The electromagnetic spectrum is the entire collection of wave energy, ranging from gamma rays, with wavelengths of 1 x 10 ⁻¹² m, to radio waves, with wavelengths of thousands of meters. All currently available dental laser devices have emission wavelengths of approximately 500 to 10,600 nm, which places them in either the visible or the invisible (infrared) nonionizing portion of the electromagnetic spectrum . Of note, the dividing line between the ionizing, cellular DNA–mutagenic portion of the spectrum and the nonionizing portion is at the junction of ultraviolet and visible-violet light. Thus all current dental lasers emit either a visible-light wavelength or an invisible, infrared-light wavelength in the portion of the nonionizing spectrum called thermal radiation. The word radiation in this context does not imply radioactive or carcinogenic but simply means the emission of electromagnetic energy .

Laser Delivery Systems

Laser energy should be delivered to the surgical site by a method that is ergonomic and precise.

Shorter-wavelength instruments, such as KTP, diode, and Nd:Y-AG lasers, have small, flexible fiberoptic systems with bare glass fibers that deliver the laser energy to the target tissue. Because the erbium and CO ₂ laser wavelengths are absorbed by water, which is a major component of conventional glass fibers, these wavelengths cannot pass through these fibers. Erbium and CO ₂ devices are therefore constructed with special fibers capable of transmitting the wavelengths, with semiflexible hollow waveguides, or with articulated arms . Some of these systems employ small quartz or sapphire tips that attach to the laser device for contact with target tissue; others employ noncontact tips . In addition, the erbium lasers incorporate a water spray for cooling hard tissues. Lasers may have different fiber diameters, handpieces, and tips. Each of these elements plays a significant role in the delivery of energy .

The fourth tissue interaction is scattering of the laser light, which weakens the intended energy. Scattering is the predominant event with use of the near-infrared lasers in healthy soft tissue. Scattering causes the photons to change directions, leading to increased absorption, with correspondingly increased chances of interacting with the predominant chromophore of those wavelengths. Scattering of the laser beam also could cause heat transfer to the tissue adjacent to the surgical site, with the potential for injury from unwanted laser effects. However, a beam that is scattered, or deflected in different directions, would be useful in facilitating laser curing of composite resin.

Laser emission modes play an important role in increasing the tissue temperature. The important principle of any laser emission mode is that the light energy strikes the tissue for a certain length of time, producing a thermal interaction. If the laser is used in a pulsed mode, the targeted tissue may have time to cool before the next pulse of laser energy is emitted. In continuous-wave mode, the operator must cease the laser emission manually so that thermal relaxation of the tissue may occur. Thin or fragile soft tissue, for example, should be treated in a pulsed mode so that the amount and rate of tissue removal are slower, but the chance of irreversible thermal damage to the target tissue and the adjacent nontarget tissue is minimal. Longer intervals between pulses also can help avoid the transfer of heat to the surrounding tissue. In addition, a gentle air stream or an air current from the high-volume suction will aid in keeping the area cooler. Similarly, with use of hard tissue lasers, a water spray will help prevent microfracturing of the crystalline structures and reduce the possibility of carbonization. Conversely, thick, dense, fibrous tissue requires more energy for removal. For the same reason, dental enamel, with its higher mineral content, requires more ablation power than softer, more aqueous carious tissue. In either case, if too much thermal energy is used, healing can be delayed, and postoperative discomfort may be greater than normal.

Conclusions

The appropriate and effective use of dental lasers in clinical practice requires a firm grasp of their scientific basis and tissue effects, proper training in techniques and applications, and sufficient operative experience to achieve and maintain proficiency. The clinician can then choose the proper laser(s) for the intended application. Although the types of tissue interaction overlap somewhat, each wavelength has specific properties that can be used to accomplish a specific treatment objective. Laser energy requires some procedures to be performed much differently from those using conventional instrumentation, but the indications for laser use continue to expand, with further benefit for patient care.