In 1917 Albert Einstein, in his paper Zur Quantentheorie der Strahlung On the Quantum Theory of Radiation, laid the foundation for the invention of the laser and its predecessor, the maser, in a ground-breaking rederivation of Max Planck's law of radiation based on the concepts of probability coefficients (later to be termed 'Einstein coefficients') for the absorption, spontaneous emission, and stimulated emission of electromagnetic radiation.
In 1928, Rudolph W. Landenburg confirmed the existence of stimulated emission and negative absorption. In 1939, Valentine A. Fabrikant predicted the use of stimulated emission to amplify "short" waves.
In 1947, Willis E. Lamb and R. C. Rutherford found apparent stimulated emission in hydrogen spectra and made the first demonstration of stimulated emission.
In 1950, Alfred Kastler (Nobel Prize for Physics 1966) proposed the method of optical pumping, which was experimentally confirmed by Brossel, Kastler .
In 1957, Charles Hard Townes and Arthur Leonard Schawlow, then at Bell Labs, began a serious study of the infrared laser. As ideas were developed, infrared frequencies were abandoned with focus on visible light instead. The concept was originally known as an "optical maser". Bell Labs filed a patent application for their proposed optical maser a year later. Schawlow and Townes sent a manuscript of their theoretical calculations to Physical Review, which published their paper that year.
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The first page of Gordon Gould's laser notebook in which he coined the acronym LASER and described the essential elements for constructing one.
At the same time Gordon Gould, a graduate student at Columbia University, was working on a doctoral thesis on the energy levels of excited thallium. Gould and Townes met and had conversations on the general subject of radiation emission. Afterwards Gould made notes about his ideas for a "laser" in November 1957, including suggesting using an open resonator, which became an important ingredient of future lasers.
In 1958, Prokhorov independently proposed using an open resonator, the first published appearance of this idea. Schawlow and Townes also settled on an open resonator design, apparently unaware of both the published work of Prokhorov and the unpublished work of Gould.
The term "laser" was first introduced to the public in Gould's 1959 conference paper "The LASER, Light Amplification by Stimulated Emission of Radiation". Gould intended "-aser" to be a suffix, to be used with an appropriate prefix for the spectrum of light emitted by the device (x-rays: xaser, ultraviolet: uvaser, etc.). None of the other terms became popular, although "raser" was used for a short time to describe radio-frequency emitting devices.
First working laser was made by Theodore H. Maiman in 1960 at Hughes Research Laboratories in Malibu, California, beating several research teams including those of Townes at Columbia University, Arthur Schawlow at Bell Labs, and Gould at a company called TRG (Technical Research Group). Maiman used a solid-state flashlamp-pumped synthetic ruby crystal to produce red laser light at 694 nanometres wavelength. Maiman's laser, however, was only capable of pulsed operation due to its three-level pumping scheme.
The concept of the semiconductor laser diode was proposed by Basov and Javan. The first laser diode was demonstrated by Robert N. Hall in 1962. Hall's device was made of gallium arsenide and emitted at 850 nm in the near-infrared region of the spectrum. The first semiconductor laser with visible emission was demonstrated later the same year by Nick Holonyak, Jr. As with the first gas lasers, these early semiconductor lasers could be used only in pulsed operation, and indeed only when cooled to liquid nitrogen temperatures (77 K).
In 1970, Zhores Alferov in the Soviet Union and Izuo Hayashi and Morton Panish of Bell Telephone Laboratories independently developed laser diodes continuously operating at room temperature, using the heterojunction structure.
Theory:-The principle of a laser is based on two separate features:
a) a light emitting/amplifying medium b) an optical resonator (usually defined by two parallel mirrors).
The physical process responsible for the light amplification is supposed to be the stimulated (induced) emission process which is assumed to occur in case of a population inversion between two atomic states in a radiation field of the corresponding frequency. This would lead to the atomic emission amplifying the incident radiation field exactly in phase (coherently) which would vastly increase the amplitude of the resultant wavetrain compared to an incoherent (random) superposition (because for N emitters it would be proportional to N rather than âË†Å¡N ).
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Now there is in my opinion a fundamental conceptual problem with the assumption of the existence of a stimulated (induced) emission: atomic physics distinguishes two different mechanisms for radiative transitions between two levels i,k of an atom:
a) Spontaneous emission that occurs with a probability given by the decay constant Ai,k, b) induced emission or absorption due to an external radiation field. Resonant scattering is for instance usually considered as absorption of a photon which lifts an electron to a higher energy level followed by the re- emission of a photon when the electron falls spontaneously back again. However, both a theoretical consideration and experimental evidence shows that this picture of a two-step process is not correct and that resonant scattering has to be described as a coherent process (i.e. a forced oscillator with damping constant Ai,k). Unlike photo ionization or excitation by electron/ion impact, scattering involves therefore no atomic energy changes as no work as being done.
With regard to the optical resonator (e.g. mirrors), classical laser theory assumes now that it (apart from focusing the light) serves as a means to enable all atoms in the light emitting medium to radiate in phase, namely if its length is a multiple of half the wavelength. In this case of optical resonance, a standing wave will be set up for a purely sinusoidal signal, and it is assumed that this circumstance enables the proposed process of stimulated emission to amplify all emissions in phase. It is usually argued here that the intrinsically spontaneous and random emission develops into a wave with a uniquely defined phase because one of the initial emissions 'overwhelms' all the others and hence defines the phase of the radiation field. However, this is only a hand waving argument which in fact defies common sense (as it would violate the superposition principle for instance). Even if a stimulated emission process exists, it is only possible that the initial 'photons' present before the situation of a population inversion are being amplified in phase separately when encountering an atom in an excited state. Although a standing wave will be set up for each of these coherently amplified 'photons' separately in the optical resonator, this has actually no effect on getting further emissions in phase as this would happen anyway (laser or maser effects do apparently also occur in natural media without any optical resonator as long as there is a significant population inversion). The only effect the optical resonator has here is to amplify each of the wavetrains by folding it back into itself if it is long enough. This, not surprisingly, is easily the case for all laser transitions as these arise typically from (quasi-) metastable states with very long life times. For a wave train with length L the resultant amplitude would therefore be L/l if l is the distance between the mirrors (provided the mirror reflectivity allows that many reflections; otherwise the amplitude would be determined .The resultant wave train is now accordingly shorter (in effect) but this will in many cases not be so important for the subsequent interaction with matter as the increased wave amplitude. Nevertheless, as all N standing waves are randomly out of phase, the amplification is obviously much less than for a coherent superposition of all emitters (L/l is usually much smaller than N).
Two coherent laser beams derived from a single source intersect at a fixed angle. The intersection volume of the two beams is positioned on the sample surface. The wavefronts of the two beams interfere in the intersection volume and form an interference pattern. The fringes of the pattern have a known distance depending on the wavelength of the laser and the angle between the two beams.
For simplicity, assume that an element with a velocity component perpendicular to the center axis is moving through the intersection volume. The element will scatter light with its amplitude modulated by the local intensity contrast. The frequency of the modulation is proportional to the velocity of the element.
Thus when recording the intensity signal the motion of elements within the observation area can be calculated. A typical sample with its natural roughness contains scattering elements everywhere on its surface which allows the measurement of the amount of material moving through the observation area.
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By simultaneous measurement at two points of the sample surface the relative
motion between these two points can be determined. This allows the use of the measuring arrangement as an extensometer.
TYPES OF LASER
[I] Based on its pumping scheme a laser can be classified as
[a] Optically pumped laser
[b]Electrically pumped laser
[II]On the basis of the operation mode, laser fall into classes of
[a]Continuous Wave Lasers
[III]According to the materials used to produce laser light, lasers can be divided into three categories :
[b]Solid State Lasers
[d]Other Laser Devices
Brief details of type
(i) Gas Laser:
Gas lasers generally have a wide variety of characteristics. For example , some gas lasers emit feeble power below 1mW, but other commercial gas lasers emit power of the order of kilowatts. Some lasers can emit continuous beam for years, others emit pulses lasting a few nanoseconds. Their outputs range from deep in the ultraviolet through the visible and infrared to millimetre waves.
(a). Helium- neon (HeNe)
The laser medium is a mixture of helium and neon gases. An electrical discharge, in the form of direct current or radio frequency current, is used to excite the medium to a higher energy level. The pumping action takes place in a complex and indirect manner. First the helium atoms are excited by the discharge to two of the excited energy levels These two levels happen to be very close to the 3s and 2s levels of the neon atoms. When the excited helium atoms collide with the neon atoms, energy is exchanged, pumping the neon atoms to the respective levels. The atoms at the neon 3s level eventually drops down to the 2p level, as a result of stimulated emission, and light of wavelength 632.8 nm is emitted. The atoms at the 2s level, on the other hand, drops to the 2p level by emitting light at 1.15 nm. However , the atoms at the 3s level may instead drop down to the 3p level, by emitting light at 3.39 mm. 632.8nm is in the visible range.
(b). Gallium Arsenate (GaAs) Laser
(ii). Solid state laser:
A solid state laser is one in which the atoms that emit light are fixed within a crystal or a glassy material. The first laser invented by Maiman in 1960 , the ruby laser, was a solid state laser. The atoms that emit light in solid state lasers are dispersed in a crystal or a piece of glass that contains many other elements. The crystal is shaped into a rod, with reflecting mirrors placed at each end. Light from an external source (such as a pulsed flash lamp, a bright continuous arc lamp, or another laser) enters the laser rod and excites the light-emitting atoms. The two mirrors form a resonant cavity and the inverted population in the laser rod, provided the feedback needed to generate a laser beam that emerges through the output mirror.
As the photons traverse the crystal, they stimulate the emission of additional photons until enough energy is available for a pulse of photons to break through the thinly mirrored end on the right of the laser crystal.
The Nd:YAG laser is a good example of the most commonly used solid state lasers. The laser medium is made up of yttrium- aluminium-garnet, with trivalent neodymium ions present as impurities. The laser transition involved corresponds to a wavelength of 1.06 mm, in the near infrared region
(iii). Semiconductor Laser:
A unique ,and perhaps the most important, type of laser in terms of opto- electronics applications is the semiconductor laser. It is unique because of its small dimensions (mm x mm x mm) , and its natural integration capabilities with micro electronic circuitry, Furthermore, the light amplification by the process of stimulated emission is not exactly in the form that we have described before.
A semiconductor laser uses special properties of the transition region at the junction of a p-type semiconductor in contact with an n-type semiconductor. In semiconductor materials, because of the extensive interaction of energy between atoms, the energy levels form bands. Energy band diagrams for an n-type and a p-type semiconductor are depicted in The energy gap between the valence band and the conduction band is designated by Eg and is measured in electronvolts,e.g., the Fermi level Ef is the level that divides the occupied from the unoccupied levels.
In a p-n junction, as shown in the energy levels readjust in accordance with thermodynamics so that the Ef band is the same through the junction. The valence band Evv and the conduction band Ec of the p-type semiconductor are higher than the corresponding bands of the n-type semiconductor. If a positive voltage is applied on the p side (the so-called positive bias), the electrons on the n side will be attracted by the applied voltage and will cross into the junction region. There they recombine with the holes that have been pushed into the junction region by the positive bias. This process will continue as long as the external circuit is on, because the electrons and holes that have recombined are continuously replenished.
When the electrons and holes recombine, they emit energy in the form of photons. The junction transition region in which this takes place is therefore the source of radiation, and may be viewed as equivalent to the E2 and E1 transition levels which we discussed earlier.
To obtain stimulated emission and amplification from this region, the equivalent of the population inversion needs to be created, for which a high density of electrons and a high density of holes must exist simultaneously in the junction region. To achieve this, heavily doped p-n junctions are used in semiconductor lasers.
shows the resultant energy levels. When a positive bias is applied on the p-side, there is a transition region with a high concentration of electrons and holes, as shown in This region serves as a population inverted medium, which amplifies the radiation emitted within it through electron - hole recombination. A p-n junction semiconductor laser is illustrated schematically in The shaded area is the transition region where the laser action takes place. This region is about 1-2 mm thick, and tens of micrometers long. As a result, the emission is squeezed into a thin plane, leading to an elliptical cross-section of the beam.
(iv) Other Laser devices:
(a). Ion and Metal Vapour Laser:-Operated at high temperatures to keep the metals (e.g., copper, gold etc.) vaporized and produced laser light in the infrared, visible or ultraviolet regions. They are excellent sources of short, high-intensity laser pulses at very high pulse-repetition rates. The mixture of metal vapour and noble gases are excited by electrical discharges. Example is copper vapour lasers which produces green and yellow light from a mixture of copper vapour with helium and neon.
(b). Carbon Dioxide Laser.
(c). The excimer Laser.
(d). The liquid (dye) Laser : Dye lasers use liquid organic dyes.
Dye lasers can produce a broad and almost continuous range of colors, mainly in the visible part of the spectrum. With proper optical system any colour can be selected or tuning from one colour to another can be done. That is why dye lasers are particularly suited for applications in which a precise colour is required. Usually another laser source e.g., copper vapour lasers are used to excite the dye.
(e). The Free Electron laser.
Applications of laser:-
There are a large no of applications of laser in our daily life
1:-Spectroscopy:- Most types of laser are an inherently pure source
of light; they emit nearmonochromatic light with a very well defined range of wavelengths. By careful, 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).
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.
(a).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).
LASIK (laser vision correction)
LASEK (laser-assisted sub-epithelial keratectomy)
PRK (photorefractive keratectomy)
(c).Soft tissue surgery: CO2, Er:YAG laser
(d).Laser scalpel (General surgery, gynecological, urology, laparoscopic)
(f)Photobiomodulation (i.e. laser therapy)
(g)"No-Touch" removal of tumors, especially of the brain and spinal cord.
(h)In dentistry for caries removal, endodontic/periodontic proc es, tooth whiteningedur, and oral surgery.
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 aiming of the laser and reducing its efficiency.
In the April 2008 edition of Popular Science, there is an article showcasing a new combat laser, the Boeing Advanced Tactical Laser Beam, which will be carried in a large aircraft (it is shown carried in a C-130) and fired at large targets (vehicles or buildings.) It is currently being tested at Kirtland Air Force Base in New Mexico. The laser itself is a chemical laser, and weighs 40,000 pounds. The range is reported to be 5 miles, and it can rapidly strike targets (it uses rapid-fire rather than a continuous beam to minimize the risk of friendly fire.) However, the prototype cost $200 million, making it doubtful that this will be put to widespread use. Barring the cost, it is expected to be in battle within five years. The recently introduced FIRESTRIKE laser system is small enough (400lbs) to fit into light vehicles.