A laser diode is a laser where the active medium is a semiconductor similar to that found in a light-emitting diode. A laser is a device that emits Light ( Electromagnetic radiation) through a process called Stimulated emission. A semiconductor' is a Solid material that has Electrical conductivity in between a conductor and an insulator; it can vary over that The most common and practical type of laser diode is formed from a p-n junction and powered by injected electrical current. A p-n junction is a junction formed by combining P-type and N-type Semiconductors together in very close contact Electric current is the flow (movement of Electric charge. The SI unit of electric current is the Ampere. 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.
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Principle of operation
A laser diode, like many other semiconductor devices, is formed by doping a very thin layer on the surface of a crystal wafer. In Semiconductor production doping is the process of intentionally introducing impurities into an extremely pure (also referred to as intrinsic) semiconductor to 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. An N-type semiconductor (N for Negative) is obtained by carrying out a process of doping, that is by adding an impurity of valence -five elements to A P-type semiconductor (P for Positive) is obtained by carrying out a process of doping, that is adding a certain type of atoms to the semiconductor in order Dioden2jpg|thumb|right|150px|Figure 2 Various semiconductor diodes
The many, many types of diode lasers known today collectively form a subset of the larger classification of semiconductor p-n junction diodes. Just as in any semiconductor p-n junction diode, forward electrical bias causes the two species of charge carrier, holes and electrons, to be "injected" from opposite sides of the p-n junction into the depletion region, situated at its heart. Holes are injected from the p-doped, and electrons from the n-doped, semiconductor. (A depletion region, devoid of any charge carriers, forms automatically and unavoidably as a result of the difference in chemical potential between n- and p-type semiconductors wherever they are in physical contact. )
As charge injection is a distinguishing feature of diode lasers as compared to all other lasers, diode lasers are traditionally and more formally called "injection lasers. " (This terminology differentiates diode lasers, e. g. , from flashlamp-pumped solid state lasers, such as the ruby laser. A solid-state laser is a Laser that uses a gain medium that is a Solid, rather than a Liquid such as in Dye lasers or a Gas Interestingly, whereas the term "solid-state" was extremely apt in differentiating 1950s-era semiconductor electronics from earlier generations of vacuum electronics, it would not have been adequate to convey unambiguously the unique characteristics defining 1960s-era semiconductor lasers. ) When an electron and a hole are present in the same region, they may recombine or "annihilate" with the result being spontaneous emission — i. In the Solid state physics of Semiconductors carrier generation and recombination are processes by which mobile Electrons and Electron holes Spontaneous emission is the process by which a light source such as an Atom, Molecule, Nanocrystal or nucleus in an Excited state e. , 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. (In a conventional semiconductor junction diode, the energy released from the recombination of electrons and holes is carried away as phonons, i. e. , lattice vibrations, rather than as photons. ) Spontaneous emission gives the laser diode below lasing threshold similar properties to an LED. The lasing threshold is the lowest excitation level at which a laser's output is dominated by Stimulated emission rather than by Spontaneous emission. Spontaneous emission is necessary to initiate laser oscillation, but it is one among several sources of inefficiency once the laser is oscillating.
The difference between the photon-emitting semiconductor laser (or LED) and conventional phonon-emitting (non-light-emitting) semiconductor junction diodes lies in the use of a different type of semiconductor, one whose physical and atomic structure confers the possibility for photon emission. These photon-emitting semiconductors are the so-called "direct bandgap" semiconductors. It is the nature of silicon and germanium, which are single-element semiconductors, that the bandgap does not align in such as way as to be considered "direct. " However, the so-called compound semiconductors, which have virtually the identical crystal structure as silicon or germanium but use alternating arrangements of two different atomic species in a checkerboard-like pattern break the symmetry and in doing so create the critical direct bandgap. Examples of compound semiconductors are gallium arsenide, indium phosphide, gallium antimonide, gallium nitride and so forth, and junction diodes fabricated from these materials emit light. Gallium arsenide ( GaAs) is a compound of two elements Gallium and Arsenic. Indium phosphide ( is a binary Semiconductor composed of Indium and Phosphorus. Gallium antimonide (GaSb is a semiconducting compound of Gallium and Antimony of the III-V family Gallium nitride ( is a very hard material commonly used in bright LEDs since the 1990s
In the absence of stimulated emission (e. g. , lasing) conditions, electrons and holes may coexist in proximity to one another, without recombining, for a certain time (termed the "upper-state lifetime" or "recombination time," about a nanosecond for typical diode laser materials before they recombine. Then a nearby photon with energy equal to the recombination energy can cause recombination by stimulated emission. In Optics, stimulated emission is the process by which an electron perturbed by a Photon having the correct energy may drop to a lower Energy level resulting This generates another photon of the same frequency, travelling in the same direction, with the same polarization and phase as the first photon. Polarization ( ''Brit'' polarisation) is a property of Waves that describes the orientation of their oscillations The phase of an oscillation or wave is the fraction of a complete cycle corresponding to an offset in the displacement from a specified reference point at time t = 0 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. In Semiconductor Physics, a direct Bandgap means that the minimum energy of the Conduction band lies directly above the maximum energy of the In Semiconductor Physics, an indirect bandgap is a Bandgap in which the minimum energy in the Conduction band is shifted by a Silicon (ˈsɪlɪkən or /ˈsɪlɪkɒn/ silicium is the Chemical element that has the symbol Si and Atomic number 14
As in other lasers, the gain region is surrounded with an optical cavity to form a laser. An optical cavity or optical resonator is an arrangement of Mirrors that forms a Standing wave Cavity resonator for Light waves Optical 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. A waveguide is a structure which guides waves such as Electromagnetic waves Light, or Sound waves 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. In Optics, stimulated emission is the process by which an electron perturbed by a Photon having the correct energy may drop to a lower Energy level resulting Finally, if there is more amplification than loss, the diode begins to "lase". The lasing threshold is the lowest excitation level at which a laser's output is dominated by Stimulated emission rather than by Spontaneous emission.
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. Laser pumping is the act of energy transfer from an external source into the Gain medium of a Laser.
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. Diffraction is normally taken to refer to various phenomena which occur when a wave encounters an obstacle A lens must be used in order to form a collimated beam like that produced by a laser pointer. A lens is an optical device with perfect or approximate Axial symmetry which transmits and refracts Light, converging or diverging 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. A laser pointer is a portable pen-sized Laser designed to be held in the hand and most commonly used to project a point of light to highlight items of interest during a
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.
Laser diode types
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.
Double heterostructure lasers
In these devices, a layer of low bandgap material is sandwiched between two high bandgap layers. In Solid state physics and related applied fields a band gap, also called an energy gap or bandgap, is an energy range in a solid where no electron states One commonly-used pair of materials is gallium arsenide (GaAs) with aluminium gallium arsenide (AlxGa(1-x)As). Gallium arsenide ( GaAs) is a compound of two elements Gallium and Arsenic. Aluminium gallium arsenide (also aluminum gallium arsenide) ( Al x Ga 1-x As) is a Semiconductor material with very Each of the junctions between different bandgap materials is called a heterostructure, hence the name "double heterostructure laser" or DH laser. A heterojunction is the interface that occurs between two layers or regions of dissimilar Crystalline Semiconductors These semiconducting materials have unequal 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. The active laser medium or gain medium is the source of optical Gain within a Laser. 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.
Quantum well lasers
If the middle layer is made thin enough, it acts as a quantum well. A quantum well is a Potential well that confines particles which were originally free to move in three dimensions to two dimensions forcing them to occupy a planar region This means that the vertical variation of the electron's wavefunction, and thus a component of its energy, is quantised. A wave function or wavefunction is a mathematical tool used in Quantum mechanics to describe any physical system 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. Origin of the concept of quantum wells In 1972 Charles H Henry, a physicist and newly-appointed Head of the Semiconductor Electronics Research Department at Bell Laboratories In statistical and Condensed matter physics, the density of states ( DOS) of a system describes the number of states at each energy level that are available
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. A waveguide is a structure which guides waves such as Electromagnetic waves Light, or Sound waves A normal mode of an oscillating system is a pattern of motion in which all parts of the system move sinusoidally with the same frequency
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 Condensed matter physics, a quantum wire is an electrically conducting Wire, in which Quantum effects are affecting transport A quantum dot is a Semiconductor whose Excitons are confined in all three Spatial dimensions.
In a quantum cascade laser, the difference between quantum well energy levels is used for the laser transition instead of the bandgap. Quantum cascade lasers (QCLs are Semiconductor lasers that emit in the mid- to far- Infrared portion of the Electromagnetic spectrum and were first demonstrated This enables laser action at relatively long wavelengths, which can be tuned simply by altering the thickness of the layer. In Physics wavelength is the distance between repeating units of a propagating Wave of a given Frequency.
Separate confinement heterostructure lasers
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. The refractive index (or index of Refraction) of a medium is a measure for how much the speed of light (or other waves such as sound waves is reduced inside the medium 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
Distributed feedback lasers (DFB) are the most common transmitter type in DWDM-systems. A distributed feedback laser ( DFB) is a type of Laser diode or Quantum cascade laser where the active region of the device is structured as a In Fiber-optic communications wavelength-division multiplexing ( WDM) is a technology which multiplexes multiple optical carrier signals on a 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. Anti-reflective or antireflection (AR coatings are a type of Optical coating applied to the surface of lenses and other optical devices to reduce 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.
VCSELs
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 vertical-cavity surface-emitting laser ( VCSEL;) is a type of semiconductor Laser diode with Laser beam emission perpendicular from the top surface 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. A dielectric mirror is a type of a Mirror composed of multiple thin layers of Dielectric material typically deposited on a substrate of Glass
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.
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.
VECSELs
Vertical external-cavity surface-emitting lasers, or VECSELs, are similar to VCSELs. A vertical-external-cavity surface-emitting-laser ( VECSEL) is a small Semiconductor laser similar to a Vertical-cavity surface-emitting laser (VCSEL 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. Epitaxy refers to the method of depositing a Monocrystalline film on a monocrystalline substrate 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. Second harmonic generation ( SHG; also called frequency doubling) is a nonlinear optical process in which Photons interacting with a nonlinear
Failure modes
Laser diodes have the same reliability and failure issues as light emitting diodes. Reliability engineering is an Engineering field that deals with the study of Reliability: the ability of a System or component to perform its required In addition they are subject to catastrophic optical damage (COD) when operated at higher power. Catastrophic optical damage ( COD) is a failure mode of high-power Semiconductor lasers It occurs when the Semiconductor junction is overloaded by exceeding
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. Reverse engineering (RE is the process of discovering the technological principles of a device object or system through analysis of its structure function and operation
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. Cleavage, in Mineralogy, is the tendency of crystalline materials to split along definite planes creating smooth surfaces of which there are several named types This approach is facilitated by the weakness of the [110] crystallographic plane in III-V semiconductor crystals (such as GaAs, InP, GaSb, etc. Crystallography is the experimental science of determining the arrangement of Atoms in Solids In older usage it is the scientific study of Crystals The Gallium arsenide ( GaAs) is a compound of two elements Gallium and Arsenic. Indium phosphide ( is a binary Semiconductor composed of Indium and Phosphorus. Gallium antimonide (GaSb is a semiconducting compound of Gallium and Antimony of the III-V family ) 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. Surface states are electronic states found at the surface of materials and are part of Condensed matter physics.
Essentially, as a result when light propagates through the cleavage plane and transits to free space from within the semiconductor crystal, a fraction of the light energy is absorbed by the surface states whence it is converted to heat by phonon-electron interactions. In Physics, a phonon is a quantized mode of vibration occurring in a rigid crystal lattice, such as the Atomic lattice of a Solid The electron is a fundamental Subatomic particle that was identified and assigned the negative charge in 1897 by J 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-than-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. Thermal runaway refers to a situation where an increase in temperature changes the conditions in a way that causes a further increase in temperature leading to a destructive result Positive feedback, sometimes referred to as "cumulative causation" is a Feedback loop system in which the system responds to perturbation in the same direction
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. RCA Corporation, founded as Radio Corporation of America, was an electronics company in existence from 1919 to 1986 Sarnoff Corporation, with headquarters in West Windsor New Jersey, is the former RCA Laboratories See also Princeton Township New Jersey, Borough of Princeton New Jersey Princeton Borough New Jersey Princeton Township New Jersey this 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. Anti-reflective or antireflection (AR coatings are a type of Optical coating applied to the surface of lenses and other optical devices to reduce 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. SPIE is a Not-for-profit international society for the exchange collection and dissemination of knowledge in Optics, Photonics, and Imaging engineering 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. Switzerland (English pronunciation; Schweiz Swiss German: Schwyz or Schwiiz Suisse Svizzera Svizra officially the Swiss Confederation 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 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.
Applications of laser diodes
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. "MMIV" redirects here For the Modest Mouse album see " Baron von Bullshit Rides Again " [2]
Laser diodes find wide use in telecommunication as easily modulated and easily coupled light sources for fiber optics communication. An optical fiber (or fibre) is a Glass or Plastic fiber that carries Light along its length They are used in various measuring instruments, eg. rangefinders. A rangefinder is a device that measures distance from the observer to a target for the purposes of Surveying, determining focus in Photography, or Another common use is in barcode readers. A barcode reader (or barcode scanner) is an electronic device for reading printed Barcodes Like a Flatbed scanner, it consists of a light source a lens Visible lasers, typically red but later also green, are common as laser pointers. For other uses see Visibility. Visible is billed as a not-for-profit free quarterly magazine dedicated to the history of Red is any of a number of similar Colors evoked by light consisting predominantly of the longest wavelengths of Light discernible by the human eye in the wavelength Green is a Color, the perception of which is evoked by light having a spectrum dominated by energy with a Wavelength of roughly 520–570- nm. A laser pointer is a portable pen-sized Laser designed to be held in the hand and most commonly used to project a point of light to highlight items of interest during a 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. Infrared ( IR) radiation is Electromagnetic radiation whose Wavelength is longer than that of Visible light, but shorter than that of A Compact Disc player (often written as compact disc player) or CD player, is an electronic device which plays audio Compact Discs CD players are often CD-ROM (an initialism of "Compact Disc Read-Only Memory " is a pre-pressed Compact Disc that contains data accessible to but not writable DVD (also known as " Digital Versatile Disc " or " Digital Video Disc " - see Etymology)is Violet lasers are used in HD DVD and Blu-ray technology. As the name of a color violet (named after the flower violet) is used in two senses first referring to the color of Light at the short- Wavelength not insert the publicly disclosed HD DVD key into this article for the time being Diode lasers have also found many applications in laser absorption spectrometry (LAS) for high-speed, low-cost assessment or monitoring of the concentration of various species in gas phase. Laser absorption spectrometry ( LAS) refers to techniques that utilize lasers to assess the concentration or amount of a species in gas phase by Absorption spectrometry 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. Diode-pumped solid-state (DPSS lasers are Solid-state lasers made by pumping a solid Gain medium, for example a Ruby or a Crystal
Applications of laser diodes can be categorized in various ways. Most applications could be served by larger solid state lasers or optical parametric oscillators, but the low cost of mass-produced diode lasers makes them essential for mass-market applications. Diode lasers can be used in a great many fields; 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. A laser printer is a common type of Computer printer that rapidly produces high quality text and graphics on plain paper A barcode reader (or barcode scanner) is an electronic device for reading printed Barcodes Like a Flatbed scanner, it consists of a light source a lens Document Scanning or Image Scanning is the action or process of converting text and graphic Paper documents Photographic film, Photographic paper Laser surgery is Surgery using a Laser to cut tissue instead of a Scalpel. Some of these applications are emerging while others are well-established.
Applications which may today or in the future make use of the coherence 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). Tunable diode laser absorption spectroscopy ( TDLAS) is a technique for measuring the concentration of certain species such as Methane, Water vapor A porphyrin is a heterocyclic Macrocycle derived from four Pyrroline subunits interconnected via their α carbon atoms via Methine bridges (=CH-
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.
History
The first to demonstrate coherent light emission from a semiconductor diode (the first laser diode), is widely acknowledged to have been Robert N. Hall and his team at the General Electric research center in 1962. In Physics, coherence is a property of waves that enables stationary (i Robert N Hall (b December 25, 1919) is an American engineer He demonstrated the first semiconductor laser, and invented a type of Magnetron [3]
The first visible wavelength laser diode was demonstrated by Nick Holonyak, Jr. Nick Holonyak Jr (born in Zeigler Illinois on November 3, 1928) invented the first visible LED in 1962 while working as a consulting , later in 1962[4]
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.
In the early 1960s liquid phase epitaxy (LPE) was invented by Herbert Nelson of RCA Laboratories. By layering the highest quality crystals of varying compositions, it enabled the demonstration of the highest quality heterojunction semiconductor laser materials for many years. LPE was adopted by all the leading laboratories, worldwide and used for many years. It was finally supplanted in the 1970s by molecular beam epitaxy and organometallic chemical vapor deposition. Chemical vapor deposition (CVD is a Chemical process used to produce high-purity high-performance solid materials
Diode lasers of that era operated with threshold current densities of 1000 Amperes per square centimeter at 77K temperatures. Such performance enabled continuous-lasing to be demonstrated in the earliest days. However, when operated at room temperature, about 300K, threshold current densities were two orders of magnitude greater, or 100,000 Amperes per square centimeter in the best devices. The dominant challenge for the remainder of the 1960s was to obtain low threshold current density at 300K and thereby to demonstrate continuous-wave lasing at room temperature from a diode laser.
The first diode lasers were homojunction diodes. That is, the material (and thus the bandgap) of the waveguide core layer and that of the surrounding clad layers, were identical. It was recognized that there was an opportunity, particularly afforded by the use of liquid phase epitaxy using aluminum gallium arsenide, to introduce heterojunctions. Heterostructures consist of layers of semiconductor crystal having varying bandgap and refractive index. Heterojunctions (formed from heterostructures) had been recognized by Herbert Kroemer, while working at RCA Laboratories in the mid-1950s, as having unique advantages for several types of electronic and optoelectronic devices including diode lasers. Herbert Kroemer (born August 25, 1928) a professor of electrical and computer engineering at the University of California Santa Barbara, received his LPE afforded the technology of making heterojunction diode lasers.
The first heterojunction diode lasers were single-heterojunction lasers. These lasers utilized aluminum gallium arsenide p-type injectors situated over n-type gallium arsenide layers grown on the substrate by LPE. An admixture of aluminum replaced gallium in the semiconductor crystal and raised the bandgap of the p-type injector over that of the n-type layers beneath. It worked; the 300K threshold currents went down by 10× to 10,000 amperes per square centimeter. Unfortunately, this was still not in the needed range and these single-heterostructure diode lasers did not function in continuous wave operation at room temperature.
The innovation that broke the room temperature challenge was the double heterostructure laser. The trick was to quickly move the wafer in the LPE apparatus between different "melts" of aluminum gallium arsenide (p- and n-type) and a third melt of gallium arsenide. It had to be done rapidly since the gallium arsenide core region needed to be significantly under 1 µm in thickness. This may have been the earliest true example of "nanotechnology. " The first laser diode to achieve continuous wave operation was a double heterostructure demonstrated in 1970 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. A continuous wave or continuous waveform ( CW) is an Electromagnetic wave of constant Amplitude and Frequency; and in Mathematical A double heterostructure is formed when two Semiconductor materials one with an energy gap less than the other are joined together Zhores Ivanovich Alferov (Жоре́с Ива́нович Алфёров ʐɐˈrʲɛs ɪˈvanəvʲɪtɕ ɐlˈfʲorəf (born March 15 1930) is a Russian Dmitri Z Garbuzov ( 1940, Sverdlovsk Russia - August 2006, Princeton New Jersey was one of the pioneers and inventors of room temperature continuous-wave-operating The Union of Soviet Socialist Republics (USSR was a constitutionally Socialist state that existed in Eurasia from 1922 to 1991 ( May 1, 1922 - September 26, 2005) was a Japanese Physicist. However, it is widely accepted that Zhores I. Alferov and team reached the milestone first.
For their accomplishment and that of their co-workers, Alferov and Kroemer shared the 2000 Nobel Prize in Physics.
See also
References
- ^ Steele, Robert V. The Laser diode rate equations model the electrical and optical performance of a laser diode A collimator is a device that narrows a beam of particles or waves A superluminescent diode (SLD is an edge-emitting Semiconductor light source based on Superluminescence. Millstone River Photonickers is an informal affinity association of those individuals associated with the development of semiconductor diode Laser technology at RCA (2005). "Diode-laser market grows at a slower rate". Laser Focus World 41 (2).
- ^ Kincade, Kathy; Stephen Anderson (2005). "Laser Marketplace 2005: Consumer applications boost laser sales 10%". Laser Focus World 41 (1).
- ^ Hall, Robert N.; G. Robert N Hall (b December 25, 1919) is an American engineer He demonstrated the first semiconductor laser, and invented a type of Magnetron E. Fenner, J. D. Kingsley, T. J. Soltys, and R. O. Carlson (Nov. 1962). "Coherent Light Emission From GaAs Junctions". Physical Review Letters 9 (9): 366–369. doi:. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document.
- ^ "After Glow", Illinois Alumni Magazine, May-June 2007. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Retrieved on 2007-08-03. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 8 - Roman Empire General Tiberius defeats Dalmatians on the river Bathinus.
- Zheludev, N. (2007). The life and times of the LED - a 100-year history. Nature Photonics 1(4), 189-192 ( For LED )
- B. Van Zeghbroeck's Principles of Semiconductor Devices( for direct and indirect band gaps)
- Saleh, Bahaa E. A. and Teich, Malvin Carl (1991). Fundamentals of Photonics. New York: John Wiley & Sons. ISBN 0-471-83965-5. ( For Stimulated Emission )
- Koyama et al, Fumio (1988), "Room temperature cw operation of GaAs vertical cavity surface emitting laser", Trans. IEICE, E71(11): 1089-1090( for VCSELS)
- Iga, Kenichi (2000), "Surface-emitting laser—Its birth and generation of new optoelectronics field", IEEE Journal of Selected Topics in Quantum Electronics 6(6): 1201–1215(for VECSELS)
External links
- Sam's Laser FAQ by Samuel M. Goldwasser
- Britney Spears Guide to Semiconductor Physics Edge-emitting lasers
- Application and technical notes explaining current and temperature control of laser diodes
- Power supplies for driving laser diodes
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