Summary of Expertise:
-- Primary Expertise -- -- Description --
laser
particle acceleration
particle accelerator
As Vice President of Research and Development, Expert has had extensive experience in the design, modeling, fabrication, and operation of lasers, using them for measurements and other applications and with laser safety. He is most familiar with excimer lasers. Other lasers of familiarity include Nd:YAG, argon ion, CO2, dye, HeNe, and FEL. His PhD is in quantum electronics, which includes coherent sources such as lasers.
pulsed ultraviolet laser
ultraviolet laser
excimer laser
Expert has conducted extensive research on both discharge and electron-beam pumped excimer lasers, including KrF, XeCl, and XeF. He is very familiar with the devices (laser chamber, optics, and pulsed power), kinetics, and applications.
laser optics
optics
In performing research on and using lasers, Expert is, of course, very familiar with the optics associated with these devices. This includes the laser cavity mirrors, other optics within the cavity (e.g. Q-switch, polarizer), and external optics, including mirrors, gratings, lenses, filters, and nonlinear crystals. He is well versed in optical damage issues, and has performed research on glancing incidence mirrors. In addition, he has been involved in the design of very specialized optics.
laser-induced damage
laser-induced optical component damage
Familiar with optical damage from his work with high-peak power lasers (excimer, Nd:YAG, and CO2), Expert has studied laser surface cleaning of mirrors.
optical diagnostics
Expert is knowledgeable of the optical diagnostics used to detect and characterize optical radiation from laser beams, incoherent sources, and linear accelerators. This includes diagnostic instruments, commercial devices (see Summary under Photonics Technology for examples), custom devices (designed by Expert), proper usage of these devices, and analysis and interpretation of their outputs.
laser application
Expert's experience in laser applications is quite diverse. It includes using lasers to measure physical phenomena (interferometry, spectroscopy, imagery, reflectometry), laser surface cleaning, laser particle acceleration, photoacoustic calorimetry, laser-triggered spark gaps, and endoscopic photocoagulation. Typically, his involvement covers everything, including concept, design, engineering, safety, testing, and usage.
laser beam
Expert has performed research to create innovative laser beams, such as ones with radial polarization, Bessel beams, and flattop beams required for effective laser cleaning applications. Measuring the characteristics of these beams is also an area of his expertise.
photonics technology
Besides lasers and optics, Expert is adept with the other devices and physics associated with the photonics field. These include optical detectors (e.g. PMT, photodiode, video cameras), optical instruments (e.g. spectrometers, power meters), optical hardware (e.g. mounts, translation stages), electronics (e.g. oscilloscopes, delay generators, video frame grabber boards), and optical fibers. He also maintains a large library of vendor brochures, catalogs, and other printed literature.
photonics research-and-development management
As Vice President of Research and Development, Expert is intimately familiar with all issues related to managing photonics R&D This work includes planning, coordinating, and managing activities as well as resolving technical and managerial issues, customer or end-user interactions, market analysis, and strategic planning.
laser particle acceleration
linear accelerator
electron accelerator
Expert has been performing research for nearly 30 years on accelerating relativistic electrons using a high-power laser beam. Since an electron linear accelerator is used during this process, he is also familiar with these devices and their associated physics. He is familiar with the basic fundamental principles of accelerators, their associated hardware and equipment, and their applications both in research and industry.
electron beam
Expert's experience includes relativistic electron beams, which stems from his laser particle acceleration research, and non-relativistic electron beams, which stems from his electron-beam pumped excimer laser research. He is knowledgeable of electron-beam sources (cold, hot, and photoexcited cathodes), electron beam physics, and electron beam interactions with matter.
electron beam pumped excimer laser
Expert has performed pioneering research on electron-beam pumped excimer lasers, including KrF, XeCl, and XeF. He is proficient in the devices (laser chamber, optics, and pulsed power), kinetics, and applications of these lasers.
laser surface cleaning
Expert has performed research in this area, with special emphasis on using a UV laser. This research includes optimization of the technique, developing engineering designs for cleaning systems, and cost analysis. He is also familiar with alternative cleaning techniques, such as CO(2) snow. Since 1990, he has performed research and development on using a pulsed laser beam to remove both molecular and particulate contamination from optical surfaces without damaging the optical coating on the surface. Such a process can also be used to clean non-optical surfaces and remote surfaces, and it can be more effective and have less total cost (capital plus operational) than conventional methods.
surface cleanliness
As part of his laser surface cleaning research, Expert is familiar with issues related to determining the cleanliness of a surface. He is also experienced with corrosion or tarnishing of surfaces and has performed research on inhibiting corrosion of mirror surfaces.
cost analysis
cost estimation
As the principal investigator and program manager for both small and multimillion-dollar programs, Expert has extensive experience in cost estimation and cost/benefit analysis, especially for experimental R&D projects. These costs include labor and materials for all phases of the project.
Sunday, April 1, 2007
4-22 High intensity laser application laboratory
Overview
The MBI high-field laser application laboratory develops, applies and provides femto- and picosecond laser systems operating in a broad intensity range up to 1019W/cm2 and beyond, complemented by short-pulse, high-average-power lasers for special applications. Apart from the research towards highest possible intensities at high pulse contrast, part of the activities is focussed on diagnostics development for the on-line characterisation of the laser parameters.
The HFL is located in a separate building with restricted access due to radiation safety and cleanliness considerations. Its structure and equipment allows to perform laser-matter interaction experiments such as single atom ionisation as well as complex laser-plasma interaction studies. The latter include incoherent and coherent x-ray emission (collisional x-ray laser) as well as generation and acceleration of charged particles, with focussing on protons and highly charged ions and their applications. A diversity of diagnostic equipment with high energetic (spectral), spatial and temporal resolution, consisting of optical and x-ray streak cameras, CCD cameras, x-ray and EUV-spectrometers, and Thomson spectrometers is available.
According to the general mission of the MBI these facilities are not only used for the in-house research (mainly projects 1-02, 2-01, 2-02 and 3-04), but also offered to external users who are interested in research collaborations with MBI groups. A broad field of disciplinary and interdisciplinary studies is addressed, ranging from atomic, laser and plasma physics to material science, metrology up to industrial relevant applications. The laboratory is also open to external users in within the Transnational Access Activity of the 5th and 6th Framework Programs of the EU (Integrated Laser Infrastructure Network LASERLAB-EUROPE). The following systems are in operation:
Two high-peak power lasers, capable of delivering intensities between 1018 and more than 1019 W/cm2 , in particular, a 10 Hz CPA 20 TW (35 fs, 700 mJ) Ti:Sapphire laser and a single shot ~10 TW (0.8 ps, ~8 J) glass laser. Presently, synchronization of the two system for unique proton imaging experiments in laser-based plasma physics is under development.
Additionally, a prototype of a collisionally excited nickel-like Ag X-ray laser at 13.9 nm with output energy of several microJ in 30 ps, using a shaped 3J picoscond pump pulse, has been successfully demonstrated. While this laser is, in principle, available for applications, it is still subject to intensive research efforts with the medium-term goal of developing a novel table-top, high-repetition rate and high average power EUV laser.
The following supporting systems and infrastructure are available in the high field laser application laboratory:
SPIDER for a quasi-on-line control of the duration of the Ti:Sa laser pulse at full energy (10 fs resolution)
Implementation of an adaptive mirror- feedback with wavefront controlling Hartmann sensor, that resulted in a improvement of the focus intensity, leading to an intensity of about 1019W/cm2.
Auto-correlator for on-line pulse duration measurement of CPA-glass laser pulse
Update of the beam propagation system for five interaction chambers in separate laboratories, surrounding the central laser hall
Implementation of radiation protection system for highly energetic charged particles and x-rays (dosemeters)
Peak intensity determination by single atom ionisation measurement in inert gases (Xe, Kr)4 channel Thomson parabola for ion spectra measurements and 4 channel neutron TOF developed
3rd order correlator for the Ti:Sapphire laser with high dynamics range as well as a sigle shor 3rd order correlator for the glass laser system. Both for monitoring of shape and contrast of the compressed highly energetic pulses.
System for on-line monitoring of the spectral content of the glass laser pulses
Experimental arrangement for guiding experiments at relativistic intensities (see also access experiments).
Furthermore the HFL-laboratory is equipped with a variety of commercial diagnostics enabling measurements with high spectral, spatial and temporal resolution (optical and x-ray streak and CCD cameras, different spectrometers from optical down to x-ray range).
Overview
The MBI high-field laser application laboratory develops, applies and provides femto- and picosecond laser systems operating in a broad intensity range up to 1019W/cm2 and beyond, complemented by short-pulse, high-average-power lasers for special applications. Apart from the research towards highest possible intensities at high pulse contrast, part of the activities is focussed on diagnostics development for the on-line characterisation of the laser parameters.
The HFL is located in a separate building with restricted access due to radiation safety and cleanliness considerations. Its structure and equipment allows to perform laser-matter interaction experiments such as single atom ionisation as well as complex laser-plasma interaction studies. The latter include incoherent and coherent x-ray emission (collisional x-ray laser) as well as generation and acceleration of charged particles, with focussing on protons and highly charged ions and their applications. A diversity of diagnostic equipment with high energetic (spectral), spatial and temporal resolution, consisting of optical and x-ray streak cameras, CCD cameras, x-ray and EUV-spectrometers, and Thomson spectrometers is available.
According to the general mission of the MBI these facilities are not only used for the in-house research (mainly projects 1-02, 2-01, 2-02 and 3-04), but also offered to external users who are interested in research collaborations with MBI groups. A broad field of disciplinary and interdisciplinary studies is addressed, ranging from atomic, laser and plasma physics to material science, metrology up to industrial relevant applications. The laboratory is also open to external users in within the Transnational Access Activity of the 5th and 6th Framework Programs of the EU (Integrated Laser Infrastructure Network LASERLAB-EUROPE). The following systems are in operation:
Two high-peak power lasers, capable of delivering intensities between 1018 and more than 1019 W/cm2 , in particular, a 10 Hz CPA 20 TW (35 fs, 700 mJ) Ti:Sapphire laser and a single shot ~10 TW (0.8 ps, ~8 J) glass laser. Presently, synchronization of the two system for unique proton imaging experiments in laser-based plasma physics is under development.
Additionally, a prototype of a collisionally excited nickel-like Ag X-ray laser at 13.9 nm with output energy of several microJ in 30 ps, using a shaped 3J picoscond pump pulse, has been successfully demonstrated. While this laser is, in principle, available for applications, it is still subject to intensive research efforts with the medium-term goal of developing a novel table-top, high-repetition rate and high average power EUV laser.
The following supporting systems and infrastructure are available in the high field laser application laboratory:
SPIDER for a quasi-on-line control of the duration of the Ti:Sa laser pulse at full energy (10 fs resolution)
Implementation of an adaptive mirror- feedback with wavefront controlling Hartmann sensor, that resulted in a improvement of the focus intensity, leading to an intensity of about 1019W/cm2.
Auto-correlator for on-line pulse duration measurement of CPA-glass laser pulse
Update of the beam propagation system for five interaction chambers in separate laboratories, surrounding the central laser hall
Implementation of radiation protection system for highly energetic charged particles and x-rays (dosemeters)
Peak intensity determination by single atom ionisation measurement in inert gases (Xe, Kr)4 channel Thomson parabola for ion spectra measurements and 4 channel neutron TOF developed
3rd order correlator for the Ti:Sapphire laser with high dynamics range as well as a sigle shor 3rd order correlator for the glass laser system. Both for monitoring of shape and contrast of the compressed highly energetic pulses.
System for on-line monitoring of the spectral content of the glass laser pulses
Experimental arrangement for guiding experiments at relativistic intensities (see also access experiments).
Furthermore the HFL-laboratory is equipped with a variety of commercial diagnostics enabling measurements with high spectral, spatial and temporal resolution (optical and x-ray streak and CCD cameras, different spectrometers from optical down to x-ray range).
Laser applications
From Wikipedia, the free encyclopedia
Jump to: navigation, search
The benefits of lasers in various applications stems from their properties such as coherency, high monochromaticity, and ability to reach extremely high powers. For instance, a highly coherent laser beam can be focused down to its diffraction limit, which at visible wavelengths is only a few hundred nanometers. This property allows:-
A laser to record gigabytes of information in the microscopic pits of a DVD.
A laser of modest power to be focused to very high intensities and used for cutting, burning, or vaporizing materials. For example, a frequency doubled neodymium yttrium aluminium garnet (Nd:YAG) laser emitting 532 nanometer (green) light at 10 watts output power can theoretically achieve a focused intensity of megawatts per square centimeter.
In reality however, perfect focusing of a beam to its diffraction limit is somewhat difficult.
Contents [hide]
1 Scientific
1.1 Spectroscopy
1.2 Lunar laser ranging
1.3 Photochemistry
1.4 Laser cooling
1.5 Nuclear fusion
1.6 Finderscope for amateur telescopes
1.7 Microscopy
2 Military
2.1 Defensive applications
2.2 Strategic Defense Initiative
2.3 Laser sight
2.4 Satellite obstruction
2.5 Illuminator
2.6 Rangefinder
2.7 Target designator
2.8 Death ray
2.8.1 Fictional military use
2.9 Recent real developments
2.9.1 Flexible mirror
2.9.2 Powerful laser
2.9.3 Tracking
2.10 To affect eyesight
3 Medical
4 Industrial & commercial
5 In consumer products
5.1 Law enforcement
6 Links
7 Images
8 References
[edit] Scientific
In science, lasers are used in many ways, including:-
A wide variety of interferometric techniques
Raman spectroscopy
Laser induced breakdown spectroscopy.
Atmospheric remote sensing
Investigating nonlinear optics phenomena
Holographic techniques employing lasers also contribute to a number of measurement techniques.
Laser (LIDAR) technology has application in geology, seismology, remote sensing and atmospheric physics.
Lasers have been used aboard spacecraft such as in the Cassini-Huygens mission.
In astronomy, lasers have been used to create artificial laser guide stars, used as reference objects for adaptive optics telescopes.
[edit] Spectroscopy
Most types of laser are an inherently pure source of light; they emit near-monochromatic light with a very well defined range of wavelengths. By careful design of the laser components, the purity of the laser light (measured as the "linewidth") can be improved more than the purity of any other light source. This makes the laser a very useful source for spectroscopy. The high intensity of light that can be achieved in a small, well collimated beam can also be used to induce a nonlinear optical effect in a sample, which makes techniques such as Raman spectroscopy possible. Other spectroscopic techniques based on lasers can be used to make extremely sensitive detectors of various molecules, able to measure molecular concentrations in the parts-per-trillion (ppt) level. Due to the high power densities achievable by lasers, beam-induced atomic emission is possible: this technique is termed Laser induced breakdown spectroscopy (LIBS).
Lasers may also be indirectly used in spectroscopy as a micro-sampling system, a technique termed Laser ablation (LA), which is typically applied to ICP-MS apparatus resulting in the powerful LA-ICP-MS.
[edit] Lunar laser ranging
Main article: Lunar laser ranging experiment
When the Apollo astronauts visited the moon, they planted retroreflector arrays to make possible the Lunar Laser Ranging Experiment. Laser beams are focused through large telescopes on Earth aimed toward the arrays, and the time taken for the beam to be reflected back to Earth measured to determine the distance between the Earth and Moon with high precision.
[edit] Photochemistry
Some laser systems, through the process of modelocking, can produce extremely brief pulses of light - as short as picoseconds or femtoseconds (10-12 - 10-15 seconds). Such pulses can be used to initiate and analyse chemical reactions, a technique known as photochemistry. The short pulses can be used to probe the process of the reaction at a very high temporal resolution, allowing the detection of short-lived intermediate molecules. This method is particularly useful in biochemistry, where it is used to analyse details of protein folding and function.
[edit] Laser cooling
A technique that has had recent success is laser cooling. This involves ion trapping or atom trapping, a method where a number of ions or atoms are confined in a specially shaped arrangement of electric and magnetic fields. Shining particular wavelengths of laser light at the ions or atoms slows them down, thus cooling them. If this process is continued, eventually they all are slowed and have the same energy level, forming an unusual arrangement of matter known as a Bose-Einstein condensate.
[edit] Nuclear fusion
Lasers are used in certain types of thermonuclear fusion reactors. Perhaps the most extravagant use of lasers in science is in the field of fusion research. Some of the world's most powerful and complex arrangements of multiple lasers and optical amplifiers are used to produce extremely high intensity pulses of light of extremely short duration. These pulses are arranged such that they impact pellets of tritium-deuterium simultaneously from all directions, hoping that the squeezing effect of the impacts will induce atomic fusion in the pellets. This technique, known as "inertial confinement fusion", so far has not been able to achieve "breakeven", that is, so far the fusion reaction generates less power than is used to power the lasers, but research continues.
[edit] Finderscope for amateur telescopes
With the advent of higher power green laser pointers came a new product line for observational astronomers -- the laser finder. Akin to the laser sighting device used in military rifles, a green laser mounted on the tube of a telescope and properly collimated can reveal precisely where in the sky the telescope is pointed. Mostly outdoors at night the green laser beam is visible. By moving the telescope (and the laser beam) to the proper location in the sky, observers can more easily find the intended celestial target.
[edit] Microscopy
Confocal laser scanning microscopy and Two-photon excitation microscopy make use of lasers to obtain blur-free images of thick specimens at various depths.
[edit] Military
Military uses of lasers include use as target designators for other weapons; their use as directed-energy weapons is currently under research. Laser weapon systems under development include the airborne laser, the advanced tactical laser, the Tactical High Energy Laser, the High Energy Liquid Laser Area Defense System, and the MIRACL, or Mid-Infrared Advanced Chemical Laser.
[edit] Defensive applications
Recently, some progress has been made in the use of the laser as a directed energy weapon, mostly in defensive applications. By using a chemical laser, one in which the laser operation is powered by an energetic chemical reaction, the requirement for generating and storing a large amount of electrical energy (which directly or indirectly is used to power most high-power lasers) is removed. This makes the laser system much more compact, and easier to transport. One example is a laser system designed to destroy missiles in flight. It is mounted in a converted commercial airliner, and could be used, for example, to protect assets such as AWACS aircraft or to destroy ballistic missiles (see Airborne Laser). However, the practical problems of reliably generating and aiming the laser beam remain formidable.
The Mobile Tactical High-Energy Laser (MTHEL) is another defensive laser system under development; this is envisioned as a field-deployable weapon system able to track incoming artillery projectiles and cruise missiles by radar and destroy them with a powerful deuterium fluoride laser.
For information and a list of laser-based weapon systems, see also Directed Energy Weapon (Lasers).
[edit] Strategic Defense Initiative
Another example of direct use of a laser as a defensive weapon was researched for the Strategic Defense Initiative (SDI, nicknamed "Star Wars"), and its successor programs. This project would use ground-based or space-based laser systems to destroy incoming intercontinental ballistic missiles (ICBMs). Again, the practical problems of using and aiming these systems would be many; particularly the problem of destroying ICBMs at the most opportune moment, the boost phase just after launch. This would involve directing a laser through a large distance in the atmosphere, which, due to optical scattering and refraction, would bend and distort the laser beam, complicating the aiming of the laser and reducing its efficiency.
Another idea to come from the SDI project was the nuclear-pumped X-ray laser. This was essentially an orbiting atomic bomb, surrounded by laser media in the form of glass rods; when the bomb exploded, the rods would be bombarded with highly-energetic gamma-ray photons, causing spontaneous and stimulated emission of X-ray photons in the atoms making up the rods. This would lead to optical amplification of the X-ray photons, producing an X-ray laser beam that would be minimally affected by atmospheric distortion and capable of destroying ICBMs in flight. The X-ray laser would be a strictly one-shot device, destroying itself on activation. Some initial tests of this concept were performed with underground nuclear testing, however, the results were not encouraging. Research into this approach to missile defense was discontinued after the cancellation of the SDI program.
In recent years, the United States Air Force has begun experimenting with using lasers combined with high-altitude airships as a potential means for a missile defense shield but also as a means to destroy enemy spacecraft or satellites in low-earth orbit. For more information, see Evolutionary Air and Space Global Laser Engagement.
[edit] Laser sight
The laser has in most military applications been used as a tool to enhance the targeting of other weapon systems. For example, a laser sight is a small, usually visible-light laser placed on a handgun or rifle aligned to emit a beam parallel to the barrel. Since a laser beam typically has low divergence, the laser light appears as a small spot even at long distances; the user simply places the spot on the desired target and the barrel of the gun is aligned. Recent studies (2001) have shown that laser sight has become an effective deterring tool for law enforcement. Criminals are more likely to surrender without resistance when they find a red laser dot on their chest.
Most laser sights use a red (670–633 nm) diode; the 633–635 nm diodes are about 10 times as bright as 670 nm diodes. Some laser sights used an infrared diode, which produced a dot invisible to the naked human eye, but would show up when the user used special optics. In the late 1990s, green diode pumped solid state laser (DPSS) laser sights (532 nm) became available.
Modern laser sights are so small that they can be installed below the barrel as part of the gun, instead of being a separate attachment. Some laser sights, such as those made by LaserMax, are integrated into a unit that replaces a pistol's guide rod, thus adding no bulk to the gun. Others, like Crimson Trace Lasergrips, are built into the grip so that activation is automatic when the gun is held.
Another type of optical sight is the reflex, or red dot sight. This gives the illusion of a red dot projected onto the target when the user looks through the sight, but only the user can see the dot. This has advantages where more than one weapon is aimed at the same target, and it is also more visible than a conventional laser in bright sunlight.
[edit] Satellite obstruction
According to a report issued by the Pentagon, China is developing a laser that could blind low Earth orbit satellites. [1]
[edit] Illuminator
Saber 203 Laser Illuminator (U.S. Air Force)This non-lethal laser weapon, shown in the accompanying photo attached to an M16 rifle, was developed by the U.S. Air Force to temporarily impair an adversary’s ability to fire a weapon or to otherwise threaten enemy forces. The Saber 203 briefly illuminates an opponent with harmless low-power laser light. Realizing he has been targeted, the aggressor hides or flees rather than risk death by aiming his weapon and attracting defensive fire.
[edit] Rangefinder
Main article: Laser range-finder
A laser range-finder is a device consisting of a pulsed laser and a light detector. By measuring the time taken for light to reflect off a far object, and knowing the speed of light, the range to the object can be found. A laser rangefinder is thus a simple form of LIDAR. The distance to the target can then be used to aim a weapon such as a tank's main gun.
[edit] Target designator
Main article: Laser designator
Another military use of lasers is as a laser target designator. This is a low-power laser pointer used to indicate a target for a precision-guided munition, typically launched from an aircraft. The guided munition adjusts its flight-path to home in to the laser light reflected by the target, enabling a great precision in aiming. The beam of the laser target designator is set to a pulse rate that matches that set on the guided munition to ensure munitions strike their designated targets and do not follow other laser beams which may be in use in the area. The laser designator can be shone onto the target by an aircraft or nearby infantry. Lasers used for this purpose are usually infrared lasers, so the enemy cannot easily detect the guiding laser light.
[edit] Death ray
Not yet made practical: The first role envisioned for the laser in military applications was as a "death ray": a hand-held device that might replace the gun as a weapon for infantry, or a vehicle-mounted "laser cannon" able to destroy tanks, ships and aircraft. However, practical considerations have severely constrained these ideas; any laser that can seriously wound a human would (with its needed power supply) be too heavy for one man to lift, and a high-power laser that can burn through tank armor would be extremely complex and very sensitive to misalignment from any knocks or vibration it might suffer, making it unsuitable for field deployment.
[edit] Fictional military use
Such uses are very common in fiction: see raygun.
[edit] Recent real developments
In 1980, the US Air Force managed to shoot down missiles which were configured as target practice drones. Results were sporadic at best, but they showed definitively that aircraft could be shot down by a static ground laser. Further tests and calculations showed that getting such a laser wespon working would be possible but not easy. A modern real military "ray gun" would need three or more lasers and an initial track acquisition device:-
0: The initial track device gives the approximate position of the target, and other relevant ,information, such as speed, altitude, predicted impact point, estimated launch position, etc.
1: The first laser (low power) then starts scanning there until it finds its exact position.
2: The second laser (low power) then measures the target, including:-
Atmospheric distortion measurements. This lets the shooter adjust a flexible mirror to counter-deviate the third laser to compensate for the atmospheric deviations.
Vibration analysis. This finds the target's mechanical resonance vibration modes.
3: The third laser sends powerful repetitive pulses at the natural frequency of the target, complete with distortion correction. This causes structural shock to the body of the missile, and also may cause damage by the powerful laser's heat. Usually flying machines are very fragile,and sudden thermal and physical shocks cause them to break apart.
Given these capacities, or similar, one sort of "ray-guns" is technically possible, such as the Boeing YAL-1A Airborne Laser [2].
The only major limitation is that heavy vehicles such as tanks would not suffer any significant effect when hit with a laser. Most probably the operator would notice that the side armour was vibrating, feel an increase in ambient temperature, and no more. The only truly noticeable effect would be that a tank equipped with, reactive armour would start burning, with an outer extreme being low-power non-damaging explosions.
It would need three technological developments to work:-
[edit] Flexible mirror
A laser beam can be diverted or distorted by refraction caused by distorting atmospheric conditions. These exist already: in the 1990s astronomers managed to compensate for atmospheric distortion by placing a deformable mirror or variable refractive index device between light emitter and optical sensor (eye, CCD, etc).
[edit] Powerful laser
Lasers are becoming more powerful. Open literature typically does not discuss the real power of modern lasers, as this is usually a closely guarded military secret. It can be assumed that they are very powerful: not enough to vaporise an army tank, perhaps, but powerful and practical enough to cut aluminium panels of aircraft from long range, or to penetrate modern body armour and inflict fatal wounds; else the military would have realized the technological limitations and stopped this expensive research.
Some lasers can emit a very brief but extremely powerful pulse of light energy, which vaporises a layer of the target material, ejecting gases so fast that they are like an explosion on the surface of the target. (This method has been used for some time medically to break apart bladder stones without surgery, by sending a small probe up the urinary tract, which could guide the laser to the surface of the stone.
Lasers can function continuously if properly cooled, and continuous lasers are used to cut metals and other materials.
[edit] Tracking
Well-explored radar technology can easily track a high-speed aircraft to the nearest ten meters or so; thus it is likely that laser tracking can track to the nearest meter, because an accurate tracking beam can become very narrow, as may be seen in toy lasers.
High-speed cameras and image processing algorithms allow an "acquisition track", which means getting a rough position near enough for high-precision laser tracking to start. (This is like when using a binocular: first get a rough position of the target by looking at it naked eye, then get a precision observation by putting the binocular in the same axis as your observed target. If you were to look for something ("scanning") with the binoculars directly, you would waste much time (and would end up with a headache.)}
[edit] To affect eyesight
There remains the possibility of using lasers to blind, since this requires much lower power levels, and is easily achievable in a man portable unit. However, most nations regard the deliberate blinding of the enemy as forbidden by the rules of war. Russia, China, and Jordan possess such weapons[citation needed], which were banned in most countries in 1980. See Protocol on Blinding Laser Weapons.
[edit] Medical
Cosmetic surgery (removing tattoos, scars, stretch marks, sunspots, wrinkles, birthmarks, and hairs): see laser hair removal. Laser types used in dermatology include ruby (694 nm), alexandrite (755 nm), pulsed diode array (810 nm), Nd:YAG (1064 nm), Ho:YAG (2090 nm), and Er:YAG (2940 nm).
Eye surgery:-
LASIK (laser vision correction)
LASEK (laser-assisted sub-epithelial keratectomy)
PRK (photorefractive keratectomy)
Laser scalpel (gynecological, urology, laparoscopic)
Dental procedures
Photobiomodulation (i.e. laser therapy)
Imaging
"No-Touch" removal of tumors, especially of the brain and spinal cord.
Acupuncture.
In dentistry for caries removal, endodontic/periodontic procedures, tooth whitening, and oral surgery.
See laser scalpel.
[edit] Industrial & commercial
Cutting and peening of metals and other material, welding, marking, etc
Guidance systems (e.g., ring laser gyroscopes)
Rangefinder / surveying,
LIDAR / pollution monitoring,
Digital minilabs
Barcode readers
Laser engraving of printing plates
Laser pointers
Holography
Photolithography
Optical communications (over optical fiber or in free space)
Optical tweezers
Writing subtitles onto motion picture films. [3]
Space elevator, a possible solution transfer energy to the climbers by laser or microwave power beaming
3D laser scanners for accurate 3D measurement.
Laser line levels are used in surveying and construction. Lasers are also used for guidance for aircraft.
Extensively in both consumer and industrial imaging equipment.
In laser printers: gas and diode lasers play a key role in manufacturing high resolution printing plates and in image scanning equipment.
Diode lasers are used as a lightswitch in industry, with a laser beam and a receiver which will switch on or off when the beam is interrupted, and because a laser can keep the light intensity over larger distances than a normal light, and is more precise than a normal light it can be used for product detection in automated production.
Laser accelerometer
From Wikipedia, the free encyclopedia
Jump to: navigation, search
The benefits of lasers in various applications stems from their properties such as coherency, high monochromaticity, and ability to reach extremely high powers. For instance, a highly coherent laser beam can be focused down to its diffraction limit, which at visible wavelengths is only a few hundred nanometers. This property allows:-
A laser to record gigabytes of information in the microscopic pits of a DVD.
A laser of modest power to be focused to very high intensities and used for cutting, burning, or vaporizing materials. For example, a frequency doubled neodymium yttrium aluminium garnet (Nd:YAG) laser emitting 532 nanometer (green) light at 10 watts output power can theoretically achieve a focused intensity of megawatts per square centimeter.
In reality however, perfect focusing of a beam to its diffraction limit is somewhat difficult.
Contents [hide]
1 Scientific
1.1 Spectroscopy
1.2 Lunar laser ranging
1.3 Photochemistry
1.4 Laser cooling
1.5 Nuclear fusion
1.6 Finderscope for amateur telescopes
1.7 Microscopy
2 Military
2.1 Defensive applications
2.2 Strategic Defense Initiative
2.3 Laser sight
2.4 Satellite obstruction
2.5 Illuminator
2.6 Rangefinder
2.7 Target designator
2.8 Death ray
2.8.1 Fictional military use
2.9 Recent real developments
2.9.1 Flexible mirror
2.9.2 Powerful laser
2.9.3 Tracking
2.10 To affect eyesight
3 Medical
4 Industrial & commercial
5 In consumer products
5.1 Law enforcement
6 Links
7 Images
8 References
[edit] Scientific
In science, lasers are used in many ways, including:-
A wide variety of interferometric techniques
Raman spectroscopy
Laser induced breakdown spectroscopy.
Atmospheric remote sensing
Investigating nonlinear optics phenomena
Holographic techniques employing lasers also contribute to a number of measurement techniques.
Laser (LIDAR) technology has application in geology, seismology, remote sensing and atmospheric physics.
Lasers have been used aboard spacecraft such as in the Cassini-Huygens mission.
In astronomy, lasers have been used to create artificial laser guide stars, used as reference objects for adaptive optics telescopes.
[edit] Spectroscopy
Most types of laser are an inherently pure source of light; they emit near-monochromatic light with a very well defined range of wavelengths. By careful design of the laser components, the purity of the laser light (measured as the "linewidth") can be improved more than the purity of any other light source. This makes the laser a very useful source for spectroscopy. The high intensity of light that can be achieved in a small, well collimated beam can also be used to induce a nonlinear optical effect in a sample, which makes techniques such as Raman spectroscopy possible. Other spectroscopic techniques based on lasers can be used to make extremely sensitive detectors of various molecules, able to measure molecular concentrations in the parts-per-trillion (ppt) level. Due to the high power densities achievable by lasers, beam-induced atomic emission is possible: this technique is termed Laser induced breakdown spectroscopy (LIBS).
Lasers may also be indirectly used in spectroscopy as a micro-sampling system, a technique termed Laser ablation (LA), which is typically applied to ICP-MS apparatus resulting in the powerful LA-ICP-MS.
[edit] Lunar laser ranging
Main article: Lunar laser ranging experiment
When the Apollo astronauts visited the moon, they planted retroreflector arrays to make possible the Lunar Laser Ranging Experiment. Laser beams are focused through large telescopes on Earth aimed toward the arrays, and the time taken for the beam to be reflected back to Earth measured to determine the distance between the Earth and Moon with high precision.
[edit] Photochemistry
Some laser systems, through the process of modelocking, can produce extremely brief pulses of light - as short as picoseconds or femtoseconds (10-12 - 10-15 seconds). Such pulses can be used to initiate and analyse chemical reactions, a technique known as photochemistry. The short pulses can be used to probe the process of the reaction at a very high temporal resolution, allowing the detection of short-lived intermediate molecules. This method is particularly useful in biochemistry, where it is used to analyse details of protein folding and function.
[edit] Laser cooling
A technique that has had recent success is laser cooling. This involves ion trapping or atom trapping, a method where a number of ions or atoms are confined in a specially shaped arrangement of electric and magnetic fields. Shining particular wavelengths of laser light at the ions or atoms slows them down, thus cooling them. If this process is continued, eventually they all are slowed and have the same energy level, forming an unusual arrangement of matter known as a Bose-Einstein condensate.
[edit] Nuclear fusion
Lasers are used in certain types of thermonuclear fusion reactors. Perhaps the most extravagant use of lasers in science is in the field of fusion research. Some of the world's most powerful and complex arrangements of multiple lasers and optical amplifiers are used to produce extremely high intensity pulses of light of extremely short duration. These pulses are arranged such that they impact pellets of tritium-deuterium simultaneously from all directions, hoping that the squeezing effect of the impacts will induce atomic fusion in the pellets. This technique, known as "inertial confinement fusion", so far has not been able to achieve "breakeven", that is, so far the fusion reaction generates less power than is used to power the lasers, but research continues.
[edit] Finderscope for amateur telescopes
With the advent of higher power green laser pointers came a new product line for observational astronomers -- the laser finder. Akin to the laser sighting device used in military rifles, a green laser mounted on the tube of a telescope and properly collimated can reveal precisely where in the sky the telescope is pointed. Mostly outdoors at night the green laser beam is visible. By moving the telescope (and the laser beam) to the proper location in the sky, observers can more easily find the intended celestial target.
[edit] Microscopy
Confocal laser scanning microscopy and Two-photon excitation microscopy make use of lasers to obtain blur-free images of thick specimens at various depths.
[edit] Military
Military uses of lasers include use as target designators for other weapons; their use as directed-energy weapons is currently under research. Laser weapon systems under development include the airborne laser, the advanced tactical laser, the Tactical High Energy Laser, the High Energy Liquid Laser Area Defense System, and the MIRACL, or Mid-Infrared Advanced Chemical Laser.
[edit] Defensive applications
Recently, some progress has been made in the use of the laser as a directed energy weapon, mostly in defensive applications. By using a chemical laser, one in which the laser operation is powered by an energetic chemical reaction, the requirement for generating and storing a large amount of electrical energy (which directly or indirectly is used to power most high-power lasers) is removed. This makes the laser system much more compact, and easier to transport. One example is a laser system designed to destroy missiles in flight. It is mounted in a converted commercial airliner, and could be used, for example, to protect assets such as AWACS aircraft or to destroy ballistic missiles (see Airborne Laser). However, the practical problems of reliably generating and aiming the laser beam remain formidable.
The Mobile Tactical High-Energy Laser (MTHEL) is another defensive laser system under development; this is envisioned as a field-deployable weapon system able to track incoming artillery projectiles and cruise missiles by radar and destroy them with a powerful deuterium fluoride laser.
For information and a list of laser-based weapon systems, see also Directed Energy Weapon (Lasers).
[edit] Strategic Defense Initiative
Another example of direct use of a laser as a defensive weapon was researched for the Strategic Defense Initiative (SDI, nicknamed "Star Wars"), and its successor programs. This project would use ground-based or space-based laser systems to destroy incoming intercontinental ballistic missiles (ICBMs). Again, the practical problems of using and aiming these systems would be many; particularly the problem of destroying ICBMs at the most opportune moment, the boost phase just after launch. This would involve directing a laser through a large distance in the atmosphere, which, due to optical scattering and refraction, would bend and distort the laser beam, complicating the aiming of the laser and reducing its efficiency.
Another idea to come from the SDI project was the nuclear-pumped X-ray laser. This was essentially an orbiting atomic bomb, surrounded by laser media in the form of glass rods; when the bomb exploded, the rods would be bombarded with highly-energetic gamma-ray photons, causing spontaneous and stimulated emission of X-ray photons in the atoms making up the rods. This would lead to optical amplification of the X-ray photons, producing an X-ray laser beam that would be minimally affected by atmospheric distortion and capable of destroying ICBMs in flight. The X-ray laser would be a strictly one-shot device, destroying itself on activation. Some initial tests of this concept were performed with underground nuclear testing, however, the results were not encouraging. Research into this approach to missile defense was discontinued after the cancellation of the SDI program.
In recent years, the United States Air Force has begun experimenting with using lasers combined with high-altitude airships as a potential means for a missile defense shield but also as a means to destroy enemy spacecraft or satellites in low-earth orbit. For more information, see Evolutionary Air and Space Global Laser Engagement.
[edit] Laser sight
The laser has in most military applications been used as a tool to enhance the targeting of other weapon systems. For example, a laser sight is a small, usually visible-light laser placed on a handgun or rifle aligned to emit a beam parallel to the barrel. Since a laser beam typically has low divergence, the laser light appears as a small spot even at long distances; the user simply places the spot on the desired target and the barrel of the gun is aligned. Recent studies (2001) have shown that laser sight has become an effective deterring tool for law enforcement. Criminals are more likely to surrender without resistance when they find a red laser dot on their chest.
Most laser sights use a red (670–633 nm) diode; the 633–635 nm diodes are about 10 times as bright as 670 nm diodes. Some laser sights used an infrared diode, which produced a dot invisible to the naked human eye, but would show up when the user used special optics. In the late 1990s, green diode pumped solid state laser (DPSS) laser sights (532 nm) became available.
Modern laser sights are so small that they can be installed below the barrel as part of the gun, instead of being a separate attachment. Some laser sights, such as those made by LaserMax, are integrated into a unit that replaces a pistol's guide rod, thus adding no bulk to the gun. Others, like Crimson Trace Lasergrips, are built into the grip so that activation is automatic when the gun is held.
Another type of optical sight is the reflex, or red dot sight. This gives the illusion of a red dot projected onto the target when the user looks through the sight, but only the user can see the dot. This has advantages where more than one weapon is aimed at the same target, and it is also more visible than a conventional laser in bright sunlight.
[edit] Satellite obstruction
According to a report issued by the Pentagon, China is developing a laser that could blind low Earth orbit satellites. [1]
[edit] Illuminator
Saber 203 Laser Illuminator (U.S. Air Force)This non-lethal laser weapon, shown in the accompanying photo attached to an M16 rifle, was developed by the U.S. Air Force to temporarily impair an adversary’s ability to fire a weapon or to otherwise threaten enemy forces. The Saber 203 briefly illuminates an opponent with harmless low-power laser light. Realizing he has been targeted, the aggressor hides or flees rather than risk death by aiming his weapon and attracting defensive fire.
[edit] Rangefinder
Main article: Laser range-finder
A laser range-finder is a device consisting of a pulsed laser and a light detector. By measuring the time taken for light to reflect off a far object, and knowing the speed of light, the range to the object can be found. A laser rangefinder is thus a simple form of LIDAR. The distance to the target can then be used to aim a weapon such as a tank's main gun.
[edit] Target designator
Main article: Laser designator
Another military use of lasers is as a laser target designator. This is a low-power laser pointer used to indicate a target for a precision-guided munition, typically launched from an aircraft. The guided munition adjusts its flight-path to home in to the laser light reflected by the target, enabling a great precision in aiming. The beam of the laser target designator is set to a pulse rate that matches that set on the guided munition to ensure munitions strike their designated targets and do not follow other laser beams which may be in use in the area. The laser designator can be shone onto the target by an aircraft or nearby infantry. Lasers used for this purpose are usually infrared lasers, so the enemy cannot easily detect the guiding laser light.
[edit] Death ray
Not yet made practical: The first role envisioned for the laser in military applications was as a "death ray": a hand-held device that might replace the gun as a weapon for infantry, or a vehicle-mounted "laser cannon" able to destroy tanks, ships and aircraft. However, practical considerations have severely constrained these ideas; any laser that can seriously wound a human would (with its needed power supply) be too heavy for one man to lift, and a high-power laser that can burn through tank armor would be extremely complex and very sensitive to misalignment from any knocks or vibration it might suffer, making it unsuitable for field deployment.
[edit] Fictional military use
Such uses are very common in fiction: see raygun.
[edit] Recent real developments
In 1980, the US Air Force managed to shoot down missiles which were configured as target practice drones. Results were sporadic at best, but they showed definitively that aircraft could be shot down by a static ground laser. Further tests and calculations showed that getting such a laser wespon working would be possible but not easy. A modern real military "ray gun" would need three or more lasers and an initial track acquisition device:-
0: The initial track device gives the approximate position of the target, and other relevant ,information, such as speed, altitude, predicted impact point, estimated launch position, etc.
1: The first laser (low power) then starts scanning there until it finds its exact position.
2: The second laser (low power) then measures the target, including:-
Atmospheric distortion measurements. This lets the shooter adjust a flexible mirror to counter-deviate the third laser to compensate for the atmospheric deviations.
Vibration analysis. This finds the target's mechanical resonance vibration modes.
3: The third laser sends powerful repetitive pulses at the natural frequency of the target, complete with distortion correction. This causes structural shock to the body of the missile, and also may cause damage by the powerful laser's heat. Usually flying machines are very fragile,and sudden thermal and physical shocks cause them to break apart.
Given these capacities, or similar, one sort of "ray-guns" is technically possible, such as the Boeing YAL-1A Airborne Laser [2].
The only major limitation is that heavy vehicles such as tanks would not suffer any significant effect when hit with a laser. Most probably the operator would notice that the side armour was vibrating, feel an increase in ambient temperature, and no more. The only truly noticeable effect would be that a tank equipped with, reactive armour would start burning, with an outer extreme being low-power non-damaging explosions.
It would need three technological developments to work:-
[edit] Flexible mirror
A laser beam can be diverted or distorted by refraction caused by distorting atmospheric conditions. These exist already: in the 1990s astronomers managed to compensate for atmospheric distortion by placing a deformable mirror or variable refractive index device between light emitter and optical sensor (eye, CCD, etc).
[edit] Powerful laser
Lasers are becoming more powerful. Open literature typically does not discuss the real power of modern lasers, as this is usually a closely guarded military secret. It can be assumed that they are very powerful: not enough to vaporise an army tank, perhaps, but powerful and practical enough to cut aluminium panels of aircraft from long range, or to penetrate modern body armour and inflict fatal wounds; else the military would have realized the technological limitations and stopped this expensive research.
Some lasers can emit a very brief but extremely powerful pulse of light energy, which vaporises a layer of the target material, ejecting gases so fast that they are like an explosion on the surface of the target. (This method has been used for some time medically to break apart bladder stones without surgery, by sending a small probe up the urinary tract, which could guide the laser to the surface of the stone.
Lasers can function continuously if properly cooled, and continuous lasers are used to cut metals and other materials.
[edit] Tracking
Well-explored radar technology can easily track a high-speed aircraft to the nearest ten meters or so; thus it is likely that laser tracking can track to the nearest meter, because an accurate tracking beam can become very narrow, as may be seen in toy lasers.
High-speed cameras and image processing algorithms allow an "acquisition track", which means getting a rough position near enough for high-precision laser tracking to start. (This is like when using a binocular: first get a rough position of the target by looking at it naked eye, then get a precision observation by putting the binocular in the same axis as your observed target. If you were to look for something ("scanning") with the binoculars directly, you would waste much time (and would end up with a headache.)}
[edit] To affect eyesight
There remains the possibility of using lasers to blind, since this requires much lower power levels, and is easily achievable in a man portable unit. However, most nations regard the deliberate blinding of the enemy as forbidden by the rules of war. Russia, China, and Jordan possess such weapons[citation needed], which were banned in most countries in 1980. See Protocol on Blinding Laser Weapons.
[edit] Medical
Cosmetic surgery (removing tattoos, scars, stretch marks, sunspots, wrinkles, birthmarks, and hairs): see laser hair removal. Laser types used in dermatology include ruby (694 nm), alexandrite (755 nm), pulsed diode array (810 nm), Nd:YAG (1064 nm), Ho:YAG (2090 nm), and Er:YAG (2940 nm).
Eye surgery:-
LASIK (laser vision correction)
LASEK (laser-assisted sub-epithelial keratectomy)
PRK (photorefractive keratectomy)
Laser scalpel (gynecological, urology, laparoscopic)
Dental procedures
Photobiomodulation (i.e. laser therapy)
Imaging
"No-Touch" removal of tumors, especially of the brain and spinal cord.
Acupuncture.
In dentistry for caries removal, endodontic/periodontic procedures, tooth whitening, and oral surgery.
See laser scalpel.
[edit] Industrial & commercial
Cutting and peening of metals and other material, welding, marking, etc
Guidance systems (e.g., ring laser gyroscopes)
Rangefinder / surveying,
LIDAR / pollution monitoring,
Digital minilabs
Barcode readers
Laser engraving of printing plates
Laser pointers
Holography
Photolithography
Optical communications (over optical fiber or in free space)
Optical tweezers
Writing subtitles onto motion picture films. [3]
Space elevator, a possible solution transfer energy to the climbers by laser or microwave power beaming
3D laser scanners for accurate 3D measurement.
Laser line levels are used in surveying and construction. Lasers are also used for guidance for aircraft.
Extensively in both consumer and industrial imaging equipment.
In laser printers: gas and diode lasers play a key role in manufacturing high resolution printing plates and in image scanning equipment.
Diode lasers are used as a lightswitch in industry, with a laser beam and a receiver which will switch on or off when the beam is interrupted, and because a laser can keep the light intensity over larger distances than a normal light, and is more precise than a normal light it can be used for product detection in automated production.
Laser accelerometer
Monday, March 26, 2007
Creating a Population Inversion
Finding substances in which a population inversion can be set up is central to the develpment of new kinds of laser. The first material used was synthetic ruby. Ruby is crystalline alumina (Al2O3) in which a small fraction of the Al3+ ions have been replaced by chromium ions, Cr3+. It is the chromium ions that give rise to the characteristic pink or red color of ruby and it is in these ions that a population inversion is set up in a ruby laser.
In a ruby laser, a rod of ruby is irradiated with the intense flash of light from xenon-filled flashtubes. Light in the green and blue regions of the spectrum is absorbed by chromium ions, raising the energy of electrons of the ions from the ground state level to the broad F bands of levels. Electrons in the F bands rapidly undergo non-radiative transitions to the two metastable E levels. A non-radiative transition does not result in the emission of light; the energy released in the transition is dissipated as heat in the ruby crystal. The metastable levels are unusual in that they have a relatively long lifetime of about 4 milliseconds (4 x 10-3 s), the major decay process being a transition from the lower level to the ground state. This long lifetime allows a high proportion (more than a half) of the chromium ions to build up in the metastable levels so that a population inversion is set up between these levels and the ground state level. This population inversion is the condition required for stimulated emission to overcome absorption and so give rise to the amplification of light. In an assembly of chromium ions in which a population inversion has been set up, some will decay spontaneously to the ground state level emitting red light of wavelength 694.3 nm in the process. This light can then interact with other chromium ions that are in the metastable levels causing them to emit light of the same wavelength by stimulated emission. As each stimulating photon leads to the emission of two photons, the intensity of the light emitted will build up quickly. This cascade process in which photons emitted from excited chromium ions cause stimulated emission from other excited ions is indicated below:
The ruby laser is often referred to as an example of a three-level system. More than three energy levels are actually involved but they can be put into three categories.These are; the lower level form which pumping takes place, the F levels into which the chromium ions are pumped, and the metastable levels from which stimulated emission occurs. Other types of laser operate on a four level system and , in general, the mechanism of amplification differs for different lasing materials. However, in all cases, it is necessary to set up a population inversion so that stimulated emission occurs more often than absorption.
Finding substances in which a population inversion can be set up is central to the develpment of new kinds of laser. The first material used was synthetic ruby. Ruby is crystalline alumina (Al2O3) in which a small fraction of the Al3+ ions have been replaced by chromium ions, Cr3+. It is the chromium ions that give rise to the characteristic pink or red color of ruby and it is in these ions that a population inversion is set up in a ruby laser.
In a ruby laser, a rod of ruby is irradiated with the intense flash of light from xenon-filled flashtubes. Light in the green and blue regions of the spectrum is absorbed by chromium ions, raising the energy of electrons of the ions from the ground state level to the broad F bands of levels. Electrons in the F bands rapidly undergo non-radiative transitions to the two metastable E levels. A non-radiative transition does not result in the emission of light; the energy released in the transition is dissipated as heat in the ruby crystal. The metastable levels are unusual in that they have a relatively long lifetime of about 4 milliseconds (4 x 10-3 s), the major decay process being a transition from the lower level to the ground state. This long lifetime allows a high proportion (more than a half) of the chromium ions to build up in the metastable levels so that a population inversion is set up between these levels and the ground state level. This population inversion is the condition required for stimulated emission to overcome absorption and so give rise to the amplification of light. In an assembly of chromium ions in which a population inversion has been set up, some will decay spontaneously to the ground state level emitting red light of wavelength 694.3 nm in the process. This light can then interact with other chromium ions that are in the metastable levels causing them to emit light of the same wavelength by stimulated emission. As each stimulating photon leads to the emission of two photons, the intensity of the light emitted will build up quickly. This cascade process in which photons emitted from excited chromium ions cause stimulated emission from other excited ions is indicated below:
The ruby laser is often referred to as an example of a three-level system. More than three energy levels are actually involved but they can be put into three categories.These are; the lower level form which pumping takes place, the F levels into which the chromium ions are pumped, and the metastable levels from which stimulated emission occurs. Other types of laser operate on a four level system and , in general, the mechanism of amplification differs for different lasing materials. However, in all cases, it is necessary to set up a population inversion so that stimulated emission occurs more often than absorption.
Absorption and emission of light by atoms
So far, nothing has been written about how an amplifying medium amplifies light. This will be dealt with here and in the next section. We must begin with an account of how light can interact with individual atoms within an amplifying medium ("atoms" will be used to include molecules and ions). Atoms consist of a positively charged core (nucleus) which is surrounded by negatively charged electrons. According to the quantum mechanical description of an atom, the energy of an atomic electron can have only certain values and these are represented by energy levels. The electrons can be thought of as orbiting the nucleus, those with the largest energy orbiting at greater distances from the nuclear core. There are many energy levels that an electron within an atom can occupy, but here we will consider only two. Also, we will consider only the electrons in the outer orbits of the atom as these can most easily be raised to higher unfilled energy states.
Absorption and Spontaneous Emission
The processes of the absorption and spontaneous emission of light are illustrated below:
A photon of light is absorbed by an atom in which one of the outer electrons is initially in a low energy state denoted by 0. The energy of the atom is raised to the upper energy level, 1, and remains in this excited state for a period of time that is typically less than 10-6 second. It then spontaneously returns to the lower state, 0, with the emission of a photon of light. Absorption is referred to as a resonant process because the energy of the absorbed photon must be equal to the difference in energy between the levels 0 and 1. This means that only photons of a particular frequency (or wavelength) will be absorbed. Similarly, the photon emitted will have energy equal to the difference in energy between the two energy levels. These common processes of absorption and spontaneous emission cannot give rise to the amplification of light. The best that can be achieved is that for every photon absorbed, another is emitted.
Stimulated Emission
Stimulated emission is a very uncommon process in nature but it is central to the operation of lasers.
Above it was stated that an atom in a high energy, or excited, state can return to the lower state spontaneously. However, if a photon of light interacts with the excited atom, it can stimulate a return to the lower state. One photon interacting with an excited atom results in two photons being emitted. Furthermore, the two emitted photons are said to be in phase, i.e. thinking of them as waves, the crest of the wave associated with one photon occurs at the same time as on the wave associated with the other. This feature ensures that there is a fixed phase relationship between light radiated from different atoms in the amplifying medium and results in the laser beam produced having the property of coherence. Stimulated emission is the process that can give rise to the amplification of light. As with absorption, it is a resonant process; the energy of the incoming photon of light must match the difference in energy between the two energy levels. Furthermore, if we consider a photon of light interacting with a single atom, stimulated emission is just as likely as absorption; which process occurs depends upon whether the atom is initially in the lower or the upper energy level. However, under most conditions, stimulated emission does not occur to a significant extent. The reason is that, under most conditions, that is, under conditions of thermal equilibrium, there will be far more atoms in the lower energy level, 0, than in the upper level, 1, so that absorption will be much more common than stimulated emission. If stimulated emission is to predominate, we must have more atoms in the higher energy state than in the lower one. This unusual condition is referred to as a population inversion and it is necessary to create a population inversion for laser action to occur
So far, nothing has been written about how an amplifying medium amplifies light. This will be dealt with here and in the next section. We must begin with an account of how light can interact with individual atoms within an amplifying medium ("atoms" will be used to include molecules and ions). Atoms consist of a positively charged core (nucleus) which is surrounded by negatively charged electrons. According to the quantum mechanical description of an atom, the energy of an atomic electron can have only certain values and these are represented by energy levels. The electrons can be thought of as orbiting the nucleus, those with the largest energy orbiting at greater distances from the nuclear core. There are many energy levels that an electron within an atom can occupy, but here we will consider only two. Also, we will consider only the electrons in the outer orbits of the atom as these can most easily be raised to higher unfilled energy states.
Absorption and Spontaneous Emission
The processes of the absorption and spontaneous emission of light are illustrated below:
A photon of light is absorbed by an atom in which one of the outer electrons is initially in a low energy state denoted by 0. The energy of the atom is raised to the upper energy level, 1, and remains in this excited state for a period of time that is typically less than 10-6 second. It then spontaneously returns to the lower state, 0, with the emission of a photon of light. Absorption is referred to as a resonant process because the energy of the absorbed photon must be equal to the difference in energy between the levels 0 and 1. This means that only photons of a particular frequency (or wavelength) will be absorbed. Similarly, the photon emitted will have energy equal to the difference in energy between the two energy levels. These common processes of absorption and spontaneous emission cannot give rise to the amplification of light. The best that can be achieved is that for every photon absorbed, another is emitted.
Stimulated Emission
Stimulated emission is a very uncommon process in nature but it is central to the operation of lasers.
Above it was stated that an atom in a high energy, or excited, state can return to the lower state spontaneously. However, if a photon of light interacts with the excited atom, it can stimulate a return to the lower state. One photon interacting with an excited atom results in two photons being emitted. Furthermore, the two emitted photons are said to be in phase, i.e. thinking of them as waves, the crest of the wave associated with one photon occurs at the same time as on the wave associated with the other. This feature ensures that there is a fixed phase relationship between light radiated from different atoms in the amplifying medium and results in the laser beam produced having the property of coherence. Stimulated emission is the process that can give rise to the amplification of light. As with absorption, it is a resonant process; the energy of the incoming photon of light must match the difference in energy between the two energy levels. Furthermore, if we consider a photon of light interacting with a single atom, stimulated emission is just as likely as absorption; which process occurs depends upon whether the atom is initially in the lower or the upper energy level. However, under most conditions, stimulated emission does not occur to a significant extent. The reason is that, under most conditions, that is, under conditions of thermal equilibrium, there will be far more atoms in the lower energy level, 0, than in the upper level, 1, so that absorption will be much more common than stimulated emission. If stimulated emission is to predominate, we must have more atoms in the higher energy state than in the lower one. This unusual condition is referred to as a population inversion and it is necessary to create a population inversion for laser action to occur
amplifiers are often incorporated into laser systems. However, except in a few exceptional cases, light amplifiers would not be regarded as lasers. A laser consists of a pumped amplifying medium positioned between two mirrors as indicated below. The purpose of the mirrors is to provide what is described as 'positive feedback'. This means simply that some of the light that emerges from the amplifying medium is reflected back into it for further amplification. Laser mirrors usually do not reflect all wavelengths or colors of light equally well - their reflectivity is matched to the wavelength or color at which the laser operates. In appearance, they do not look like ordinary mirrors and are transparent at some wavelengths. An amplifier with positive feedback is known as an oscillator.
The space between the two mirrors is known as the laser cavity. The beam within the cavity undergoes multiple reflections between the mirrors and is amplified each time it passes through the amplifying medium. One of the mirrors reflects almost all of the light that falls upon it (total reflector in the above diagram). The other mirror reflects between 20% and 98% of the incident light depending upon the type of laser, the light that is not reflected being transmitted through the mirror. This transmitted portion constitutes the output beam of the laser.
The laser cavity has several important functions. Following pumping, spontaneous emission of light from excited atoms within the amplifying medium initiates the emission of low intensity light into the laser cavity. This light is increased in intensity by multiple passes through the amplifying medium so that it rapidly builds up into an intense beam. In the absence of cavity mirrors, this self-starting process, or oscillation, would not occur.
The cavity ensures that the divergence of the beam is small. Only light that travels in a direction closely parallel to the axis of the cavity can undergo multiple reflections at the mirrors and make multiple passes through the amplifying medium. More divergent rays execute a zig-zag path within the cavity and wander out of it.
The laser cavity also improves the spectral purity of the laser beam. Usually, the amplifying medium will amplify light within a narrow range of wavelengths. However, within this narrow range, only light of particular wavelengths can undergo repeated reflection up and down the cavity. The characteristics that a light beam within the cavity must possess in order to undergo repeated reflections define what is referred to as a cavity mode. Light which may still be amplified by the amplifying medium but which does not belong to one of these special modes of oscillation is rapidly attenuated and will not be present in the output beam. This behaviour is similar to that of a vibrating guitar string in that a particular string will only vibrate at certain frequencies. In a similar way, an optical cavity will only sustain repeated reflections for particular well-defined wavelengths of light.
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The space between the two mirrors is known as the laser cavity. The beam within the cavity undergoes multiple reflections between the mirrors and is amplified each time it passes through the amplifying medium. One of the mirrors reflects almost all of the light that falls upon it (total reflector in the above diagram). The other mirror reflects between 20% and 98% of the incident light depending upon the type of laser, the light that is not reflected being transmitted through the mirror. This transmitted portion constitutes the output beam of the laser.
The laser cavity has several important functions. Following pumping, spontaneous emission of light from excited atoms within the amplifying medium initiates the emission of low intensity light into the laser cavity. This light is increased in intensity by multiple passes through the amplifying medium so that it rapidly builds up into an intense beam. In the absence of cavity mirrors, this self-starting process, or oscillation, would not occur.
The cavity ensures that the divergence of the beam is small. Only light that travels in a direction closely parallel to the axis of the cavity can undergo multiple reflections at the mirrors and make multiple passes through the amplifying medium. More divergent rays execute a zig-zag path within the cavity and wander out of it.
The laser cavity also improves the spectral purity of the laser beam. Usually, the amplifying medium will amplify light within a narrow range of wavelengths. However, within this narrow range, only light of particular wavelengths can undergo repeated reflection up and down the cavity. The characteristics that a light beam within the cavity must possess in order to undergo repeated reflections define what is referred to as a cavity mode. Light which may still be amplified by the amplifying medium but which does not belong to one of these special modes of oscillation is rapidly attenuated and will not be present in the output beam. This behaviour is similar to that of a vibrating guitar string in that a particular string will only vibrate at certain frequencies. In a similar way, an optical cavity will only sustain repeated reflections for particular well-defined wavelengths of light.
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Energizing the amplifying medium
Increasing the intensity of a light beam that passes through an amplifying medium amounts to putting additional energy into the beam. This energy comes from the amplifying medium which must in turn have energy fed into it in some way. In laser terminology, the process of energizing the amplifying medium is known as "pumping".
There are several ways of pumping an amplifying medium. When it is a solid, pumping is usually achieved by irradiating it with intense light. This light is absorbed by atoms or ions within the medium raising them into higher energy states. Xenon-filled flashtubes positioned as shown below are used as a simple source of pumping light. Passing a high voltage electric discharge through the flashtubes causes them to emit an intense flash of white light, some of which is absorbed by the amplifying medium. The assembly of flashtubes is enclosed within a polished metal reflector (not shown in the diagram below) to concentrate as much light as possible on the amplifying medium. A laser that is pumped in this way will have a pulsed output.
Pumping an amplifying medium by irradiating it with intense light is referred to as optical pumping. The source of pumping light can be another laser. Some types of laser that were originally pumped using xenon-filled flashtubes are now pumped by laser diodes.
Gaseous amplifying media have to be contained in some form of enclosure or tube and are often pumped by passing an electric discharge through the medium itself. The mechanism by which this elevates atoms or molecules in the gas to higher energy states depends upon the gas that is being excited and is often complex. In many gas lasers, the end windows of the laser tube are inclined at an angle and they are referred to as brewster windows. Brewster windows are able to transmit a beam that is polarized in the plane of the diagram without losses due to reflection. Such a laser would have an output beam that is polarized.
The diagram illustrates pumping by passing a discharge longitudinally through the gaseous amplifying medium but, in some cases, the discharge takes place transversely from one side of the medium to the other. Many lasers that are pumped by an electric discharge can produce either a pulsed output or a continuous output depending upon whether the discharge is pulsed or continuous.
Various other methods of pumping the amplifying medium in a laser are used. For example, laser diodes are pumped by passing an electric current across the junction where the two types of semiconductor within the diode come together.
Increasing the intensity of a light beam that passes through an amplifying medium amounts to putting additional energy into the beam. This energy comes from the amplifying medium which must in turn have energy fed into it in some way. In laser terminology, the process of energizing the amplifying medium is known as "pumping".
There are several ways of pumping an amplifying medium. When it is a solid, pumping is usually achieved by irradiating it with intense light. This light is absorbed by atoms or ions within the medium raising them into higher energy states. Xenon-filled flashtubes positioned as shown below are used as a simple source of pumping light. Passing a high voltage electric discharge through the flashtubes causes them to emit an intense flash of white light, some of which is absorbed by the amplifying medium. The assembly of flashtubes is enclosed within a polished metal reflector (not shown in the diagram below) to concentrate as much light as possible on the amplifying medium. A laser that is pumped in this way will have a pulsed output.
Pumping an amplifying medium by irradiating it with intense light is referred to as optical pumping. The source of pumping light can be another laser. Some types of laser that were originally pumped using xenon-filled flashtubes are now pumped by laser diodes.
Gaseous amplifying media have to be contained in some form of enclosure or tube and are often pumped by passing an electric discharge through the medium itself. The mechanism by which this elevates atoms or molecules in the gas to higher energy states depends upon the gas that is being excited and is often complex. In many gas lasers, the end windows of the laser tube are inclined at an angle and they are referred to as brewster windows. Brewster windows are able to transmit a beam that is polarized in the plane of the diagram without losses due to reflection. Such a laser would have an output beam that is polarized.
The diagram illustrates pumping by passing a discharge longitudinally through the gaseous amplifying medium but, in some cases, the discharge takes place transversely from one side of the medium to the other. Many lasers that are pumped by an electric discharge can produce either a pulsed output or a continuous output depending upon whether the discharge is pulsed or continuous.
Various other methods of pumping the amplifying medium in a laser are used. For example, laser diodes are pumped by passing an electric current across the junction where the two types of semiconductor within the diode come together.
Amplification of light
All lasers contain an energized substance that can increase the intensity of light passing through it. This substance is called the amplifying medium or, sometimes, the gain medium, and it can be a solid, a liquid or a gas. Whatever its physical form, the amplifying medium must contain atoms, molecules or ions, a high proportion of which can store energy that is subsequently released as light. How the amplifying medium increases the intensity of light passing through it will be explained in sections 4 and 5. For the moment, we will just assume that light amplification is possible.
In a neodymium YAG (Nd:YAG) laser, the amplifying medium is a rod of yttrium aluminium garnate (YAG) containing ions of the lanthanide metal neodymium (Nd). In a dye laser, it is a solution of a fluorescent dye in a solvent such as methanol. In a helium-neon laser, it is a mixture of the gases helium and neon. In a laser diode, it is a thin layer of semiconductor material sandwiched between other semiconductor layers. The factor by which the intensity of the light is increased by the amplifying medium is known as the gain. The gain is not a constant for a particular type of medium. It's magnitude depends upon the wavelength of the incoming light, the intensity of the incoming light, the length of the amplifying medium and also upon the extent to which the amplifying medium has been energized.
All lasers contain an energized substance that can increase the intensity of light passing through it. This substance is called the amplifying medium or, sometimes, the gain medium, and it can be a solid, a liquid or a gas. Whatever its physical form, the amplifying medium must contain atoms, molecules or ions, a high proportion of which can store energy that is subsequently released as light. How the amplifying medium increases the intensity of light passing through it will be explained in sections 4 and 5. For the moment, we will just assume that light amplification is possible.
In a neodymium YAG (Nd:YAG) laser, the amplifying medium is a rod of yttrium aluminium garnate (YAG) containing ions of the lanthanide metal neodymium (Nd). In a dye laser, it is a solution of a fluorescent dye in a solvent such as methanol. In a helium-neon laser, it is a mixture of the gases helium and neon. In a laser diode, it is a thin layer of semiconductor material sandwiched between other semiconductor layers. The factor by which the intensity of the light is increased by the amplifying medium is known as the gain. The gain is not a constant for a particular type of medium. It's magnitude depends upon the wavelength of the incoming light, the intensity of the incoming light, the length of the amplifying medium and also upon the extent to which the amplifying medium has been energized.
Laser
From Wikipedia, the free encyclopedia
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For other uses, see Laser (disambiguation).
A laser (acronym for Light Amplification by Stimulated Emission of Radiation) is an optical source that emits light in a coherent beam. The back-formed verb lase means "to produce laser light" or "to apply laser light to".[1]
In analogy with optical lasers, a device which produces any particles or electromagnetic radiation in a coherent state is also called a "laser", usually with indication of type of particle as prefix (for example, atom laser.) In most cases, "laser" refers to a source of coherent photons, i.e. light or other electromagnetic radiation.
Laser light is typically near-monochromatic, i.e., consisting of a very small band of wavelengths or a very specific color, and emitted in a narrow beam. This contrasts with common light sources, such as the incandescent light bulb, which emit incoherent photons in almost all directions, usually over a wide spectrum of wavelengths.
Laser action is explained by the theories of quantum mechanics and thermodynamics. Many materials have been found to have the required characteristics to form the laser gain medium needed to power a laser, and these have led to the invention of many types of lasers with different characteristics suitable for different applications.
The laser was proposed as a variation of the maser principle in the late 1950s, and the first laser was demonstrated in July 1960 by Theodore Maiman at Hughes Research Laboratories. Since that time, laser manufacture has become a multi-billion dollar industry, and the laser has found applications in fields including science, the defense industry, medicine, and consumer electronics.
Experiment using a (likely argon) laser. (US military)For lasers in fiction, see also raygun.
Contents [hide]
1 Physics
2 History
2.1 Foundations: Einstein
2.2 The maser
2.3 The laser
2.4 Recent innovations
3 Uses
3.1 Example uses by typical output power
4 FASOR
5 Laser safety
6 Types and operating principles
6.1 Gas lasers
6.2 Chemical lasers
6.2.1 Excimer lasers
6.3 Solid-state lasers
7 Popular misconceptions
8 Fictional predictions
9 Hobby uses
[edit] Physics
Principal components:
1. Active laser medium
2. Laser pumping energy
3. High reflector
4. Output coupler
5. Laser beam
A helium-neon laser demonstration at the Kastler-Brossel Laboratory at Univ. Paris 6. The glowing ray in the middle is an electric discharge producing light in much the same way as a neon light. It is the gain medium through which the laser passes, not the laser beam itself, which is visible there. The laser beam crosses the air and marks a red point on the screen to the right.See also: Laser science and Laser construction
A laser is composed of an active laser medium, or gain medium, and a resonant optical cavity. The gain medium transfers external energy into the laser beam. It is a material of controlled purity, size, concentration, and shape, which amplifies the beam by the quantum mechanical process of stimulated emission, discovered by Albert Einstein while researching the photoelectric effect. The gain medium is energized, or pumped, by an external energy source. Examples of pump sources include electricity and light, for example from a flash lamp or from another laser. The pump energy is absorbed by the laser medium, placing some of its particles into high-energy ("excited") quantum states. When the number of particles in one excited state exceeds the number of particles in some lower-energy state, population inversion is achieved. In this condition, an optical beam passing through the medium produces more stimulated emission than the stimulated absorption, so the beam is amplified. An excited laser medium can also function as an optical amplifier.
The light generated by stimulated emission is very similar to the input signal in terms of wavelength, phase, and polarization. This gives laser light its characteristic coherence, and allows it to maintain the uniform polarization and monochromaticity established by the optical cavity design.
The optical cavity, an example of a type of cavity resonator, contains a coherent beam of light between reflective surfaces so that each photon passes through the gain medium more than once before it is emitted from the output aperture or lost to diffraction or absorption. As light circulates through the cavity, passing through the gain medium, if the gain (amplification) in the medium is stronger than the resonator losses, the power of the circulating light can rise exponentially. But each stimulated emission event returns a particle from its excited state to the ground state, reducing the capacity of the gain medium for further amplification. When this effect becomes strong, the gain is said to be saturated. The balance of pump power against gain saturation and cavity losses produces an equilibrium value of the intracavity laser power; this equilibrium determines the operating point of the laser. If the chosen pump power is too small, the gain is not sufficient to overcome the resonator losses, and the laser will emit only very small light powers. The minimum pump power needed to begin laser action is called the lasing threshold. The gain medium will amplify any photons passing through it, regardless of direction; but only the photons aligned with the cavity manage to pass more than once through the medium and so have significant amplification.
The beam in the cavity and the output beam of the laser, if they occur in free space rather than waveguides (as in an optical fiber laser), are often Gaussian beams. If the beam is not a pure Gaussian shape, the transverse modes of the beam may be analyzed as a superposition of Hermite-Gaussian or Laguerre-Gaussian beams. The beam may be highly collimated, that is, having a very small beam divergence, but a perfectly collimated beam cannot be created, due to diffraction. But a laser beam will spread much less than a beam of incoherent light. The beam remains collimated over a distance which varies with the square of the beam diameter, and eventually diverges at an angle which varies inversely with the beam diameter. Thus, a beam generated by a small laboratory laser such as a helium-neon laser spreads to about 1.6 kilometres (1 mile) diameter if shone from the Earth to the Moon. By comparison, the output of a typical semiconductor laser, due to its small diameter, diverges almost as soon as it leaves the aperture, at an angle of anything up to 50°. However, such a divergent beam can be transformed into a collimated beam by means of a lens. In contrast, the light from non-laser light sources cannot be collimated by optics as well or much.
The output of a laser may be a continuous constant-amplitude output (known as CW or continuous wave); or pulsed, by using the techniques of Q-switching, modelocking, or gain-switching. In pulsed operation, much higher peak powers can be achieved.
Some types of lasers, such as dye lasers and vibronic solid-state lasers can produce light over a broad range of wavelengths; this property makes them suitable for generating extremely short pulses of light, on the order of a femtosecond (10-15 s).
Though the laser phenomenon was discovered with the help of quantum physics, it is not essentially more quantum mechanical than are other sources of light. The operation of a free electron laser can be explained without reference to quantum mechanics.
It is understood that the word light in the acronym Light Amplification by Stimulated Emission of Radiation is typically used in the expansive sense, as photons of any energy; it is not limited to photons in the visible spectrum. Hence there are infrared lasers, ultraviolet lasers, X-ray lasers, etc. For example, a source of atoms in a coherent state can be called an atom laser.
Because the microwave equivalent of the laser, the maser, was developed first, devices that emit microwave and radio frequencies are usually called masers. In early literature, particularly from researchers at Bell Telephone Laboratories, the laser was often called the optical maser. This usage has since become uncommon, and as of 1998 even Bell Labs uses the term laser. [2]
[edit] History
[edit] Foundations: Einstein
In 1916, Albert Einstein laid the foundation for the invention of the laser and its predecessor, the maser, in a ground-breaking rederivation of Max Planck's law of radiation based on the concepts of spontaneous and induced emission.
In 1928, Rudolph W. Landenburg confirmed the existence of stimulated emission and negative absorption. [3]
In 1939, Valentin A. Fabrikant (USSR) predicted the use of stimulated emission to amplify "short" waves.[4]
In 1947, Willis E. Lamb and R. C. Retherford found apparent stimulated emission in hydrogen spectra and made the first demonstration of stimulated emission. [5]
In 1950, Alfred Kastler (Nobel Prize for Physics 1966) proposed the method of optical pumping, which was experimentally confirmed by Brossel, Kastler and Winter two years later.[6]
[edit] The maser
In 1953, Charles H. Townes and graduate students James P. Gordon and Herbert J. Zeiger produced the first maser, a device operating on similar principles to the laser, but producing microwave rather than optical radiation. Townes's maser was incapable of continuous output. Nikolay Basov and Aleksandr Prokhorov of the Soviet Union worked independently on the quantum oscillator and solved the problem of continuous output systems by using more than two energy levels. These systems could release stimulated emission without falling to the ground state, thus maintaining a population inversion.
Townes, Basov, and Prokhorov shared the Nobel Prize in Physics in 1964 "For fundamental work in the field of quantum electronics, which has led to the construction of oscillators and amplifiers based on the maser-laser principle".
[edit] The laser
In 1957 Charles Hard Townes and Arthur Leonard Schawlow, then at Bell Labs, began a serious study of the infrared maser. As ideas were developed, infrared frequencies were abandoned with focus on visible light instead. The concept was originally known as an "optical maser". Bell Labs filed a patent application for their proposed optical maser a year later. Schawlow and Townes sent a manuscript of their theoretical calculations to Physical Review, which published their paper that year (Volume 112, Issue 6).
The first page of Gordon Gould's laser notebook, in which he coined the acronym LASER and described the essential elements for constructing one.At the same time Gordon Gould, a graduate student at Columbia University, was working on a doctoral thesis on the energy levels of excited thallium. Gould and Townes met and had conversations on the general subject of radiation emission. Afterwards Gould made notes about his ideas for a "laser" in November 1957, including suggesting using an open resonator, which became an important ingredient of future lasers.
In 1958, Prokhorov independently proposed using an open resonator, the first published appearance of this idea. Schawlow and Townes also settled on an open resonator design, apparently unaware of both the published work of Prokhorov and the unpublished work of Gould.
The term "laser" was first introduced to the public in Gould's 1959 conference paper "The LASER, Light Amplification by Stimulated Emission of Radiation"[7]. Gould intended "-aser" to be a suffix, to be used with an appropriate prefix for the spectra of light emitted by the device (x-ray laser = xaser, ultraviolet laser = uvaser, etc.). None of the other terms became popular, although "raser" was used for a short time to describe radio-frequency emitting devices.
Gould's notes included possible applications for a laser, such as spectrometry, interferometry, radar, and nuclear fusion. He continued working on his idea and filed a patent application in April 1959. The U.S. Patent Office denied his application and awarded a patent to Bell Labs in 1960. This sparked a legal battle that ran 28 years, with scientific prestige and much money at stake. Gould won his first minor patent in 1977, but it was not until 1987 that he could claim his first significant patent victory when a federal judge ordered the government to issue patents to him for the optically pumped laser and the gas discharge laser.
The first working laser was made by Theodore H. Maiman in 1960[8] at Hughes Research Laboratories in Malibu, California, beating several research teams including those of Townes at Columbia University, Arthur L. Schawlow at Bell Labs,[9] and Gould at a company called TRG (Technical Research Group). Maiman used a solid-state flashlamp-pumped synthetic ruby crystal to produce red laser light at 694 nanometres wavelength. Maiman's laser, however, was only capable of pulsed operation due to its three energy level pumping scheme.
Later in 1960 the Iranian physicist Ali Javan, working with William Bennet and Donald Herriot, made the first gas laser using helium and neon. Javan later received the Albert Einstein Award in 1993.
The concept of the semiconductor laser diode was proposed by Basov and Javan. The first laser diode was demonstrated by Robert N. Hall in 1962. Hall's device was made of gallium arsenide and emitted at 850 nm in the near-infrared region of the spectrum. The first semiconductor laser with visible emission was demonstrated later the same year by Nick Holonyak, Jr. As with the first gas lasers, these early semiconductor lasers could be used only in pulsed operation, and indeed only when cooled to liquid nitrogen temperatures (77 K).
In 1970, Zhores Alferov in the Soviet Union and Izuo Hayashi and Morton Panish of Bell Telephone Laboratories independently developed laser diodes continuously operating at room temperature, using the heterojunction structure.
[edit] Recent innovations
This section is a stub. You can help by expanding it.
Graph showing the history of maximum laser pulse intensity throughout the past 40 years.Since the early period of laser history, laser research has produced a variety of improved and specialized laser types, optimized for different performance goals, including:-
new wavelength bands
maximum average output power
maximum peak output power
minimum output pulse duration
maximum power efficiency
maximum charging
maximum firing
and this research continues to this day.
Lasing without maintaining the medium excited into a population inversion, was discovered in 1992 in sodium gas and again in 1995 in rubidium gas by various international teams. This was accomplished by using an external maser to induce "optical transparency" in the medium by introducing and destructively interfering the ground electron transitions between two paths, so that the likelihood for the ground electrons to absorb any energy has been cancelled.
In 1985 at the University of Rochester's Laboratory for Laser Energetics a breakthrough in creating ultrashort-pulse, very high-intensity (terawatts) laser pulses became available using a technique called chirped pulse amplification, or CPA, discovered by Gérard Mourou. These high intensity pulses can produce filament propagation in the atmosphere.
[edit] Uses
Lasers range in size from microscopic diode lasers (top) with numerous applications, to football field sized neodymium glass lasers (bottom) used for inertial confinement fusion, nuclear weapons research and other high energy density physics experiments.Main article: Laser applications
When lasers were invented in 1960, they were called "a solution looking for a problem". Since then, they have become ubiquitous, finding utility in thousands of highly varied applications in every section of modern society, including consumer electronics, information technology, science, medicine, industry, law enforcement, entertainment, and the military.
The first application of lasers visible in the daily lives of the general population was the supermarket barcode scanner, introduced in 1974. The laserdisc player, introduced in 1978, was the first successful consumer product to include a laser, but the compact disc player was the first laser-equipped device to become truly common in consumers' homes, beginning in 1982, followed shortly by laser printers.
In 2004, excluding diode lasers, approximately 131,000 lasers were sold world-wide, with a value of US$2.19 billion.[10] In the same year, approximately 733 million diode lasers, valued at $3.20 billion, were sold.[11]
[edit] Example uses by typical output power
Different uses need lasers with different output powers. Many lasers are designed for a higher peak output with an extremely short pulse, and this requires different technology from a continuous wave (constant output) lasers, as are used in communication, or cutting. Output power is always less than the input power needed to generate the beam.
The peak power required for some uses:
5 mW - CD-ROM drive
5-10 mW - DVD player
100 mW - CD-R drive
250 mW - output power of Sony SLD253VL red laser diode, used in consumer 48-52 speed CD-R burner.[12]
1 W - green laser in current Holographic Versatile Disc prototype development.
100 to 3000 W (peak output 1.5 kW) - typical sealed CO2 lasers used in industrial Beam Laser Machines (laser cutting). These are usually compact, extremely reliable, inexpensive to run and can provide over 20,000 hours of cutting before requiring service.[13]
1 kW - Output power expected to be achieved by "a single 1 cm diode laser bar"[14]
700 terawatts (TW) - The National Ignition Facility is working on a system that, when complete, will contain a 192-beam, 1.8-megajoule laser system adjoining a 10-meter-diameter target chamber.[15] The system is expected to be completed in April of 2009.
1.25 petawatts (PW) - world's most powerful laser (claimed on 23 May 1996 by Lawrence Livermore Laboratory).
[edit] FASOR
A 50W FASOR used at the Starfire Optical RangeFor other meanings of "FASOR" or "fasor", see FASOR.
FASOR is Frequency Addition Source of Optical Radiation. The example in this image is used for laser guide star experiments. It is tuned to D2A hyperfine component of the sodium D line and used to excite sodium atoms in the mesospheric upper atmosphere. It consists of 1064nm and 1319nm Nd:YAG single frequency injection locked lasers that are both resonant in a cavity containing a Lithium Triborate (LBO) crystal which sums the frequencies yielding 589.158nm light. (1/1064 + 1/1319 = 1/588.93).
[edit] Laser safety
Main articles: Laser safety and Lasers and aviation safety
Even the first laser was recognized as being potentially dangerous. Theodore Maiman characterized the first laser as one "Gillette"; as it could burn through one Gillette razor blade. Today, it is accepted that even low-power lasers with only a few milliwatts of output power can be hazardous to human eyesight.
At wavelengths which the cornea and the lens can focus well, the coherence and low divergence of laser light means that it can be focused by the eye into an extremely small spot on the retina, resulting in localized burning and permanent damage in seconds or even less time. Lasers are classified into safety classes numbered I (inherently safe) to IV (even scattered light can cause eye and/or skin damage). Laser products available for consumers, such as CD players and laser pointers are usually in class I, II, or III. Certain infrared lasers with wavelengths beyond about 1.4 micrometres are often referred to as being "eye-safe". This is because the intrinsic molecular vibrations of water molecules very strongly absorb light in this part of the spectrum, and thus a laser beam at these wavelengths is attenuated so completely as it passes through the eye's cornea that no light remains to be focused by the lens onto the retina. The label "eye-safe" can be misleading, however, as it only applies to relatively low power continuous wave beams and any high power or q-switched laser at these long wavelengths will burn the cornea, causing severe eye damage.
This effect is used in a surgical treatment for detached retina: the resulting pinpoint damage heals with scarring, which "spot-welds" the retina onto its backing to try to stop more detachment.
[edit] Types and operating principles
For a more complete list of laser types see this list of laser types.
Spectral output of several types of lasers.
[edit] Gas lasers
Gas lasers using many gases have been built and used for many purposes. They are one of the oldest types of laser.
Spectrum of a helium neon laser showing the very high spectral purity intrinsic to nearly all lasers. Compare with the relatively broad spectral emittance of a light emitting diode.The helium-neon laser (HeNe) emits 543 nm and 633 nm and is very common in education because of its low cost.
Carbon dioxide lasers emit up to 100 kW at 9.6 µm and 10.6 µm, and are used in industry for cutting and welding.
Argon-ion lasers emit 458 nm, 488 nm or 514.5 nm.
Carbon monoxide lasers must be cooled but can produce up to 500 kW.
A nitrogen transverse electrical discharge in gas at atmospheric pressure (TEA) laser is an inexpensive gas laser producing UV Light at 337.1 nm.
Metal ion lasers are gas lasers that generate deep ultraviolet wavelengths. Helium-silver (HeAg) 224 nm and neon-copper (NeCu) 248 nm are two examples. These lasers have particularly narrow oscillation linewidths of less than 3 GHz (0.5 picometers),[16] making them candidates for use in fluorescence suppressed Raman spectroscopy.
[edit] Chemical lasers
Chemical lasers are powered by a chemical reaction, and can achieve high powers in continuous operation. For example, in the Hydrogen fluoride laser (2700-2900 nm) and the Deuterium fluoride laser (3800 nm) the reaction is the combination of hydrogen or deuterium gas with combustion products of ethylene in nitrogen trifluoride.
[edit] Excimer lasers
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)).
[edit] Solid-state lasers
Solid state laser materials are commonly made by doping a crystalline solid host with ions that provide the required energy states. For example, the first working laser was a ruby laser, made from ruby, or (chromium-doped sapphire).
Another common type is made from Neodymium-doped yttrium aluminium garnet (YAG), known as Nd:YAG. Nd:YAG lasers can produce high powers in the infrared spectrum at 1064 nm. They are used for cutting, welding and marking of metals and other materials, and also in spectroscopy and for pumping dye lasers. Nd:YAG lasers are also commonly frequency doubled to produce 532 nm when a visible (green) coherent source is required.
Ytterbium, holmium, thulium, and erbium are other common dopants in solid state lasers. Ytterbium is used in crystals such as Yb:YAG, Yb:KGW, Yb:KYW, Yb:SYS, Yb:BOYS, Yb:CaF2, typically operating around 1020-1050 nm. They are potentially very efficient and high powered due to a small quantum defect. Extremely high powers in ultrashort pulses can be achieved with Yb:YAG. Holmium-doped YAG crystals emit at 2097 nm and form an efficient laser operating at infrared wavelengths strongly absorbed by water-bearing tissues. The Ho-YAG is usually operated in a pulsed mode, and passed through optical fiber surgical devices to resurface joints, remove rot from teeth, vaporize cancers, and pulverize kidney and gall stones.
Titanium-doped sapphire (Ti:sapphire) produces a highly tunable infrared laser, commonly used for spectroscopy as well as the most common ultrashort pulse laser.
Thermal limitations in solid-state lasers arise from unconverted pump power that manifests itself as heat and phonon energy. This heat, when coupled with a high thermo-optic coefficient (dn/dT) can give rise to thermal lensing as well as reduced quantum efficiency. These types of issues can be overcome by another novel diode-pumped solid state laser, the diode-pumped thin disk laser. The thermal limitations in this laser type are mitigated by utilizing a laser medium geometry in which the thickness is much smaller than the diameter of the pump beam. This allows for a more even thermal gradient in the material. Thin disk lasers have been shown to produce up to kiloWatt levels of power.[17]
[edit] Popular misconceptions
The representation of lasers in popular culture, especially in science fiction and action movies, is often misleading. Contrary to their portrayal in many science fiction movies, a laser beam would not be visible (at least to the naked eye) in the near vacuum of space as there would be insufficient matter to scatter off.
In air, however, moderate intensity (tens of mW/cm²) laser beams of shorter green and blue wavelengths and high intensity beams of longer orange and red wavelengths can be visible due to Rayleigh scattering. With even higher intensity pulsed beams, the air can be heated to the point where it becomes a plasma, which is also visible. This causes rapid heating and explosive expansion of the surrounding air, which makes a popping noise analogous to the thunder which accompanies lightning. This phenomenon can cause retro-reflection of the laser beam back into the laser source, possibly damaging its optics. When this phenomenon occurs in certain scientific experiments it is referred to as a "plasma mirror" or "plasma shutter".
Some action movies depict security systems using lasers of visible light (and their foiling by the hero, typically using mirrors); the hero may see the path of the beam by sprinkling some dust in the air. It is far easier and cheaper to build infrared laser diodes rather than visible light laser diodes, and such systems almost never use visible light lasers. Additionally, putting enough dust in the air to make the beam visible is likely to be enough to "break" the beam and trigger the alarm (as proven on an episode of Mythbusters on the Discovery Channel).
Science fiction films special effects often depict laser beams propagating at only a few metres per second—slowly enough to see their progress, in a manner reminiscent of conventional tracer ammunition—whereas in reality a laser beam travels at the speed of light and would seem to appear instantly to the naked eye from start to end.
Several of these misconceptions can be found in the James Bond film Goldfinger, the first film to feature a laser. In one of the most famous scenes in the Bond films, Bond, played by Sean Connery, faces a laser beam approaching his groin while melting the solid gold table to which he is strapped. The director Guy Hamilton found that a real laser beam would not show up on camera so it was added as an optical effect. In fact the table was precut up the center, and then painted over. Then as the laser appears to cut right through it, it is melted away by the man underneath with an oxyacetylene torch, while a real laser would have produced a fairly heat-free and silent cut. [18]
In addition to movies and popular culture, laser misconceptions are present in some popular science publications or simple introductory explanations.[citation needed] For example, laser light is not perfectly parallel as is sometimes claimed; all laser beams spread out to some degree as they propagate due to diffraction. In addition, no laser is perfectly monochromatic (i.e. coherent); most operate at several closely spaced frequencies (colors) and even those that nominally operate a single frequency still exhibit some variation in frequency. Furthermore, mode locked lasers are designed to operate with thousands or millions of frequencies locked together to form a short pulse.
[edit] Fictional predictions
Before stimulated emission was discovered, novelists used to describe machines that we can identify as "lasers".
The first fictional device similar to a military CO2 laser (see Heat-Ray) appears in the sci-fi novel The War of the Worlds by H. G. Wells in 1898.
A laser-like device was described in Alexey Tolstoy's sci-fi novel The Hyperboloid of Engineer Garin in 1927: see Raygun#In specific scenarios (scroll down to alphabetical order 'H' in the left column).
Mikhail Bulgakov exaggerated the biological effect (laser biostimulation) of intensive red light in his sci-fi novel Fatal Eggs (1925), without any reasonable description of the source of this red light. (In that novel, the red light first appears occasionally from the illuminating system of an advanced microscope; then the protagonist Prof. Persikov arranges the special set-up for generation of the red light.)
The descriptions of laser-like devices in the early sci-fi novels are realistic (scientific) in comparison with "lasers" as they appear in later confusing Hollywood movies typical for years 1950 to 2000. See raygun.
[edit] Hobby uses
In recent years, some hobbyists have taken interests in lasers. Lasers used by hobbyists are generally of class IIIa or IIIb, although some have made their own class IV types.[19] However, compared to other hobbyists, laser hobbyists are far less common, due to the cost and potential dangers involved.
Popping balloons, lighting matches, and doing laser shows appear to be popular among laser enthusiasts. Laser show enthusiasts often use Electronica music, such as techno and trance.
Due to the cost of lasers, some hobbyists use cheaper means to obtain lasers, such as extracting diodes from DVD burners. [1]
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For other uses, see Laser (disambiguation).
A laser (acronym for Light Amplification by Stimulated Emission of Radiation) is an optical source that emits light in a coherent beam. The back-formed verb lase means "to produce laser light" or "to apply laser light to".[1]
In analogy with optical lasers, a device which produces any particles or electromagnetic radiation in a coherent state is also called a "laser", usually with indication of type of particle as prefix (for example, atom laser.) In most cases, "laser" refers to a source of coherent photons, i.e. light or other electromagnetic radiation.
Laser light is typically near-monochromatic, i.e., consisting of a very small band of wavelengths or a very specific color, and emitted in a narrow beam. This contrasts with common light sources, such as the incandescent light bulb, which emit incoherent photons in almost all directions, usually over a wide spectrum of wavelengths.
Laser action is explained by the theories of quantum mechanics and thermodynamics. Many materials have been found to have the required characteristics to form the laser gain medium needed to power a laser, and these have led to the invention of many types of lasers with different characteristics suitable for different applications.
The laser was proposed as a variation of the maser principle in the late 1950s, and the first laser was demonstrated in July 1960 by Theodore Maiman at Hughes Research Laboratories. Since that time, laser manufacture has become a multi-billion dollar industry, and the laser has found applications in fields including science, the defense industry, medicine, and consumer electronics.
Experiment using a (likely argon) laser. (US military)For lasers in fiction, see also raygun.
Contents [hide]
1 Physics
2 History
2.1 Foundations: Einstein
2.2 The maser
2.3 The laser
2.4 Recent innovations
3 Uses
3.1 Example uses by typical output power
4 FASOR
5 Laser safety
6 Types and operating principles
6.1 Gas lasers
6.2 Chemical lasers
6.2.1 Excimer lasers
6.3 Solid-state lasers
7 Popular misconceptions
8 Fictional predictions
9 Hobby uses
[edit] Physics
Principal components:
1. Active laser medium
2. Laser pumping energy
3. High reflector
4. Output coupler
5. Laser beam
A helium-neon laser demonstration at the Kastler-Brossel Laboratory at Univ. Paris 6. The glowing ray in the middle is an electric discharge producing light in much the same way as a neon light. It is the gain medium through which the laser passes, not the laser beam itself, which is visible there. The laser beam crosses the air and marks a red point on the screen to the right.See also: Laser science and Laser construction
A laser is composed of an active laser medium, or gain medium, and a resonant optical cavity. The gain medium transfers external energy into the laser beam. It is a material of controlled purity, size, concentration, and shape, which amplifies the beam by the quantum mechanical process of stimulated emission, discovered by Albert Einstein while researching the photoelectric effect. The gain medium is energized, or pumped, by an external energy source. Examples of pump sources include electricity and light, for example from a flash lamp or from another laser. The pump energy is absorbed by the laser medium, placing some of its particles into high-energy ("excited") quantum states. When the number of particles in one excited state exceeds the number of particles in some lower-energy state, population inversion is achieved. In this condition, an optical beam passing through the medium produces more stimulated emission than the stimulated absorption, so the beam is amplified. An excited laser medium can also function as an optical amplifier.
The light generated by stimulated emission is very similar to the input signal in terms of wavelength, phase, and polarization. This gives laser light its characteristic coherence, and allows it to maintain the uniform polarization and monochromaticity established by the optical cavity design.
The optical cavity, an example of a type of cavity resonator, contains a coherent beam of light between reflective surfaces so that each photon passes through the gain medium more than once before it is emitted from the output aperture or lost to diffraction or absorption. As light circulates through the cavity, passing through the gain medium, if the gain (amplification) in the medium is stronger than the resonator losses, the power of the circulating light can rise exponentially. But each stimulated emission event returns a particle from its excited state to the ground state, reducing the capacity of the gain medium for further amplification. When this effect becomes strong, the gain is said to be saturated. The balance of pump power against gain saturation and cavity losses produces an equilibrium value of the intracavity laser power; this equilibrium determines the operating point of the laser. If the chosen pump power is too small, the gain is not sufficient to overcome the resonator losses, and the laser will emit only very small light powers. The minimum pump power needed to begin laser action is called the lasing threshold. The gain medium will amplify any photons passing through it, regardless of direction; but only the photons aligned with the cavity manage to pass more than once through the medium and so have significant amplification.
The beam in the cavity and the output beam of the laser, if they occur in free space rather than waveguides (as in an optical fiber laser), are often Gaussian beams. If the beam is not a pure Gaussian shape, the transverse modes of the beam may be analyzed as a superposition of Hermite-Gaussian or Laguerre-Gaussian beams. The beam may be highly collimated, that is, having a very small beam divergence, but a perfectly collimated beam cannot be created, due to diffraction. But a laser beam will spread much less than a beam of incoherent light. The beam remains collimated over a distance which varies with the square of the beam diameter, and eventually diverges at an angle which varies inversely with the beam diameter. Thus, a beam generated by a small laboratory laser such as a helium-neon laser spreads to about 1.6 kilometres (1 mile) diameter if shone from the Earth to the Moon. By comparison, the output of a typical semiconductor laser, due to its small diameter, diverges almost as soon as it leaves the aperture, at an angle of anything up to 50°. However, such a divergent beam can be transformed into a collimated beam by means of a lens. In contrast, the light from non-laser light sources cannot be collimated by optics as well or much.
The output of a laser may be a continuous constant-amplitude output (known as CW or continuous wave); or pulsed, by using the techniques of Q-switching, modelocking, or gain-switching. In pulsed operation, much higher peak powers can be achieved.
Some types of lasers, such as dye lasers and vibronic solid-state lasers can produce light over a broad range of wavelengths; this property makes them suitable for generating extremely short pulses of light, on the order of a femtosecond (10-15 s).
Though the laser phenomenon was discovered with the help of quantum physics, it is not essentially more quantum mechanical than are other sources of light. The operation of a free electron laser can be explained without reference to quantum mechanics.
It is understood that the word light in the acronym Light Amplification by Stimulated Emission of Radiation is typically used in the expansive sense, as photons of any energy; it is not limited to photons in the visible spectrum. Hence there are infrared lasers, ultraviolet lasers, X-ray lasers, etc. For example, a source of atoms in a coherent state can be called an atom laser.
Because the microwave equivalent of the laser, the maser, was developed first, devices that emit microwave and radio frequencies are usually called masers. In early literature, particularly from researchers at Bell Telephone Laboratories, the laser was often called the optical maser. This usage has since become uncommon, and as of 1998 even Bell Labs uses the term laser. [2]
[edit] History
[edit] Foundations: Einstein
In 1916, Albert Einstein laid the foundation for the invention of the laser and its predecessor, the maser, in a ground-breaking rederivation of Max Planck's law of radiation based on the concepts of spontaneous and induced emission.
In 1928, Rudolph W. Landenburg confirmed the existence of stimulated emission and negative absorption. [3]
In 1939, Valentin A. Fabrikant (USSR) predicted the use of stimulated emission to amplify "short" waves.[4]
In 1947, Willis E. Lamb and R. C. Retherford found apparent stimulated emission in hydrogen spectra and made the first demonstration of stimulated emission. [5]
In 1950, Alfred Kastler (Nobel Prize for Physics 1966) proposed the method of optical pumping, which was experimentally confirmed by Brossel, Kastler and Winter two years later.[6]
[edit] The maser
In 1953, Charles H. Townes and graduate students James P. Gordon and Herbert J. Zeiger produced the first maser, a device operating on similar principles to the laser, but producing microwave rather than optical radiation. Townes's maser was incapable of continuous output. Nikolay Basov and Aleksandr Prokhorov of the Soviet Union worked independently on the quantum oscillator and solved the problem of continuous output systems by using more than two energy levels. These systems could release stimulated emission without falling to the ground state, thus maintaining a population inversion.
Townes, Basov, and Prokhorov shared the Nobel Prize in Physics in 1964 "For fundamental work in the field of quantum electronics, which has led to the construction of oscillators and amplifiers based on the maser-laser principle".
[edit] The laser
In 1957 Charles Hard Townes and Arthur Leonard Schawlow, then at Bell Labs, began a serious study of the infrared maser. As ideas were developed, infrared frequencies were abandoned with focus on visible light instead. The concept was originally known as an "optical maser". Bell Labs filed a patent application for their proposed optical maser a year later. Schawlow and Townes sent a manuscript of their theoretical calculations to Physical Review, which published their paper that year (Volume 112, Issue 6).
The first page of Gordon Gould's laser notebook, in which he coined the acronym LASER and described the essential elements for constructing one.At the same time Gordon Gould, a graduate student at Columbia University, was working on a doctoral thesis on the energy levels of excited thallium. Gould and Townes met and had conversations on the general subject of radiation emission. Afterwards Gould made notes about his ideas for a "laser" in November 1957, including suggesting using an open resonator, which became an important ingredient of future lasers.
In 1958, Prokhorov independently proposed using an open resonator, the first published appearance of this idea. Schawlow and Townes also settled on an open resonator design, apparently unaware of both the published work of Prokhorov and the unpublished work of Gould.
The term "laser" was first introduced to the public in Gould's 1959 conference paper "The LASER, Light Amplification by Stimulated Emission of Radiation"[7]. Gould intended "-aser" to be a suffix, to be used with an appropriate prefix for the spectra of light emitted by the device (x-ray laser = xaser, ultraviolet laser = uvaser, etc.). None of the other terms became popular, although "raser" was used for a short time to describe radio-frequency emitting devices.
Gould's notes included possible applications for a laser, such as spectrometry, interferometry, radar, and nuclear fusion. He continued working on his idea and filed a patent application in April 1959. The U.S. Patent Office denied his application and awarded a patent to Bell Labs in 1960. This sparked a legal battle that ran 28 years, with scientific prestige and much money at stake. Gould won his first minor patent in 1977, but it was not until 1987 that he could claim his first significant patent victory when a federal judge ordered the government to issue patents to him for the optically pumped laser and the gas discharge laser.
The first working laser was made by Theodore H. Maiman in 1960[8] at Hughes Research Laboratories in Malibu, California, beating several research teams including those of Townes at Columbia University, Arthur L. Schawlow at Bell Labs,[9] and Gould at a company called TRG (Technical Research Group). Maiman used a solid-state flashlamp-pumped synthetic ruby crystal to produce red laser light at 694 nanometres wavelength. Maiman's laser, however, was only capable of pulsed operation due to its three energy level pumping scheme.
Later in 1960 the Iranian physicist Ali Javan, working with William Bennet and Donald Herriot, made the first gas laser using helium and neon. Javan later received the Albert Einstein Award in 1993.
The concept of the semiconductor laser diode was proposed by Basov and Javan. The first laser diode was demonstrated by Robert N. Hall in 1962. Hall's device was made of gallium arsenide and emitted at 850 nm in the near-infrared region of the spectrum. The first semiconductor laser with visible emission was demonstrated later the same year by Nick Holonyak, Jr. As with the first gas lasers, these early semiconductor lasers could be used only in pulsed operation, and indeed only when cooled to liquid nitrogen temperatures (77 K).
In 1970, Zhores Alferov in the Soviet Union and Izuo Hayashi and Morton Panish of Bell Telephone Laboratories independently developed laser diodes continuously operating at room temperature, using the heterojunction structure.
[edit] Recent innovations
This section is a stub. You can help by expanding it.
Graph showing the history of maximum laser pulse intensity throughout the past 40 years.Since the early period of laser history, laser research has produced a variety of improved and specialized laser types, optimized for different performance goals, including:-
new wavelength bands
maximum average output power
maximum peak output power
minimum output pulse duration
maximum power efficiency
maximum charging
maximum firing
and this research continues to this day.
Lasing without maintaining the medium excited into a population inversion, was discovered in 1992 in sodium gas and again in 1995 in rubidium gas by various international teams. This was accomplished by using an external maser to induce "optical transparency" in the medium by introducing and destructively interfering the ground electron transitions between two paths, so that the likelihood for the ground electrons to absorb any energy has been cancelled.
In 1985 at the University of Rochester's Laboratory for Laser Energetics a breakthrough in creating ultrashort-pulse, very high-intensity (terawatts) laser pulses became available using a technique called chirped pulse amplification, or CPA, discovered by Gérard Mourou. These high intensity pulses can produce filament propagation in the atmosphere.
[edit] Uses
Lasers range in size from microscopic diode lasers (top) with numerous applications, to football field sized neodymium glass lasers (bottom) used for inertial confinement fusion, nuclear weapons research and other high energy density physics experiments.Main article: Laser applications
When lasers were invented in 1960, they were called "a solution looking for a problem". Since then, they have become ubiquitous, finding utility in thousands of highly varied applications in every section of modern society, including consumer electronics, information technology, science, medicine, industry, law enforcement, entertainment, and the military.
The first application of lasers visible in the daily lives of the general population was the supermarket barcode scanner, introduced in 1974. The laserdisc player, introduced in 1978, was the first successful consumer product to include a laser, but the compact disc player was the first laser-equipped device to become truly common in consumers' homes, beginning in 1982, followed shortly by laser printers.
In 2004, excluding diode lasers, approximately 131,000 lasers were sold world-wide, with a value of US$2.19 billion.[10] In the same year, approximately 733 million diode lasers, valued at $3.20 billion, were sold.[11]
[edit] Example uses by typical output power
Different uses need lasers with different output powers. Many lasers are designed for a higher peak output with an extremely short pulse, and this requires different technology from a continuous wave (constant output) lasers, as are used in communication, or cutting. Output power is always less than the input power needed to generate the beam.
The peak power required for some uses:
5 mW - CD-ROM drive
5-10 mW - DVD player
100 mW - CD-R drive
250 mW - output power of Sony SLD253VL red laser diode, used in consumer 48-52 speed CD-R burner.[12]
1 W - green laser in current Holographic Versatile Disc prototype development.
100 to 3000 W (peak output 1.5 kW) - typical sealed CO2 lasers used in industrial Beam Laser Machines (laser cutting). These are usually compact, extremely reliable, inexpensive to run and can provide over 20,000 hours of cutting before requiring service.[13]
1 kW - Output power expected to be achieved by "a single 1 cm diode laser bar"[14]
700 terawatts (TW) - The National Ignition Facility is working on a system that, when complete, will contain a 192-beam, 1.8-megajoule laser system adjoining a 10-meter-diameter target chamber.[15] The system is expected to be completed in April of 2009.
1.25 petawatts (PW) - world's most powerful laser (claimed on 23 May 1996 by Lawrence Livermore Laboratory).
[edit] FASOR
A 50W FASOR used at the Starfire Optical RangeFor other meanings of "FASOR" or "fasor", see FASOR.
FASOR is Frequency Addition Source of Optical Radiation. The example in this image is used for laser guide star experiments. It is tuned to D2A hyperfine component of the sodium D line and used to excite sodium atoms in the mesospheric upper atmosphere. It consists of 1064nm and 1319nm Nd:YAG single frequency injection locked lasers that are both resonant in a cavity containing a Lithium Triborate (LBO) crystal which sums the frequencies yielding 589.158nm light. (1/1064 + 1/1319 = 1/588.93).
[edit] Laser safety
Main articles: Laser safety and Lasers and aviation safety
Even the first laser was recognized as being potentially dangerous. Theodore Maiman characterized the first laser as one "Gillette"; as it could burn through one Gillette razor blade. Today, it is accepted that even low-power lasers with only a few milliwatts of output power can be hazardous to human eyesight.
At wavelengths which the cornea and the lens can focus well, the coherence and low divergence of laser light means that it can be focused by the eye into an extremely small spot on the retina, resulting in localized burning and permanent damage in seconds or even less time. Lasers are classified into safety classes numbered I (inherently safe) to IV (even scattered light can cause eye and/or skin damage). Laser products available for consumers, such as CD players and laser pointers are usually in class I, II, or III. Certain infrared lasers with wavelengths beyond about 1.4 micrometres are often referred to as being "eye-safe". This is because the intrinsic molecular vibrations of water molecules very strongly absorb light in this part of the spectrum, and thus a laser beam at these wavelengths is attenuated so completely as it passes through the eye's cornea that no light remains to be focused by the lens onto the retina. The label "eye-safe" can be misleading, however, as it only applies to relatively low power continuous wave beams and any high power or q-switched laser at these long wavelengths will burn the cornea, causing severe eye damage.
This effect is used in a surgical treatment for detached retina: the resulting pinpoint damage heals with scarring, which "spot-welds" the retina onto its backing to try to stop more detachment.
[edit] Types and operating principles
For a more complete list of laser types see this list of laser types.
Spectral output of several types of lasers.
[edit] Gas lasers
Gas lasers using many gases have been built and used for many purposes. They are one of the oldest types of laser.
Spectrum of a helium neon laser showing the very high spectral purity intrinsic to nearly all lasers. Compare with the relatively broad spectral emittance of a light emitting diode.The helium-neon laser (HeNe) emits 543 nm and 633 nm and is very common in education because of its low cost.
Carbon dioxide lasers emit up to 100 kW at 9.6 µm and 10.6 µm, and are used in industry for cutting and welding.
Argon-ion lasers emit 458 nm, 488 nm or 514.5 nm.
Carbon monoxide lasers must be cooled but can produce up to 500 kW.
A nitrogen transverse electrical discharge in gas at atmospheric pressure (TEA) laser is an inexpensive gas laser producing UV Light at 337.1 nm.
Metal ion lasers are gas lasers that generate deep ultraviolet wavelengths. Helium-silver (HeAg) 224 nm and neon-copper (NeCu) 248 nm are two examples. These lasers have particularly narrow oscillation linewidths of less than 3 GHz (0.5 picometers),[16] making them candidates for use in fluorescence suppressed Raman spectroscopy.
[edit] Chemical lasers
Chemical lasers are powered by a chemical reaction, and can achieve high powers in continuous operation. For example, in the Hydrogen fluoride laser (2700-2900 nm) and the Deuterium fluoride laser (3800 nm) the reaction is the combination of hydrogen or deuterium gas with combustion products of ethylene in nitrogen trifluoride.
[edit] Excimer lasers
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)).
[edit] Solid-state lasers
Solid state laser materials are commonly made by doping a crystalline solid host with ions that provide the required energy states. For example, the first working laser was a ruby laser, made from ruby, or (chromium-doped sapphire).
Another common type is made from Neodymium-doped yttrium aluminium garnet (YAG), known as Nd:YAG. Nd:YAG lasers can produce high powers in the infrared spectrum at 1064 nm. They are used for cutting, welding and marking of metals and other materials, and also in spectroscopy and for pumping dye lasers. Nd:YAG lasers are also commonly frequency doubled to produce 532 nm when a visible (green) coherent source is required.
Ytterbium, holmium, thulium, and erbium are other common dopants in solid state lasers. Ytterbium is used in crystals such as Yb:YAG, Yb:KGW, Yb:KYW, Yb:SYS, Yb:BOYS, Yb:CaF2, typically operating around 1020-1050 nm. They are potentially very efficient and high powered due to a small quantum defect. Extremely high powers in ultrashort pulses can be achieved with Yb:YAG. Holmium-doped YAG crystals emit at 2097 nm and form an efficient laser operating at infrared wavelengths strongly absorbed by water-bearing tissues. The Ho-YAG is usually operated in a pulsed mode, and passed through optical fiber surgical devices to resurface joints, remove rot from teeth, vaporize cancers, and pulverize kidney and gall stones.
Titanium-doped sapphire (Ti:sapphire) produces a highly tunable infrared laser, commonly used for spectroscopy as well as the most common ultrashort pulse laser.
Thermal limitations in solid-state lasers arise from unconverted pump power that manifests itself as heat and phonon energy. This heat, when coupled with a high thermo-optic coefficient (dn/dT) can give rise to thermal lensing as well as reduced quantum efficiency. These types of issues can be overcome by another novel diode-pumped solid state laser, the diode-pumped thin disk laser. The thermal limitations in this laser type are mitigated by utilizing a laser medium geometry in which the thickness is much smaller than the diameter of the pump beam. This allows for a more even thermal gradient in the material. Thin disk lasers have been shown to produce up to kiloWatt levels of power.[17]
[edit] Popular misconceptions
The representation of lasers in popular culture, especially in science fiction and action movies, is often misleading. Contrary to their portrayal in many science fiction movies, a laser beam would not be visible (at least to the naked eye) in the near vacuum of space as there would be insufficient matter to scatter off.
In air, however, moderate intensity (tens of mW/cm²) laser beams of shorter green and blue wavelengths and high intensity beams of longer orange and red wavelengths can be visible due to Rayleigh scattering. With even higher intensity pulsed beams, the air can be heated to the point where it becomes a plasma, which is also visible. This causes rapid heating and explosive expansion of the surrounding air, which makes a popping noise analogous to the thunder which accompanies lightning. This phenomenon can cause retro-reflection of the laser beam back into the laser source, possibly damaging its optics. When this phenomenon occurs in certain scientific experiments it is referred to as a "plasma mirror" or "plasma shutter".
Some action movies depict security systems using lasers of visible light (and their foiling by the hero, typically using mirrors); the hero may see the path of the beam by sprinkling some dust in the air. It is far easier and cheaper to build infrared laser diodes rather than visible light laser diodes, and such systems almost never use visible light lasers. Additionally, putting enough dust in the air to make the beam visible is likely to be enough to "break" the beam and trigger the alarm (as proven on an episode of Mythbusters on the Discovery Channel).
Science fiction films special effects often depict laser beams propagating at only a few metres per second—slowly enough to see their progress, in a manner reminiscent of conventional tracer ammunition—whereas in reality a laser beam travels at the speed of light and would seem to appear instantly to the naked eye from start to end.
Several of these misconceptions can be found in the James Bond film Goldfinger, the first film to feature a laser. In one of the most famous scenes in the Bond films, Bond, played by Sean Connery, faces a laser beam approaching his groin while melting the solid gold table to which he is strapped. The director Guy Hamilton found that a real laser beam would not show up on camera so it was added as an optical effect. In fact the table was precut up the center, and then painted over. Then as the laser appears to cut right through it, it is melted away by the man underneath with an oxyacetylene torch, while a real laser would have produced a fairly heat-free and silent cut. [18]
In addition to movies and popular culture, laser misconceptions are present in some popular science publications or simple introductory explanations.[citation needed] For example, laser light is not perfectly parallel as is sometimes claimed; all laser beams spread out to some degree as they propagate due to diffraction. In addition, no laser is perfectly monochromatic (i.e. coherent); most operate at several closely spaced frequencies (colors) and even those that nominally operate a single frequency still exhibit some variation in frequency. Furthermore, mode locked lasers are designed to operate with thousands or millions of frequencies locked together to form a short pulse.
[edit] Fictional predictions
Before stimulated emission was discovered, novelists used to describe machines that we can identify as "lasers".
The first fictional device similar to a military CO2 laser (see Heat-Ray) appears in the sci-fi novel The War of the Worlds by H. G. Wells in 1898.
A laser-like device was described in Alexey Tolstoy's sci-fi novel The Hyperboloid of Engineer Garin in 1927: see Raygun#In specific scenarios (scroll down to alphabetical order 'H' in the left column).
Mikhail Bulgakov exaggerated the biological effect (laser biostimulation) of intensive red light in his sci-fi novel Fatal Eggs (1925), without any reasonable description of the source of this red light. (In that novel, the red light first appears occasionally from the illuminating system of an advanced microscope; then the protagonist Prof. Persikov arranges the special set-up for generation of the red light.)
The descriptions of laser-like devices in the early sci-fi novels are realistic (scientific) in comparison with "lasers" as they appear in later confusing Hollywood movies typical for years 1950 to 2000. See raygun.
[edit] Hobby uses
In recent years, some hobbyists have taken interests in lasers. Lasers used by hobbyists are generally of class IIIa or IIIb, although some have made their own class IV types.[19] However, compared to other hobbyists, laser hobbyists are far less common, due to the cost and potential dangers involved.
Popping balloons, lighting matches, and doing laser shows appear to be popular among laser enthusiasts. Laser show enthusiasts often use Electronica music, such as techno and trance.
Due to the cost of lasers, some hobbyists use cheaper means to obtain lasers, such as extracting diodes from DVD burners. [1]
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