Laser technologies

Summary

The term “laser” is an acronym for “light amplification by stimulated emission of radiation.” A laser device emits light via a process of optical amplification. The process is based on the stimulated emission of photons (particles of electromagnetic energy). Laser beams exhibit a high degree of spatial and temporal coherence, which means that the beam does not widen or deteriorate over a distance. Laser applications are numerous and include surgery, skin treatment, eye treatment, kidney-stone treatment, light displays, optical disks, bar-code scanners, welding, and printers.

Definition and Basic Principles

A laser consists of a highly reflective optical cavity, which contains mirrors and a gain medium. The gain medium is a substance with light-amplification properties. Energy is applied to the gain medium via an electrical discharge or an optical source such as another laser or a flash lamp. The process of applying energy to the gain medium is known as pumping. Light of a specific wavelength is amplified in the optical cavity. The mirrors ensure that the light bounces repeatedly back and forth in the chamber. With each bounce, the light is further amplified. One mirror in the chamber is partially transparent. Amplified light escapes through this mirror as a light beam.

Many types of gain media are employed in lasers, including gases (carbon dioxide, carbon monoxide, nitrogen, argon, and helium/neon mixtures), silicate or phosphate glasses, certain crystals (yttrium aluminum garnet), and semiconductors (gallium arsenide and indium gallium arsenide). A basic concept of laser technology is “population inversion.” Normally, most of the particles that make up a gain medium lack energy and are in the ground state. However, pumping the medium puts most or all the particles in an excited state. This results in a powerful, focused laser beam. Lasers are classified as operating in either continuous or pulsed mode. In continuous mode, the power output is continuous and constant. In pulse mode, the laser output takes on the form of intermittent pulses of light.

Background and History

In 1917, Albert Einstein theorized about the process of stimulated emission, which makes lasers possible. The precursor of the laser was the maser (microwave amplification by stimulated emission of radiation). A patent for the maser was granted to American physicists Charles Hard Townes and Arthur L. Schawlow on March 24, 1959. The maser did not emit light but amplified radio signals for space research.

In 1958, Townes and Schawlow published scientific papers that theorized about visible lasers. Also in 1958, Gordon Gould, a doctoral student at Columbia University under Townes, began building an optical laser. He did not obtain a patent until 1977.

In 1960, the first gas laser (helium-neon) was invented by Iranian American physicist Ali Javan. This type of laser, which converts electrical energy to light, has many practical applications, including laser surgery. In 1962, American engineer Robert N. Hall invented the semiconductor laser, which has many applications for communications systems and electronic appliances. In 1969, Gary Starkweather, a Xerox researcher, demonstrated the use of a laser beam in printing. Laser printers were a marked improvement over the dot-matrix printer. The print quality was much better, and they could print on a single sheet of paper rather than a continuous pile of fan-folded paper.

In September 1976, Sony first demonstrated an optical audio disk. This was the precursor to compact discs (CDs) and digital versatile discs (DVDs). In 1977, telephone companies began trials using fiber-optic cables to carry telephone traffic. In 1987, New York City ophthalmologist Stephen Trokel performed the first laser surgery on a patient's eyes.

How It Works

Laser Ablation. Laser ablation involves removing material from a surface (usually a solid but occasionally a liquid). A pulsed laser is most commonly used. However, at a high intensity, continuous laser can ablate material. At lower levels of laser energy, the material is heated and evaporates. At higher levels, the material is converted to plasma. Plasma is similar to gas, but it differs from gas in that some of the particles are ionized (electrically charged) due to the loss of electrons. Because of this ionization, plasma is electrically conductive.

Laser Cutting. Laser cutting uses laser energy to cut materials either by melting, burning, or vaporizing. Laser cutting is extremely focusable to about 25 microns (one-quarter the width of a human hair). Thus, a minimal amount of material is removed. The three common types of lasers used for cutting are carbon dioxide (CO₂), neodymium (Nd), and neodymium-yttrium aluminum garnet (Nd-YAG).

Laser Guidance. Laser guidance involves the use of a laser beam to guide a projectile (a bomb or a bullet) to a target. In its simplest form, such as a beam emitted from a rifle, the shooter points the laser beam so that the bullet will hit the target. A much more complex process is the guidance of a missile or a bomb. In some cases, the missile contains a laser-homing device in which the projectile “rides” the laser beam to the target. More commonly, a technique referred to as semi-active laser homing (SALH) is employed. With SALH, the laser beam is kept pointed at the target after the projectile is launched. Laser energy is scattered from the target, and as the missile approaches the target, heat sensors home in on this energy. If a target does not reflect laser energy well, the beam is aimed at a reflective source near the target.

Laser Lighting Displays. The focused beam emitted by a laser makes it useful for light shows. The bright, narrow beam is highly visible in the night sky. The beam can also be used to draw images on a variety of surfaces, such as walls or ceilings, or even theatrical smoke. The image can be reflected from mirrors to produce laser sculptures. The beam can be moved at any speed in different directions by the use of a galvanometer, which deflects the beam via an electrical current. The variety of vivid colors available with lasers enhances the visual effects.

Laser Printing. Laser printing involves projecting an image onto an electrically charged rotating drum that is coated with a photoconductor. When exposed to light, the electrical conductivity of the photoconductor increases. The drum then picks up particles of dry ink (toner). These particles are picked up by varying degrees depending on the amount of charge. The toner is then applied to a sheet of paper. The process involves rapidly “painting” the image line by line. Printers can be either monochrome (one color, usually black) or color. Color printers contain toners in four colorscyan, magenta, yellow, and black. A separate drum is used for each toner. The mixture of the colors on the toners produces a crisp, multicolored image. Duplex laser printers are available, which print on both sides of a sheet of paper. Some duplex printers are manual devices, which require the operator to flip one or more pages manually when indicated. Automatic duplexers mechanically turn each sheet of paper and feed it past the drum twice.

Optical Disks. Optical disks are flat, circular disks that contain binary data in the form of microscopic pits, which are non-reflective and form a binary value of 0. Smooth areas are reflective and form a binary value of 1. Optical disks are both created and read with a laser beam. The disks are encoded in a continuous spiral running from the center of the disk to the perimeter. Some disks are dual-layered. With these disks, after reaching the perimeter, a second spiral track is etched back to the center. The amount of data storage is dependent on the wavelength of the laser beam. The shorter the wavelength, the greater the storage capacity (shorter-wavelength lasers can read a smaller pit on the disk surface). For example, the high-capacity Blu-ray Disc (BD) uses short-wavelength blue light. Laser can be used to create a master disk from which duplicates can be made by a stamping process.

Laser 3-D Scanners. Laser three-dimensional (3-D) scanners analyze an object via a laser beam. The collected data is used to construct a digital, 3-D object of the model. In addition to shape, some scanners can replicate color.

Applications and Products

Laser technology includes many business, entertainment, industrial, medical, and military applications. The reflective ability of a laser beam has one major drawback—it can inadvertently strike an unintended target. For example, a reflected laser beam could damage an eye, so laser operators wear protective goggles. Products range from inexpensive laser pointers and CDs to surgical, industrial, and military devices costing hundreds of thousands of dollars. The following applications and products are a representative sample and are not comprehensive.

Optical Disks. Most optical disks are read-only, but some are rewritable. They store data, computer programs, music, graphic images, and video games. Since the first CD in 1982, this technology has evolved markedly. Optical data storage has, in large part, supplanted storage on magnetic tape. Although optical storage media can degrade over time due to environmental factors, they are more durable than magnetic tape, which continuously loses its magnetic charge and is subject to wear as it passes through the rollers and recording head. This is not the case for optical media because only the laser beam has contact with the recording surface. CDs are primarily used to store music. A five-inch CD can hold an entire album, replacing the vinyl record, which was subject to wear and degradation. A limitation of the CD is its 700 megabytes of data (eighty minutes of music) storage capacity. Three years before the introduction of the CD, the larger LaserDisc appeared for home video use. However, it never attained popularity in the United States. The DVD, which appeared in 1996, rapidly gained popularity and soon outpaced VHS tape to store feature-length movies. The DVD can store 4.7 gigabytes of data in single-layer format and 8.5 gigabytes in dual-layer format. The development of high-definition (HD) television fueled the development of higher-capacity storage media. After several years of format war, the Blu-ray DVD won out over the HD disc. The Blu-ray disc can store about six times the amount of data as a standard DVD, around 25 gigabytes in single-layer format and 50 gigabytes in dual-layer format.

Medical Uses. Medical applications of laser technology exist for abdominal, ophthalmic, and vascular surgeries and in dermatology. The narrow laser beam can cut through tissue and cauterize small blood vessels at the same time. Laser can be used with an open surgical incision as well as laparoscopic procedures in which the surgery is performed through small incisions for passage of the laparoscope and surgical instruments. The surgeon and surgical assistants can view images on a video monitor. Mirrors can be used to deflect the laser beam in the desired direction. However, the surgeon must be extremely careful when directing a laser beam at an internal structure–inadvertent reflection of the beam can damage tissue (for example, puncture the bowel).

A common ophthalmic procedure is the radial keratotomy. In this procedure, fine laser cuts are made in a radial fashion around the cornea. These precision cuts can correct myopia (nearsightedness) and hyperopia (farsightedness). Laser is also used to treat a number of eye problems, such as a detached retina (separation of the imaging surface of the retina from the back of the eye), glaucoma (increased pressure within the eyeball), and cataracts (clouding of the lens).

Atherosclerosis is a narrowing of the blood vessels due to plaque formation. Lasers can be used to vaporize the plaque. It also can be used to bore small holes within the heart to improve circulation within the heart muscle.

Dermatology procedures, cosmetic and medical, can be performed with a laser. Laser dermatologic applications include acne treatment and acne scar removal, removal of age spots, skin resurfacing, scar removal, tattoo removal, spider vein removal, and hair removal.

French researchers reported a noninvasive technique to diagnose cystic fibrosis prenatally. Cystic fibrosis causes thick mucus secretions to form in the lungs and glands like the pancreas, resulting in the progressive deterioration of the affected organs. The technology involves the identification of affected fetuses by laser microdissection of a fetal cell circulating in the mother's bloodstream. The procedure avoids the risk of a miscarriage when chorionic villus sampling or amniocentesis is used.

Medical devices are made of plastic or metal and are surgically placed in the body, such as stents inserted in the heart to improve blood flow or hip-replacement prosthetics. These devices can become coated with biofilm, which is of bacterial origin and resistant to antibiotics. Compared to hand curettes, ultrasonic devices, and nylon brushes, the Er:YAG laser (erbium-doped yttrium aluminum garnet laser) is a much better removal tool for biofilm. Additionally, it has an antibacterial effect.

Industrial Uses. Lasers can make precision cuts on metal or other materials. A minimal amount of material is removed, leaving a smooth, polished surface on both sides of the cut. It also can weld and heat-treat materials. Other industrial uses include marking and measuring parts, laser cutting, and engraving in metals, glass, ceramics, and plastics. The aerospace, automotive, and healthcare fields also benefit from laser-based three-dimensional printing.

Business Uses. The supermarket bar-code scanner was one of the earliest laser applications, first appearing in 1974. Laser printers are ubiquitous in all but the smallest business offices. They range in price from less than USD$100 to more than USD$10,000. The higher-priced models have features such as duplexing capability, color, high-speed printing, and collating.

Military Uses. In addition to marking targets and weapons guidance, the laser is used by the military for defensive countermeasures. It also can produce temporary blindness, which can temporarily impair an enemy's ability to fire a weapon or engage in another harmful activity. The military also uses laser guidance systems. Defensive countermeasure applications include small infrared lasers, which confuse heat-seeking missiles to intercept lasers, and power boost-phase intercept laser systems, which contain a complex system of lasers that can locate, track, and destroy intercontinental ballistic missiles (ICBMs). Intercept systems are powered by chemical lasers. When deployed, a chemical reaction results in the quick release of large amounts of energy.

Surveying and Ranging. A 3-D laser scanner emits beams of laser lights around the area to be surveyed. The created point clouds can be mapped in software to develop a 3-D digital map. It can be used to survey inaccessible areas safely.

Robotics. Robots use lasers for remote sensing and to calculate distances. It can help them find dimensions of obstacles, long-range data for surveys and help them navigate autonomously.

Laser Lighting Displays. Laser light displays are popular worldwide. The displays range from simple to complex and are often choreographed to music. A popular laser multimedia display is Hong Kong's Symphony of Lights, which is presented nightly. The display is accentuated during Tet, the Vietnamese New Year. The exteriors of 44 buildings on either side of Victoria Harbour are illuminated with a synchronized, multicolored laser display, which is accompanied by music. More than four million visitors and locals have viewed the display.

Careers and Course Work

The laser technology industry offers careers ranging from entry-level positions, such as lighting display operators, to high-level and highly technical positions, which require a scientific or engineering degree. The high-level positions require at least a bachelor's degree with coursework in several fields related to laser technology—engineering, physics, computer science, mathematics, and robotics. Many of the positions require a master's degree or doctorate. Positions are available for individuals with a degree in laser engineering in both the government and private sectors. The ability to be a team player is often of value for these positions because ongoing research is often a collaborative effort. University research positions are also available. In this arena, the employee is expected to divide his or her time between research and teaching. The Board of Laser Safety (BLS) offers certifications for medical and non-medical laser safety officers. Postgraduate courses include photonics technologies, lasers in dentistry, biophysics, and biophotonics.

Laser technicians are needed in various fields, including medical, business, and entertainment. Many of these positions require some training beyond high school at a community college or trade school. Dermatologists and other medical specialists often employ technicians to perform laser procedures. If a company employs several laser technicians, supervisory positions may be available. Aspirants can work as optical design engineers, principal optical engineers, laser processing engineers, and laser processing engineers.

Social Context and Future Prospects

Laser applications have become innumerable and ubiquitous. Many aspects of daily living taken for granted are possible because of laser technology, such as ringing up groceries or making a phone call. Research in the field continues to find limitless applications for lasers and develop new technology. The research on military applications is particularly vigorous. Developing lasers as weapons has been hindered by the ability to generate enough power to produce a laser blast with sufficient destruction power. In March 2009, Northrop Grumman launched their 100 kilowatt (100,000 watts) weapons-grade laser. (100 kilowatts is enough to power about six U.S. homes for a month). Tests were conducted by the U.S. Army on the device at the White Sands Missile Range in New Mexico in 2010. Further tests were conducted by the U.S. Navy in 2011 before the project was canceled, again because of the impracticality of the amount of electrical power needed. By 2014, the Navy’s AN/SEQ-3 Laser Weapon System (LaWS) was operational as an anti-drone weapon. Further development led to the creation of the high energy laser with integrated optical-dazzler and surveillance (HELIOS), which is a 60-kilowatt laser weapon developed by Lockheed Martin to temporarily disorient or blind its target.

Medical applications for lasers are expanding rapidly. The consumer, however, must be aware that adding the “laser” adjective to a procedure does not necessarily make it superior to previous techniques. The laser scalpel, for example, is another means of cutting tissue. Scientists in the United Arab Emirates developed laser beam technology for mass COVID-19 testing with instant results. Scientists continue to investigate advancements in the field, including applications in quantum computing and communication, sensing capabilities, imaging techniques, space exploration, and sustainability.

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