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Core Messages

— A significant understanding of lasers and light sources is required for optimal use of these technologies

— A basic understanding of laser physics is at the core of good laser treatments

— Laser safety and minimizing patient risk is at least as important as an understanding of laser physics.

History

What Is Light?

Light is a very complex system of radiant energy that is composed of waves and energy packets known as photons. It is arranged into the electromagnetic spectrum (EMS) according to the length of those waves. The distance between two successive troughs or crests of these waves, measured in meters, determines the wavelength. For the visible portion of the EMS, the wavelength determines the color of the laser light. The number of wave crests (or troughs) that pass a given point in a second determines the frequency for each source of EMS energy. The wavelength and frequency of light are inversely related to one another. Thus, shorter wavelengths of light have higher frequencies and more energetic photons than longer wavelengths of light which have lower frequencies and less energetic photons.

■ When Was Light First Used for Medical Purposes?

One must go back to about 4000 B.C. in ancient Egypt to find the earliest recorded use of light. It was at that time that sunlight coupled with a topical photosensitizer, like parsley or other herbs containing psoralen, to help repigment individuals suffering from vitiligo, where the skin becomes depigmented through a presumed autoimmune reaction. In Europe in the 19th century, sunlight was used as a treatment for cutaneous tuberculosis. However, it wasn't until 1961 that Dr. Leon Goldman, a dermatologist at the University of Cincinnati, first employed a ruby laser for the removal of tattoos and other pigmented cutaneous lesions. For his continuous efforts in promoting the use of lasers for medical purposes and for co-founding the American Society for Laser Medicine and Surgery, Dr. Goldman (Goldman et al. 1963) has been called the "Father of Lasers in Medicine and Surgery." Since those earliest days, many physicians in different specialties have played key roles in the advancement of the use of lasers in medicine such that today most specialties use lasers in either diagnosing or treating a number of different disorders and diseases (Wheeland 1995).

■ Who Invented the Laser?

Professor Albert Einstein (Einstein 1917) published all of the necessary formulas and theoretical concepts to build a laser in his 1917 treatise called The Quantum Theory of Radiation. In this treatise, he described the interaction of atoms and molecules with electromagnetic energy in terms of the spontaneous absorption and emission of energy. By applying principles of thermodynamics he concluded that stimulated emission of energy was also possible. However, it wasn't until 1959 that Drs. Charles H. Townes and Arthur L. Schalow (Schalow and Townes 1958) developed the first instrument based on those concepts, known as the MASER (Microwave Amplification through the Stimulated Emission of Radiation). Then, in 1960, the first true laser, a ruby laser, was operated by Dr. Theodore H. Maiman (Maiman 1960). The development of additional lasers occurred rapidly, with the helium-neon laser appearing in 1961, the argon laser in 1962, the carbon dioxide and Nd:YAG laser in 1964, the dye laser in 1966, the excimer laser in 1975, the copper vapor laser in 1981, and the gold vapor laser in 1982.

What Is a Laser?

The word "LASER" is an acronym that stands for Light Amplification by the Stimulated Emission of Radiation. For this reason, a laser is not just an instrument but also a physical process of amplification (Table 1.1). The last word in the acronym, "radiation," is a common source of patient anxiety since it is associated with the high energy ionizing radiation often associated with cancer radiotherapy. However, in the case of lasers, the word is employed to describe how the laser light is propagated through space as "radiant" waves. Patients should be assured that all currently approved medical lasers are incapable of ionizing tissue and have none of the risks associated with the radiation used in cancer therapy.

All lasers are composed of the same four primary components. These include the laser medium (usually a solid, liquid, or gas), the optical cavity or resonator which surrounds the laser medium and contains the amplification process, the power supply or "pump" that excites the atoms and creates population inversion, and a delivery system (usually a fiber optic or articulating arm with mirrored joints) to precisely deliver the light to the target.

Lasers are usually named for the medium contained within their optical cavity (Table 1.2). The gas lasers consist of the argon, excimers, copper vapor, helium-neon, krypton, and car-

bon dioxide devices. One of the most common liquid lasers contains a fluid with rhodamine dye and is used in the pulsed dye laser. The solid lasers are represented by the ruby, neo-dymium:yttrium-aluminum-garnet (Nd:YAG), alexandrite, erbium, and diode lasers. All of these devices are used to clinically treat a wide variety of conditions and disorders based on their wavelength, nature of their pulse, and energy.

The excitation mechanism, i. e., power supply or "pump," is a necessary component of every laser in order to generate excited electrons and create population inversion (Arndt and Noe 1982). This can be accomplished by direct electrical current, optical stimulation by another laser (argon), radiofrequency excitation, white light from a flashlamp, or even (rarely) chemical reactions that either make or break chemical bonds to release energy, as in the hydrogen-fluoride laser.

To understand stand how laser light is created it is important to recall the structure of an atom. All atoms are composed of a central nucleus surrounded by electrons that occupy discrete energy levels or orbits around the nucleus and give the atom a stable configuration (Fig. 1.1). When an atom spontaneously absorbs a photon of light, the outer orbital electrons briefly move to a higher energy orbit, which is an unstable configuration (Fig. 1.2). This configuration is very evanescent and the atom quickly releases a photon of light spontaneously so the electrons can return to their normal, lower energy, but stable inner orbital configuration (Fig. 1.3). Under normal circumstances, this spontaneous absorption and release of light occurs in a disorganized and random fashion and results in the production of incoherent light.

When an external source of energy is supplied to a laser cavity containing the laser medium, usually in the form of electricity, light, microwaves, or even a chemical reaction, the resting atoms are stimulated to drive their electrons to unstable, higher energy, outer orbits. When more atoms exist in this unstable high energy configuration than in their usual resting configuration, a condition known as population inversion is created, which is necessary for the subsequent step in light amplification (Fig. 1.4).

Table 1.1. Laser terminology

Absorption

The transformation of radiant energy to another form of energy (usually heat) by interacting with matter

Coherence

All waves are in phase with one another in both time and space

Collimation

All waves are parallel to one another with little divergence or convergence

Electromagnetic radiation

A complex system of radiant energy composed of waves and energy bundles that is organized according to the length of the propagating wave

Energy

The product of power (watts) and pulse duration (seconds) which is expressed in joules

Extinction length

The thickness of a material necessary to absorb 98% of the incident energy

Focus

The exact point at which the laser energy is at peak power

Irradiance (power density)

The quotient of incident laser power on a unit surface area, expressed as watts/cm2

Joule

A unit of energy which equals one watt-second

Laser

An instrument that generates a beam of light of a single wavelength or color that is both highly collimated and coherent; an acronym that stands for light amplification by the stimulated emission of radiation

Laser medium

A material or substance of solid, liquid, or gaseous nature that is capable of producing laser light due to stimulated electron transition from an unstable high energy orbit to a lower one with release of collimated, coherent, monochromatic light

Meter

A unit length based on the spectrum of krypton-86; frequently subdivided into millimeters (io-3m), micrometers (io-6m), and nanometers (io-9m)

Monochromatic

Light energy emitted from a laser optical cavity of only a single wavelength

Optically pumped laser

A laser where electrons are excited by the absorption of light energy from an external source

Photoacoustic effect

The ability of Q-switched laser light to generate a rapidly moving wave within living tissue that destroys melanin pigment and tattoo ink particles

Population inversion

The state present within the laser optical cavity (resonator) where more atoms exist in unstable high energy levels than their normal resting energy levels

Power

The rate at which energy is emitted from a laser

Power density (irradiance)

The quotient of incident laser power on a unit surface area, expressed as watts/cm2

Pump

The electrical, optical, radiofrequency or chemical excitation that provides energy to the laser medium

Q-switch

An optical device (Pockels cell) that controls the storage or release of laser energy from a laser optical cavity

Reflectance

The ratio of incident power to absorbed power by a given medium

Scattering

Imprecise absorption of laser energy by a biologic system resulting in a diffuse effect on tissue

Selective photothermolysis

A concept used to localize thermal injury to a specific target based on its absorption characteristics, the wavelength of light used, the duration of the pulse, and the amount of energy delivered

Thermomodulation

The ability of low energy light to upregulate certain cellular biologic activities without producing an injury

Transmission

The passage of laser energy through a biologic tissue without producing any effect

Amplification of light occurs in the optical cavity or resonator of the laser. The resonator typically consists of an enclosed cavity that allows the emitted photons of light to reflect back and forth from one mirrored end of the chamber to the other many times until a sufficient intensity has been developed for complete amplification to occur. Through a complex process of absorption and emission of photons of energy, the prerequisite for the development of a laser beam of light has been met and amplification occurs. The photons are then allowed to escape through a small perforation in the partially reflective mirror. The emerging beam of light has three unique characteristics that allow it to be delivered to the appropriate target by fiber optics or an articulated arm.

■ What Are the Unique Characteristics of Laser Light?

By stimulating the emission of light from a laser, laser light has three unique characteristics that differentiate it from nonlaser light. The first of these characteristics is that laser light is

Table 1.2. Types of lasers

Fig. 1.1. Normal configuration of an atom with central nucleus and surrounding electrons in stable orbits

Table 1.2. Types of lasers

Name

Type

Wavelength

ArFl

Excimer

193 nm

KrCl

Excimer

222 nm

KrFl

Excimer

248 nm

XeCl

Excimer

308 nm

XeFl

Excimer

351 nm

Argon

Gas

488 and 514 nm

Copper vapor

Gas

511 and 578 nm

Krypton

Gas

521-530 nm

Frequency-

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