LASER OF APPLICATIONS
LASER OF APPLICATIONS
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INTRODUCTION :
The word laser is an acronym derived from “light amplification by stimulated emission of radiation. These are any of a class of devices that produces an intense beam of light of a very pure single colour. This light beam may be intense enough to vaporize the hardest and most heat-resistant materials.
A laser is a device that emits light (electromagnetic radiation) through a process called stimulated emission. The term laser is an acronym for light amplification by stimulated emission of radiation. Laser light is usually spatially coherent, which means that the light either is emitted in a narrow, low-divergence beam, or can be converted into one with the help of optical components such as lenses. Typically, lasers are thought of as emitting light with a narrow wavelength spectrum (“monochromatic” light). This is not true of all lasers, however: some emit light with a broad spectrum, while others emit light at multiple distinct wavelengths simultaneously. The coherence of typical laser emission is distinctive. Most other light sources emit incoherent light, which has a phase that varies randomly with time and position.
The first laser was born on January 28, 1958, by Schawlow and Townes who presented their work in “Infrared and optical masers.” Masers are the predecessors of the lasers which are also produced by stimulated emissions and are of longer wavelengths than light. When atoms-icons or molecule absorb energy electrons in the atom get excited i.e. they shift to higher level. But this excited state of the electron is unstable and to become stable, it emits extra energy in the form of light and falls to its ground state. This emitted light is called situated emission. If a collection of excited atoms is prepared. Then the incident light will stimulate more light than emission and there is a net amplification of the incident light beam known as laser. Any laser device consists of three main components. The active medium, the pumping source and the optical resonator. Radiation that is directed straight along the axis bounces back and forth between mirrors many times to build up a strong oscillation. This resultant wave is a plane wave. A plane wave is nothing but coherent wave, highly monochromatic produced due to the resonance pro`cess which makes all et wavelengths produced a single phase output, which is the laser radiation.
Atoms and molecules exist at low and high energy levels. Those at low levels can be excited to higher levels, usually by heat, and after reaching the higher levels they give off light when they return to a lower level. In ordinary light sources the many excited atoms or molecules emit light independently and in many different colours (wavelengths). If, however, during the brief instant that an atom is excited, light of a certain wavelength impinges on it, the atom can be stimulated to emit radiation that is in phase (in step) with the wave that stimulated it. The new emission thus augments or amplifies the passing wave; if the phenomenon can be multiplied sufficiently, the resulting beam, made up of wholly coherent light (i.e., light of a single frequency or colour in which all the components are in step with each other), will be tremendously powerful.
Albert Einstein recognized the existence of stimulated emission in 1917, but not until the 1950s were ways found to use it in devices. The American physicists Charles H. Townes and A.L. Schawlow showed that it was possible to construct such a device using optical light. Two Soviet physicists proposed related ideas independently. The first laser, constructed in 1960 by Theodore H. Maiman of the United States, used a rod of ruby. Since then many types of lasers have been built.
World’s Strongest Laser Unveiled at California Lab
The world’s most powerful laser, created to help keep tabs on the nation’s nuclear weapons stockpile while also studying the heavens, has been unveiled.
The super laser, known officially as the National Ignition Facility, was unveiled Friday (29th May) at the Lawrence Livermore National Laboratory about 50 miles east of San Francisco. The NIF, which is the size of a football field, consists of 192 separate laser beams, each traveling 1,000 feet in one-thousandth of a second to converge simultaneously on a target the size of a pencil eraser. : The laser will be used in astrophysics, allowing scientists to mimic conditions inside planets and new solar systems, something the lab’s officials said would allow for conducting experiments that could never be undertaken on Earth before.
TYPES OF LASER
Of the several different types of lasers produced by different means and used for different purposes, the following are most important.
Optically Pumped Solid-State Lasers
One way to achieve the excitation of atoms to the higher energy level for laser action to take place is by illuminating the laser material with light of a frequency higher than that which the laser is to emit. This process is called optical pumping; the light pump must be of high intensity, as the process is usually rather inefficient.
An optically pumped solid-state laser consists of a rod of the material chosen, with its ends polished flat and parallel and coated with mirrors to reflect the laser light. The sides are left clear to admit the light from the pumping bmp, which may be a pulsed gas discharge, flashing on and off like a photographer’s electronic flash bulb. It may be wound around the laser rod, positioned alongside, or focused on it by a. The first operating laser employed a rod of pink ruby, an artificial crystal of sapphire (aluminum oxide). Many other rare-earth elements have since been employed, the most widely used being neodymium. Power outputs in the form of brilliant flashes of light of thousands of watts can be obtained.
Liquid Lasers
Solid-state lasers have the disadvantage of occasional breakdown and damage at higher power levels because of the intense heat generated within the material and by the pumping lamp. The liquid laser is not susceptible to such damage; the crystalline or glassy rod is replaced by a transparent cell containing a suitable liquid, such as a i solution of neodymium oxide or chloride in selenium oxychloride. Such cells can be made as large as desired to increase power output. Only a small number of inorganic liquids, however, will function as lasers.
Dye Lasers
Certain organic dyes are capable of fluorescing-i.e., re-radiating light of a different colour. Though the excited state if their atoms lasts only a small fraction of a second and the light emitted is not concentrated in a narrow band, many such dyes have been made to exhibit laser action, with the advantage that they can be tuned to a wide range)f frequencies.
Dyes such as rhodamine 6G, which emits orange-yellow light, can be made to laser (provide laser action) by excitation by another laser. Rhodamine 6G was the first dye for which continuous, rather than pulsed, operation was achieved, making possible the production of a continuous beam of tunable laser light. Another dye, , methylumbelliferone, with the addition of hydrochloric acid, can be made to laser at wavelengths varying across the light spectrum from ultraviolet to yellow, producing laser light of almost any desired frequency within this range.
Gas-Discharge Lasers
Atoms in a gas discharge can be excited to radiate and produce light, as in a neon sign. Occasionally, a particular ‘ energy level will cause an exceptionally high number of atoms to accumulate within it. If mirrors are positioned at ‘ the ends of the discharge tube, laser action results. Though the conditions are unusual and occur for only a few of ‘ the many wavelengths at which the discharge emits, most gases can be made to exhibit laser action at some wavelength under certain discharge conditions. Gas-discharge lasers commonly use a helium-neon mixture, though those designed to produce laser action at infrared wavelengths employ such gases as carbon monoxide and hydrogen cyanide.
Gas Dynamic Lasers
If a hot gas is allowed to cool rapidly, the number of molecules in a low-energy state may decrease more rapidly and fall below the number in a higher energy state, thus permitting laser action. This condition can be achieved by expanding burning carbon monoxide mixed with nitrogen through jet nozzles. High power outputs of more than 30,000 watts can be obtained.
Chemical Lasers
Certain chemical reactions produce enough high-energy atoms to permit laser action to take place. Laser action can occur in carbon dioxide, for example, if it is present when the elements hydrogen and fluorine are reacting to produce hydrogen fluoride. Large amounts of energy can be released when only moderate amounts of the appropriate materials react.
Semiconductor Lasers
A semiconductor laser consists of a flat junction of two pieces of semiconductor material, each of which has been treated with a different type of impurity. Aluminum gallium arsenide and gallium arsenide typically are used in lasers of this type, though pairs of other so-called III-V compound semiconductors may be employed. When a large electrical current is passed through such a device, laser light emerges from the junction region. Power output is limited, but the low cost, small size, and comparatively high efficiency make these devices suitable for use as light sources in optical fibre communications systems (see below) and in compact digital audio disc players.
Free-Electron Lasers
Lasers of this type are more efficient than any other variety in producing beams of very high power radiation. Furthermore, these devices are tunable, so that they can be made to operate at microwave to ultraviolet wavelengths. (Theoretically they have the potential of generating laser radiation of X-ray wavelength, though present technology is still incapable of such short wavelengths.) In a free-electron laser, free electrons (i.e., those not bound to nuclei) from a particle accelerator or some other source are passed through an undulator (commonly called a “wiggler”), a device consisting of a linear array of electromagnets. An alternating magnetic field in the undulator bends the electrons into a spiral path around the lines of force, whereby they are accelerated to velocities approaching the speed of light and emit energy in the form of synchrotron radiation (q.v.). The intensity and wavelength of this radiation can be adjusted by modifying certain parameters of the magnetic field. Because of this ability to produce laser light tunable over a broad range of wavelengths and high efficiency, researchers believe that the free-electron laser, with further development, will prove especially suitable in such applications as isotope separation, semiconductor research, and ballistic missile defence (namely, as a laser beam weapon).
LASER APPLICATIONS
The light produced by lasers is in general far more monochromatic, directional, powerful, and coherent than that from any other light sources. Nevertheless, the individual kinds of lasers differ greatly in these properties as well as in wavelength, size, and efficiency. There is no single laser suitable for all purposes, but some of the combinations of properties can do things that were difficult or impossible before lasers were developed.
A continuous visible beam from a laser using a gas, such as the helium-neon combination, provides a nearly ideal straight line for all kinds of alignment applications. The beam from such a laser typically diverges by less than one part in a thousand, approaching the theoretical limit. The beam’s divergence can be reduced by passing it backward through a telescope, although fluctuations in the atmosphere then limit the sharpness of a beam over a long path. Lasers have come to be widely used for alignment in large construction-e.g., to guide machines for drilling tunnels and for laying pipelines.
A pulsed laser can be used in light radar, sometimes called LIDAR, and the narrowness of its beam permits sharp definition of targets. As with radar, the distance to an object is measured by the time taken for the light to reach and return from it, since the speed of light is known. LIDAR echoes have been returned from the Moon, facilitated by a multiprism reflector that was placed there by the first astronauts to land there. Distances can be measured from an observatory on Earth to the lunar mirror with an accuracy of several centimetres. Simultaneous measurements of the mirror’s distance and direction from two observatories on different parts of the Earth could give an accurate value for the distance between the two observatories. A series of such measurements can tell the rate at which continents are drifting relative to each other.
A vertically directed laser radar in an airplane can serve as a fast, high-resolution device for mapping fine details, such as the contours of steps in a stadium or the shape of the roof of a house. With pulsed laser radar, returns can be obtained from dust particles and even from air molecules at higher altitudes. Thus air densities can be measured and air currents can sometimes be traced.
The high coherence of a laser’s output is very helpful in measurement and other applications involving interference of light beams. If a light beam is divided into two parts that travel different paths, when the beams come together again they may be either in step so that they reinforce each other or out of step so that they cancel one another. Thus the brightness of the recombined wave changes from light to dark, producing interference fringes, when the difference in path lengths is changed by one-half of a wavelength. Such devices are called laser interferometers. Very small displacements can be detected, and larger distances can be measured with precision. With lasers, these measurements can be carried out over extremely long distances. Laser interferometers are used to monitor small displacements in the Earth’s crust across geological faults. In manufacturing, such devices are employed to gauge fine wires, to monitor the products of automated machine tools, and to test optical compo-nents.
Lasers can be so monochromatic that a small shift in the light frequency can be detected. Light reflected from an object that is moving toward the laser is raised in frequency by an amount depending on the velocity of the object (Doppler effect). For a receding object, the frequency is lowered. In either case, if some of the original and tie shifted light are recombined at a photodetector, a signal at the difference frequency (the difference in fre-quency between the original and the shifted light) is observed, and even small velocities can be measured.
The brightness and coherence of laser light make it especially suitable for visual effects and photography that simulate third dimensional depth-e.g., holography (q.v.).
The light from many lasers is relatively powerful and can be focused by a conventional lens system to a small spot of great intensity. Thus even a moderately small pulsed laser can vaporize a small amount of any substance and drill narrow holes in the hardest materials. Ruby lasers, for example, are used to drill holes in diamonds and in sapphires for watch bearings. For biological research, a finely focused laser can vaporize parts of a single cell, thus permitting microsurgery of chromosomes.
Strong heating can be produced by a laser at a place where no mechanical contact is possible. Thus one of the earliest applications of lasers was for surgery on the retina of the eye.
Lasers are also used for small-scale cutting and welding. They can trim resistors to exact values by removing, material and can alter connections within integrated arrays of microcircuit elements. The high brightness, pure colour, and directionality of laser light make it ideally suited for experiments on light scattering. Even a small amount of light that is scattered with a change of wavelength or direction can be readily identified. In particular, a type of scattering known as the Raman Effect produces characteristic wavelength shifts which molecular species can be identified. With laser sources and sensitive spectrography, small samples of transparent liquids, gases, or solids can be analyzed. It is even possible to measure contaminants in the atmosphere at a considerable distance by the Raman scattering of light from a laser beam.
Lasers also are used in a major type of computer printer. Laser printers employ a laser beam and a system of ‘ optical devices to etch images on a photoconductor drum. The images are carried from the drum to paper by leans of electrostatic photocopying.
Medical Uses of Lasers
The highly collimated beam of a laser can be further focused to a microscopic dot of extremely high energy density. This makes it useful as a cutting and cauterizing instrument. Lasers are used for photocoagulation of the retina to halt retinal hemorrhaging and for the tacking of retinal tears. Higher power lasers are used after cataract surgery the supportive membrane surrounding the implanted lens becomes milky. Photodisruption of the membrane often can cause it to draw back like a shade, almost instantly restoring vision. A focused laser can act as an extremely sharp scalpel for delicate surgery, cauterizing as it cuts. (“Cauterizing” refers to long-standing medical radices of using a hot instrument or a high frequency electrical probe to singe the tissue around an incision, eating off tiny blood vessels to stop bleeding.) The cauterizing action is particularly important for surgical procedures in blood-rich tissue such as the liver.
Lasers have been used to make incisions half a micron wide, compared to about 80 microns for the diameter of a human hair.
” Cosmetic surgery (removing tattoos, scars, stretch marks, sunspots, wrinkles, birthmarks, and hairs) Laser types used in dermatology include ruby, alexandrite, pulsed diode array), Nd:YAG ,Ho:YAGa and Er:YAG.
- Eye surgery and refractive surgery
- Soft tissue surgery: C02, Er:YAG laser
- Laser scalpel (General surgery, gynecological, urology, laparoscopic)
- Dental procedures
- Photobiomodulation (i.e. laser therapy)
- “No-Touch” removal of tumors, especially of the brain and spinal cord.
- In dentistry for caries removal, endodontic/periodontic procedures, tooth whitening, and oral surgery.
Material Processing
The highly collimated beam of a laser can be further focused to a microscopic dot of extremely high energy density for welding and cutting.
The automobile industry makes extensive use of carbon dioxide lasers with powers up to several kilowatts for computer controlled welding on auto assembly lines.
Garmire points out an interesting application of C02 lasers to the welding of stainless steel handles on copper cooking pots. A nearly impossible task for conventional welding because of the great difference in thermal conductivities between stainless steel and copper, it is done so quickly by the laser that the thermal conductivities are irrelevant.
It is also used for processes like Laser brazing, laser bending, laser engraving or marking, laser cleaning etc.
Lasers in the Garment Industry
Laser cutters are credited with keeping the garment industry competitive in the market. Computer-controlled laser garment cutters can be programmed to cut garments – and that might involve just a few seconds. The programmed cutter can cut dozens to hundreds of thicknesses of cloth, and can cut out every piece of the garment in a single run.
The usefulness of the laser for such cutting operations comes from the fact that the beam is highly collimated and can be further focused to a microscopic dot of extremely high energy density for cutting.
Lasers in Communication
Fiber optic cables are a major mode of communication partly because multiple signals can be sent with high quality and low loss by light propagating along the fibers. The light signals can be modulated with the information to be sent by either light emitting diodes or lasers. The lasers have significant advantages because they are more nearly monochromatic and this allows the pulse shape to be maintained better over long distances. If a better pulse shape can be maintained, then the communication can be sent at higher rates without overlap of the pulses.
In consumer electronics, telecommunications, and data communications, lasers are used as the transmitters in optical communications over optical fibre and free space.
To store and retrieve data in optical discs
Laser lighting displays (pictured) accompany many music concerts.
Heat Treatment
Heat treatments for hardening or annealing have been long practiced in metallurgy. But lasers offer some new possibilities for selective heat treatments of metal parts. For example, lasers can provide localized heat treatments such as the hardening of the surfaces of automobile camshafts. These shafts are manufactured to high precision, and if the entire camshaft is heat treated, some warping will inevitably occur. But the working surfaces of the cams can be heated quickly with a carbon dioxide laser and hardened without appreciably affecting the remainder of the shaft, preserving the precision of manufacture.
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 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. 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 spec-troscopy (LIBS).
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.
Laser Cooling
A technique that has had recent success is laser cooling. This involves atom trapping, a method where a number of 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. As this process is continued, they all are slowed and have the same energy level, forming an unusual arrangement of matter known as a Bose-Einstein condensate.
Nuclear Fusion
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.
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. Laser capture microdissection use lasers to procure specific cell populations from a tissue section under microscopic visualization.
Additional laser microscopy techniques include harmonic microscopy, four-wave mixing microscopy and interferometric microscopy.
Military
Military uses of lasers include applications such as target designation and ranging, defensive countermeasures, communications and directed energy weapons. Directed energy weapons such as Boeing’s Airborne Laser which can be mounted on a 747 jet is able to burn the skin off enemy missiles.
Defensive Countermeasures
Defensive countermeasure applications can range from compact, low power infrared countermeasures to high-power, airborne laser systems. IR countermeasure systems use lasers to confuse the seeker heads on heat- seeking anti-aircraft missiles. High power boost-phase intercept laser systems use a complex system of lasers to, find, track and destroy intercontinental ballistic missiles. In this type of system a chemical laser, one in which the, laser operation is powered by an energetic chemical reaction, is used as the main weapon beam (see Airborne Laser). 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.
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). The practical problems of using and aiming these systems were 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 timing 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 SDI program was canceled.
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