The Fundamentals of Laser Technology
The first successful laser was built in 1960 by Theodore H. Maiman at the Hughes Aircraft Research Laboratory in Palo Alto, California. Since then, many applications have been developed for lasers. These include the compact disc player, telephone systems with fiber
optics, guidance systems for military weapons, supermarket checkout scanners, quality control in industry, entertainment with laser light shows, and numerous medical applications.
Ordinary light sources such as flashlights, flames, and the sun do not emit laser light. The individual atoms emit their light waves in a random, uncoordinated manner, in the same way that water waves spread out at random when a handful of pebbles is thrown into a pool. In contrast, a laser beam consists of light waves that are all synchronized; they all have the same wavelength and remain in step as they travel in the same direction. Synchronizing the light emission from billions of atoms in a light source is the chief difficulty in building a laser.
The key idea for solving this problem had been proposed in an article on the general theory of light absorption and emission by atoms written by the famous physicist Albert Einstein in the 1920s. When an atom absorbs a burst of light energy (a photon), an electron in the atom is raised from a lower energy level to a higher one. A short time later, the electron spontaneously falls back down to the lower energy level, emitting a photon of light in the process. Einstein’s contribution was to suggest a third mechanism in addition to ordinary absorption and emission. Based on theoretical arguments of symmetry and the conservation of energy, he proposed a new process called “stimulated emission of radiation.” (The word “laser” is an acronym for light amplification by stimulated emission of radiation.)
To understand stimulated emission, consider an atom whose electron has been raised to a higher energy level, called an “excited state.” This state is unstable, and the electron ordinarily will fall back down to the lower energy level in a short time. Suppose, however, that a photon of precisely the right energy strikes the atom while the electron is still in its temporary excited state. This photon cannot be absorbed because the electron is already in its excited state. Einstein reasoned that the incoming photon would cause the excited electron to fall to the lower energy level. A photon would then be emitted from the atom and would join the incoming photon. The two photons would be exactly synchronized in wavelength and direction.
If there were many atoms whose electrons had previously been raised to excited states, the process of stimulated emission would continue. The two photons could strike two other excited atoms and stimulate them to emit light energy, making a total of four photons. These four would trigger four more atoms, making eight photons, and so forth. Eventually, a so-called photon avalanche consisting of a huge number of synchronized light waves would be generated, which is the desired laser beam.
To make a successful laser, some additional requirements must be met. For one thing, a source of energy must be provided to raise most of the electrons to their excited states. For a gas laser, this energy is normally supplied by a high voltage. Examples of gas lasers are those using carbon dioxide, argon, or a mixture of helium and neon. A solid crystal, such as a clear ruby rod, would be excited by a bright burst of light from a device similar to a camera flash attachment. A solid-state diode laser is energized by a flow of electric current across a diode junction. In each case, it is necessary to “pump” the laser so that many atoms are in their excited state, ready and waiting to be triggered by an incoming photon to release their energy.
Another requirement for a successful laser is that the electrons must remain in their excited state for a longer-than-normal time. The problem with most materials is that electrons fall spontaneously to their lower energy level almost instantaneously, in less than one-millionth of a second. The photon avalanche effect requires that a substantial majority of the atoms be in their excited state. For very short-lived excited states, it is not possible to maintain this condition. Experimenters have no way to control the lifetime of excited states, so they must search for those atoms and molecules that already have the appropriate longer lifetime supplied by nature. It has not been possible to build a laser using hydrogen gas, for example, because hydrogen does not have any long-lived excited states. The spontaneous emission of light takes place so quickly that there is no time for a photon avalanche to develop.
Another condition for laser action is to have two parallel mirrors at the ends of the laser material. The laser beam bounces back and forth many times between the mirrors at the speed of light, gaining or maintaining its energy from the excited atoms of the laser material. One of the mirrors is made slightly less than 100 percent reflecting, so that a small portion of the laser energy is allowed to exit in a narrow beam. For most medical applications, a thin optical fiber is joined directly to the end of the laser in order to transmit the beam to the desired location in the body.
Much research has been done to develop good optical fibers. The fiber should transmit a laser beam with very little loss of energy along the way. The technology of drawing thin glass fibers with few impurities or imperfections has become quite sophisticated. A fiber must be thin and uniform so that the laser beam will be forced to travel down its center, thus avoiding the loss of light energy through the walls.
For an ultraviolet laser, glass fibers cannot be used because of absorption. (The sun’s ultraviolet radiation is absorbed by ordinary eyeglasses.) Special quartz fibers with low absorption have been developed for lasers in the ultraviolet region of the spectrum. In the infrared region of the spectrum, new optical materials are still under continuing investigation.
The wavelength (color) of a laser is determined entirely by the energy levels of the atoms or molecules being used. For example, a carbon dioxide laser always has a wavelength of 10.6 microns, which is infrared. The helium-neon gas laser always produces visible red light at a wavelength of 0.63 microns. A wide range of laser wavelengths has become available as a result of extensive research efforts by physicists and optical engineers. Since 1960, lasers have become much more rugged and dependable in construction. Some lasers operate with a continuous beam, while others produce very short pulses, depending on the desired application. Also, lasers can be designed to operate at a low power level for diagnostic purposes or a high power level for surgery.
Safety precautions must be followed when working with a laser. Not only the patient but also the surgical team must be protected from possible harmful radiation. The eyes must be protected from laser beam reflections from a shiny surface. Ultraviolet light is a special hazard because its high-energy photons can cause cell damage and genetic mutations. The great benefits of laser surgery can be negated by an inexperienced or careless surgeon.
Uses and Complications
The first medical use of a laser beam was for surgery on the retina of the eye, in 1963. Diabetic patients in particular frequently develop excessive blood vessels in the retina that give their eyes a typically reddish color. In advanced cases, the blood vessels can hemorrhage and eventually cause blindness. The green light of an argon laser will pass through the clear cornea and lens of the eye, but when it hits the dark brown melanin pigment of the retina, it will be absorbed and cause a tiny hot spot. The physician uses a series of laser pulses, carefully focused on affected areas of the retina, to burn away the extra blood vessels. The remarkable property of the laser beam in this procedure is that its energy penetrates to the rear of the eye, leaving the clear fluid unaffected.
In a greenhouse, light from the sun comes in through the glass but infrared radiation cannot get back out. This example illustrates that some materials, such as glass, are transparent for visible light but opaque for infrared light. Similarly, the front of the eye is transparent for visible light but absorbs infrared light. Therefore, an infrared laser such as the YAG (yttrium-aluminum-garnet) laser can be used for surgery near the front of the eye because its light energy is selectively absorbed there.
A particular problem following
cataract surgery is that a secondary cataract may develop on the membrane behind the implanted artificial lens. About one-third of patients with such a lens implant require a second surgery to remove the secondary cataract. A YAG laser beam will pass through the cornea and artificial lens but can be focused to produce a hot spot at the site of the secondary cataract to destroy it. More than 200,000 such procedures are performed each year in the United States, and the result is a dramatic, almost instantaneous improvement of vision.
A third form of laser eye surgery, laser-assisted in situ keratomileusis (LASIK), became quite popular at the end of the twentieth century; it has, in fact, become the most commonly performed surgery in the country. In LASIK surgery, the cornea of the eyeball is reshaped to help patients overcome myopia (nearsightedness), hyperopia (farsightedness), or astigmatism. The procedure is done with a cool beam laser that can be used to remove thin layers of tissue from selected sites on the cornea to change its curvature. Success rates for the surgery are high: 95 to 98 percent of the patients get 20/40 vision. The surgery is not without its problems, however. Because of its popularity and the large fee charged for a quick and fairly simple procedure, unqualified or less experienced physicians are oftentimes performing the delicate procedure; approximately 5 percent of patients receiving LASIK surgery receive less-than-satisfactory results. Two other corrective procedures for eyesight involving lasers are the use of Keravision Intacs (intrastomal corneal rings), which involves placing a lens in the cornea, and PRK (photorefractive keratectomy), in which the cornea is scraped without actual incision into the cornea.
Another dramatic medical application of the laser is the breaking up of stones in the kidney, ureter, or gallbladder. Such calcified, hard deposits previously could be removed only by surgery. It is now possible, for example, to insert an optical fiber of less than half a millimeter in diameter through the urethra and then transmit the laser beam to the site of the stone. High-power light pulses of very short duration (less than one-billionth of a second) create a shock wave that breaks the stone into small fragments that the body can eliminate.
Another promising application of laser surgery that has received much publicity is laser
angioplasty, which is used to open up a blood vessel near the heart that is partially or wholly blocked by a deposit of plaque. The hope is that the laser procedure may be able to replace heart bypass surgery, but the results are still preliminary.
Laser angioplasty involves inserting a catheter that contains a fiberscope, an inflation cuff, and an optical fiber into the artery of the arm and advancing it into the coronary artery. The fiberscope enables the physician to see the blockage, the cuff is used to stop the blood flow temporarily, and the optical fiber transmits the laser energy that vaporizes the plaque. Sometimes, the laser method is used only to open up a small channel, after which balloon angioplasty is used to stretch the walls of the blood vessel.
The main risk in using the laser beam is that the alignment of the optical fiber inside the artery may be deflected and cause a puncture of the blood vessel wall. Improvements in the imaging system are needed. Also, further work must be done to see which light wavelengths are most effective in removing plaque and preventing recurrence of the obstruction.
The heating effect of a laser beam has been used by surgeons to control bleeding. For example, bleeding
ulcers in the stomach, intestine, and colon have been successfully cauterized with laser light transmitted through an optical fiber. A similar procedure has been used to treat
emphysema patients. An optical fiber is inserted through the wall of the chest, and the laser’s heat is used to shrink the small blisters that are present on the surface of the lungs. Also, the heat from a laser has been used during internal surgery to seal the surrounding capillaries that contribute to bleeding.
For the preceding procedures, a carbon dioxide gas laser that emits infrared light is normally used. The reason is that water molecules in the tissue absorb the infrared wavelengths most efficiently. The optical fibers used to transmit infrared light are not as efficient and reliable as those used for visible or ultraviolet light, however, so further research on fiber materials is in progress.
One notable success of laser surgery has been the removal of birthmarks. Because of their typically reddish-purple coloration, birthmarks are commonly called port-wine stains. They can be quite unsightly, especially when located on the face of a person with an otherwise light complexion. To remove such a birthmark, the laser beam has to burn out the network of extra blood vessels under the skin. A similar procedure can be used to remove unwanted tattoos.
The color of the laser must be chosen so that its wavelength will be absorbed efficiently by the dark purple stain. What would happen if a purple laser beam were used? Its light would be reflected rather than absorbed by a purple object, and it would not produce the desired heating. Yellow or orange is most effectively absorbed by a purple object. The surgeon must be careful that the laser light is absorbed primarily by the purple birthmark without harming the normal, healthy tissue around it.
Experimental work is being done to determine whether laser surgery can be applied to
cancer. Small malignant tumors in the lungs, bladder, and trachea have been treated with a technique called photodynamic therapy. The patient is injected with a colored dye that is preferentially absorbed in the tumor. Porphyrin, the reddish-brown pigment in blood, is one substance that has been known for many years to become concentrated in malignant tissue. The suspected site is irradiated with ultraviolet light from a krypton laser, causing it to glow like fluorescent paint, which allows the surgeon to determine the outline of the tumor. To perform the surgery, an intense red laser is focused on the tumor to kill the malignant tissue. Two separate optical fibers must be used for this procedure, one for the ultraviolet diagnosis and one for the red laser therapy.
The three traditional cancer treatments of surgery, chemotherapy, and radiation therapy all seek to limit damage to healthy tissue surrounding a tumor. Photodynamic surgery by laser must develop its methodology further to accomplish the same goal.
Much has already been accomplished in applying laser surgery to various body organs. Future developments are likely to emphasize microsurgery on smaller structures, such as individual cells or even genetic material in DNA molecules.
Perspective and Prospects
Some inventions, such as the printing press, the steam engine, and the electric lightbulb, were made by innovators who were trying to solve a particular practical problem of their time. Other inventions, such as the microscope, artificial radio waves, and low-temperature superconductivity, initially were scientific curiosities arising from basic research, and their later applications were not at all anticipated. The laser belongs to this second category.
The first successful laser, built by Theodore Maiman, consisted of a small cylindrical ruby rod with shiny mirrored ends and a bright flash lamp to excite the atoms in the rod. Maiman’s goal was to determine whether the separately excited atoms could be made to release their absorbed energy almost simultaneously in one coordinated burst of monochromatic (single-wavelength) light.
No one could have foreseen the wide range of technological applications that resulted from Maiman’s experiment. It is important to appreciate that he did not set out to improve telephone communication or eye surgery; those developments came about after the laser became available.
It is worthwhile here to summarize some of the general uses for lasers in modern technology. The tremendous advances in medical applications could not have happened without the concurrent development of new types of lasers, with a variety of wavelengths and power levels, needed for other industrial products.
Among laser applications are the following: the compact disc (CD) player, in which the laser beam replaces the LP record needle; optical fibers that can carry several thousand simultaneous telephone calls; supermarket checkout scanners, in which a laser beam reads the universal product code on each item; three-dimensional pictures, called holograms, that are displayed at many art museums; military applications, such as guided weapons and Star Wars technology; surveying, bridge building, and tunneling projects in which exact alignment is critical; and nuclear fusion research, in which high-power lasers can produce nuclear reactions that may become a future source of energy as coal and oil resources are depleted. Sophisticated advances in using lasers to control specific chemical reactions and to predict earthquakes are under development.
Laser technology in medicine will continue to advance as biologists, electrical and optical engineers, physicians, biophysicists, and people from related disciplines share this common interest.
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