eMedicine Specialties > Otolaryngology and Facial Plastic Surgery > Cosmetic Surgery
Lasers, General Principles and Physics
Updated: Feb 13, 2008
Introduction
The word laser is an acronym derived from the phrase “light amplification by stimulated emission of radiation.” A laser-equipped device can generate a high-intensity light that is monochromatic, unidirectional, and parallel. These unique characteristics make the laser useful for commercial and medical applications. An understanding of the general properties of lasers allows the physician and layperson to better appreciate the technology and its capabilities and limitations.
History
The theoretic principles behind the laser were developed as early as 1917, when Einstein laid the groundwork for stimulated emission in his treatise "On the Quantum Theory of Radiation."1 In 1955, Gordon produced the maser, the microwave predecessor of the laser.2 In 1958, Schwalow and Townes of Bell Laboratories described the physical operating principles of the laser, which Maiman applied in 1960 to produce the first operating laser—a red beam generated by the excitation of a ruby crystal with intense pulses of light from a flash lamp.3,4 The ruby laser became the first medical laser when it was used in 1963 to coagulate retinal lesions.
Other substances were ultimately found that could serve as the active medium of a laser device. Many substrates, especially rare earth elements such as erbium (Er), gadolinium (Gd), holmium (Ho), praseodymium (Pr), thulium (Tm), uranium (Ur), and ytterbium (Yb), have been used successfully.
In 1961, a laser generated from crystals of yttrium-aluminum-garnet treated with 1-3% neodymium (Nd:YAG) was developed. This laser emitted energy in the near infrared (IR) spectrum at a wavelength of 1060 nm. Researchers found its high-penetration emission to be useful for vaporizing tissues and thermally coagulating large blood vessels (<3 mm). Today, the Nd:YAG laser is mainly used to ablate tattoos and tumors of the genitourinary and gastrointestinal tracts, although it has many other uses, including ophthalmic surgery (eg, peripheral iridectomies/iridotomies, postcataract capsulotomies) and hair removal.
The first gas laser, which was also developed in 1961, used a mixture of helium and neon. Although its beam was not powerful enough to trigger a thermal reaction, its red color allowed it to be used as an aiming beam for invisible lasers such as the carbon dioxide (CO2) laser. Currently, it is marketed in Europe and Asia as a biostimulator to relieve pain and accelerate wound healing.
In 1962, the argon laser was developed. This laser emitted energy in the blue-green portion of the electromagnetic spectrum. Energy with wavelengths in this part of the spectrum is more readily absorbed by 2 naturally occurring chromophores (absorbing pigments) in the human body—melanin and hemoglobin—than by the surrounding tissue. Today, the argon laser is used to photocoagulate (ie, thermally obliterate without vaporization) blood vessels in the treatment of diabetic retinopathy and port-wine stains.
In 1964, Patel at Bell Laboratories developed the CO2 laser. It emitted spectral energy in the far IR portion of the electromagnetic spectrum at 10,600 nm. At this wavelength, energy is heavily absorbed by water, which is the primary constituent and chromophore of cells in living tissue. Thus, the energy generated by this laser can be used for cutting or volume ablation by means of tissue vaporization. This unique characteristic makes the CO2 laser the most widely used medical laser today.
Physics
Atoms at their resting energy state, or ground state (E0), can be excited to a higher energy state (E*) when they absorb electrical, optical, or thermal energy (see Image 1). At the E* level, atoms are unstable and spontaneously return to their E0 ground state, which liberates the absorbed energy as light or photons. This process is referred to as spontaneous emission of radiation (see Image 2).
If, on their brief descent from E* to E0, the excited atoms or molecules at E* are further bombarded with the same energy that caused the initial transition from E0 to E* or a proportional amount, the net result is the liberation of an amount of energy twice the original (see Image 3). Thus, if a photon strikes an atom at E0, causing it to go to its E* level, and if a second photon strikes the atom as it returns to E0, the atom emits 2 photons of the same frequency.
This emission occurs in phase (coherence) with and in the same direction as the first bombarding photon. This process is called stimulated emission. The 2 emitted photons may then each strike other excited atoms, further stimulating emission of photons with the same phase and frequency. As more atoms are excited to the upper energy level E* to the extent that the number of atoms in the active medium at E* is greater than those at E0, a population inversion occurs in the system. This chain reaction rapidly produces a powerful eruption of a coherent beam of radiation—a laser.
Basic Components
The basic laser device consists of 3 components: (1) an active medium, or lasing medium; (2) an optical cavity, or resonator; and (3) an energizing source, or pump. The active medium in lasers may be a solid, liquid, or gas. Different active media emit different energies or wavelengths of light. However, they all operate with the same basic principles.
The resonator contains an active medium. At each end of the resonator, parallel reflectors or mirrors are placed facing each other. The front of the output mirror is designed to be partially reflective. It reflects only a portion of the light impinging on it, allowing some portion of the total energy or light to escape. The rear mirror is a total reflector that reflects 100% of the energy impinging on it. The pump source provides the energy (thermal, electric, or optical [eg, a flash lamp]) for absorption by the active medium.
When the active medium is pumped with sufficient energy, a population inversion occurs, causing the spontaneous emission of photons. Some of these photons are reflected back and forth between the 2 mirrors (others are dissipated as heat) and then collide with atoms in the excited state; these collisions subsequently stimulate the emission of radiation. As other photons collide with excited atoms, energy within the resonator builds and is amplified by reflections between the parallel mirrors. At the front output mirror, a portion of the energy is permitted to escape. This energy is in the form of an intense beam of monochromatic (same wavelength), collimated (parallel, nondiverging), and coherent (same direction) light (see Image 4).
Laser Properties
Power density
When the laser beam exits the resonator, its diameter is often too large and diffuse, and the beam itself may have inadequate power to be useful. Therefore, the laser beam is passed through a focusing lens to reduce its diameter, which increases its intensity and energy so that it is of more suitable size for manipulation and practicality. Its intensity, referred to as its power density (Pd) or irradiance (E), is defined as the energy delivered per unit area of incident tissue. It is measured in terms of wattage of laser per diameter of the beam. That is, Pd varies inversely with the square of the diameter of the laser beam, as follows:
Pd = (100 W)/d 2, where W is the laser power in watts, and d is the diameter of the laser beam in centimeters (ie, 100W/cm2).
For a given wattage, a wide or unfocused beam has less penetration ability and is more useful for procedures such as skin resurfacing, vaporization of tissue, and coagulation of blood vessels. A focused beam penetrates to a greater depth and is more useful in procedures involving delicate cutting and volume ablation.
Fluence
To accurately determine the total amount of energy delivered to the tissue by the laser, the duration of exposure is vital. Prolonged exposures result in tissue destruction, and too short an exposure results in an inadequate effect. The dose, or fluence, is a measure of the total energy. It is determined by multiplying Pd by the exposure time (t) and is expressed in terms of energy per unit area of incident tissue, as follows:
Fluence = Pd(t) = J/A, where J is the energy in joules and A is the cross-sectional area of beam in square centimeters.
Wavelength
The effect of light on skin depends on the wavelength of the light. Light in the UV region (100-400 nm), which is invisible to the human eye, is known to cause deleterious effects such as erythema, hyperpigmentation, and cutaneous carcinoma. Light energy in the visible spectrum (380-700 nm) is mostly innocuous, but it can be absorbed and cause thermal damage when it is delivered to the skin at a high intensity. Light in the near IR region of the spectrum (780-3000 nm), which is also invisible to the human eye, causes skin and retinal defects. In general, the effects of light in the mid-to-far IR region of the spectrum (3-1000 µm) are limited to the superficial layers.
The degree of absorption and its thermal effect on skin vary with the amount and type of chromophores that are present in the recipient. As stated earlier, hemoglobin and melanin are natural endogenous chromophores. An example of an exogenous chromophore is tattoo ink. Different chromophores have different absorption coefficients. The absorption coefficient is a measure of the degree of absorption by the chromophores at a particular wavelength. Because the laser is monochromatic and because it has a very narrow bandwidth, it permits selective targeting of chromophores in the tissue for treatment. This property is one of the underlying principles of selective photothermolysis (SP).
Selective photothermolysis
The aforementioned laser parameters—power density, fluence, and wavelength—are the fundamental principles in the operation of medical lasers in the concept known as SP. Anderson and Parrish described SP in 1983, when they outlined the essential factors necessary for discrete laser-induced tissue damage to occur. SP is a method for localizing tissue damage to specific chromophore targets at the cellular level; therefore, it can be used to minimize undesired thermal damage to the surrounding tissue caused by thermal diffusion.
The rate of thermal diffusion of a given tissue is known as the thermal relaxation time (T R) and is defined as the time required for a given heated tissue to lose 50% of its heat through diffusion. It is measured in terms of the area affected and the thermal diffusivity (D) of the target tissue, as follows:
T R = r 2/4 D, where r is the radius of target tissue.
Therefore, significant thermal diffusion (and hence thermal damage) is minimized if the duration of the laser pulse is shorter than the TR of the target tissue.
For example, water (the primary constituent by weight of living cells) has a high absorption coefficient of 230 cm-1 at 10,600 nm, the wavelength emission of a CO2 laser, and a T R of 326 µs. With these properties, if a CO2 laser contacts the skin for less than 326 µs, most of the radiation is absorbed by the water in the targeted skin, with almost no thermal diffusion. However, if the duration of the laser impingement on the tissue is longer than 326 µs, heat is transmitted to the surrounding nontargeted tissue and results in undesirable thermal injury.
Therefore, for proper SP to occur, the target tissue (through its chromophores) must possess greater optical absorption than the nontargeted surrounding tissue does, and the laser of choice must have a pulse duration shorter than the T R of the target tissue. Because soft tissues in humans generally have a T R of less than 1 ms, the laser pulse must be extremely short and high-powered to be medically beneficial and minimally destructive. Because many types of lasers exist, selection is crucial and must be tailored to the specific procedure.
Modes: Continuous Wave, Pulsed, and Q Switching
The light generated with a laser, in general, can be delivered in 2 ways: as a constant flow of energy (continuous-wave [CW] laser or as multiple discrete pulses (pulsed laser). The 2 types of lasers are fundamentally different in design, light delivery, and operation.
A CW laser is generated by continuously pumping energy into the active medium to achieve an equilibrium between the number of atoms raised to the excited state and the number of photons emitted. At such an equilibrium, continuous laser output results. The duration of a CW laser pulse is approximately 0.25 s. With this duration and with relatively constant power delivery to tissues, significant thermal damage occurs. To minimize their destructive effects, CW lasers have been modified to emit beams in a pulsatile fashion by adding electronically controlled, mechanically gated, timed shutters to interrupt the output beam at preset intervals. This system is not a true pulsed laser per se because the laser beam is physically chopped off to produce the pulse effect.
Pulsed lasers, in contrast, deliver high-energy beams in very short pulses in the range of milliseconds without the use of a shutter. Emissions are produced when the pump is modulated to create discrete laser pulses, which usually are broad and randomly shaped.
Both types of lasers can be further modified to produce even shorter pulses, usually in the range of 10-250 ns, by using a method referred to as Q switching. With this technique, a large population inversion builds before emission is stimulated. This population inversion is accomplished by using a mechanical opaque shutter or by inserting a high-speed, electrically sensitive, polarizable optical shutter known as a Pockel cell between the 2 mirrors of the laser.
Because this shutter effectively blocks the photons' path between the 2 mirrors and prevents resonation, stimulated emission does not occur. When the population inversion reaches its maximal level, the opaque mechanical shutter opens. When Pockel cells are used, an electrical pulse is applied to the cell that changes it from opaque to transparent, allowing the photons to reflect back and forth and subsequently generate an intense beam.
Since their initial use to coagulate ophthalmic lesions in 1963, laser-equipped devices are now used in almost every specialty of medicine. They have revolutionized the science of medicine and have become valuable and indispensable medical tools. With the rapid pace of technologic advances, other novel applications are likely to be discovered.
Clinical and Diagnostic Applications
Fields of Practice
Laser technology is widely used in medical research, diagnostics, and treatment. The clinical efficacy of laser therapy is well known, and lasers are used in many fields and settings.
Dentistry
Lasers have a wide range of applications in various fields of dentistry, especially when conventional treatments are not effective. In endodontics, laser technology is being considered for dentin structure modification, cleaning and shaping of the root canal system, pulp diagnosis, and endodontic surgery. Lasers such as the Nd:YAG, CO2, and semiconductor Diode lasers have been used in soft tissue treatment of the oral cavity. In periodontics, the Er:YAG laser has potential for clinical application in hard tissue treatment due to its ability to cut or contour bone with minimal damage and fast healing.5 In orthodontics, lasers have been used in laser surface scanning of craniofacial anomalies.
Dermatology
Advances in laser technology have provided dermatologists with more choices and have contributed to improved clinical results.6 Lasers are widely used in the treatment of dermatologic conditions, including acne vulgaris, pseudofolliculitis barbae, and vascular and pigmented lesions.7,8 They are also used for removal of unwanted hair (by the Nd:YAG laser), tattoos, and scars.9 Diagnostically, they are used in the laser-capture microdissection technique, which provides faster identification of infectious agents than standard diagnostic techniques based on the polymerase chain reaction (PCR).
Oncology
Lasers are used in photodynamic therapy of patients with both malignant tumors and non-tumoral illnesses.10 Photodynamic therapy involves the absorption of a photosensitizing agent (which interacts with visible light), retention of the agent in tumor tissue, and selective laser irradiation of tumor tissues, which were previously sensitized by dyes. This technique plays a large role in preserving healthy organ issue. Lasers, particularly the laser-capture microdissection technique, are an important part of proteomic technology for cancer diagnosis and the development of markers for early detection.11 In oncologic surgery, CO2 lasers have been used for the surgical treatment of carcinomas of the oral cavity, pharynx, and larynx.
Ophthalmology
Laser energy sources, including the Nd:YAG and CO2 lasers, are playing an increasingly important role in ophthalmic surgery. The Nd:YAG, which can be used to power a quartz-laser scalpel, is particularly useful because of its high penetration of optical tissue and its hemostatic properties. Laser in situ keratomileusis (LASIK), is one of the most common laser surgical techniques. LASIK is used to correct myopia, hyperopia, and astigmatism. Its advantages include less pain and discomfort and decreased rehabilitation time. Its drawbacks, however rare, include infection of the stroma after surgery.12
Otolaryngology
Lasers have enhanced the precision and the ability to photocoagulate tissue in airway surgery, particularly in the pediatric population.13 Due to the increased risk of scarring by other types of lasers, the carbon dioxide laser is used most frequently for this type of procedure.
Plastic surgery
Lasers play an important role in skin rejuvenation. A recently developed combination of the Er:YAG and CO2 lasers has diminished some of the clinical differences between the results of skin resurfacing using these 2 lasers (such as effects on patients with different skin laxities, and differences in recovery periods).14 This new technique is still being investigated, but seems to have a promising future.
A recent development to skin rejuvenation and resurfacing is the fractionated laser modality. In this system, only a fraction of the treated skin surface is actually affected. The laser "hits" are evenly distributed throughout the treated surface, whereby each laser hit is surrounding by a microscopic zone of "un-lasered," unaffected tissue. The theory behind this modality is that it would allow a quicker healing of the treated surface and promote shorter downtime for patients.15Applications
Laser scanning cytometry
Lasers have an application in the analysis of cells and their compartments. Laser scanning cytometry (LSC) is a new technique that has a significant advantage over other cytometric methods because of its use of the minimal sample volume and its ability to connect fluorescence and morphologic data. Possible applications of this technique include tissue biopsy, cytogenetic profiling, and determining ploidy and immunophenotype.16,17,18
Laser tissue welding
Laser tissue welding is an alternative method for closure. This technique is becoming important in many surgical specialties, including urology, cardiothoracic surgery, plastic surgery, and neurosurgery.19 The method is used especially in instances when stapling or suturing is difficult. The introduction of additional technology, such as protein solders, chromophore enhancement, and temperature-controlled feedback systems, has aided the recognition of this technique in clinical medicine.
Low-level laser therapy
The low-intensity laser, with an energy of less than 10-100 W, has recently been scrutinized as a possible therapeutic tool.20 Although the evidence is conflicting, low-level laser therapy (LLLT) may be useful in cutaneous wound healing and pain relief. Although its use is not widespread in the United States, the low-intensity laser is widely used in other countries for the treatment of dermatologic, dental, neurologic, and chiropractic disorders.
Pain research
Laser pulses (from argon, CO2, Nd:YAG, and other lasers) have been used in experimental and clinical studies to induce pain in both humans and animals. Activation of skin nociceptors with heat provides research data regarding the mechanisms of injury and sensitization and the therapeutic effects of various interventions.21
Multimedia
 | Media file 1: Atoms at their resting energy state, or ground state (E0), can be excited to a higher energy state (E*) when they absorb electrical, optical, or thermal energy. |
 | Media file 2: At the E* level, atoms are unstable and spontaneously return to their E0 ground state; this process liberates the absorbed energy as light or photons and is referred to as spontaneous emission of radiation. |
 | Media file 3: If, on their brief descent from E* to E0, the excited atoms or molecules at E* are further bombarded with the same energy that caused the initial transition from E0 to E* or a proportional amount, the net result is the liberation of an amount of energy twice the original. |
|
This feature requires the newest version of Flash. You can download it here. | Media file 4: Schematic representation of a ruby laser (animation). |
Keywords
general principles and physics of lasers, laser, laser physics, principles, carbon dioxide laser, chromophore, population inversion, continuous wave, pulse laser, Q-switching, light amplification by stimulated emission of radiation, stimulated emission, CO2 laser, laser tissue welding, laser scanning cytometry, low-level laser therapy, Nd:YAG
More on Lasers, General Principles and Physics |
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Further Reading
Keywords
general principles and physics of lasers, laser, laser physics, principles, carbon dioxide laser, chromophore, population inversion, continuous wave, pulse laser, Q-switching, light amplification by stimulated emission of radiation, stimulated emission, CO2 laser, laser tissue welding, laser scanning cytometry, low-level laser therapy, Nd:YAG
