INTRODUCTION
The name LASER is an acronym for Light Amplification by the Stimulated Emission of Radiation.
Light is really an electromagnetic wave. Each wave has brightness and color, and vibrates at a certain angle, so-called polarization. This is also true for laser light but it is more parallel than any other light source. Every part of the beam has (almost) the exact same direction and the beam will therefore diverge very little. With a good laser an object at a distance of 1 km (0.6 mile) can be illuminated with a dot about 60 mm (2.3 inches) in radius.
As it is so parallel it can also be focused to very small diameters where the concentration of light energy becomes so great that you can cut, drill or turn with the beam. It also makes it possible to illuminate and examine very tiny details. It is this property that is used in surgical appliances and in CD players.
It can also be made very monochromic, so that just one light wavelength is present. This is not the case with ordinary light sources. White light contains all the colors in the spectrum, but even a colored light, such as a red LED (light emitting diode) contains a continuous interval of red wavelengths. On the other hand, laser emissions are not usually very strong when it comes to energy content. A very powerful laser of the kind that is used in a laser show does not give off more light than an ordinary streetlight; the difference is in how parallel it is.

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Before the Laser there was the Maser
In 1954, Charles Townes and Arthur Schawlow invented the maser (microwave amplification by stimulated emission of radiation), using ammonia gas and microwave radiation – the maser was invented before the (optical) laser. The technology is very close but does not use a visible light. The maser was used to amplify radio signals and as an ultrasensitive detector for space research.
Many different materials can be used as lasers. Some, like the ruby laser, emit short pulses of laser light. Others, like helium-neon gas lasers or liquid dye lasers emit a continuous beam of light.
Laser action
Lasers are possible because of the way light interacts with electrons. Electrons exist at specific energy levels or states characteristic of that particular atom or molecule. The energy levels can be imagined as rings or orbits around a nucleus. Electrons in outer rings are at higher energy levels than those in inner rings. Electrons can be bumped up to higher energy levels by the injection of energy-for example, by a flash of light. When an electron drops from an outer to an inner level, “excess” energy is given off as light. The wavelength or color of the emitted light is precisely related to the amount of energy released. Depending on the particular lasing material being used, specific wavelengths of light are absorbed (to energize or excite the electrons) and specific wavelengths are emitted (when the electrons fall back to their initial level).
In a cylinder a fully reflecting mirror is placed on one end and a partially reflecting mirror on the other. A high-intensity lamp is spiraled around the ruby cylinder to provide a flash of white light that triggers the laser action. The green and blue wavelengths in the flash excite electrons in the atoms to a higher energy level. Upon returning to their normal state, the electrons emit their characteristic ruby-red light. The mirrors reflect some of this light back and forth inside the ruby crystal, stimulating other excited chromium atoms to produce more red light, until the light pulse builds up to high power and drains the energy stored in the crystal. High-voltage electricity causes the quartz flash tube to emit an intense burst of light, exciting some of the atoms in the ruby crystal to higher energy levels. At a specific energy level, some atoms emit particles of light called photons. At first the photons are emitted in all directions. Photons from one atom stimulate emission of photons from other atoms and the light intensity is rapidly amplified. Mirrors at each end reflect the photons back and forth, continuing this process of stimulated emission and amplification. The photons leave through the partially silvered mirror at one end. This is laser light.
EINSTEIN’S THEORY OF LASER
Although Einstein did not invent the laser his work laid the foundation. It was Einstein who pointed out that stimulated emission of radiation could occur along with spontaneous emission & absorption. He used his photon mathematics to examine the case of a large collection of atoms full of excess energy and ready to emit a photon at some random time in a random direction. If a stray photon passes by, then the atoms are stimulated by its presence to emit their photons early. More remarkably, the emitted photons go in the same direction and have exactly the same frequency as the original photon ! Later, as the small crowd of identical photons moves through the rest of the atoms, more and more photons will leave their atoms early to join in the subatomic parade.
All it took to invent the laser was for someone to find the right kind of atoms and to add reflecting mirrors to help the stimulated emission along .The acronym LASER means Light Amplification by (using Einstein’s ideas about) Stimulated Emission of Radiation.
Stimulated Emission
A third type of photon-related electron transitions in an atom is stimulated emission. Suppose an electron is in a higher energy level and a photon comes along with an energy equal to the difference between the electron’s energy and a lower energy.Then the photon will stimulate the electron to fall into the lower energy state, thereby emitting a photon.
The emitted photon will have the same energy as the original photon, and viewed as waves we will then have two waves emerging from the atom in phase with the same frequency. Such waves will constructively interfere, leading to a more intense wave.
This is the principle behind the laser, which stands for Light Amplification by Stimulated Emission of Radiation. In a laser atoms are kept in an excited state by “pumping” the laser, and some photons are inserted. This causes some atoms to undergo stimulated emission, and the resulting photons cause other atoms to undergo stimulated emission, leading to a chain reaction. The resultant light is very intense and coherent (composed of one frequency), and can be easily focused.
TYPES OF LASER
ON THE BASIS OF ENERGY LEVEL

Two level: In this photon from mata stable state jumps to second level on excitation
Three level: In this photon from mata stable state jumps to third level on excitation
Four level: In this photon from mata stable state jumps to fourth level on excitation

ON THE BASIS OF MATERIAL USED
Gas lasers
Helium Neon Laser – It is used in Interferometer, holography, spectroscopy, barcode scanning, alignment, optical demonstrations.
Argon Laser – It is used in Retinal phototherapy (for diabetes), lithography, confocal microscopy,spectroscopy pumping other lasers.
Krypton Laser – This laser is used in Scientific research, mixed with argon to create “white-light” lasers, light shows.
Xenon ion Laser – used in scientific research. In this type of laser many lines throughout visible spectrum extend into the UV and IR.
Nitrogen Laser – It is used in Pumping of dye lasers, measuring air pollution, scientific research. Nitrogen lasers can operate superradiantly (without a resonator cavity). Amateur laser construction.
Carbon Dioxide laser – It is used in Material processing (cutting, welding, etc.), surgery. It is produced by Transverse (high power) or longitudinal (low power) electrical discharge.
Excimer Laser – This type of laser is produced by excimer recombination via electrical discharge.It is used in Ultraviolet lithography for semiconductor manufacturing, laser surgery, LASIK
Chemical lasers
These type of lasers are used as directed-energy weapons like:
Hydrogen Fluoride laser: It is formed by Chemical reaction in a burning jet of ethylene and nitrogen trifluoride (NF3) and is used in research for laser weaponry by the U.S. DOD, operated in continuous wave mode, can have power in the megawatt range.
Deuterium fluoride laser: It is used in MIRACL, Pulsed Energy Projectile & Tactical High Energy Laser
COIL (Chemical oxygen-iodine laser): It is produced by Chemical reaction in a jet of singlet delta oxygen and iodine and is used in Laser weaponry, scientific and materials research, laser used in the U.S. military’s Airborne laser, operated in continuous wave mode, can have power in the megawatt range.
DYE LASER:
These type of lasers are used in Research, spectroscopy, birthmark removal, isotope separation. The tuning range of the laser depends on which dye is used.
Metal-vapor lasers
Helium-cadmium (HeCd) metal-vapor laser: They are used in Printing and typesetting applications, fluorescence excitation examination (ie. in U.S. paper currency printing), scientific research.
Helium-mercury (HeHg) metal-vapor laser: They are used in Rare, scientific research, amateur laser construction.
Helium-silver (HeAg) metal-vapor laser, Neon-copper (NeCu) metal-vapor laser:
They are used in scientific research.
Copper vapor laser: It is used in Dermatological uses, high speed photography, pump for dye lasers.
Gold vapor laser: This type of laser is used in Rare, dermatological and photodynamic therapy uses.
Solid-state laser
Ruby laser: Used in Holography, tattoo removal. The first type of visible light laser invented; May 1960.
Nd:YAG laser : used in Material processing, rangefinding, laser target designation, surgery, research, pumping other lasers (combined with frequency doubling to produce a green 532 nm beam). One of the most common high power lasers. Usually pulsed (down to fractions of a nanosecond)
Er:YAG laser: used in Periodontal scaling, Dentistry
Neodymium YLF (Nd:YLF) solid-state laser: Mostly used for pulsed pumping of certain types of pulsed Ti:sapphire lasers, combined with frequency doubling.
Neodymium doped Yttrium orthovanadate (Nd:YVO4) laser: Mostly used for continuous pumping of mode-locked Ti:sapphire or dye lasers, in combination with frequency doubling. Also used pulsed for marking and micromachining. A frequency doubled nd:YVO4 laser is also the normal way of making a green laser pointer.
Neodymium glass (Nd:Glass) laser: Used in extremely high power (terawatt scale), high energy (megajoules) multiple beam systems for inertial confinement fusion. Nd:Glass lasers are usually frequency tripled to the third harmonic at 351 nm in laser fusion devices.
Cerium doped lithium strontium(or calcium) aluminum fluoride (Ce:LiSAF, Ce:LiCAF): Used in Remote atmospheric sensing, LIDAR, optics research.
Erbium doped and erbium-ytterbium codoped glass lasers: These are made in rod, plate/chip, and optical fiber form. Erbium doped fibers are commonly used as optical amplifiers for telecommunications.
Semiconductor laser
Semiconductor laser diode has working wavelength between 0.4-20µm, depending on active region and are used in Telecommunications, holography, printing, weapons, machining, welding, pump. There are many different types of semiconductor lasers like:
AlGaAs : used in Optical discs, laser pointers, data communications. 780 nm Compact Disc player laser is the most common laser type in the world. Solid-state laser pumping, machining, medical.
InGaAsP : used in Telecommunications, solid-state laser pumping, machining, medical.
Quantum cascade laser : used in Research,Future applications may include collision-avoidance radar, industrial-process control and medical diagnostics such as breath analyzers.
Other types of lasers
Free electron laser : This type of laser is having a broad wavelength range (about 100 nm – several mm); one free electron laser may be tunable over a wavelength range and is basically used in atmospheric research, material science, medical applications.
Gas dynamic laser : used In Military applications; can operate in CW mode at several megawatts optical power and is produced by Spin state population inversion in carbon dioxide molecules caused by supersonic adiabatic expansion of mixture of nitrogen and carbon dioxide.
“Nickel-like” Samarium laser : It has pumping source Lasing in ultra-hot samarium plasma formed by double pulse terawatt scale irradiation fluences created by Rutherford Appleton Laboratory’s Nd:glass Vulcan laser and first demonstration of efficient “saturated” operation of a sub-10 nm X-ray laser, possible applications in high resolution microscopy and holography, operation is close to the “water window” at 2.2 to 4.4 nm where observation of DNA structure and the action of viruses and drugs on cells can be examined.
Raman laser, uses inelastic stimulated Raman scattering in a nonlinear media, mostly fiber, for amplification. It finds its applications in Complete 1-2 µm wavelength coverage; distributed optical signal amplification for telecommunications; optical solitons generation and amplification
APPLICATIONS
Industrial Applications of Laser
Today, laser can be found in a broad range of applications within industry, where it can be used for such things as pointing and measuring. In the manufacturing industry, laser is used to measure the ball cylindricity in bearings by observing the dispersion of a laser beam when reflected on the ball.Laser also works as a spirit level and can be used to indicate a flat surface by just sweeping the laser beam along the surface. This is, for instance, used when making walls at building sites. In the mining industry, laser is used to point out the drilling direction.
Laser technologies have also been used within environmental areas. One example is the ability to determine from a distance the environmental toxins in a column of smoke. Other examples are being able to predict and measure the existence of photochemical smog and ozone, both at ground level where it isn’t wanted and in the upper layers of the atmosphere where it is needed. Laser is also used to supervise wastewater purification.
Laser works as a light source in all fiber optics in use. It has greater bandwidth. It is insensitive to interference from external electrical and magnetic fields. Fiber optics is used increasingly often in data and telecommunications around the world.
Medicine
Laser is used in medicine to improve precision work like surgery. Brain surgery is an example of precision surgery that calls for the surgeon to reach the intended area precisely. To make sure of this, lasers are used both to measure and to point in the area in question. Birthmarks, warts and discoloring of the skin can easily be removed with an unfocused laser. The operations are quick and heal quickly and, best of all, they are less painful than ordinary surgery performed with a scalpel.
RECENT APPLICATIONS
DVD
A DVD player contains laser that is used not because it produces a parallel beam, but rather because the light emerges from a tiny point, which enables it to be focused on the different layers of the disc. The information, ones and zeros, is stored in several layers, and only one layer is to be read at a time. Every point on a particular layer is read during every revolution of the disc. In order to make room for a lot of information on every disc, the beam has to be focused on as small an area as possible. This cannot be done with any other light source than a laser.
Laser Pointers
Lasers pointers are made from inexpensive semiconductors laser as together with lens produce a parallel beam of light that can be used to make a bright spot to point with. Their range is very large. If one points at a surface 200 meters (220 yards) distant in the dark, a person standing close to the object being pointed at will have no trouble seeing the shining spot (of course, someone else has to hold the laser). On the other hand, the one holding the pointer will have difficulty seeing the spot. The eternal question of range has more to do with the light’s behavior on its way back to the sender than with the length of the beam.
Laser Sights
Laser sights for rifles and guns can be based on several different principles. Some send a laser beam parallel to the trajectory so that the point of impact becomes visible. This method exposes the marksman. Some project a red dot inside a telescopic sight (instead of cross hairs). In both cases, the dot can be produced with a ring around it.
Speed Measurement Using Laser
The method the police use to measure car speed is based on a laser signal that is sent towards the target. This beam bounces back and is mixed with light that has not hit the car. The result is an oscillation – the same as when you tune a guitar – with higher frequency (more treble) the faster the target moves. The speed has to be measured straight from the front or from the back. If it is measured at an angle, the speed is underrated. This means that you cannot get false values that are too high.
The measurement is dependent on the car having something that reflects well. The license plate is perfect, as are different types of reflecting objects. Fogged surfaces are okay, but reduce the maximum distance.
Laser Distance Meter
The primary use of laser distance meters today is surveyors and constructors, Least spectacular is the so-called parking assistance that helps the driver to estimate the distance to the car behind when parking. A more recent application measures the distance to the car in front of the driver when driving on highways or other roads. You simply lock in the distance to the car in front of you in order to maintain that distance. This makes driving more efficient and faster as long as it all works. This kind of laser is found in most robots with mechanical vision.
LASER DISCOVERIES
1971
Gabor (alone) was given the prize, having founded the basic ideas of the holographic method, which is a famous and spectacular application of laser technology. At first “just” a method of creating 3-D pictures, it has since become a useful tool for the observation of vibrating objects. Much of what we today know about how musical instruments produce their tones is due to the use of holograms.
In addition to holograms that can be bought and hung on a wall, simpler holograms can be found on many other things where you might not expect to find them. Small holograms are present on many credit cards and identity cards in order to make them more difficult to forge.
1981
Bloembergen and Schawlow received the prize for their contribution to the development of laser spectroscopy. One typical application of this is nonlinear optics which means methods of influencing one light beam with another and permanently joining several laser beams (not just mixing them – compare the difference between mixing two substances and making them chemically react with one another).
These phenomena mean that a light beam can in principle be steered by another light beam. If in the future someone intends to build an optical computer (that could be much faster and much more efficient in storing data), it would have to be based on a nonlinear optic.
When using optical fibers, for example in broadband applications, several of the switches and amplifiers that are used require nonlinear optical effects.
1997
Chu, Cohen-Tannoudji and Phillips et al. received the prize for their developments of methods to cool and trap atoms with laser light which is a method for inducing atoms to relinquish their heat energy to laser light and thus reach lower and lower temperatures.
When their temperature sinks very close to absolute zero, atoms form aggregates (make clumps) in a way that reveals some of the innermost aspects of nature. And that is the important application of laser cooling, namely to make us understand more of nature. Very soon after the discovery other scientists started to use the technique to further develop closely related areas.
2000
Alferov and Kroemer were given the prize for their development within the field of semiconductor physics, where they had studied the type of substances that was first used to build semiconductor lasers, that is, the kind of miniature lasers that today have become the cheapest, lightest and smallest. The idea is to produce both the light source and energy supply and place the mirrors in one crystal (less than 1 mm facet, with many sequences). This has become not only the basis for many cheap and portable appliances, but also the foundation in optical information networks.
The CD player, laser writer, laser pointer and the bar code reader the cashier at the supermarket uses, are all based on their discovery.
REFRENCES

NEWAGE PUBLISHER PVT. LTD.,LASERANDNONLINEAROPTICS,P.B LAUD
macmillan publisher,laser theory and application,k.dhyacagrajan,ak.ghatak
universities publishers,laser,e.a siegman
http://www.nobel.org