BASICS OF LASERS

Physicians treating glaucoma surgically with lasers benefit if they understand some of the properties of visible, near-visible, and invisible laser radiation in a vacuum, in air, and, especially, in tissue. This requires understanding several aspects of light:

•Radiation as a form of energy

•Laser sources of radiation

•How we evaluate energy, power, and duration

•Focusing and defocusing

•Light delivery from the laser to the target tissue

•The interaction of light with the tissue target

Laser safety is a related issue, and will be discussed at the close of this chapter.

RADIATION AND LIGHT ENERGY

Hold your hand near a glowing iron poker and you will feel the heat. This is your tangible indication of radiant transportation of energy from the metal, through the air, to your hand. Heating the iron atoms in the poker in a fire caused the outer shell electrons of the atoms to jump to a higher-energy orbit. After the poker was removed from the fire, some of these electrons started falling to their resting orbits, releasing the stored energy as electromagnetic radiation packets, called photons. Photons travel in all directions from their origin. They have both particlelike and wave-like energy properties, which account for absorption and refraction of radiation, respectively.

For all colors, the velocity of light waves in a medium is a constant characteristic of that medium. In a vacuum, it is about 3 * 108 meters>second. At all wavelengths, velocity (V) is equal to frequency (v) multiplied by the wavelength ( ).

V = v *

Max Planc showed in 1900 that the energy in a photon is proportionate to how fast it vibrates (i.e., its frequency). Light from the shorter-wavelength ultraviolet or blue end of the spectrum has a higher frequency of vibration, and more energy per photon, than the longer-wave- length red or infrared light. This differential photon energy is especially useful in laser surgery, as we shall see in the section on target tissue effects.

LASER SOURCES

When atoms or molecules are subjected to an external source of energy, a photon discharged from one excited atom can induce the release of another photon when it passes near a second excited atom. This occurs in some, but not all, transparent media, a good example of which is an yttrium-aluminum-garnet crystal, containing some (doped with) neodymium atoms. This process of induced photon release, called stimulated emission, was first postulated by Einstein in 1917. Both photons, arising from the same type of atom, will have the same color (wavelength), will vibrate in phase (coherence), and will travel in the same direction. This amplification doubles the light energy that was present before the interaction. It can repeat again and again, provided the media contains many excited atoms (a population inversion) and the pathway through the media is long enough for the photons to encounter them.

One way to elongate this pathway is to place the excited medium between parallel mirrors. Those few photons arising from spontaneous decay and moving perpendicular to the mirrors will oscillate back and forth through the medium and quickly recruit a large number of ancillary photons.

The parallel mirrors form the boundary of a space containing the medium and numerous oscillating photons, called the laser cavity (Fig. 39–1). If one mirror is partially, instead of fully, reflective, then some of the photons escape

Completely silvered mirror

Active medium

Power

LASER

BEAM

CHAPTER 39 BASICS OF LASERS • 427

FIGURE 39–1 Schematic illustration of the basic laser cavity, power coil for input energy, and Q-switch. (Reproduced with permission from Regillo CD, Brown GC, Flynn HW, eds. Vitreoretinal Disease: The Essentials, New York: Thieme; 1999.)

with each oscillation and form a beam of monochromatic, unidirectional laser light. The wavelength of this output light is a characteristic of the laser medium (Table 39–1).

The name laser is an acronym denoting the physical principles on which this device is based: light amplification by stimulated emission of radiation. Lasers have three basic characteristics: (1) a source of input energy; (2) a solid, liquid, or gas medium; and (3) a cavity. For most lasers, the input energy is either light, as from a flashlamp, or an electric spark or current. Important characteristics of laser beams are monochromicity, collimation (minimal divergence of light rays), and coherence.

Some atoms or molecules used in lasers decay from the excited, high-energy state to the ground state in more than one step. Upon decay, they emit a photon with one wavelength during the first step and another photon of a different wavelength during the second step, and so on. A laser device based on such media may put out intermixed beams of more than one color. An example of this is the argon atom, which emits several colors, the clinically important ones having blue (488 nm) and green (514 nm) wavelengths. A bandpass filter can remove all but one of the beams from its output, allowing the operator to select the green output over the blue, or vice versa. Another method to separate two intermixed beams is to pass them through a prism, taking advantage of the fact that light of one wavelength will be refracted (bent) differently from light of another wavelength.

TABLE 39–1 WAVELENGTHS OF OUTPUT LIGHT FROM

VARIOUS OPHTHALMIC LASERS

Another important characteristic of laser instruments is whether the output is continuous [referred to as continuous wave (CW) output] or pulsed. A CW laser has continuous output while it is turned on, like a flashlight that is illuminated. Pulsed output occurs in the form of a single burst of laser light or as a series of many brief small pulses arising within a relatively brief time envelope, usually measured in milliseconds. The latter is called a free-running laser.

Q-switching is a method to delay laser output by placing a transiently opaque shutter within the cavity, allowing widespread excitation to build in the atoms within the medium. Abrupt clearing of the light blocking shutter (the switch) allows the output of a single, brief, “giant” laser pulse. Because the opaque switch “spoils” the “quality” of the cavity, this technique is termed Q-switched. The duration of Q-switched pulses is usually 5 to 20 nsec. By contrast, a pulsed output with many thousands of small pulses per second and an infinitely long time envelope may mimic the effect on tissue of a CW laser.

ENERGY, POWER, AND DURATION

Our terminology for describing laser output arises from Newtonian physics. These terms are helpful for predicting and quantitating laser–tissue interactions (Table 39–2).

TABLE 39–2 COMMON TERMINOLOGY DESCRIBING LASER

OUTPUT

428 • SECTION VI LASER THERAPY OF GLAUCOMA

TABLE 39–3 FREQUENTLY USED PREFIXES FOR LASER

PARAMETERS AND THEIR EQUIVALENT SCIENTIFIC NOTATION

From Newton’s law, force = (mass * acceleration). By definition, a newton is the force required to accelerate a mass of 1 kg at a rate of one m>sec each second (1 m>sec2). At Earth’s surface, the acceleration due to gravity is about 9.8 m>sec2.

Work is energy expended by applying a force over a distance. A joule is the work of 1 newton (1 N) of force through 1 m. At Earth’s surface a 1 kg mass weighs 9.8 N, or 2.2 lb. Lifting a 1 kg mass for a distance of 1 m requires 9.8 joules (9.8 J) of energy. Remembering that a calorie, the energy required to heat a 1 gm mass of water by 1°C, is nearly 4.2 J, one can calculate that slowly lifting 22 lb from resting on the floor to overhead (a distance of about 2 m) requires almost 200 J, or 47 calories of work.

Power is the rate of work, or energy production. It equals work divided by the duration of time over which the work is done. A watt is power at a rate of 1 J>sec. Thus a 100 watt (100 W) light bulb has output of 1 J in 0.01 seconds, and 100 J>sec. In other words, a 1 W CW laser puts out 1 J>sec.

Irradiance is the radiant flux density at a surface and is expressed in W>cm2. The radiant density is the light energy within a volume, expressed in J>m3.

When used in medicine, laser energy, power, and duration assume various measures, ranging from minuscule to massive. We use a number of prefixes, most of them familiar, to denote multiples for the magnitudes of these quantities. These prefixes are listed in Table 39–3.

FOCUSING AND DEFOCUSING

The focused spot diameter (d) for an ideal laser beam with wavelength ( ) having a uniform energy distribution in its cross section diameter (D) and traversing a uniform, thin lens of focal length (f) may be approximated by the following equation1:

d = 2.44D f

An ideal laser output beam has a gaussian, rather than a uniform, energy distribution in its cross section, because the minimum spot diameter is larger when the energy distribution is not uniform.

A laser output with a shorter wavelength, an output lens with a shorter focal length, and an expanded laser beam with a larger diameter all cause a smaller-diameter focal spot. If other parameters are equal, the beam intensity is greater in a smaller focused spot, which is useful for cutting or ablating tissue.

The convergence of a laser beam coming to focus is equal to its divergence after it passes the focal point. If the beam focus is located in space in front of or behind the target, then the laser beam cross section on the target will be larger than at focus. Such defocusing allows the surgeon to adjust the treatment beam diameter on the target tissue (e.g., the retina or the trabecular meshwork) for appropriate tissue effect. Generally, it is better to defocus with the focal point behind the target plane, because it avoids a “hot spot” in the transparent media in front of the target.

LIGHT DELIVERY FROM THE LASER

TO TARGET TISSUE

A surgical laser system contains, in addition to a laser, a means to deliver the laser output to the tissue target. Such delivery could be by direct contact of the output window of the laser system with the tissue. However, systems for eye surgery incorporate some intervening pathway for optical delivery. The light may travel through air to a focusing device, or there may be a fiberoptic light guide. Some commercial systems use both, with the output first traveling through a fiberoptic, then delivery optics, and then air, as it approaches the eye. Fiberoptic devices for tissue contact delivery of laser energy generally have handpieces containing terminal optics for shaping the beam.

Whatever the delivery medium, most laser beams lose photon coherence over a comparatively short distance from the laser output window. However, the beam retains monochromaticity and much of the original directionality.

Commercial laser systems provide an aiming spot, composed of either a highly attenuated portion of the treatment beam or a superimposed, coincident, lowpower beam. These coincident aiming beams are usually the red output of helium: neon lasers (632 nm) or visible diode lasers.

INTERACTION OF LIGHT

WITH OCULAR TISSUES

Photons traveling from a laser source through delivery optics to encounter a tissue surface may (1) reflect back toward the source, (2) bend (refract) upon passing the

surface, (3) scatter while traversing the tissue, (4) be absorbed by tissue components, or (5) traverse the tissue without interaction. Reflection from the eye surface usually involves a small proportion of the incident beam and is limited by using antireflection-coated treatment contact lenses. Refraction at the surface is affected by the angle of incidence and the difference between the index of refraction of the tissue and that of the media containing the incident beam. Quartz glass has an index of refraction about 1.45, air about 1.0, and the cornea and sclera about 1.37. Because the difference between the index of refraction between glass and tissue is relatively small (n = 0.08), laser delivery using contact fiberoptic devices produces less refraction than delivery through air (n = 0.37). Scatter occurs when the pathway of light waves traversing a tissue encounters local variations in density of the media. This causes some of the photons to bend away from the direction of travel of the beam.

Photon absorption by tissue components results in local change of molecules and atoms. When photon energy is absorbed slowly, as during treatment with CW or free-running lasers, the local effect is thermal. Photocoagulation occurs with a local temperature between 55° and 85°C, at a temperature that causes proteins to denature. The thermal effect may spread outward from the irradiated target. Local disruption occurs when the target temperature reaches 90° to 100°C, at which tissue water boils. Above 100°C, there is tissue charring, burning, and evaporation.

Photodisruption occurs when photons in the target are so tightly packed in space and time that they cause local ionization. Q-switched lasers provide this interaction, which is useful for performing iridotomies and capsulotomies. A visible spark may occur at the target site during this laser–tissue interaction. Photochemical interactions occur during the absorption of blue or ultraviolet photons, which have high enough energy to break molecular bonds. These interactions produce either ablation, as during excimer laser treatment, or cytotoxicity, when used with supplemental chromophores.

The practitioner must be mindful of healing responses that occur after laser surgery. Tissue responses are modified by preoperative medicines, inflammation, altered blood supply, aqueous humor changes, race, and postoperative antifibrotics and anti-inflammatory drugs. These can either enhance or reduce the intended effect of the laser treatment.

LASER SAFETY

Ophthalmic laser systems can cause diverse, unintended effects on the eye, cardiovascular system, and skin.2 In ophthalmic systems, the beam emitted to air is either con-

CHAPTER 39 BASICS OF LASERS • 429

verging or diverging. A collimated beam, found inside the laser console or with laser pointers, is particularly dangerous for the emmetropic eye, which can focus such a beam perfectly on the fovea.3

Electronic components housed within the laser console present another potential danger. Some of these can store electric charges that can deliver potentially lethal cardiovascular shock if contacted inadvertently. Some of these charges may remain in the capacitors even after the laser system is unplugged. The obvious advice to practitioners who have not had specific training in laser maintenance is to leave the covers of all laser systems in place.

Inadvertent exposure to certain laser wavelengths can damage skin, even if unfocused. This applies particularly to carbon dioxide, erbium, holmium, and excimer lasers. Because these beams have invisible wavelengths, the surgeon should always be aware of where the beam is located in the room, to avoid inadvertent skin exposure.

By the time an ophthalmic treatment laser output beam has traveled several meters from the delivery optics, the luminance has usually decreased, due to beam divergence, to a level unlikely to harm the eye of a casual observer. However, ocular laser surgery often uses treatment contact lenses with a flat front surface, raising the potential for a potentially harmful specular reflection.4 Fortunately, the surgeon is usually protected by the delivery design of the system. Nevertheless, observers should always stay out of the 2 m zone and, for additional protection, wear attenuating goggles specific to the wavelength of the system. Even better, all but required personnel should be excluded from the laser treatment suite.5

REFERENCES

1.Sliney D, Wolbarsht M. Laser beam diagnostics. In: Sliney D, Wolbarsht M, eds. Safety with Lasers and Other Optical Sources. A Comprehensive Handbook. New York: Plenum Press; 1980:411–412.

2.Sliney DH. Laser safety. Lasers Surg Med 1995;16: 215–225.

3.Mainster MA, Timberlake GT, Warren KA, Sliney DH. Pointers on laser pointers [editorial]. Ophthalmology 1997;104:1213–1214.

4.Wood RL Jr, Sliney DH, Basye RA. Laser reflections from surgical instruments. Lasers Surg Med 1992;12: 675–678.

5.Sliney D, Wolbarsht M. Laser safety in research laboratories and medical facilities. In: Sliney D, Wolbarsht M, eds. Safety with Lasers and Other Optical Sources. A Comprehensive Handbook. New York: Plenum Press; 1980:563–590.