Atomic vapor laser isotope separation, or AVLIS, is a method by which specially tuned lasers are used to separate isotopes of uranium using selective ionization of hyperfine transitions.[1][2]
As compared to gas centrifuges the AVLIS process provides high energy efficiency, high separation factors, and a low volume of radioactive waste.
A similar technology, using molecules instead of atoms, is molecular laser isotope separation (MLIS).
Principle[edit]
The absorption lines of 235U and 238U differ slightly due to hyperfine structure; for example, the 238U absorption peak of 502.74 nanometers shifts to 502.73 nm in 235U. AVLIS uses tunable dye lasers, which can be precisely tuned, so that only 235U absorbs the photons and selectively undergoes excitation and then photoionization. The ions are then electrostatically deflected to a collector, while the neutral unwanted uranium-238 passes through.
The AVLIS system consists of a vaporizer and a collector, forming the separation system, and the laser system. The vaporizer produces a stream of pure gaseous uranium.
Laser excitation[edit]
The laser commonly used is a two-stage tunable pulsed dye laser usually pumped by a copper vapor laser;[3][4] the master oscillator is tunable, narrow-linewidth, low noise, and highly precise.[5] Its power is significantly increased by a dye laser amplifier acting as optical amplifier. Three frequencies ("colors") of lasers are used for full ionization of uranium-235.[6]
For AVLIS in other elements, such as lithium, tunable narrow-linewidth diode lasers are used.[7]
Commercialization and international significance[edit]
In the largest technology transfer in U.S. government history, in 1994 the AVLIS process was transferred to the United States Enrichment Corporation for commercialization. However, on June 9, 1999, after a $100 million investment, USEC cancelled its AVLIS program.
AVLIS continues to be developed by some countries and it presents some specific challenges to international monitoring.[8] Iran is now known to have had a secret AVLIS program. However, since it was uncovered in 2003, Iran has claimed to have dismantled it.[9][10]
Brief history[edit]
The history of AVLIS, as recorded in the open refereed literature, began in the early-mid 1970s in the former Soviet Union and the United States.[11] In the US, AVLIS research was mainly carried out at the Lawrence Livermore National Laboratory although some industrial laboratories were early players. Tunable laser development for AVLIS, applicable to uranium, has also been reported from several countries including Pakistan (1974), Australia (1982-1984), France (1984), India (1994), and Japan (1996).[11]
See also[edit]
- Australian Atomic Energy Commission
- Calutron
- Chemical reaction by isotope selective laser (CRISLA)
- Gaseous diffusion
- List of laser articles
- Separation of isotopes by laser excitation (SILEX)
- Nuclear fuel cycle
- Nuclear power
https://en.wikipedia.org/wiki/Atomic_vapor_laser_isotope_separation
A gas laser is a laser in which an electric current is discharged through a gas to produce coherent light. The gas laser was the first continuous-light laser and the first laser to operate on the principle of converting electrical energy to a laser light output. The first gas laser, the Helium–neon laser (HeNe), was co-invented by Iranian-American physicist Ali Javan and American physicist William R. Bennett, Jr., in 1960. It produced a coherent light beam in the infrared region of the spectrum at 1.15 micrometres.[1]
Types of gas laser[edit]
Gas lasers using many gases have been built and used for many purposes.
Carbon dioxide lasers, or CO2 lasers can emit hundreds of kilowatts[2] at 9.6 µm and 10.6 µm, and are often used in industry for cutting and welding. The efficiency of a CO2 laser is over 10%.
Carbon monoxide or "CO" lasers have the potential for very large outputs, but the use of this type of laser is limited by the toxicity of carbon monoxide gas. Human operators must be protected from this deadly gas. Furthermore, it is extremely corrosive to many materials including seals, gaskets, etc.
Helium–neon (HeNe) lasers can be made to oscillate at over 160 different wavelengths by adjusting the cavity Q to peak at the desired wavelength. This can be done by adjusting the spectral response of the mirrors or by using a dispersive element (Littrow prism) in the cavity. Units operating at 633 nm are very common in schools and laboratories because of their low cost and near-perfect beam qualities.
Nitrogen lasers operate in the ultraviolet range, typically 337.1 nm, using molecular nitrogen as its gain medium, pumped by an electrical discharge.
TEA lasers are energized by a high voltage electrical discharge in a gas mixture generally at or above atmospheric pressure. The acronym "TEA" stands for Transversely Excited Atmospheric.
Chemical lasers[edit]
Chemical lasers are powered by a chemical reaction and can achieve high powers in continuous operation. For example, in the hydrogen fluoride laser(2.7–2.9 µm) and the deuterium fluoride laser (3.8 µm) the reaction is the combination of hydrogen or deuterium gas with combustion products of ethylene in nitrogen trifluoride. They were invented by George C. Pimentel.
Chemical lasers are powered by a chemical reaction permitting a large amount of energy to be released quickly. Such very high power lasers are especially of interest to the military. Further, continuous-wave chemical lasers at very high power levels, fed by streams of gasses, have been developed and have some industrial applications.
Excimer lasers[edit]
Excimer lasers are powered by a chemical reaction involving an excited dimer, or excimer, which is a short-lived dimeric or heterodimeric molecule formed from two species (atoms), at least one of which is in an excited electronic state. They typically produce ultraviolet light, and are used in semiconductor photolithography and in LASIK eye surgery. Commonly used excimer molecules include F2 (fluorine, emitting at 157 nm), and noble gas compounds (ArF [193 nm], KrCl [222 nm], KrF [248 nm], XeCl [308 nm], and XeF [351 nm]).[3]
Ion lasers[edit]
Argon-ion lasers emit light in the range 351–528.7 nm. Depending on the optics and the laser tube a different number of lines is usable but the most commonly used lines are 458 nm, 488 nm and 514.5 nm.
Metal-vapor lasers[edit]
Metal-vapor lasers are gas lasers that typically generate ultraviolet wavelengths. Helium-silver (HeAg) 224 nm neon-copper (NeCu) 248 nm and helium-cadmium (HeCd) 325 nm are three examples. These lasers have particularly narrow oscillation linewidths of less than 3 GHz (500 femtometers),[4] making them candidates for use in fluorescence suppressed Raman spectroscopy.
The Copper vapor laser, with two spectral lines of green (510.6 nm) and yellow (578.2 nm), is the most powerful laser with the highest efficiency in the visible spectrum.[5]
Advantages[edit]
- High volume of active material
- Active material is relatively inexpensive
- Almost impossible to damage the active material
- Heat can be removed quickly from the cavity
Applications[edit]
- He-Ne laser is mainly used in making holograms.
- In laser printing He-Ne laser is used as a source for writing on the photosensitive material.
- He-Ne lasers were used in reading Bar Codes, which are imprinted on products in stores. They have been largely replaced by laser diodes.
- Nitrogen lasers and excimer laser are used in pulsed dye laser pumping.[6]
- Ion lasers, mostly argon, are used in CW dye laser pumping.[6]
See also[edit]
Advantages and applications[edit]
An advantage of fiber lasers over other types of lasers is that the laser light is both generated and delivered by an inherently flexible medium, which allows easier delivery to the focusing location and target. This can be important for laser cutting, welding, and folding of metals and polymers. Another advantage is high output power compared to other types of laser. Fiber lasers can have active regions several kilometers long, and so can provide very high optical gain. They can support kilowatt levels of continuous output power because of the fiber's high surface area to volume ratio, which allows efficient cooling. The fiber's waveguide properties reduce or eliminate thermal distortion of the optical path, typically producing a diffraction-limited, high-quality optical beam. Fiber lasers are compact compared to solid-state or gas lasers of comparable power, because the fiber can be bent and coiled, except in the case of thicker rod-type designs, to save space. They have lower cost of ownership.[1][2][3] Fiber lasers are reliable and exhibit high temperature and vibrational stability and extended lifetime. High peak power and nanosecond pulses improve marking and engraving. The additional power and better beam quality provide cleaner cut edges and faster cutting speeds.[4][5]
Other applications of fiber lasers include material processing, telecommunications, spectroscopy, medicine, and directed energy weapons.[6]
Design and manufacture[edit]
Unlike most other types of lasers, the laser cavity in fiber lasers is constructed monolithically by fusion splicing different types of fiber; fiber Bragg gratingsreplace conventional dielectric mirrors to provide optical feedback. They may also be designed for single longitudinal mode operation of ultra-narrow distributed feedback lasers (DFB) where a phase-shifted Bragg grating overlaps the gain medium. Fiber lasers are pumped by semiconductor laser diodesor by other fiber lasers.
Double-clad fibers[edit]
Many high-power fiber lasers are based on double-clad fiber. The gain medium forms the core of the fiber, which is surrounded by two layers of cladding. The lasing mode propagates in the core, while a multimode pump beam propagates in the inner cladding layer. The outer cladding keeps this pump light confined. This arrangement allows the core to be pumped with a much higher-power beam than could otherwise be made to propagate in it, and allows the conversion of pump light with relatively low brightness into a much higher-brightness signal. There is an important question about the shape of the double-clad fiber; a fiber with circular symmetry seems to be the worst possible design.[7][8][9][10][11][12] The design should allow the core to be small enough to support only a few (or even one) modes. It should provide sufficient cladding to confine the core and optical pump section over a relatively short piece of the fiber.
Tapered double-clad fiber (T-DCF) has tapered core and cladding which enables power scaling of amplifiers and lasers without thermal lensing mode instability.[13][14]
Power scaling[edit]
Recent developments in fiber laser technology have led to a rapid and large rise in achieved diffraction-limited beam powers from diode-pumped solid-state lasers. Due to the introduction of large mode area (LMA) fibers as well as continuing advances in high power and high brightness diodes, continuous-wavesingle-transverse-mode powers from Yb-doped fiber lasers have increased from 100 W in 2001 to over 20 kW.[citation needed] In 2014 a combined beam fiber laser demonstrated power of 30 kW.[15]
High average power fiber lasers generally consist of a relatively low-power master oscillator, or seed laser, and power amplifier (MOPA) scheme. In amplifiers for ultrashort optical pulses, the optical peak intensities can become very high, so that detrimental nonlinear pulse distortion or even destruction of the gain medium or other optical elements may occur. This is generally avoided by employing chirped-pulse amplification (CPA). State of the art high-power fiber laser technologies using rod-type amplifiers have reached 1 kW with 260 fs pulses [16] and made outstanding progress and delivered practical solutions for the most of these problems.
However, despite of the attractive characteristics of fiber lasers, several problems arise when power scaling. The most significant are thermal lensing and material resistance, nonlinear effects such as stimulated Raman scattering (SRS), stimulated Brillouin scattering (SBS), mode instabilities, and poor output beam quality.
The main approach to solving the problems related to increasing the output power of pulses has been to increase the core diameter of the fiber. Special active fibers with large modes were developed to increase the surface-to-active-volume ratio of active fibers and, hence, improve heat dissipation enabling power scaling.
Moreover, specially developed double cladding structures have been used to reduce the brightness requirements of the high-power pump diodes by controlling pump propagation and absorption between the inner cladding and the core.
Several types of active fibers with a large effective mode area (LMA) have been developed for high power scaling including LMA fibers with a low-aperture core,[17] micro-structured rod-type fiber [16][18] helical core [19] or chirally-coupled fibers,[20] and tapered double-clad fibers (T-DCF).[13] The mode field diameter (MFD) achieved with these low aperture technologies [16][17][18][19][20] usually does not exceed 20–30 μm. The micro-structured rod-type fiber has much larger MFD (up to 65 μm [21]) and good performance. An impressive 2.2 mJ pulse energy was demonstrated by a femtosecond MOPA [22] containing large-pitch fibers (LPF). However, the shortcoming of amplification systems with LPF is their relatively long (up to 1.2 m) unbendable rod-type fibers meaning a rather bulky and cumbersome optical scheme.[22] LPF fabrication is highly complex requiring significant processing such as precision drilling of the fiber pre-forms. The LPF fibers are highly sensitive to bending meaning robustness and portability is compromised.
Mode locking[edit]
In addition to the types of mode locking used with other lasers, fiber lasers can be passively mode locked by using the birefringence of the fiber itself.[23] The non-linear optical Kerr effect causes a change in polarization that varies with the light's intensity. This allows a polarizer in the laser cavity to act as a saturable absorber, blocking low-intensity light but allowing high intensity light to pass with little attenuation. This allows the laser to form mode-locked pulses, and then the non-linearity of the fiber further shapes each pulse into an ultra-short optical soliton pulse.
Semiconductor saturable-absorber mirrors (SESAMs) can also be used to mode lock fiber lasers. A major advantage SESAMs have over other saturable absorber techniques is that absorber parameters can be easily tailored to meet the needs of a particular laser design. For example, saturation fluence can be controlled by varying the reflectivity of the top reflector while modulation depth and recovery time can be tailored by changing the low temperature growing conditions for the absorber layers. This freedom of design has further extended the application of SESAMs into modelocking of fiber lasers where a relatively high modulation depth is needed to ensure self-starting and operation stability. Fiber lasers working at 1 µm and 1.5 µm were successfully demonstrated.[24][25][26][27]
Graphene saturable absorbers have also been used for mode locking fiber lasers.[28][29][30] Graphene's saturable absorption is not very sensitive to wavelength, making it useful for mode locking tunable lasers.
Dark soliton fiber lasers[edit]
In the non-mode locking regime, a dark soliton fiber laser was successfully created using an all-normal dispersion erbium-doped fiber laser with a polarizer in-cavity. Experimental findings indicate that apart from the bright pulse emission, under appropriate conditions the fiber laser could also emit single or multiple dark pulses. Based on numerical simulations the dark pulse formation in the laser may be a result of dark soliton shaping.[31]
Multi-wavelength fiber lasers[edit]
Multi-wavelength emission in a fiber laser demonstrated simultaneous blue and green coherent light using ZBLAN optical fiber. The end-pumped laser was based on an upconversion optical gain media using a longer wavelength semiconductor laser to pump a Pr3+/Yb3+ doped fluoride fiber that used coated dielectric mirrors on each end of the fiber to form the cavity.[32]
Fiber disk lasers[edit]
Another type of fiber laser is the fiber disk laser. In such lasers, the pump is not confined within the cladding of the fiber, but instead pump light is delivered across the core multiple times because it is coiled in on itself. This configuration is suitable for power scaling in which many pump sources are used around the periphery of the coil.[33][34][35][36]
See also[edit]
A laser diode (LD, also injection laser diode or ILD, or diode laser) is a semiconductor device similar to a light-emitting diode in which a diode pumped directly with electrical current can create lasing conditions at the diode's junction.[1]: 3
Driven by voltage, the doped p-n-transition allows for recombination of an electron with a hole. Due to the drop of the electron from a higher energy level to a lower one, radiation, in the form of an emitted photon is generated. This is spontaneous emission. Stimulated emission can be produced when the process is continued and further generate light with the same phase, coherence and wavelength.
The choice of the semiconductor material determines the wavelength of the emitted beam, which in today's laser diodes range from infra-red to the UV spectrum. Laser diodes are the most common type of lasers produced, with a wide range of uses that include fiber optic communications, barcode readers, laser pointers, CD/DVD/Blu-ray disc reading/recording, laser printing, laser scanning and light beam illumination. With the use of a phosphor like that found on white LEDs, Laser diodes can be used for general illumination.
 A packaged laser diode shown with a penny for scale | |
| Type | semiconductor, light-emitting diode |
|---|---|
| Working principle | semiconductor, Carrier generation and recombination |
| Invented | Robert N. Hall, 1962; Nick Holonyak, Jr., 1962 |
| Pin configuration | Anode and cathode |
Theory[edit]
 | This section needs additional citations for verification. (July 2011) |
A laser diode is electrically a PIN diode. The active region of the laser diode is in the intrinsic (I) region, and the carriers (electrons and holes) are pumped into that region from the N and P regions respectively. While initial diode laser research was conducted on simple P-N diodes, all modern lasers use the double-hetero-structure implementation, where the carriers and the photons are confined in order to maximize their chances for recombination and light generation. Unlike a regular diode, the goal for a laser diode is to recombine all carriers in the I region, and produce light. Thus, laser diodes are fabricated using direct band-gap semiconductors. The laser diode epitaxial structure is grown using one of the crystal growth techniques, usually starting from an N dopedsubstrate, and growing the I doped active layer, followed by the P doped cladding, and a contact layer. The active layer most often consists of quantum wells, which provide lower threshold current and higher efficiency.[1][page needed]
Electrical and optical pumping[edit]
Laser diodes form a subset of the larger classification of semiconductor p-n junction diodes. Forward electrical bias across the laser diode 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. Holes are injected from the p-doped, and electrons from the n-doped, semiconductor. (A depletion region, devoid of any charge carriers, forms as a result of the difference in electrical potential between n- and p-type semiconductors wherever they are in physical contact.) Due to the use of charge injection in powering most diode lasers, this class of lasers is sometimes termed "injection lasers," or "injection laser diode" (ILD). As diode lasers are semiconductor devices, they may also be classified as semiconductor lasers. Either designation distinguishes diode lasers from solid-state lasers.
Another method of powering some diode lasers is the use of optical pumping. Optically pumped semiconductor lasers (OPSL) use a III-V semiconductor chip as the gain medium, and another laser (often another diode laser) as the pump source. OPSL offer several advantages over ILDs, particularly in wavelength selection and lack of interference from internal electrode structures.[2][3] A further advantage of OPSLs is invariance of the beam parameters - divergence, shape, and pointing - as pump power (and hence output power) is varied, even over a 10:1 output power ratio.[4]
Generation of spontaneous emission[edit]
When an electron and a hole are present in the same region, they may recombine or "annihilate" producing a spontaneous emission — i.e., the electron may re-occupy the energy state of the hole, emitting a photon with energy equal to the difference between the electron's original state and hole's state. (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 below the lasing threshold produces similar properties to an LED. Spontaneous emission is necessary to initiate laser oscillation, but it is one among several sources of inefficiency once the laser is oscillating.
Direct and indirect bandgap semiconductors[edit]
The difference between the photon-emitting semiconductor laser and a conventional phonon-emitting (non-light-emitting) semiconductor junction diode lies in the type of semiconductor used, one whose physical and atomic structure confers the possibility for photon emission. These photon-emitting semiconductors are the so-called "direct bandgap" semiconductors. The properties of silicon and germanium, which are single-element semiconductors, have bandgaps that do not align in the way needed to allow photon emission and are not considered "direct." Other materials, the so-called compound semiconductors, have virtually identical crystalline structures as silicon or germanium but use alternating arrangements of two different atomic species in a checkerboard-like pattern to break the symmetry. The transition between the materials in the alternating pattern creates the critical "direct bandgap" property. Gallium arsenide, indium phosphide, gallium antimonide, and gallium nitride are all examples of compound semiconductor materials that can be used to create junction diodes that emit light.
Generation of stimulated emission[edit]
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. A nearby photon with energy equal to the recombination energy can cause recombination by stimulated emission. This generates another photon of the same frequency, polarization, and phase, travelling in the same direction as the first photon. This means that stimulated emission will cause 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; therefore silicon is not a common material for laser diodes.
Optical cavity and laser modes[edit]
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's 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–Pérot 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 exit. 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, the light is contained within a very thin layer, and the structure supports only a single optical mode in the direction perpendicular to the layers. In the transverse direction, if the waveguide is wide compared to the wavelength of light, then the waveguide can support multiple transverse optical modes, and the laser is known as "multi-mode". These transversely multi-mode lasers are adequate in cases where one needs a very large amount of power, but not a small diffraction-limited TEM00 beam; for example in printing, activating chemicals, microscopy, 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 transverse 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 material 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 peak of the gain curve will lase most strongly. The width of the gain curve will determine the number of additional "side modes" that may also lase, depending on the operating conditions. Single spatial mode lasers that can support multiple longitudinal modes are called Fabry Perot (FP) lasers. An FP laser will lase at multiple cavity modes within the gain bandwidth of the lasing medium. The number of lasing modes in an FP laser is usually unstable, and can fluctuate due to changes in current or temperature.
Single spatial mode diode lasers can be designed so as to operate on a single longitudinal mode. These single frequency diode lasers exhibit a high degree of stability, and are used in spectroscopy and metrology, and as frequency references. Single frequency diode lasers are classed as either distributed feedback (DFB) lasers or distributed Bragg reflector (DBR) lasers.
Formation of laser beam[edit]
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.
History[edit]
As early as 1953 John von Neumann described the concept of semiconductor laser in an unpublished manuscript.[citation needed] In 1957, Japanese engineer Jun-ichi Nishizawa filed a patent for the first semiconductor laser.[5][6] It was an advancement of his earlier inventions, the PIN diode in 1950 and the solid-state maser in 1955.[6]
Following theoretical treatments of M.G. Bernard, G. Duraffourg and William P. Dumke in the early 1960s coherentlight emission from a gallium arsenide (GaAs) semiconductor diode (a laser diode) was demonstrated in 1962 by two US groups led by Robert N. Hall at the General Electric research center[7] and by Marshall Nathan at the IBM T.J. Watson Research Center.[8] There has been ongoing debate as to whether IBM or GE invented the first laser diode which was largely based on theoretical work by William P. Dumke at IBM's Kitchawan Lab (currently known as the Thomas J. Watson Research Center) in Yorktown Heights, NY. The priority is given to General Electric group who have obtained and submitted their results earlier; they also went further and made a resonant cavity for their diode.[9] It was initially speculated, by MIT's Ben Lax among other leading physicists, that silicon or germanium could be used to create a lasing effect, but theoretical analyses convinced William P. Dumke that these materials would not work. Instead, he suggested Gallium Arsenide as a good candidate. The first visible wavelength GaAs laser diode was demonstrated by Nick Holonyak, Jr. later in 1962.[10]
Other teams at MIT Lincoln Laboratory, Texas Instruments, and RCA Laboratories were also involved in and received credit for their historic initial demonstrations of efficient light emission and lasing in semiconductor diodes in 1962 and thereafter. GaAs lasers were also produced in early 1963 in the Soviet Union by the team led by Nikolay Basov.[11]
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.
Diode lasers of that era operated with threshold current densities of 1000 A/cm2 at 77 K temperatures. Such performance enabled continuous-lasing to be demonstrated in the earliest days. However, when operated at room temperature, about 300 K, threshold current densities were two orders of magnitude greater, or 100,000 A/cm2 in the best devices. The dominant challenge for the remainder of the 1960s was to obtain low threshold current density at 300 K 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. LPE afforded the technology of making heterojunction diode lasers. In 1963 he proposed the double heterostructure laser.
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 300 K 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 met 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. 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. However, it is widely accepted that Zhores I. Alferov and team reached the milestone first.[12]
For their accomplishment and that of their co-workers, Alferov and Kroemer shared the 2000 Nobel Prize in Physics.
Types[edit]
The simple laser diode structure, described above, is 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[edit]
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 homojunctionlaser, 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 within the heterojunction; hence, the light is confined to the region where the amplification takes place.
Quantum well lasers[edit]
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 quantized. 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 welllasers. 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.
Quantum cascade lasers[edit]
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. They are heterojunction lasers.
Interband cascade lasers[edit]
An Interband cascade laser (ICL) is a type of laser diode that can produce coherent radiation over a large part of the mid-infrared region of the electromagnetic spectrum.
Separate confinement heterostructure lasers[edit]
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.[citation needed]
Distributed Bragg reflector lasers[edit]
A distributed Bragg reflector laser (DBR) is a type of single frequency laser diode.[13] It is characterized by an optical cavity consisting of an electrically or optically pumped gain region between two mirrors to provide feedback. One of the mirrors is a broadband reflector and the other mirror is wavelength selective so that gain is favored on a single longitudinal mode, resulting in lasing at a single resonant frequency. The broadband mirror is usually coated with a low reflectivity coating to allow emission. The wavelength selective mirror is a periodically structured diffraction grating with high reflectivity. The diffraction grating is within a non-pumped, or passive region of the cavity . A DBR laser is a monolithic single chip device with the grating etched into the semiconductor. DBR lasers can be edge emitting lasers or VCSELs. Alternative hybrid architectures that share the same topology include extended cavity diode lasers and volume Bragg grating lasers, but these are not properly called DBR lasers.
Distributed feedback lasers[edit]
A distributed feedback laser (DFB) is a type of single frequency laser diode.[13] DFBs 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. DFB lasers are widely used in optical communication applications where a precise and stable wavelength is critical.
The threshold current of this DFB laser, based on its static characteristic, is around 11 mA. The appropriate bias current in a linear regime could be taken in the middle of the static characteristic (50 mA).Several techniques have been proposed in order to enhance the single-mode operation in these kinds of lasers by inserting a onephase-shift (1PS) or multiple-phase-shift (MPS) in the uniform Bragg grating.[14] However, multiple-phase-shift DFB lasers represent the optimal solution because they have the combination of higher side-mode suppression ratio and reduced spatial hole-burning.
Vertical-cavity surface-emitting laser[edit]
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 the figure. The reflectors at the ends of the cavity are dielectric mirrors made from alternating high and low refractive index quarter-wave thick multilayer.
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 = λ/2 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. However, they normally show a lower power output level.
Vertical-external-cavity surface-emitting-laser[edit]
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 small thickness 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. However, because of their lack of p-n junction, optically-pumped VECSELs are not considered "diode lasers", and are classified as semiconductor lasers.[citation needed]
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.
External-cavity diode lasers[edit]
External-cavity diode lasers are tunable lasers which use mainly double heterostructures diodes of the AlxGa(1-x)As type. The first external-cavity diode lasers used intracavity etalons[15] and simple tuning Littrow gratings.[16] Other designs include gratings in grazing-incidence configuration and multiple-prism grating configurations.[17]
Reliability[edit]
|
 | This section may be too technical for most readers to understand. (July 2011) |
Laser diodes have the same 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. Reverse engineering is not always able to reveal the differences between more-reliable and less-reliable diode laser products.
Semiconductor lasers can be surface-emitting lasers such as VCSELs, or in-plane edge-emitting lasers. For edge-emitting lasers, the edge facet mirror is often formed by cleaving the semiconductor wafer to form a specularly reflecting plane.[1]: 24 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.
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.
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 where 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-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.
In the 1970s, this problem, which is particularly nettlesome for GaAs-based lasers emitting between 0.630 µm and 1 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 were still undisclosed 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, was undisclosed as of June 2006.
Reliability of high-power diode laser pump bars (used 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[edit]
Laser diodes are numerically the most common laser type, with 2004 sales of approximately 733 million units,[18] as compared to 131,000 of other types of lasers.[19]
Telecommunications, scanning and spectrometry[edit]
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, such as 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. 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. 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.
Uses 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 and beam quality, polarization, etc.) it is useful 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, barcode 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 well-established while others are emerging.
Medical uses[edit]
Laser medicine: medicine and especially dentistry have found many new uses for diode lasers.[20][21][22][23] The shrinking size and cost[24] of the units and their increasing user friendliness makes them very attractive to clinicians for minor soft tissue procedures. Diode wavelengths range from 810 to 1,100 nm, are poorly absorbed by soft tissue, and are not used for cutting or ablation.[25][26][27][28] Soft tissue is not cut by the laser's beam, but is instead cut by contact with a hot charred glass tip.[27][28] The laser's irradiation is highly absorbed at the distal end of the tip and heats it up to 500 °C to 900 °C.[27]Because the tip is so hot, it can be used to cut soft-tissue and can cause hemostasis through cauterization and carbonization.[27][28] Diode lasers when used on soft tissue can cause extensive collateral thermal damage to surrounding tissue.[27][28]
As laser beam light is inherently coherent, certain applications utilize the coherence of laser diodes. These include interferometric distance measurement, holography, coherent communications, and coherent control of chemical reactions.
Laser diodes are used for their "narrow spectral" properties in the areas of 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).
Laser diodes are used for their ability to generate ultra-short pulses of light by the technique known as "mode-locking." Areas of use 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.
Common wavelengths[edit]
Visible light[edit]
- 405 nm: InGaN blue-violet laser, in Blu-ray Disc and HD DVD drives
- 445–465 nm: InGaN blue laser multimode diode recently introduced (2010) for use in mercury-free high-brightness data projectors
- 510–525 nm: InGaN Green diodes recently (2010) developed by Nichia and OSRAM for laser projectors.[29]
- 635 nm: AlGaInP better red laser pointers, same power subjectively twice as bright as 650 nm
- 650–660 nm: GaInP/AlGaInP CD and DVD drives, cheap red laser pointers
- 670 nm: AlGaInP bar code readers, first diode laser pointers (now obsolete, replaced by brighter 650 nm and 671 nm DPSS)
Infrared[edit]
- 760 nm: AlGaInP gas sensing: O
2 - 785 nm: GaAlAs Compact Disc drives
- 808 nm: GaAlAs pumps in DPSS Nd:YAG lasers (e.g., in green laser pointers or as arrays in higher-powered lasers)
- 848 nm: laser mice
- 980 nm: InGaAs pump for optical amplifiers, for Yb:YAG DPSS lasers
- 1,064 nm: AlGaAs fiber-optic communication, DPSS laser pump frequency
- 1,310 nm: InGaAsP, InGaAsN fiber-optic communication
- 1,480 nm: InGaAsP pump for optical amplifiers
- 1,512 nm: InGaAsP gas sensing: NH
3 - 1,550 nm: InGaAsP, InGaAsNSb fiber-optic communication
- 1,625 nm: InGaAsP fiber-optic communication, service channel
- 1,654 nm: InGaAsP gas sensing: CH
4 - 1,877 nm: GaInAsSb gas sensing: H
2O - 2,004 nm: GaInAsSb gas sensing: CO
2 - 2,330 nm: GaInAsSb gas sensing: CO
- 2,680 nm: GaInAsSb gas sensing: CO
2 - 3,030 nm: GaInAsSb gas sensing: C
2H
2 - 3,330 nm: GaInAsSb gas sensing: CH
4
See also[edit]
https://en.wikipedia.org/wiki/Laser_diode
No comments:
Post a Comment