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A solid-state device that consists of two outer semiconductor layers separated by a middle layer and generates laser radiation when charge carriers of opposite polarity, one each from the top and bottom layers, meet in the middle layer. Also called diode laser.
Background
A laser, which is an acronym for Light Amplification by Stimulated Emission of Radiation, is a device that converts energy into light. Electrical or optical energy is used to excite atoms or molecules, which then emit light. A laser consists of a cavity, with plane or spherical mirrors at the ends, that is filled with lasable material. This material can be excited to a semistable state by light or an electric discharge. The material can be a crystal, glass, liquid, dye, or gas as long as it can be excited in this way.
The simplest cavity has two mirrors, one that totally reflects and one that reflects between 50 and 99%. As the light bounces between these mirrors, the intensity increases. Since the laser light travels as an intense beam, the laser produces very bright light. Laser beams can also be projected over great distances, and can be focused to a very small spot.
The type of mirror determines the type of beam. A very bright, highly monochromatic (one wavelength or one color) and coherent beam is produced when one mirror transmits only 1-2% of the light. If plane mirrors are used, the beam is highly collimated (made parallel). The beam comes out near one end of the cavity when concave mirrors are used. The type of beam in the first case makes lasers very useful in medicine since these properties allow the doctor to target the desired area more accurately, avoiding damage to surrounding tissue.
A semiconductor laser converts electrical energy into light. This is made possible by using a semiconductor material, whose ability to conduct electricity is between that of conductors and insulators. By doping a semiconductor with specific amounts of impurities, the number of negatively charged electrons or positively charged holes can be changed.
Compared to other laser types, semiconductor lasers are compact, reliable and last a long time. Such lasers consist of two basic components, an optical amplifier and a resonator. The amplifier is made from a direct-bandgap semiconductor material based on either gallium arsenide (GaAs) or InP substrates. These are compounds based on the Group III and Group V elements in the periodic table. Alloys of these materials are formed onto the substrates as layered structures containing precise amounts of other materials.
The resonator continuously recirculates light through the amplifier and helps to focus it. This component usually consists of a waveguide and two plane-parallel mirrors. These mirrors are coated with a material to increase or decrease reflectivity and to improve resistance to damage from the high power densities.
The performance and cost of a semiconductor depends on its output power, brightness, and operating lifetime. Power is important because it determines the maximum throughput or feed rate of a process. High brightness, or the ability to focus laser output to a small spot, determines power efficiency. Lifetime is important because the longer a laser lasts, the less it costs to operate, which is especially critical in industrial applications.
The simplest semiconductor lasers consist of a single emitter that produces over one watt of continuous wave power. To increase power, bars and multibar modules or stacks have been developed. A bar is an array of 10 to 50 side-by-side individual semiconductor lasers integrated into a single chip and a stack is a two-dimensional array of multiple bars. Bars can produce 50 watts of output power and last over 5,000 hours. Because such high powers produce a lot of heat, cooling systems must be incorporated into the design.
History
The concept behind lasers was first proposed by Albert Einstein, who showed that light consists of wave energies called photons. Each photon has an energy that corresponds to the frequency of the waves. The higher the frequency, the greater the energy carried by the waves. Einstein and another scientist named S. N. Bose then developed the theory behind the phenomenon of photons' tendency to travel together.
Laser action was first demonstrated in the microwave region in 1954 by Nobel Prize winner Charles Townes and his co-workers. They projected a beam of ammonia molecules through a system of focusing electrodes. When microwave power of appropriate frequency was passed through the cavity, amplification occurred and the term Microwave Amplification by Stimulated Emission of Radiation (M.A.S.E.R.) was born. The term laser was first coined in 1957 by physicist Gordon Gould.
Townes also worked with Arthur Schawlow and the two proposed the laser in 1958, receiving a patent in 1960. The first practical laser was invented that same year by a physicist named Theodore Maiman, while he was employed at Hughes Research Laboratories. This laser used a pink ruby crystal surrounded by a flash tube enclosed within a polished aluminum cylindrical cavity cooled by forced air. Two years later, a continuous lasing ruby was made by replacing the flash lamp with an arc lamp.
In 1962, laser action in a semiconductor material was demonstrated by Robert Hall and researchers at General Electric, with other United States researchers soon following. It took about another decade for the first semiconductor diode laser to be developed that could operate at room temperature, which was first demonstrated by Russian researchers. Bell Labs followed the Russian researchers' success, while also improving laser lifetimes. In 1975, Diode Laser Labs of New Jersey introduced the first commercial room-temperature semiconductor laser.
Despite this progress, these lasers still were inadequate for telecommunications applications. Instead they found wide use (after other performance and lifetime improvements) in audio compact disks after Philips (The Netherlands) and Sony (Japan) developed a CD in 1980 using a diode laser. By the end of the decade, tens of millions of CD players were being sold every year. More recently, digital video disks have become available for optical storage, which are also based on diode lasers.
As power has increased, semiconductor lasers have expanded into other applications. Since 1995, performance of high-power diode lasers has jumped by a factor of 25. With this higher reliability, large groups of diode lasers can now be combined to create "stacks" of up to 25 individual diode lasers.
In 1999, laser-diode revenues represented 64% of all lasers sold, up from 57% in 1996, and were projected to reach 69% in 2000. In terms of units sold, semiconductor lasers have accounted for about 99% of the total (over 400 million units), which means most laser light is now produced directly or indirectly (via diode pumping) by semiconductor lasers. In addition to industrial applications, semiconductor lasers are being used as pump sources for solid state lasers and fiber lasers, in graphics applications such as color proofing and digital direct-to-plate printing, and for various medical and military applications (target illumination and ranging). In 2000, Laser Focus World estimated that about 34% of medical therapy lasers were of the semiconductor type.
Raw Materials
The conventional semiconductor laser consists of a compound semiconductor, gallium arsenide. This material comes in the form of ingots that are then further processed into substrates to which layers of other materials are added. The materials used to form these layers are precisely weighed according to a specific formula. Other materials that are used to make this type of laser include certain metals (zinc, gold, and copper) as additives (dopants) or electrodes, and silicon dioxide as an insulator.
Design
The basic design of a semiconductor laser consists of a "double heterostructure." This consists of several layers that have different functions. An active or light amplification layer is sandwiched between two cladding layers. These cladding layers provide injection of electrons into the active layer. Because the active layer has a refractive index larger than those of the cladding layers, light is confined in the active layer.
The performance of the laser can be improved by changing the junction design so that diffraction loss in the optical cavity is reduced. This is made possible by modifying the laser material to control the index of refraction of the cavity and the width of the junction. The index of refraction of the material depends upon the type and quantity of impurity. For instance, if part of the gallium in the positively-charged layer is replaced by aluminum, the index of refraction is reduced and the laser light is better confined to the optical cavity.
The width of the junction can also affect the performance. A narrow dimension confines the current to a single line along the length of the laser, increasing the current density. Peak power output must be limited to no more than 400 watts per cm (0.4 in) length of the junction and current density to less than 6,500 amperes per centimeter squared at the junction to extend the life of the laser.
The Manufacturing Process
Making the substrate
Growing the layers
2 The most common method for growing the layers onto the substrate is called liquid-phase epitaxy (LPE). Layers that have the same or fixed crystal-growth direction as that of the substrate can be grown on the substrate when the substrate comes into contact with a solution of the desired composition. As the temperature is decreased, the semiconductor compound (such as GaAs) comes out of the solution in crystalline form and is deposited onto the substrate.
An LPE system consists of a reactor (where the layers are grown), a substrate loading system, a pump and exhaust system (for removing air or impure gases after the materials are put in or taken out), a gas flow system (to move hydrogen gas through the reactor to remove impure gases) and a temperature control system. Pure materials are used for making the reactor so that the layers are not contaminated. The loading box is usually filled with nitrogen gas to purge the air while opening the reactor. The reactor typically consists of a quartz tube, in which a graphite boat and boat holder are placed. The graphite boat consists of an outer frame, a substrate holder, a spacer and a melt box.
Fabricating the laser device
Quality Control
The substrate onto which the semiconductor structure is grown must meet certain requirements regarding crystal direction, etch pit density (EPD), impurity concentration, substrate thickness and wafer size. The crystal direction must be within several degrees. Etch pits, which are rectangular hills or holes, are revealed by selectively etching the substrate with some type of acid solution. The etch pit density (number of etch pits per square centimeter) is used for estimating dislocation density, which affects the laser lifetime. An EPD of 103 per centimeter squared or less is required. Impurity concentrations are around 1018 per cubic centimeter. Substrates can range in size up to 3 in (7.6 cm) in diameter and typically are sliced into 350-micron thick pieces.
After the growth process, the surface of the semiconductor wafer is examined by an optical microscope. To examine the layered structure, a ground or cleaved cross section of wafer is stained and etched to increase the contrast of the layers using a scanning electron microscope. X-ray diffraction is used to determine the compositions of the layers and to measure the lattice patterns of the structure. The impurity concentration and refractive index of the layers is also measured using several analytical methods. After the laser device is fabricated, such operating parameters as voltage/current curves, threshold current density and spectral characteristics are measured.
The Future
Industry analysts from Frost & Sullivan predict that the diode laser systems market will reach nearly $4.6 billion by 2005. This growth is partially due to expanding applications in materials processing as high-power diode lasers become less expensive than solid state lasers. The compact size and electrical efficiency also make high-power semiconductor lasers attractive for industrial applications such as heat treating and welding. New material compositions and processing methods are also being developed to expand the applications.
Where to Learn More
Books
Iga, Kenichi, and Susumu Kinoshita. Process Technology for Semiconductor Lasers: Crystal Growth and Microprocesses. New York: Springer-Verlag, 1996.
The Photonics Dictionary, 4th International Edition. Pittsfield, MA: Laurin Publishing, 2000
Periodicals
Anderson, Stephen. "Diodes Dominate Laser Applications." Laser Focus World (April 2000). http://lfw.pennnet.com (January 2001).
Lang, Robert. "Semiconductor Lasers: What's Vital to Commercial Devices." The Photonics Design and Applications Handboook 2000 Pittsfield, MA: Laurin Publishing, 2000.
"Making Photons: Lasers and Light Sources." Photonics Spectra (January 2000): 90-92.
Matthews, Steve. "Out of the Lab and into the Home." Laser Focus World (May 2000). http://lfw.pennnet.com (January 2001).
Matthews, Steve. "Semiconductor Lasers 2000: The Early Years: Promise and Problems." Laser Focus World (April 2000). http://lfw.pennnet.com (January 2001).
McComb, Stephen, and Michael Atchley. "High-Power Laser Diodes: Reliable, Multikilowatt Semiconductor Lasers Mature." Laser Focus World (December 1999). http://lfw.pennnet.com (January 2001).
[Article by: Laurel M. Sheppard]
A laser diode is a laser where the active medium is a semiconductor similar to that found in a light-emitting diode. The most common and practical type of laser diode is formed from a p-n junction and powered by injected electrical current. These devices are sometimes referred to as injection laser diodes to distinguish them from optically pumped laser diodes, which are more easily produced in the laboratory.
A laser diode, like many other semiconductor devices, is formed by doping a very thin layer on the surface of a crystal wafer. The crystal is doped to produce an n-type region and a p-type region, one above the other, resulting in a p-n junction, or diode.
As in other diodes, when this structure is forward biased, holes from the p-region are injected into the n-region, where electrons are the majority carrier. Similarly, electrons from the n-region are injected into the p-region, where holes are the majority carrier. When an electron and a hole are present in the same region, they may recombine by spontaneous emission—that is, the electron may re-occupy the energy state of the hole, emitting a photon with energy equal to the difference between the electron and hole states involved. These injected electrons and holes represent the injection current of the diode, and spontaneous emission gives the laser diode below lasing threshold similar properties to an LED. Spontaneous emission is necessary to initiate laser oscillation, but it is a source of inefficiency once the laser is oscillating.
Under suitable conditions, the electron and the hole may coexist in the same area for quite some time (on the order of microseconds) before they recombine. Then a nearby photon with energy equal to the recombination energy can cause recombination by stimulated emission. This generates another photon of the same frequency, travelling in the same direction, with the same polarization and phase as the first photon. This means that stimulated emission causes gain in an optical wave (of the correct wavelength) in the injection region, and the gain increases as the number of electrons and holes injected across the junction increases. The spontaneous and stimulated emission processes are vastly more efficient in direct bandgap semiconductors than in indirect bandgap semiconductors, thus silicon is not a common material for laser diodes.
As in other lasers, the gain region is surrounded with an optical cavity to form a laser. In the simplest form of laser diode, an optical waveguide is made on that crystal surface, such that the light is confined to a relatively narrow line. The two ends of the crystal are cleaved to form perfectly smooth, parallel edges, forming a Fabry-Perot resonator. Photons emitted into a mode of the waveguide will travel along the waveguide and be reflected several times from each end face before they are emitted. As a light wave passes through the cavity, it is amplified by stimulated emission, but light is also lost due to absorption and by incomplete reflection from the end facets. Finally, if there is more amplification than loss, the diode begins to "lase".
Some important properties of laser diodes are determined by the geometry of the optical cavity. Generally, in the vertical direction, the light is contained in a very thin layer, and the structure supports only a single optical mode in the direction perpendicular to the layers. In the lateral direction, if the waveguide is wide compared to the wavelength of light, then the waveguide can support multiple lateral optical modes, and the laser is known as "multi-mode". These laterally multi-mode lasers are adequate in cases where one needs a very large amount of power, but not a small diffraction-limited beam; for example in printing, activating chemicals, or pumping other types of lasers.
In applications where a small focused beam is needed, the waveguide must be made narrow, on the order of the optical wavelength. This way, only a single lateral mode is supported and one ends up with a diffraction limited beam. Such single spatial mode devices are used for optical storage, laser pointers, and fiber optics. Note that these lasers may still support multiple longitudinal modes, and thus can lase at multiple wavelengths simultaneously.
The wavelength emitted is a function of the band-gap of the semiconductor and the modes of the optical cavity. In general, the maximum gain will occur for photons with energy slightly above the band-gap energy, and the modes nearest the gain peak will lase most strongly. If the diode is driven strongly enough, additional side modes may also lase. Some laser diodes, such as most visible lasers, operate at a single wavelength, but that wavelength is unstable and changes due to fluctuations in current or temperature.
Due to diffraction, the beam diverges (expands) rapidly after leaving the chip, typically at 30 degrees vertically by 10 degrees laterally. A lens must be used in order to form a collimated beam like that produced by a laser pointer. If a circular beam is required, cylindrical lenses and other optics are used. For single spatial mode lasers, using symmetrical lenses, the collimated beam ends up being elliptical in shape, due to the difference in the vertical and lateral divergences. This is easily observable with a red laser pointer.
The simple diode described above has been heavily modified in recent years to accommodate modern technology, resulting in a variety of types of laser diodes, as described below.
The simple laser diode structure, described above, is extremely inefficient. Such devices require so much power that they can only achieve pulsed operation without damage. Although historically important and easy to explain, such devices are not practical.
In these devices, a layer of low bandgap material is sandwiched between two high bandgap layers. One commonly-used pair of materials is gallium arsenide (GaAs) with aluminium gallium arsenide (AlxGa(1-x)As). Each of the junctions between different bandgap materials is called a heterostructure, hence the name "double heterostructure laser" or DH laser. The kind of laser diode described in the first part of the article may be referred to as a homojunction laser, for contrast with these more popular devices.
The advantage of a DH laser is that the region where free electrons and holes exist simultaneously—the active region—is confined to the thin middle layer. This means that many more of the electron-hole pairs can contribute to amplification—not so many are left out in the poorly amplifying periphery. In addition, light is reflected from the heterojunction; hence, the light is confined to the region where the amplification takes place.
If the middle layer is made thin enough, it acts as a quantum well. This means that the vertical variation of the electron's wavefunction, and thus a component of its energy, is quantised. The efficiency of a quantum well laser is greater than that of a bulk laser because the density of states function of electrons in the quantum well system has an abrupt edge that concentrates electrons in energy states that contribute to laser action.
Lasers containing more than one quantum well layer are known as multiple quantum well lasers. Multiple quantum wells improve the overlap of the gain region with the optical waveguide mode.
Further improvements in the laser efficiency have also been demonstrated by reducing the quantum well layer to a quantum wire or to a "sea" of quantum dots.
In a quantum cascade laser, the difference between quantum well energy levels is used for the laser transition instead of the bandgap. This enables laser action at relatively long wavelengths, which can be tuned simply by altering the thickness of the layer. As of 2005, quantum cascade lasers have not yet been widely commercialized.
The problem with the simple quantum well diode described above is that the thin layer is simply too small to effectively confine the light. To compensate, another two layers are added on, outside the first three. These layers have a lower refractive index than the centre layers, and hence confine the light effectively. Such a design is called a separate confinement heterostructure (SCH) laser diode.
Almost all commercial laser diodes since the 1990s have been SCH quantum well diodes.
Distributed feedback lasers (DFB) are the most common transmitter type in DWDM-systems. To stabilize the lasing wavelength, a diffraction grating is etched close to the p-n junction of the diode. This grating acts like an optical filter, causing a single wavelength to be fed back to the gain region and lase. Since the grating provides the feedback that is required for lasing, reflection from the facets is not required. Thus, at least one facet of a DFB is anti-reflection coated. The DFB laser has a stable wavelength that is set during manufacturing by the pitch of the grating, and can only be tuned slightly with temperature. Such lasers are the workhorse of demanding optical communication.
Vertical-cavity surface-emitting lasers (VCSELs) have the optical cavity axis along the direction of current flow rather than perpendicular to the current flow as in conventional laser diodes. The active region length is very short compared with the lateral dimensions so that the radiation emerges from the surface of the cavity rather than from its edge as shown in Fig. 2. The reflectors at the ends of the cavity are dielectric mirrors made from alternating high and low refractive index quarter-wave thick multilayer.
There are several advantages to producing VCSELs when compared with the production process of edge-emitting lasers. Edge-emitters cannot be tested until the end of the production process. If the edge-emitter does not work, whether due to bad contacts or poor material growth quality, the production time and the processing materials have been wasted. Additionally, because VCSELs emit the beam perpendicular to the active region of the laser as opposed to parallel as with an edge emitter, tens of thousands of VCSELs can be processed simultaneously on a three inch Gallium Arsenide wafer. Furthermore, even though the VCSEL production process is more labor and material intensive, the yield can be controlled to a more predictable outcome.
Such dielectric mirrors provide a high degree of wavelength-selective reflectance at the required free surface wavelength λ if the thicknesses of alternating layers d1 and d2 with refractive indices n1 and n2 are such that n1d1 + n2d2 = ½λ which then leads to the constructive interference of all partially reflected waves at the interfaces. But there is a disadvantage because of the high mirror reflectivities, VCSELs have lower output powers when compared to edge emitting lasers.
Vertical external-cavity surface-emitting lasers, or VECSELs, are similar to VCSELs. In VCSELs, the mirrors are typically grown epitaxially as part of the diode structure, or grown separately and bonded directly to the semiconductor containing the active region. VECSELs are distinguished by a construction in which one of the two mirrors is external to the diode structure. As a result, the cavity includes a free-space region. A typical distance from the diode to the external mirror would be 1 cm.
One of the most interesting features of any VECSEL is the thin-ness of the semiconductor gain region in the direction of propagation, less than 100 nm. In contrast, a conventional in-plane semiconductor laser entails light propagation over distances of from 250 µm upward to 2 mm or longer. The significance of the short propagation distance is that it causes the effect of "antiguiding" nonlinearities in the diode laser gain region to be minimized. The result is a large-cross-section single-mode optical beam which is not attainable from in-plane ("edge-emitting") diode lasers.
Several workers demonstrated optically pumped VECSELs, and they continue to be developed for many applications including high power sources for use in industrial machining (cutting, punching, etc.) because of their unusually high power and efficiency when pumped by multi-mode diode laser bars.
Electrically pumped VECSELs have also been demonstrated. Applications for electrically pumped VECSELs include projection displays, served by frequency doubling of near-IR VECSEL emitters to produce blue and green light.
Laser diodes have similar reliability and failure issues as light emitting diodes. In addition, they are subject to catastrophic optical damage (COD) when operated at higher power.
Many of the advances in reliability of diode lasers in the last 20 years remain proprietary to their developers. The reliability of a laser diode can make or break a product line. Moreover, "reverse engineering" is not always able to uncover the differences between more-reliable and less-reliable diode laser products.
At the edge of a diode laser, where light is emitted, a mirror is traditionally formed by cleaving the semiconductor wafer to form a specularly reflecting plane. This approach is facilitated by the weakness of the [110] crystallographic plane in III-V semiconductor crystals (such as GaAs, InP, GaSb, etc.) compared to other planes. A scratch made at the edge of the wafer and a slight bending force causes a nearly atomically perfect mirror-like cleavage plane to form and propagate in a straight line across the wafer.
But it so happens that the atomic states at the cleavage plane are altered (compared to their bulk properties within the crystal) by the termination of the perfectly periodic lattice at that plane. Surface states at the cleaved plane, have energy levels within the (otherwise forbidden) bandgap of the semiconductor.
Essentially as a result, when light propagates through the cleavage plane and transits to free space from within the semiconductor crystal, a fraction of that light energy is absorbed by the surface states whence it is converted to heat by phonon-electron interactions. This heats the cleaved mirror. In addition, the mirror may heat simply because the edge of the diode laser—which is electrically pumped—is in less-perfect contact with the mount that provides a path for heat removal. The heating of the mirror causes the bandgap of the semiconductor to shrink in the warmer areas. The bandgap shrinkage brings more electronic band-to-band transitions into alignment with the photon energy causing yet more absorption. This is thermal runaway, a form of positive feedback, and the result can be melting of the facet, known as catastrophic optical damage, or COD.
In the 1970's, this problem, which is particularly nettlesome for GaAs-based lasers emitting between 1 µm and 0.630 µm wavelengths (less so for InP based lasers used for long-haul telecommunications which emit between 1.3 µm and 2 µm), was identified. Michael Ettenberg, a researcher and later Vice President at RCA Laboratories' David Sarnoff Research Center in Princeton, New Jersey, devised a solution. A thin layer of aluminum oxide was deposited on the facet. If the aluminum oxide thickness is chosen correctly it functions as an anti-reflective coating, reducing reflection at the surface. This alleviated the heating and COD at the facet.
Since then, various other refinements have been employed. One approach is to create a so-called non-absorbing mirror (NAM) such that the final 10 µm or so before the light emits from the cleaved facet are rendered non-absorbing at the wavelength of interest.
In the very early 1990s, SDL, Inc. began supplying high power diode lasers with good reliability characteristics. CEO Donald Scifres and CTO David Welch presented new reliability performance data at, e.g., SPIE Photonics West conferences of the era. The methods used by SDL to defeat COD were considered to be highly proprietary and have still not been disclosed publicly as of June, 2006.
In the mid-1990s, IBM Research (Ruschlikon, Switzerland) announced that it had devised its so-called "E2 process" which conferred extraordinary resistance to COD in GaAs-based lasers. This process, too, has never been disclosed as of June, 2006.
Reliability of high-power diode laser pump bars (employed to pump solid state lasers) remains a difficult problem in a variety of applications, in spite of these proprietary advances. Indeed, the physics of diode laser failure is still in the process of being worked out and research on this subject remains active, if proprietary.
Extension of the lifetime of laser diodes is critical to their continued adaptation to a wide variety of applications.
Laser diodes are numerically the most common type of laser, with 2004 sales of approximately 733 million diode lasers,[1] as compared to 131,000 of other types of lasers.[2]
Laser diodes find wide use in telecommunication as easily modulated and easily coupled light sources for fiber optics communication. They are used in various measuring instruments, eg. rangefinders. Another common use is in barcode readers. Visible lasers, typically red but later also green, are common as laser pointers. Both low and high-power diodes are used extensively in the printing industry both as light sources for scanning (input) of images and for very high-speed and high-resolution printing plate (output) manufacturing. Infrared and red laser diodes are common in CD players, CD-ROMs and DVD technology. Violet lasers are used in HD-DVD and Blu-Ray technology. High-power laser diodes are used in industrial applications such as heat treating, cladding, seam welding and for pumping other lasers, such as diode pumped solid state lasers. The use of diode lasers for high-speed, low-cost, combustion spectroscopy is being explored.
In general, applications of laser diodes can be categorized in various ways. Most applications of diode lasers can be served by larger solid state lasers or optical parametric oscillators but it is the ability to mass-produce diode lasers at low cost that makes them essential for mass-market applications. Diode lasers have application to virtually every field of endeavor that attracts wide attention today. Since light has many different properties (power, wavelength & spectral quality, beam quality, polarization, etc.) it is interesting to classify applications by these basic properties.
Many applications of diode lasers primarily make use of the "directed energy" property of an optical beam. In this category one might include the laser printers, bar-code readers, image scanning, illuminators, designators, optical data recording, combustion ignition, laser surgery, industrial sorting, industrial machining, and directed energy weaponry. Some of these applications are emerging whereas many are familiar to the wider society.
Applications which may today or in the future make use of the "coherent" properties of diode-laser-generated light include interferometric distance measurement, holography, coherent communications, and coherent control of chemical reactions.
Applications which may make use of "narrow spectral" properties of diode lasers include range-finding, telecommunications, infra-red countermeasures, spectroscopic sensing, generation of radio-frequency or terahertz waves, atomic clock state preparation, quantum key cryptography, frequency doubling and conversion, water purification (in the UV), and photodynamic therapy (where a particular wavelength of light would cause a substance such as porphyrin to become chemically active as an anti-cancer agent only where the tissue is illuminated by light).
Applications where the ability to "generate ultra-short pulses of light" by the technique known as "mode-locking" include clock distribution for high-performance integrated circuits, high-peak-power sources for laser-induced breakdown spectroscopy sensing, arbitrary waveform generation for radio-frequency waves, photonic sampling for analog-to-digital conversion, and optical code-division-multiple-access systems for secure communication.
The first to demonstrate coherent light emission from a semiconductor diode (the first laser diode), included Robert N. Hall and his team at the General Electric research center in November 1962.[3]
Other teams at IBM, MIT Lincoln Laboratory, Texas Instruments, and RCA Laboratories were also involved in and receive credit for historic initial demonstrations of efficient light emission and lasing in semiconductor diodes in 1962 and thereafter.
The first visible light laser diode was demonstrated by Nick Holonyak, Jr., later in 1962[4]
The first laser diode to achieve continuous wave operation was a double heterostructure demonstrated essentially simultaneously by Zhores Alferov and collaborators (including Dmitri Z. Garbuzov) of the Soviet Union, and Morton Panish and Izuo Hayashi working in the United States.
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 | Dictionary. The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2007. Published by Houghton Mifflin Company. All rights reserved. Read more |
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| How Products are Made. How Products are Made. Copyright © 2002 by The Gale Group, Inc. All rights reserved. Read more |
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| Wikipedia. This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Laser diode". Read more |
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