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.¹ 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 to loudness. For a wave emitting light, amplitude correlates to brightness. A joule (J) is a unit of energy; a useful quantity for dentistry is a millijoule (mJ), or one thousandth (10⁻³) of a joule ( J; 0.001 J).

FIGURE 2-1 • Properties of electromagnetic waves. Amplitude is the height of the wave from the zero axis to the peak. Wavelength is the horizontal distance between two adjacent parts of a wave.

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 a nanometer (nm), one billionth (10⁻⁹) of a meter, or micrometer (also micron, [µ or µm], one millionth (10⁻⁶) of a meter.

As waves travel, they rotate around 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 hertz is a term commonly found in physics, it is also used to describe the number of pulses per second of emitted laser energy.

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

The laser beams emitted from some instruments are collimated (all waves parallel to each other) over a long distance, but beams produced from optical fibers (e.g., in Nd:YAG and diode lasers) usually diverge at the fiber tip. All can be precisely focused, and this monochromatic, coherent beam of light energy can be utilized to accomplish the treatment objective.

Using a household fixture as an example, a 100-watt lamp will produce a moderate amount of light for a room area, with some heat. On the other hand, two watts (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.²

Amplification

Amplification is the part of this process that occurs inside the laser. Identifying the components of a laser instrument shows how laser light is produced. The center of the laser is called the laser cavity. The following three components make up the laser cavity:

Active medium

Pumping mechanism

Optical resonator

The active medium is composed of chemical elements, molecules, or compounds. Lasers are generically named for the material of the active medium, which can be (1) a container of gas, such as a canister of carbon dioxide (CO₂) gas in a CO₂ laser; (2) a solid crystal, such as a crystal of yttrium, aluminum, and garnet (YAG) in an erbium (Er) YAG or a neodymium (Nd) YAG laser; (3) a solid-state semiconductor, such as the semiconductors found in diode lasers; or (4) a liquid, such as found in some medical laser devices.

Surrounding this active medium is an excitation source, such as a flashlamp strobe device, electrical circuit, electrical coil, or similar source of energy that pumps energy into the active medium. When this pumping mechanism pumps energy into the active medium, the energy is absorbed by the electrons in the outermost shell of the active medium’s atoms. These electrons have absorbed a specific amount of energy to reach the next shell farther from the nucleus, which is at a higher energy level. A “population inversion” occurs when more of the electrons from the active medium are in the higher energy level shell farther from the nucleus than are in the ground state (Figure 2-2). The electrons in this excited state then spontaneously give off that energy in the form of a photon (Figure 2-3). This is called spontaneous emission (Figure 2-4).

FIGURE 2-2 • An atom of an active medium in ground state.

FIGURE 2-3 • An atom of an active medium in excited state.

FIGURE 2-4 • An atom of an active medium spontaneously emits a photon and returns to a stable orbit, giving off the energy that it had just absorbed, according to the principle of Conservation of Energy.

Completing the laser cavity are two mirrors, one at each end of the optical cavity, placed parallel to each other; or in the case of a semiconductor, two polished surfaces at each end. These mirrors or polished surfaces act as optical resonators, reflecting the waves back and forth, and help to collimate and amplify the developing beam. A cooling system, focusing lenses, and other controlling mechanisms complete the mechanical components. Figure 2-5 shows a schematic of a gas or solid active-medium laser (e.g., CO₂, Nd:YAG). Figure 2-6 shows a schematic of a semiconductor diode device.

FIGURE 2-5 • Illustration of a gas or solid, active-medium laser, such as a carbon dioxide (CO₂) or neodymium:yttrium-aluminum-garnet (Nd:YAG) laser.

FIGURE 2-6 • Semiconductor diode device.

Stimulated Emission

Stimulated emission is the process by which laser beams are produced inside the laser cavity. The theory of stimulated emission was postulated by Albert Einstein in 1916.³ He based his work on some earlier work by physicists from Germany (Max Planck) and Denmark (Niels Bohr), who theorized a model of the atom as well as the quantum theory of physics, which described a quantum as the smallest unit of energy emitted from an atom.^4,5 Einstein used this concept and further theorized that an additional quantum of energy may be absorbed by the already-energized atom and would result in a release of two quanta (Figure 2-7). This energy is emitted, or radiated, as identical photons, traveling as a coherent wave. These photons in turn are then able to energize more atoms in a geometric progression, which further causes the emission of additional identical photons, resulting in an amplification of the light energy, thus producing a laser beam (Figure 2-8).

FIGURE 2-7 • An atom of an active medium showing stimulated emission, releasing two identical photons before returning to a stable state.

FIGURE 2-8 • Light amplification by stimulated emission of radiation.

Radiation

The light waves produced by the laser are a specific form of radiation, or electromagnetic energy.⁶ The electromagnetic spectrum is the entire collection of wave energy, ranging from gamma rays, with wavelengths of 10 to 10⁻¹² m, to radio waves, with wavelengths of thousands of meters. All currently available dental laser devices have emission wavelengths of approximately 0.5 µ, or 500 nm, to 10.6 µ, or 10,600 nm, which places them in either the visible or the invisible nonionizing portion of the electromagnetic spectrum, as shown in Figure 2-9. It is important to note that 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 dental lasers emit either a visible-light wavelength or an invisible, infrared-light wavelength in the portion of the nonionizing spectrum called thermal radiation.⁷

FIGURE 2-9 • Part of electromagnetic spectrum showing division of ionizing, visible portion and nonionizing portion.

The following four dental laser instruments emit visible light:

Argon laser: blue wavelength of 488 nm

Argon laser: blue-green wavelength of 514 nm

Frequency-doubled Nd-doped YAG laser, also called a potassium titanyl phosphate (KTP) laser: green wavelength of 532 nm

Low-level lasers: red nonsurgical wavelengths of 635 nm (for photobiomodulation) and 655 nm (for caries detection)

Argon lasers are no longer manufactured as dental surgical instruments, although they are still used for medical procedures.

Other dental lasers emit invisible laser light in the near, middle, and far infrared portion of the electromagnetic spectrum. These include photobiomodulation devices between 800 and 900 nm, as well as surgical instruments, as follows:

Diode lasers: various wavelengths between 800 and 1064 nm using a semiconductor active medium of gallium and arsenide; some devices add either aluminum or indium.

Nd:YAG laser: 1064 nm

Erbium-chromium–doped yttrium-scandium-gallium-garnet (Er,Cr:YSGG) laser: 2780 nm

Er:YAG laser: 2940 nm

CO₂ laser: 10,600 nm

Laser Delivery Systems

Laser energy should be delivered to the surgical site by a method that is ergonomic and precise.⁸ Shorter-wavelength instruments (e.g., KTP, diode, Nd:YAG) have small, flexible fiberoptic systems with bare glass fibers that deliver the laser energy to the target tissue (Figure 2-10). Because the erbium and CO₂ laser wavelengths are absorbed by water, which is a major component of conventional glass fibers, these wavelengths cannot pass through these fibers. Erbium and CO₂ devices therefore are constructed with special fibers capable of transmitting the wavelengths, with semiflexible hollow waveguides, or with articulated arms (Figure 2-11). 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 (Figure 2-12). In addition, the erbium lasers employ a water spray for cooling hard tissues. Lasers may have different fiber diameters, handpieces, and tips (Figure 2-13). The diameter of the fiber, handpiece, and tip plays a significant role in the delivery of energy (Figure 2-14).

FIGURE 2-10 • An assembled fiberoptic delivery system consisting of the bare fiber, a handpiece, and a disposable tip.

FIGURE 2-11 • Articulated-arm delivery system, typical of CO₂ lasers and some erbium devices.

FIGURE 2-12 • Quartz tip assembled onto the end of a handpiece for a fiber optically delivered erbium laser.

FIGURE 2-13 • Variety of handpieces available with most CO₂ laser systems offer a variety of spot sizes and focal distances.

FIGURE 2-14 • Fiber diameters for Nd:YAG and diode lasers, yielding different spot sizes.

All conventional dental instrumentation, either hand or rotary, must physically touch the tissue being treated, giving the operator instant feedback. As mentioned, dental lasers can be used either in contact or out of contact. The fiber tip can easily be inserted into a periodontal pocket to remove small amounts of granulomatous tissue (Figures 2-15 to 2-17). In noncontact use, the beam is aimed at the target some distance away (Figure 2-18). This modality is useful for following various tissue contours, but with the loss of tactile sensation, the surgeon must pay close attention to the tissue interaction with the laser energy. All the invisible-light dental lasers (Nd:YAG, CO₂, diode, erbium) are equipped with separate aiming beams, which can be either a laser or a conventional light. The aiming beam is delivered coaxially along the fiber or waveguide and shows the operator the exact spot where the laser energy will strike the tissue.

FIGURE 2-15 • CO₂ periodontal tip treating periodontal pocket.

FIGURE 2-16 • Nd:YAG fiber entering periodontal pocket.

FIGURE 2-18 • Nd:YAG fiber out of contact treating aphthous ulcer.

FIGURE 2-17 • Nd:YAG laser fiber contacting aphthous ulcer.

Spot Size

The active beam is focused by lenses. With hollow-waveguide or articulated-arm delivery systems, there is a precise spot at the point where the energy is the greatest. This focal point is used for incision and excision surgery. For fiberoptic delivery systems, the focal point is at or near the tip of the fiber, which again has the greatest energy. For CO₂ lasers, which are used out of contact, the focal point may be anywhere from 1 mm to 12 mm from the tissue surface, depending on the handpiece being used (Figure 2-19). When the handpiece is moved away from the tissue and away from the focal point, the beam is defocused (out of focus), becomes more divergent, and therefore delivers less energy to the surgical site (Figure 2-20). At a small divergent distance, the beam can cover a wider area, which would be useful in achieving hemostasis. At a greater distance, the beam will lose its effectiveness because the energy will dissipate.

FIGURE 2-19 • A handpiece at the correct distance from the tissue is “in focus.”

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