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.
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