Laser

Background and history

How it works

Stimulated emission

Oscillation

Solid state lasers

Gas lasers

Applications

Communications (diode lasers)

Materials processing (CO₂ and Nd:YAG)

Optical data storage (laser diodes)

Surgery (Nd:YAG, CO₂, holmium, erbium)

Resources

The laser is a device that uses the principle of stimulated emission to produce light. The qualities of the light generated by a laser are significantly different from that generated by a conventional source such as an incandescent light bulb or fluorescent light tube. These major differences include:

• Divergence: the laser generally emits a pencil thin beam of light whose divergent angle is closely related to the wavelength and limiting aperture size.
• Bandwidth: the light emitted by the laser generally consists of a very narrow range of wavelengths, or color.
• Intensity: the output from a laser is typically orders of magnitude higher in intensity (measured in watts per square meter) than a conventional light source.
• Coherence; the output from a laser is generally coherent; that is, the peaks and troughs of the light waves all correspond, allowing the light to form clear interference patterns.

Background and history

In the 1950s, there was a push by scientists to develop sources of coherent electromagnetic radiation at wavelengths shorter than vacuum tubes could provide. Charles Townes and co-workers at Columbia University, New York, developed the ammonia maser (microwave amplification by stimulated emission of radiation) in 1954, a device which produced coherent microwaves. In 1958, Townes and Art Schawlow published the principles of a maser operating in the visible region of the electromagnetic spectrum. The first successful demonstration of a laser followed in 1960 by Theodore Maiman of Hughes Laboratories, who operated a pulsed ruby laser that generated several kilowatts of optical power.

In the following few years, several different laser systems were demonstrated in gases (helium neon mixture, carbon dioxide, argon, krypton) and solids (uranium, samarium, neodymium, nickel, cobalt, and vanadium ions implanted in electrically insulating crystalline hosts). Since that time, laser action has been demonstrated in many different materials, involving all four states of matter (solid, liquid, gas, and plasma), covering the range of wavelengths from x rays to submillimeter waves. Only a few types of laser find widespread use because of issues such as efficiency, ease of use, reliability, and cost.

How it works

The laser consists of the following components: the pump, which is the source of energy to drive the laser, the active medium, which is where the stored energy is converted into the laser light through the process of stimulated emission, the optical cavity, which is usually made up of a pair of mirrors, one whose reflectivity at the wavelength of the laser is as close to 100% as possible and the other with a reflectivity less than 100%, through which the output beam propagates.

Stimulated emission

Stimulated emission is a process similar to absorption, but operates in the opposite direction. In absorption, an incoming photon is absorbed by an atom, leaving the atom in an excited state and annihilating the photon in the process. In stimulated emission, an incoming photon stimulates an excited atom to give up its stored energy in the form of a photon that is identical in wavelength, direction, polarization, and phase to the stimulus photon. If the excited atom is unable to produce a photon that matches the incoming photon, then stimulated emission cannot take place.

For laser action to occur, a majority of the atoms in the active medium must be excited into an energetic state, creating a population inversion of energized atoms ready to emit light. This is generally accomplished by pumping the atoms optically or electrically. As a photon passes through the collection of excited atoms, it can stimulate the generation of many trillions of photons, or more, creating an avalanche of light. The active medium can thus be regarded as an amplifier that takes in a small signal (one photon, for example) and delivers a large signal (many photons, all identical to the first) at the output. This amplification, or gain, is provided by stimulated emission; hence the term laser, which is actually an acronym for light amplification by stimulated emission of radiation.

To illustrate laser operation, consider the well-known helium neon (HeNe) laser, once a staple of supermarket scanners. The active medium is a mixture of helium and neon gases, enclosed in a glass tube a few inches long, with an electrode and a mirror at each end. The atoms in the gas mixture are excited, or pumped, by an electrical discharge that runs through the gas, in much the same way that a neon sign is lit. The conditions in the HeNe laser have been optimized so that the maximum number of neon atoms are in the correct state to emit light at the familiar red wavelength, 633 nm (nanometers).

Oscillation

Stimulated emission alone is not sufficient to produce laser output. The emission from the atoms occurs in all directions; to produce the highly directional output that makes the laser such a useful tool, the light must be trapped in an optical cavity that provides feedback, i.e., forces the light to travel in a desired direction. An optical cavity is free conduction band electrons recombine with holes in the valence band, emitting photons equal in energy to the band gap of the semiconductor. The only suitable semiconductors are the so-called direct gap materials, such as gallium arsenide and indium phosphide. By using alloys of different compositions, the laser band-gap can be engineered to produce light over a wide range of wavelengths (426 to 1,550 nm). Silicon, having an indirect gap, is not suitable for diode lasers.

Diode lasers are typically very small (the laser chip, with wires running into it, is mounted on a copper block only 6 mm wide). The cavity is formed by cleaved crystal facets that act as mirrors. The cavity length is typically between 100 microns and 1 mm, and the emitting volume has a cross-section of about 1 micron high by 1 to 200 microns wide. A diode laser can produce power in the range 1 mW to 1 W, depending on the size of the active volume. Standard semiconductor processing techniques can be used to form arrays of individual lasers in order to generate higher powers: an array with an emitting area of 1 cm × 1 cm can produce several kW of optical power.

Solid state lasers

Although the first laser demonstrated was a solid state ruby laser, for many years the most common commercial systems were gas lasers such as helium neon lasers and argon ion lasers, or lasers based on organic dyes. Helium neon lasers were frequently limited in output power, argon ion lasers required expensive, sophisticated power supplies and cooling sources, and the dyes used in dye lasers were messy and often toxic. In the past decade, solid state lasers and diode lasers have become the dominant players in the commercial marketplace.

In a solid state laser, the active species is distributed throughout a solid, usually crystalline, material, although glass can also be used as a host. The lasers are robust and frequently tunable, though heat dissipation can sometimes be an issue. Certain types of solid state crystals, for example neodymium-doped yttriumaluminum garnet (Nd:YAG), can be pumped by diode lasers instead of by other lasers or by flash-lamps, which is often the case for other materials. Such diode-pumped, solid state systems are reliable, economical, compact, and easy to operate–in fact, many commercial systems are turnkey, needing only to be plugged in and turned on to operate.

Solid state lasers are available from mid-infrared to ultraviolet wavelengths, and at a variety of output powers. Many solid-state lasers are tunable, providing output across a range of wavelengths. Some of the most sophisticated laser systems developed are solid-state-based, including ultrafast systems that produce light pulses just 6 femtoseconds (10^-15 seconds) long, and are used to study molecular dynamics. Other uses for solid state lasers include medical applications, materials processing, and remote sensing.

Gas lasers

Another common laser class is that of gas lasers, which includes helium neon (HeNe) lasers, carbon dioxide (CO₂) lasers, nitrogen lasers, and so on. The helium neon laser, widely used until the advent of the diode laser, was one of the first types developed and commercialized. As described above, it is a discharge-pumped gas laser, which generally produces an output measuring a few mW in power.

Like the HeNe laser, the CO₂ laser is a gas laser powered by an electrical discharge. However, the photon generated by stimulated emission arises from a vibrational transition within the molecule, rather than an electronic transition within an atom. As a result, the photon energy is much less than the laser systems described so far, and the wavelength is correspondingly longer at 10.6 æm. The CO₂ laser is one of the most efficient laser sources, having an efficiency in excess of 10%. Since the gas can flow through the discharge tube, the gas can easily take away excess heat and the laser can be cooled very effectively; this allows the CO₂ laser to operate at high average powers up to around 10 kW.

Applications

When it was first invented, the laser was called “a solution looking for a problem” because few good applications could be found for it. This is no longer the case, and the laser has found its way into many uses in every day life. In 2004, according to Laser Focus World, about 131,000 lasers (excluding diode lasers) were sold in the world, with about 733 million diode lasers sold. The major application areas for the laser are in communications, materials processing, optical data storage, surgery, defense, and scientific research.

Communications (diode lasers)

A pulsed infrared laser beam traveling down a single strand of optical fiber can carry thousands of times the information that can be carried by an electrical signal over copper wire. Not only can a single optical signal travel more rapidly than an electrical signal, but also optical signals of different wavelengths, or colors, can travel down a fiber simultaneously, and without interfering. This technique is known as wavelength division multiplexing (WDM). Using WDM, the capacity of an optical network can be increased by a factor number of wavelengths, or channels used—if two channels are used, the network capacity doubles, without the need for an additional fiber; if four channels are used, the network capacity quadruples. To offer an analogy, a WDM network is like a high-speed superhighway compared to the one-lane country road that is copper wire.

Low-loss optical fiber is lighter, more compact, and less expensive than the more traditional copper wire. Transcontinental and transoceanic optical fiber systems installed for telephone systems use directly modulated diode lasers operating at 1550 nm to generate the signals. Because even optical fiber generates some loss, the signals must be amplified periodically by repeaters, which also use laser technology. In the repeaters, a piece of optical fiber doped with erbium is pumped by a diode laser to optically amplify the original signal, which is launched back into the fiber link to continue its journey.

Materials processing (CO₂ and Nd:YAG)

Since a laser beam can be focused down to a very small spot of light that can be absorbed very well at the surface of a material (be it metal, plastic, textile, etc.), the material can reach very high temperatures up to 9,032°F (5,000°C) and melt or even vaporize. In factories, laser systems are used to measure parts, inspect them for quality, and label, cut, weld, or resurface materials ranging from plastic film to sheet steel a quarter of an inch thick.

Lasers form the basis of precision-measuring tools called interferometers that can measure distances less than 1/100th the thickness of a human hair, and are as useful on construction sites as in laboratories. Such instruments can be scanned over objects to create images, and are used on highways to identify vehicles automatically, or on NASA spacecraft to map the surface of the moon and asteroids, for instance. In semiconductor manufacturing, ultraviolet lasers provide the exposure source for optical lithography, a technique used to produce computer chips with features as small as one hundred thousandth of an inch (0.25 microns).

Optical data storage (laser diodes)

The fact that a laser beam can be focused down to a very small spot has been used by the information industry and implemented in the form of the compact disc. The area needed to store a bit of information on a disc surface is smaller when optical reading is used than when magnetic reading is used. Much more information can be stored on a compact disc (several gigabytes) than on an equivalent magnetic disc (several hundred megabytes). This represents a savings in space and cost, which has opened up new markets for optical data storage-music CDS, CD-ROM, and optical storage libraries for large computer systems.

Surgery (Nd:YAG, CO₂, holmium, erbium)

Lasers have a variety of applications in the field of medicine. Using laser beams whose wavelength is absorbed strongly; surgeons can cut and remove tissue with great precision by vaporizing it, with little damage to the surrounding tissue. The systems are used in minimally-invasive surgical techniques such as angioplasty (removing plaque from artery walls) and lithotripsy (the destruction of kidney stones in the bladder), in which a hair-thin optical fiber feeds light into the body through a tiny incision; the small opening required for the fiber results in reduced scarring and shorter healing times. In a technique called photorefractive keratectomy, lasers are routinely used to correct myopia and astigmatism. Other medical applications include removal of birthmarks, tattoos, and small varicose veins from the skin; welding tissue and sealing blood vessels during surgery; or resurfacing skin to minimize wrinkles or sun damage.

KEY TERMS

Absorption —The destruction of a photon when its energy is used by an atom to jump from a lower energy level to a higher level.

Coherent light —A light beam where the phase difference between any two points on a line perpendicular to the direction of propagation is constant with time.

Optical cavity —An optical structure formed by at least two mirrors in which a light beam can be made to circulate in such a way that it retraces the same path every round trip.

Phase —The term phase is used when comparing two or more optical waves. If the waves are at the same point in their cycle at the same moment in time, they are said to be in phase.

Photon —The particle associated with light. A photon is emitted by an atom when the atom undergoes a shift in internal energy from a high state to a lower state: the excess energy is carried off by the photon.

Stimulated emission —A process of emitting light in which one photon stimulates the generation of a second photon which is identical in all respects to the first.

Cutting-edge researchers are using visible and infrared lasers to increase blood flow to oxygen-starved regions of the heart, to visually detect cancer cells on the skin’s surface, or to image through the skin to monitor blood flow or detect cancerous regions. In a technique called photodynamic therapy, laser light activates a drug that accumulates in certain types of tumors, causing the drug to destroy the cancerous tissue. A sophisticated diagnostic technique called spectroscopy takes advantage of the fact that cancerous tissue reflects light differently than non-cancerous tissue, allowing doctors to identify skin cancer and cervical cancer. A laser-based optical mammography system is even under development.

See also Hologram and holography; Laser surgery.

Resources

BOOKS

Chang, William Shen-chie. Principles of Lasers and Optics. Cambridge, UK, and New York: Cambridge University Press, 2005.

Csele, Mark. Fundamentals of Light Sources and Lasers. Hoboken, NJ: J. Wiley, 2004.

Hecht, Jeff. Laser Pioneers. New York: Academic Press, 1992.

Hitz, C. Breck. Introduction to Laser Technology. New York: IEEE Press, 2001.

Keller, Gregory, et al. Lasers in Aesthetic Surgery. New York: Thieme Medical Publishing, 2001.

Nair, K.P. Raiappan. Atoms, Molecules, and Lasers. Oxford, UK: Alpha Science, 2006.

Niemz, Markolf H. Laser-Tissue Interactions: Fundamentals and Applications. 2nd ed. Berlin: Springer Verlag, 2002.

Iain A. McIntyre

Laser

The laser is a device that uses the principle of stimulated emission to produce light . The qualities of the light generated by a laser are significantly different from that generated by a conventional source such as an incandescent light bulb or fluorescent light tube. These major differences include: divergence: the laser generally emits a pencil thin beam of light whose divergent angle is closely related to the wavelength and limiting aperture size. bandwidth: the light emitted by the laser generally consists of a very narrow range of wavelengths, or color . intensity: the output from a laser is typically orders of magnitude higher in intensity (measured in watts per square meter) than a conventional light source. coherence; the output from a laser is generally coherent; that is, the peaks and troughs of the lightwaves all correspond, allowing the light to form clear interference patterns.

Background and history

In the 1950s, there was a push by scientists to develop sources of coherent electromagnetic radiation at wavelengths shorter than vacuum tubes could provide. Charles Townes and co-workers at Columbia University, New York, developed the ammonia maser (microwave amplification by stimulated emission of radiation) in 1954, a device which produced coherent microwaves. In 1958, Townes and Art Schawlow published the principles of a maser operating in the visible region of the electromagnetic spectrum . The first successful demonstration of a laser followed in 1960 by Theodore Maiman of Hughes Laboratories, who operated a pulsed ruby laser which generated several kilowatts of optical power.

In the following few years, several different laser systems were demonstrated in gases (helium neon mixture, carbon dioxide , argon, krypton) and solids (uranium , samarium, neodymium, nickel, cobalt, and vanadium ions implanted in electrically insulating crystalline hosts). Since that time, laser action has been demonstrated in many different materials, involving all four states of matter (solid, liquid, gas, and plasma), covering the range of wavelengths from x rays to submillimeter waves. Only a few types of laser find widespread use because of issues such as efficiency, ease of use, reliability, and cost.

How it works

The laser consists of the following components: the pump, which is the source of energy to drive the laser; the active medium, which is where the stored energy is converted into the laser light through the process of stimulated emission; the optical cavity, which is usually made up of a pair of mirrors , one whose reflectivity at the wavelength of the laser is as close to 100% as possible and the other with a reflectivity less than 100%, through which the output beam propagates.

Stimulated emission

Stimulated emission is a process similar to absorption, but operates in the opposite direction. In absorption, an incoming photon is absorbed by an atom, leaving the atom in an excited state and annihilating the photon in the process. In stimulated emission, an incoming photon stimulates an excited atom to give up its stored energy in the form of a photon that is identical in wavelength, direction, polarization, and phase to the stimulus photon. If the excited atom is unable to produce a photon that matches the incoming photon, then stimulated emission cannot take place.

For laser action to occur, a majority of the atoms in the active medium must be excited into an energetic state, creating a population inversion of energized atoms ready to emit light. This is generally accomplished by pumping the atoms optically or electrically. As a photon passes through the collection of excited atoms, it can stimulate the generation of many trillions of photons, or more, creating an avalanche of light. The active medium can thus be regarded as an amplifier that takes in a small signal (one photon, say) and delivers a large signal (many photons, all identical to the first) at the output. This amplification, or gain, is provided by stimulated emission; hence the term laser, which is actually an acronym for light amplification by stimulated eission of radiation.

To illustrate laser operation, consider the well-known helium neon (HeNe) laser, once a staple of super-market scanners. The active medium is a mixture of helium and neon gases, enclosed in a glass tube a few inches long, with an electrode and a mirror at each end. The atoms in the gas mixture are excited, or pumped, by by an electrical discharge that runs through the gas, in much the same way that a neon sign is lit. The conditions in the HeNe laser have been optimized so that the maximum number of neon atoms are in the correct state to emit light at the familiar red wavelength, 633 nm.

Oscillation

Stimulated emission alone is not sufficient to produce laser output. The emission from the atoms occurs in all directions; to produce the highly directional output that makes the laser such a useful tool, the light must be trapped in an optical cavity that provides feedback, i.e., forces the light to travel in a desired direction. An optical cavity is free conduction band electrons recombine with holes in the valence band, emitting photons equal in energy to the band gap of the semiconductor. The only suitable semiconductors are the so-called "direct gap" materials, such as gallium arsenide and indium phosphide. By using alloys of different compositions, the laser bandgap can be engineered to produce light over a wide range of wavelengths (426 nm-1,550 nm). Silicon, having an indirect gap, is not suitable for diode lasers.

Diode lasers are typically very small (the laser chip, with wires running into it, is mounted on a copper block only 6 mm wide). The cavity is formed by cleaved crystal facets that act as mirrors. The cavity length is typically between 100 microns and 1 mm, and the emitting volume has a cross-section of about 1 micron high by 1-200 microns wide. A diode laser can produce power in the range 1 mW to 1 W, depending on the size of the active volume. Standard semiconductor processing techniques can be used to form arrays of individual lasers in order to generate higher powers: an array with an emitting area of 1 cm X 1 cm can produce several kW of optical power.

Solid state lasers

Although the first laser demonstrated was a solid state ruby laser, for many years the most common commercial systems were gas lasers such as helium neon lasers and argon ion lasers, or lasers based on organic dyes. Helium neon lasers were frequently limited in output power, argon ion lasers required expensive, sophisticated power supplies and cooling sources, and the dyes used in dye lasers were messy and often toxic. In the past decade, solid state lasers and diode lasers have become the dominant players in the commercial marketplace.

In a solid state laser, the active species is distributed throughout a solid, usually crystalline, material, although glass can also be used as a host. The lasers are robust and frequently tunable, though heat dissipation can sometimes be an issue. Certain types of solid state crystals, for example neodymium-doped yttrium aluminum garnet (Nd:YAG), can be pumped by diode lasers instead of by other lasers or by flashlamps, which is often the case for other materials. Such diode-pumped, solid state systems are reliable, economical, compact, and easy to operate—in fact, many commercial systems are turnkey, needing only to be plugged in and turned on to operate.

Solid state lasers are available from mid-infrared to ultraviolet wavelengths, and at a variety of output powers. Many solid state lasers are tunable, providing output across a range of wavelengths. Some of the most sophisticated laser systems developed are solid-state-based, including ultrafast systems that produce light pulses just 6 femtoseconds (10-15 seconds) long, and are used to study molecular dynamics. Other uses for solid state lasers include medical applications, materials processing, and remote sensing .

Gas lasers

Another common laser class is that of gas lasers, which includes helium neon (HeNe) lasers, carbon dioxide (CO2) lasers, nitrogen lasers, and so on. The helium neon laser, widely used until the advent of the diode laser, was one of the first types developed and commercialized. As described above, it is a discharge-pumped gas laser, which generally produces an output measuring a few mW in power.

Like the HeNe laser, the CO2 laser is a gas laser powered by an electrical discharge. However, the photon generated by stimulated emission arises from a vibrational transition within the molecule , rather than an electronic transition within an atom. As a result, the photon energy is much less than the laser systems described so far, and the wavelength is correspondingly longer at 10.6 æm. The CO2 laser is one of the most efficient laser sources, having an efficiency in excess of 10%. Since the gas can flow through the discharge tube, the gas can easily take away excess heat and the laser can be cooled very effectively; this allows the CO2 laser to operate at high average powers up to around 10 kW.

Applications

When it was first invented, the laser was called "a solution looking for a problem" because few good applications could be found for it. This is no longer the case, and the laser has found its way into many uses in every day life. The major application areas for the laser are in communications, materials processing, optical data storage , surgery , defense, and scientific research.

Communications (diode lasers)

A pulsed infrared laser beam traveling down a single strand of optical fiber can carry thousands of times the information which can be carried by an electrical signal over copper wire. Not only can a single optical signal travel more rapidly than an electrical signal, but optical signals of different wavelengths, or colors, can travel down a fiber simultaneously, and without interfering. This technique is known as wavelength division multiplexing (WDM). Using WDM, the capacity of an optical network can be increased by a factor number of wavelengths, or channels used—if two channels are used, the network capacity doubles, without the need for an additional fiber; if four channels are used, the network capacity quadruples. To offer a good analogy, a WDM network is like a high-speed superhighway compared to the one-lane country road that is copper wire.

Low-loss optical fiber is lighter, more compact, and less expensive than the more traditional copper wire. Transcontinental and transoceanic optical fiber systems installed for telephone systems use directly modulated diode lasers operating at 1550 nm to generate the signals. Because even optical fiber generates some loss, the signals must be amplified periodically by repeaters, which also use laser technology. In the repeaters, a piece of optical fiber doped with erbium is pumped by a diode laser to optically amplify the original signal, which is launched back into the fiber link to continue its journey.

Materials processing (CO2 and Nd:YAG)

Since a laser beam can be focused down to a very small spot of light which can be absorbed very well at the surface of a material (be it metal , plastic, textile, etc.), the material can reach very high temperatures up to 9,032°F (5,000°C) and melt or even vaporize. In factories, laser systems are used to measure parts, inspect them for quality, and label, cut, weld, or resurface materials ranging from plastic film to sheet steel a quarter of an inch thick.

Lasers form the basis of precision-measuring tools called interferometers that can measure distances less than 1/100th the thickness of a human hair, and are as useful on construction sites as in laboratories. Such instruments can be scanned over objects to create images, and are used on highways to identify vehicles automatically, or on NASA spacecraft to map the surface of the Moon and asteroids. In semiconductor manufacturing, ultraviolet lasers provide the exposure source for optical lithography , a technique used to produce computer chips with features as small as one hundred thousandth of an inch (0.25 microns).

Optical data storage (laser diodes)

The fact that a laser beam can be focused down to a very small spot has been used by the information industry and implemented in the form of the compact disc . The area needed to store a bit of information on a disc surface is smaller when optical reading is used than when magnetic reading is used. Much more information can be stored on a compact disc (several gigabytes) than on an equivalent magnetic disc (several hundred megabytes). This represents a savings in space and cost, which has opened up new markets for optical data storage-music CDS, CD-ROM, and optical storage libraries for large computer systems.

Surgery (Nd:YAG, CO2, holmium, erbium)

Lasers have a variety of applications in the field of medicine. Using laser beams whose wavelength is absorbed strongly, surgeons can cut and remove tissue with great precision by vaporizing it, with little damage to the surrounding tissue. The systems are used in minimally-invasive surgical techniques such as angioplasty (removing plaque from artery walls) and lithotripsy (the destruction of kidney stones in the bladder), in which a hair-thin optical fiber feeds light into the body through a tiny incision; the small opening required for the fiber results in reduced scarring and shorter healing times. In a technique called photorefractive keratectomy, lasers are routinely used to correct myopia and astigmatism. Other medical applications include removal of birthmarks, tattoos, and small varicose veins from the skin; welding tissue and sealing blood vessels during surgery; or resurfacing skin to minimize wrinkles or sun damage.

Cutting-edge researchers are using visible and infrared lasers to increase bloodflow to oxygen-starved regions of the heart , to visually detect cancer cells on the skin's surface, or to image through the skin to monitor bloodflow or detect cancerous regions. In a technique called photodynamic therapy, laser light activates a drug that accumulates in certain types of tumors, causing the drug to destroy the cancerous tissue. A sophisticated diagnostic technique called spectroscopy takes advantage of the fact that cancerous tissue reflects light differently than non-cancerous tissue, allowing doctors to identify skin cancer and cervical cancer. A laser-based optical mammography system is even under development.

See also Hologram and holography; Laser surgery.

Resources

books

Hecht, Jeff. Laser Pioneers. New York: Academic Press, 1992.

Hecht, Jeff. Understanding Lasers. New York: IEEE Press, 1994.

Keller, Gregory, et al. Lasers in Aesthetic Surgery. New York: Thieme Medical Pub., 2001

Laurence, Clifford L. The Laser Book. Englewood Cliffs, NJ: Prentice Hall, 1986.

Niemz, Markolf H. Laser-Tissue Interactions: Fundamentals and Applications. 2nd ed. Berlin: Springer Verlag, 2002.

Stevens, Lawrence. Laser Basics. Englewood Cliffs, NJ: Prentice Hall, 1985.

Verdeyen, Joseph. Laser Electronics. Englewood Cliffs, NJ: Prentice Hall, 1994.

Iain A. McIntyre

KEY TERMS

Absorption

—The destruction of a photon when its energy is used by an atom to jump from a lower energy level to a higher level.

Coherent light

—A light beam where the phase difference between any two points on a line perpendicular to the direction of propagation is constant with time.

Optical cavity

—An optical structure formed by at least two mirrors in which a light beam can be made to circulate in such a way that it retraces the same path every round trip.

Phase

—The term phase is used when comparing two or more optical waves. If the waves are at the same point in their cycle at the same moment in time, they are said to be "in phase."

Photon

—The particle associated with light. A photon is emitted by an atom when the atom undergoes a shift in internal energy from a high state to a lower state: the excess energy is carried off by the photon.

Stimulated emission

—A process of emitting light in which one photon stimulates the generation of a second photon which is identical in all respects to the first.

Laser

Laser technologies are used for a wide range of purposes in laser-based products, including CD players, DNA screening machines, forensic tools, missile guiding devices, mapping and topographic instruments, and surgical devices. A laser is basically an intense beam of light. Ordinary light is scattered in variable wavelengths and frequencies, whereas laser beams are highly organized light with all photons traveling in the same frequency and wavelength. Laser (or light amplification by stimulated emission radiation) is a technology that allows controlled photonic release from atoms in specific wavelengths, thus producing a directional monochromatic (singlecolor) light beam of high coherence (e.g., tightly organized photons with synchronized wave fronts of the same frequency). Forensic science applications of laser technologies include a wide range of devices and techniques, such as laser spectroscopy , interferometric measurements ( laser mapping systems), laser scanning, bullet trajectory projections, and laser photography .

Laser technologies are a growing market in forensics and crime investigation, with new tools designed specifically for this field. Crime scene investigation and reconstruction ballistics can be a time-consuming task, with crucial evidence such as trace fingerprints sometimes overlooked by the human eye. Bullet trajectory calculations with tapes and traditional reconstruction methods may take several hours in complex crime scenes. The use of bullet trajectory laser rods improves precision and saves time. Laser rods are used to determine the exact point of origin and distance from which a gun was fired, or, when more than one person was shooting, the exact original location and trajectory angle of each bullet fired. Laser rods are placed in each bullet hole found in the scene and activated to emit light. Laser beams flowing from each hole will reproduce the complete trajectory pattern of all bullets fired, making visible the entire exchange in a manner that can be photographed. Therefore, forensic technicians are able to track the trajectory of each bullet back to its point of origin, as well as to identify bullets that ricocheted from objects and changed direction.

Another useful application of lasers in forensic science is spectroscopy. Spectroscopy involves the analysis of materials by studying the reflection and absorption of light for the identification of traces of substance residues such as accelerants, illegal drugs, or poisons. Laser spectroscopy determine the molecular structure of materials and chemical compounds. Infrared laser spectroscopy can determine molecular structures of polymers on surfaces and gas phase ions, and is used to detect explosive components or illegal drugs in samples. Some portable spectrometers can analyze evidence at the crime scene, inside plastic bags or glass bottles, water solutions, as well as residual particles on surfaces.

Laser fluorescence is another method of analysis that can be used at the crime scene. One practical example is the small portable lasers in the shape of narrow flashlights, which are used to scan surfaces of a scene in search of fingerprints. As the beam travels on the surface of objects, furniture, walls, or doors, fingerprints become visible due to the rapid absorption and release of light by atoms present in the printed substance. This time-saving scan allows the location of fingerprints in places where they would otherwise be hard to find, as well as the quick location of fingerprints in an entire area. Once located and mapped, fingerprints can be dusted with fluorescent powder to be photographed.

Often more than one method is used to detect toxic industrial components present in the environment, such as coupling plasma mass spectrometry with laser spectroscopy. These techniques are used by the Federal Bureau of Investigation (FBI ) to identify security dyes and gas residues in stolen cash. Laser desorption mass spectrometry (LDMS) is a technique used to identify substances in fabric, dyes, and security inks. Ink security systems are used to protect cash in ATM machines and bank safe contents. The ink is pressurized to release a concentrated red dye spray from the ATM cassettes when triggered by an anti-tampering electronic sensor, spraying an indelible stain on the currency. The skin and clothes of criminals are also marked, thus creating evidence. Other security systems use tear gas and red staining or smoke and dye for similar purposes. Smoke and dye in bank vaults release a hot cloud of red smoke that marks valuables and criminals, whereas tear gas and dye systems intend to stain evidence and temporarily disrupt the robbery, gaining time for the police arrival at the scene. LDMS is used to identify these markers in currencies and other items of evidence, and also facilitates tracking stolen currency in circulation.

Laser radars are law enforcement devices that measure the speed of vehicles. Laser speed guns are portable and can be pointed by police officers directly to a vehicle. A pulse infrared light is emitted towards the targeted vehicle, reflecting on its surface and returning to the gun where a sensor calculates the nanoseconds elapsed between emission and reflection, determining the distance to the car. As the car is in movement and the laser gun pulses laser light thousands of times per second, repeating calculations and comparing the many results, it can accurately determine the speed of the vehicle. Some laser speed devices are mounted on poles in strategic places by the roadside, in connection with high-speed photograph cameras that take a picture of the car and license number when triggered by the laser radar.

Other laser-based measurement tools, such as 3-D laser stations, are used to reconstruct the events underlying road accidents involving several vehicles or mass crime scenes such as nightclub or supermarket bombings. The scene is first photographed from all angles, and then 3-D mapping laser equipment is used to scan the entire area, registering several point positions. Some laser scanners have the capacity to capture 5,000 measurements per second, such as the one that was used in forensic analysis of the terrorist nightclub bombing in Bali in 2003. Photographs and mapping data are then downloaded into software that calculates point-to point distances and angles, automatically reconstructing three-dimensional images of the scene.

Some DNA typing machines also use laser fluorescence to identify certain molecules during the automated DNA sequencing process of certain DNA segments known as short tandem repeats (STR). STR sequences and lengths are so specific to individuals that they led to the expression "DNA fingerprinting." Another DNA technique is laser micro-dissection, used for sperm identification in semen samples. This method has high sensitivity, and permits the isolation of individual sperm cells from other cell types present in the sample. STR profiling can be accomplished from minute DNA samples with this technique, after DNA purification using high-sensitivity kits.

see also Accident reconstruction; Alternate light source analysis; Ballistics; Biosensor technologies; Bomb detection devices; Bomb (explosion) investigations; Bullet track; Chromatography; Crime scene reconstruction; Digital imaging; DNA; DNA fingerprint; DNA sequences, unique; DNA typing systems; Electromagnetic spectrum; Energy dispersive spectroscopy; FBI crime laboratory; Gas chromatograph-mass spectrometer; Geospatial Imagery; Impression evidence; Ink analysis; Laser ablation-inductively coupled plasma mass spectrometry; Latent fingerprint; Metal detectors; Monochromatic light; Paint analysis; Radiation, electromagnetic radiation injury; Scanning electron microscopy.

Laser

█ LARRY GILMAN

"Laser" is an acronym for lightwave amplification by stimulated emission of radiation. Lasers exploit the fact that electrons in atoms' outer orbitals can move between energy levels. Like a marble being shifted up and down a set of stairs, an electron can be raised to a higher energy level by giving it the right amount of energy or can give up a fixed amount of energy when it drops to a lower level. The energy given up when an electron drops to a lower level is emitted as a photon (minimal unit of light); the greater the energy lost by the electron, the shorter the wavelength of the emitted light. If the electrons in a material happen to be undergoing energy shifts corresponding to wavelengths that our eyes can see, the material is seen to "glow."

Laser light is a special type of glow. In some materials, a photon passing near an atom with an outer-orbital electron in a high-energy state can, without being absorbed or deflected, stimulate that electron to drop to a lower energy state. The electron gives up its energy in the form of a photon that is of the same wavelength as the impinging photon, in phase with it, and traveling in the same direction. (To say that two photons are "in phase" means that, if they are considered as waves extended through space, their peaks and troughs are aligned; peak matches peak and trough matches trough.) Such light is termed "coherent." Coherent light is rare in nature because atoms in most light sources (e.g., the Sun) are

emitting photons at random moments and in random directions, independently of each other. In a laser, however, a chain reaction or domino effect occurs.

The electrons in a sample of some substance, for example, a cylinder of gas or a cylindrical crystal of artificial sapphire, are first fed energy—"pumped" to high energy levels. (Pumping was accomplished in all early lasers by illuminating the laser's working substance with intense light, hence "lightwave amplification" in the acronym.) If enough of the atoms in the substance are in the excited state to begin with, a domino effect can begin when one atom emits a photon. This photon impinges on a nearby atom, causing it to release a photon having the same frequency, direction, and phase. These two photons go on to stimulate other atoms, which stimulate others, and so on. The result is that most of the energy locked up in the excited electrons of the laser's working substance is turned quickly into a burst of coherent light. A substance undergoing this process is said to "lase." The resulting light pulse, which is aligned with the long axis of the sample of lasing substance, can be very intense. Lasers that beam continuously, rather than pulsing, can also be built; the trick is to devise a means of continually reexciting the electrons in the lasing substance as their energy drains away as laser light.

Laser light has several important characteristics: (1) It forms a tight beam, that is, a beam that spreads only slightly with distance. (2) It can be very bright: it is commonplace for a laser to be brighter than the surface of the sun. (3) As all the photons in a given laser beam are produced by identical electron-orbital changes, they are all of the same frequency. That is, a laser beam is of an extremely pure color. (4) Because laser light is coherent, slight shifts in the frequency of laser light, such as those caused by the Doppler effect, are easy to detect. Also, light from a single laser source can be used to interfere with itself after following different paths to a common destination, allowing the extremely precise measurement of distances by the technique termed interferometry.

Since their invention in the 1950s, lasers have found thousands of applications in manufacturing, communications, medicine, astronomy and the other sciences, and weaponry. A few outstanding military applications of laser technology are as follows:

• Laser-guided weapons. The distinctive character of laser light—its coherence, brilliance, and purity of color—enables it to stand out from its surroundings, even during broad daylight. Thus, it is easy for a missile to home in on a target (e.g., tank or building) that has been "painted" or illuminated temporarily by a laser beam. Munitions that guide themselves to laser-painted targets are termed laser-guided weapons. Most of the precision-guided munitions in the U.S. arsenal today are laser-guided.
• Missile-defense lasers. Beginning with the Star Wars program proposed by President Ronald Reagan in the early 1980s, several schemes have been proposed for using large lasers to shoot down ballistic missiles. The Stars Wars program proposed orbital laser stations or x-ray lasers pumped by nuclear bombs to shoot down ballistic missiles; these ideas were abandoned as too expensive and, possibly, too susceptible to countermeasures. However, development of less-ambitious laser-defense schemes continues. In 2003 or 2004, the U.S. Air Force hopes to perform the first missile-shootdown tests of its YAL-1A Airborne Laser system, a powerful laser mounted on a modified Boeing 747 jetliner.
• LIDAR. LIDAR (light detection and ranging) is analogous to radar (radio detection and ranging), but has capabilities that radar does not. In its simplest form, it measures the distance from a laser transmitter to a reflective object by measuring how much time it takes for a laser pulse to make the round trip. Doppler LIDAR, like doppler radar, deduces the velocity of the target by measuring the frequency shift of the echo. LIDAR can also measure the composition of distant reflectors by sending paired laser beams having different frequencies; differing absorption by the substance reflecting the beams (e.g., smoke particles) reveals information about the chemical composition of the target. LIDAR is used by low-flying stealth aircraft to track terrain ahead of them; unlike conventional radar, LIDAR illuminates a very small area of terrain and so is difficult to detect.
• Virtual retinal displays. A virtual retinal display shines low-powered lasers mounted on a headset directly onto the retina of the human eye. The display lasers—one for each primary color—are directed at scanning mirrors that rapidly scan the lasers over the user's retina. (The eyes' own movements are tracked in real time and compensated for by a computer.) The scanning occurs so rapidly that the user perceives a solid image, not a moving dot of light. Virtual retinal displays have the advantage that they allow the user to see normally at the same time; the image produced by the virtual retinal display is superimposed over whatever else the user happens to be looking at. This can be a boon to pilots, allowing them to receive information from electronic sources without having to look away from their flight environment.

█ FURTHER READING:

ELECTRONIC:

"Lasers: Spontaneous and Stimulated Emission." Kottan Labs. 2001. <http://www.kottan-labs.bgsu.edu/teaching/workshop2001/chapter4a.pdf> (April 18, 2003).

"Virtual Retinal Display Technology." Naval Postgraduate School, Department of Computer Science. September 15, 1999. <http://www.cs.nps.navy.mil/people/faculty/capps/4473/projects/fiambolis/vrd/vrd_full.html#VRDworks> (April 18, 2003).

SEE ALSO

Laser Listening Devices

LASERS. Laser is an acronym for light amplification by stimulated emission of radiation. External energy pumped into the atoms of the lasing medium excites electrons to higher energy states; returning to their base state, they emit photons. Cascading photons produce a narrow, tightly focused beam of intense, coherent, monochromatic light.

The special properties of laser beams—intensity, coherence, directionality—held obvious promise for military purposes. Beginning promptly with the 1961 invention, mission‐oriented laser research and development centered on such practical applications as range finding and guidance.

Operational range finders began seeing field service during the Vietnam War by the mid‐1960s. Incorporated in fire control systems, they especially suited direct fire weapons like tank guns; such units for the M‐60 tank began service in 1968. Immediately successful, laser range‐finding and fire control systems rapidly became standard equipment. Laser simulators have also sharply enhanced training realism for tank gunners and infantry small arms.

Laser guidance, teaming ground‐based or airborne target designators with projectile‐borne sensors, was one of the precision methods that began revolutionizing air attack on surface targets from the late 1960s onward. The designator directs a laser beam at the target, the laser seeker picks up the reflected light, and the bomb or missile homes in on the illuminated target.

Laser‐guided bombs made their first appearance under the U.S. Air Force's Paveway program. Field modification kits for several standard bomb models began reaching Vietnam in 1971. Each included a laser seeker, guidance unit, and control canards bolted to the bomb's nose, enlarged tail fins bolted to the rear. This first Paveway generation met outstanding success in 1972 attacks on North Vietnamese bridges. Paveway II arrived in 1980, Paveway III in 1987, each kit more sophisticated and costly than its predecessor.

Augmented with an off‐the‐shelf rocket motor, Paveway II also became the basis for the navy's Skipper II air‐to‐surface missile. It entered service in 1985 as a low‐cost (though less capable) alternative to the Maverick, a 1977 version of which was the first U.S. laser‐guided missile. Superseded in 1983 by an upgraded model with a better laser seeker and larger warhead, the Maverick now largely serves a Marine close air support role.

The army fielded its first laser‐guided missile, the Hellfire, in the early 1980s. Developed specifically as an antitank missile for Apache attack helicopters, it could acquire its target after launch. Outstanding capabilities and performance led the army to adapt Hellfire for other aircraft and make it the focus of antitank tactics.

Less successful was the Copperhead cannon‐launched guided projectile, also intended as a tank killer. Production began in 1981, but persistent technical difficulties and escalating costs ended its procurement in 1990.

From the beginning, the laser's potential as a weapon excited military interest, peaking in the proposed missile defense system called the Strategic Defense Initiative. Other potential military roles for lasers, more or less speculative in the early 1990s, include laser equivalents of radar (LADAR), beam‐riding missiles, and communication systems.
[See also Antitank Weapons; Missiles.]

Bibliography

Bengt Anderberg and and Myron L. Wolbarsht , Laser Weapons: The Dawn of a New Military Age, 1992.
Guy Hartcup , The Silent Revolution: Development of Conventional Weapons, 1945–85, 1993.

Barton C. Hacker