Physics: Lasers

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Physics: Lasers

Introduction

Laser is an acronym that stands for light amplification by stimulated emission of radiation. Lasers operate in the infrared, visible, or ultraviolet regions of the electromagnetic spectrum. Who deserves credit for the invention of the laser? The answer is not a simple one. Bell Laboratories' Web site states categorically that it was invented there. Others dispute that.

The first working laser saw the light of day on July 7, 1960. At first the invention seemed to be a solution looking for applications. In fact, for the first couple of years lasers seemed able to do little more than blaze holes in razor blades for TV commercials. Then, as the advantages of the strange new light produced by lasers became clearer, the devices seemed to find application everywhere.

Today the number and variety of lasers is astonishing, and they are used in such varied fields as medicine, science, industrial production, the home, office, communications, and the military.

The main reason for this wide use is that laser light has very special qualities: It is very pure in color, it can be very intense, and it can be directed with great precision. What makes all of this possible? Because of the way it is produced, it has a quality called coherence, which means that its waves remain in phase (in step) as they are produced.

Historical Background and Scientific Foundations

Stimulated Emission

The laser and its parent, the maser (microwave amplification by stimulated emission of radiation), can be traced back 90 years to their theoretical beginnings.

The German-born American physicist Albert Einstein (1879–1955), although best known for his work in relativity theory, also did important work on that other important twentieth-century scientific masterpiece,quantum theory. In one paper, published in 1916, he showed that under certain conditions, controlled emission of light energy could be obtained from an atom.

When an atom or molecule has somehow had its energy level raised by the input of energy, it can release this stored energy in one of two ways. It might be released in the form of a photon of light energy at some moment and in a direction that cannot be known in advance. This is spontaneous emission. It can also be stimulated by subjecting the particle to a small “shot” of electromagnetic radiation of the proper frequency. This is stimulated emission.

Einstein wrote that when an incoming shot of energy caused an electron to drop from a higher to a lower orbit, the electron emitted another photon. In other words, the energy of the emitted photon would be added to that of the photon that stimulated the action in the first place. Here, potentially, was light amplification. But what is special about the laser principle is that the emitted photon would be of the same frequency, in the same direction, and in step with (having the same phase as) the one that hit it.

Some two decades later, from 1939–1940, Russian physicist Valentin A. Fabrikant (1907–1991) began to think about what conditions would be needed to produce amplification of light in this way. At about the same time, American physicist Charles H. Townes (1915–), who had a PhD in physics from the California Institute of Technology, joined Bell Laboratories. Although Townes had worked in several theoretical fields, World War II was approaching, and he was assigned to work on a radar-directed bombing system. The system used microwave frequencies with a wavelength of several centimeters.

After the war, his work with microwaves fit in nicely with a growing interest in molecular (microwave) spectroscopy, and in 1948 he moved to Columbia University. Townes took his interest in microwave spectroscopy with him, and in 1949 began working with American physicist Arthur Schawlow (1921–1999), a newly minted physics PhD from the University of Toronto who had joined Columbia University on a fellowship.

Townes and Schawlow studied the idea that as the wavelength of the microwaves used in spectroscopy grew shorter, the radiation's interaction with molecules became stronger, leading to a more sensitive spectroscopic device. But generating the smaller wavelengths was a problem. Townes had the idea that the desired frequencies could be generated somehow by the use of gaseous molecules.

Townes's interest in millimeter waves—the next major step down in wavelength from microwaves—was for more powerful spectroscopes. The military, however, was funding some of the lab's research and had other interests. Microwaves, used in radar, for example, find wide application in communications, and this explained the military's interest: The higher the frequency of electromagnetic radiation, the greater its information-carrying capacity. Systems utilizing millimeter waves—one frequency step shorter than microwaves—would be a huge advance, but researchers were stymied by the idea of building the required resonating cavity in millimeter proportions. To get around this, Townes, along with a couple of his more advanced students, began to work on a unit for microwaves that had a resonating cavity on the order of a few centimeters.

On April 26, 1951, after much thought, he figured out how to do this. Normally, more of the molecules in any substance are in low-energy states than in high ones. Townes wanted to change the natural balance and create a situation with an abnormally large number of high-energy molecules, then stimulate them to emit their energy by nudging them with microwaves. Here was amplification.

He could even take some of the emitted radiation and feed it back into the device to stimulate additional molecules, thereby creating an oscillator. This feedback arrangement, he knew, could be carried out in a cavity, which would resonate (just like an organ pipe and acoustic waves) at the proper frequency. The resonator would be a box whose dimensions were comparable with the wavelength of the radiation, that is, a few centimeters on a side.

On the back of an envelope he figured out some of the basic requirements. Three years and many experiments later, the maser was a reality. The original maser was a small metal box into which excited ammonia molecules were added. Townes later wrote: “The idea that I added … was to use a resonant cavity so that the signal would go repeatedly through the gas, bouncing back and forth, picking up energy each time. The process would provide effectively infinite amplification.”

When microwaves were beamed into the excited ammonia, the box emitted a pure, strong beam of high-frequency microwaves, far more frequency coherent (in step) than any that had ever been achieved before. The output of an ammonia maser is stable to one part in 100 billion, making it an extremely accurate atomic clock; masers' amplifying properties have also been found to be very useful for magnifying faint radio signals from space and for satellite communications.

Townes chose to work with ammonia gas because he knew that it was possible to separate the low-energy molecules from the high and to get the excited molecules into the cavity without too much trouble. This procedure for getting the majority of atoms or molecules in a container into a higher energy state is called population inversion and is basic to the operation of both masers and lasers.

It's worth noting that two Russians, Nikolay G. Basov (1922–2001) and A.M. Prokhorov (1916–2002), were working along similar lines independently of Townes. In 1952 they presented a paper at an All-Union(U.S.S.R.) Conference, in which they discussed the possibility of constructing a molecular generator, that is, a maser. Their further publications, in 1954 and 1955, were in many respects similar to Townes's, and even showed a new way to obtain the active atomic systems for a maser.

Thus on October 29, 1964, when the Nobel Prize in Physics was awarded for fundamental work in the field of quantum electronics, which led to construction of oscillators and amplifiers based on the “aser” principle, it was awarded not only to Townes, but to Basov and Prokhorov as well.

A Laser Is Born

Following the maser development, there was much speculation about the possibility of extending the principle to the optical region. The difficulty, of course, was that optical wavelengths are so tiny—about one ten-thousandth of that of microwaves. The maser depended on a physical resonator a few centimeters or even millimeters in length. But at millimeter wavelengths, such resonators are already so small that they are hard to make accurately. Making a box one ten-thousandth that size was out of the question. Another approach was necessary.

In 1958 Townes and Schawlow (who was by then working at Bell Telephone Laboratories and had become his brother-in-law) outlined the theory and proposed a structure for an optical maser. They suggested that resonance could be obtained by making the waves travel back and forth along a relatively long thin column of amplifying substance that had parallel reflectors, that is, mirrors facing inward, at each end. One of the mirrors would be only partially reflective, to permit the beam to emerge when it grew to a certain strength. They dubbed this device, which would have a working medium of potassium vapor, an optical maser and published their theory in the respected journal Physical Review on December 15, 1958.

But according to another researcher, history has gotten things all wrong. According to Gordon Gould (1920–2005), a graduate student at Columbia who was working down the hall from Professor Townes at the time, the basic insights that made it all possible should have been credited to him.

The First Working Laser

After Townes and Schawlow's theory of the optical maser was published on December 15, 1958, the race to build the first actual device began in earnest. The clear winner, in 1960, was American physicist Dr. Theodore H. Maiman (1907–2007), then at Hughes Aircraft Company. Curiously, the active substance he used was neither the potassium vapor design suggested by Townes and Schawlow nor the gas laser suggested by Gould. Rather it was a single ruby crystal, with the ends ground flat and silvered.

Ruby is an aluminum oxide in which a small fraction of the aluminum atoms in the molecular structure, or lattice, have been replaced with chromium atoms. These atoms absorb green and blue light, imparting a red color. The chromium atoms can be boosted from their ground state into excited states when they absorb green or blue light. This process, by which population inversion is achieved, is called pumping.

Pumping in a crystal laser is generally achieved by placing the ruby rod within a spiral flash lamp. When the lamp is flashed, a bright beam of red light emerges from one end of the ruby, which has been only partially silvered. The duration of this flash of red light is quite brief, lasting only some 300 millionths of a second, but it is very intense. In the early lasers, such a flash reached a peak power of 10,000 watts.

The outside world seemed to have little understanding of the significance of Maiman's accomplishment. Maiman detailed his work for quick publication in Physical Review Letters, but passed it through the Hughes Patent Office first. The patent office cleared the paper but didn't think the report of his work was important enough to warrant filing for patent protection. Thus Hughes lost any claim to foreign patent rights. Second, the editor at Physical Review Letters sent Maiman a curt letter of rejection. Maiman's paper was subsequently published in the British science journal Nature on August 6, 1960.

A Multitude of Lasers

Although the researchers at Bell Laboratories were disappointed that Maiman had reached the goal of building the optical maser first, they pointed out, happily, that Maiman's laser was “only” a pulsed laser. In other words, light energy was pumped in and a bullet of energy sped out from it. Then the whole process had to be repeated. Pulsed operation is fine for spot welding and for operations such as radar-type range finding, where pulses of energy are used anyway. But even though lasers permit use of optical wavelengths with their much greater carrying capacity, pulsed systems would not be useful for communications (or for other applications to be discussed later). Continuous-wave (CW) operation remained a major goal.

In addition, solid crystals are difficult to manufacture. Hence, it was natural for laser pioneers to look hopefully at gas lasers, which would theoretically be easier to make, once the proper conditions were satisfied. Simply fill a glass tube with the proper gas and seal it. But other advantages would accrue. For one thing the relatively sparse population of emitting atoms in a gas provides an almost ideally homogeneous medium. That is, the emitting atoms (corresponding to chromium in the ruby crystal) are not “contaminated” by

GORDON GOULD (1920–2005)

Gordon Gould (1920–2005) was born in New York City. Even as a child, his heroes were inventors like Alexander Graham Bell (1847–1942), Thomas Edison (1847–1941), and Guglielmo Marconi(1874–1937). Knowing that to invent anything worthwhile he would have to understand the physics behind it, he concentrated on this subject throughout his school years. In 1957 he was working toward a PhD at Columbia University, an important center of physics. Charles Townes (1915–), inventor of the maser, was teaching there, and they occasionally discussed their work. Gould was doing research on optical and microwave spectroscopy, but was beginning to think about lasers. There is considerable disagreement between the two men as to how much each told the other about his laser ideas.

On the night of November 9, 1957, at the age of 37, Gould came up with his concept for the laser. The maser, Townes's invention, amplified microwaves. A laser would be much more powerful, since a photon of light by its very nature has a hundred thousand times more energy than a single unit of microwave energy. Gould—at least in his own estimation—had come up with one of the most important inventions of the twentieth century. Yet he was to embark on a comedy of errors that would prevent him from profiting from that invention for almost 30 years.

Knowing that he had something important in hand, he walked away from his almost-won doctorate. He consulted a patent attorney but, misconstruing the attorney's advice, thought he had to build a prototype before he could file for a patent. Therefore he spent most of 1958 trying to refine and improve his device and did not file until 1959. In the meantime, other laser researchers, including Townes and Schawlow, had already filed for the same or similar devices. What Gould did do, fortunately for him, was write down and diagram a careful description of his ideas in a school notebook, which he then took to a corner candy store, where he had the pages notarized. The date was November 13, 1957. Gould was the first to use the term laser.

In 1958 Gould joined a newly formed New York company called TRG, where he felt there was a better chance that he could develop his laser. The company applied for and won a contract from ARPA, the Department of Defense's Advanced Research Projects Agency, to support this work, but they were not successful in developing a working laser. Later Gould became a professor at Polytechnic Institute of Brooklyn, and in 1973 he helped found the industrial firm Optelicom, Inc.

Over the years, however, he also entered upon a long and very expensive patent war. Along the way he enlisted the aid of a series of partners who helped in his battles. It was a war fought in many courtrooms—against competing scientists, against firms that he claimed were illegally using “his” designs without paying licensing fees, and against the Patent Office itself. He insisted that Townes had used his ideas, while Townes insisted that whenever they talked before that November date, Gould would rush back to his office and write down what they had discussed. Gould says their discussions merely alerted him to the fact that Townes was thinking along similar lines to his.

At first things looked bleak for Gould. In March 1964, for instance, the Patent Office ruled against him and in favor of Bell Laboratories, upholding the optical maser patent awarded to Schawlow and Townes on March 22, 1960. Gould and his partners turned to the courts, suing the Patent Office in federal court when his own patents were disallowed. After a great deal of expensive litigation and more battles with the Patent Office, things began to turn in his direction. In May 1977 Gould's application for an optically pumped amplifier was allowed. After some other successes, he was awarded patents for a gas-discharge amplifier which, with the optically pumped amplifier, covered 80 percent of the lasers made in the United States. He was granted other patents as well.

The delay, curiously, had made the patents far more lucrative than they would have been if they had been issued immediately. In other words, if they had been issued when applied for in 1959, they would have run their 17-year course before laser applications exploded into the huge industry they became in later years.

the lattice or host atoms. Since only active atoms need be used, the frequency coherence of a gas laser would probably be even better than that of the crystal laser, they reasoned.

Less than a year after the development of Maiman's ruby laser, Iranian-born physicist Ali Javan (1926–) at Bell Laboratories proposed a gas laser employing a mixture of helium and neon. This was an ingeniously contrived partnership whereby one gas did the energizing, and the other did the amplifying. Javan's laser provided the first continuous output, generally referred to as CW, or continuous wave, operation.

Gas lasers now utilize many different gases for different wavelength outputs and powers and provide the “purest” light of all. An additional advantage is that the optical pumping light can be dispensed with; an input signal of radio waves of the proper frequency can do the job.

Power and Efficiency

The two units generally used to specify the power output of a laser are watts and joules. The watt, the rate at which (electrical) work is being done, is the more familiar unit—we need only think of a 15- or a 150-watt bulb to get an idea of its magnitude. The joule is a unit of energy and can be thought of as the total capacity to do work. One joule is equivalent to 1 watt-second, or 1 watt applied for 1 second. But it can also mean a 10-watt burst of laser light lasting 0.1 second, or a billion watts lasting a billionth of a second.

In the early years, crystal lasers were the most powerful, but other materials, such as liquids and specially prepared glass, have now been made that provide solid competition. In 2003 the U.S. National Ignition Facility Project produced 10.4 kilojoules of ultraviolet light in a neodymium glass-based laser beam, setting a world record for laser performance.

One of the least satisfactory aspects of the laser has been its notoriously low efficiency. For a while the best that could be achieved was about 1%, that is, 100 watts of light had to be put in to get one watt of coherent light out. In gas lasers the efficiency was even lower, ranging from 0.01 to 0.1%.

In gas lasers this was no great problem, since high power was not the objective. But with the high-power solid lasers, pumping power could be a major undertaking. A high-powered laser pump built by Westinghouse Research Laboratories could handle 70,000 joules. In more familiar terms, the peak power input while the pump is on is about 100,000,000 watts. For a brief instant this is roughly equal to the electrical power needs of a city of 100,000 people.

Two developments changed the efficiency levels. First, the carbon dioxide (CO2) gas laser is quite efficient, with the figure having passed 15%. CO2 lasers, producing either pulses or continuous waves, can put out powerful beams and have found wide use in many applications. Special-purpose devices have been constructed that produce 20 megawatts CW.

The second is the injection or semiconductor laser, in which efficiencies of more than 40% have been achieved. In a current program supported by the U.S. Defense Advanced Research Projects Agency, Alfalight, a semiconductor laser manufacturer, has demonstrated 65% efficiency; the program is shooting for 80% in three years. Unless unforeseen difficulties arise, this figure is expected to continue to rise to a theoretical maximum of 100%.

In the semiconductor laser all the functions of the laser have been packed into a tiny semiconductor crystal. In this case, electrons and “holes” (vacancies in the crystal structure that act like positive charges) accomplish the job done by excited atoms in the other types of lasers. Although the device itself is about the size of this letter “o,” it is self-contained and can convert electric current directly into laser light. This has made possible a vast field of experiment and improvement in the world of lasers.

Modern Cultural Connections

Today lasers are in action practically everywhere we turn. In the home, for example, we have laser-based CD

IN CONTEXT:LASING—A NEW WORD

The all-important rod in the ruby laser is formed as a single large crystal. Because it must be free of extraneous material, it is grown artificially. That is, the crystal is formed as it is pulled slowly from a “melt,” after which it is ground to size and polished.

In operation, the crystal rod contains many atoms in the ground state and a few in an excited state. When the pumping lamp flashes, it raises most of the atoms to the excited state, creating the required population inversion. Lasing begins when an excited atom spontaneously emits a photon parallel to the axis of the crystal. Photons emitted in other directions merely pass out through the transparent sides of the crystal. The emitted photon stimulates another atom in its path to contribute a second photon, in step, and in the same direction.

This process continues as the photons are reflected back and forth between the ends of the crystal. (We might think of lone soldiers falling into step with a column of marching men.) The beam continues to build; when amplification is great enough, the beam flashes out through the partially silvered side of the rod—a narrow, parallel, concentrated, coherent beam of light.

and DVD players, CD-ROM drives, pointers, laser thermometers for cooking, levels and “tape” measures for the craftsman, and even a laser guide that helps guide a driver into the proper parking spot in a garage.

Laser applications can, in fact, be divided into two broad categories: (1) commercial, industrial, military, home, and medical uses; and (2) scientific research. In the first group, lasers are used to do something that has been done in another way (but perhaps not as well). For example, one of the first medical applications was in eye surgery, for “welding” a detached retina back into place. The laser is particularly useful here because laser light can penetrate transparent objects such as the eye's lens without harming it, and can do the needed surgery (“stitching”) on the retina.

The device used on the eye may have a power of only 5 watts, which is less than a typical night light. But the concentration of the beam is such that it can be focused down to the size of a single cell. Lasers have been replacing the eye surgeon's blade in treating farsightedness and astigmatism by reshaping the cornea to alter the way the eye refracts light.

In another medical application that puts the laser's unique qualities to work, urologists use its concentrated power to blast kidney stones while they watch the process through the same fiber-optic cable that is carrying the laser shots.

The laser's unusual features have also led to its wide use in cosmetic surgery to treat everything from sagging eyelids to varicose veins, and for all kinds of skin rejuvenation, including treatments for wrinkles and problems with acne, skin texture, and discoloration. When used in general surgery, lasers can perform nearly bloodless cutting by cauterizing (sealing off blood vessels) as they cut. They are being used for pain relief, in place of radiotherapy for cancer treatment.

Highly accurate laser tracking and measurement systems have been developed for precision manufacturing. Laser light's high intensity can cut or penetrate almost anything, including the hardest known material, diamond. An additional advantage is that laser drills do not get dull with use.

Scientific Research

In the second category, scientific research, the narrowness of the laser beam has made it ideal for applications requiring accurate alignment. Perhaps the ultimate such application is the 2-mile-long (3.2-km) linear accelerator built in 1966 by Stanford University for what was at the time the United States Atomic Energy Commission.

Only a laser beam could accomplish the incredible task of keeping the 7/8-inch (2.2-cm) bore of the accelerator straight along its full 2-mile (3.2 km) length. A remote monitoring scheme, based on the same laser beam, told operators when a section shifted out of line (due, for example, to small earth movements) by more than about 1/32 inch (0.79 cm) and identified the section.

The accelerator is housed at the Stanford Linear Accelerator Center (SLAC) in California. Established in 1962, it remains one of the world's leading research laboratories and is currently building what will be the world's first x-ray laser. This system will use the last 0.6mile (1 km) of the 2-mile (3.2 Km) SLAC accelerator to generate the needed input energy. The device will open up a world of new applications.

In scientific research the laser has proved its value over and over. Pulses of laser light can be powerful, but they can also be short. Laser pulses lasting a few fem-toseconds (one quadrillionth [10 -15 ] of a second) were produced in the 1980s. Current work at the University of Munich and the Max Planck Institute for Quantum Optics in Garching, Germany, has brought this down to attoseconds (one quintillionth [10 -18 ] of a second). This makes it possible to probe atomic and subatomic electron processes. For the first time researchers can “freeze” the motion of electrons in the length and time-scales of atoms.

In both scientific and medical research, the laser is often combined with some existing kind of equipment, where its special features provide an advantage. For example, researchers at Stanford University hope to treat cancer by creating carbon nanotube messengers that, combined with other substances, will latch on to cancer cells. The objective is to turn a near-infrared laser into a cancer weapon; its light would pass through normal human tissue without harming it, but the 150-nanometer-long nanotubes would strongly absorb the radiation and turn it into heat that, they hope, will destroy the cancer cell.

The most exciting probability of all, however, is that lasers will undoubtedly change your life in ways that we cannot even conceive at this time.

Primary Source Connection

One of the many ways in which lasers are now being used is by the U.S. military. Though some critics argue that lasting eye damage can occur, as Will Knight reports in the New Scientist, the U.S. Department of Defense would like to begin using them in Iraq.

US MILITARY SETS LASER PHASRs TO STUN

The US government has unveiled a “non-lethal” laser rifle designed to dazzle enemy personnel without causing them permanent harm. But the device will require close scrutiny to ensure compliance with a United Nations protocol on blinding laser weapons.

The Personnel Halting and Stimulation Response (PHASR) rifle was developed at the Air Force Research Laboratory in New Mexico, U.S., and two prototypes have been delivered to military bases in Texas and Virginia for further testing.

The U.S. Department of Defense (DoD) believes the weapon could be used, for example, to temporarily blind suspects who drive through a roadblock. However, the DoD has yet to reveal details of how the laser works and has yet to respond to New Scientist's requests for further information.

Laser weapons capable of blinding enemies have been developed in the past but were banned under a 1995 UN convention called the Protocol on Blinding Laser Weapons. The wording of this protocol, however, does not prohibit lasers that temporarily dazzle a foe.

Permanent harm

“In the past, the problem with lasers of this type has been that they often permanently blind human targets,” says Tobias Feakin, an expert at Bradford University's Non-Lethal Weapons Research Project in the UK.

But he says newer systems may avoid this problem by using less powerful laser beams. “This new wave of low-intensity laser weapons do not have a permanently damaging effect, apparently,” he told New Scientist.

Several laboratories across the world are working on such weapons. But even low power laser systems can cause eye damage if they are used at close quarters or for extended periods.

The PHASR may attempt to address safety concerns by automatically sensing its distance from a target. The limited information released by the DoD includes mention of an “eye-safe range finder”, which may mean the laser's power is adjusted depending on the distance to the target. The system is also said to incorporate a “two wavelength laser system”, which may be designed to counter goggles that can filter out certain wavelengths of laser light.

Pulsing green light

Neil Davison, another expert at Bradford University, says the situation in Iraq may encourage the U.S. to push for the development of less-than-lethal laser weapons. “They already use bright white lights at vehicle checkpoints in Iraq to dazzle drivers who are approaching too fast,” he says.

LE Systems, based in Connecticut, U.S., for example, makes the Laser Dazzler, which resembles an ordinary torch and emits a low power pulsing green laser light. The company says this device has been tested extensively and been shown to cause no lasting eye damage.

The possibility of causing lasting eye damage can be reduced by diffusing the laser beam or rapidly moving it across the target with a series of mirrors.

And the same U.S. military research lab developed another laser weapon more than a decade ago, called the Sabre 203. This device attached beneath the barrel of a normal rifle and emitted a low-power laser light over a range of 300 metres. It was used by U.S. forces in Somalia in 1995 but later shelved because of concerns over safety and effectiveness.

Will Knight

knight, will. “us military sets laser phasrs to stun.” new scientist.http://www.newscientist.com/article/dn8275.html (accessed november 14, 2007).

See Also Chemistry: Molecular Structure and Ste-reochemistry; Chemistry: States of Matter: Solids, Liquids, Gases, and Plasma; Physics: Fundamental Forces and the Synthesis of Theory; Physics: Semiconductors; Physics: Spectroscopy; Physics: The Inner World: The Search for Subatomic Particles; Physics: The Standard Model, String Theory, and Emerging Models of Fundamental Physics; Physics: Wave-Particle Duality.

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Hal Hellman

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