LASER AND APPLICATIONS

Submitted by:

ARUN T A ROLL NO: 07

Dept. InstrumentationCUSAT

INTRODUCTION• Lasers have truly been the one of the greatest

invention in 20th century.• They have found a variety of uses in electronics,

computer hardware, madicine, industry, military and experimental science.

• In 1917, Albert Einstein first theorized about the process which makes lasers possible called “stimulated emission”.

• The laser beam was invented by phycist Maiman in 1960.

• The laser stands for Light Amplification by Stimulated Emission of radiation

CONTD….

• The basic operating principles of the laser were put forth by Charles Townes and Arthur Schalow from the Bell Telephone Laboratories in 1958, and the first actual laser, based on a pink ruby crystal, was demonstrated in 1960 by Theodor Maiman at Hughes Research Laboratories.

• Laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation.

• A laser differ from other sources of light beacause it emits light coherently

• In coherent beam of electromagnetic energy, all the waves have the same frequency and phase.

PROPERTIES OF LASER• Monochromatic: It means that it consist of one color or wavelength.• Divergence and directionality: it means that the beam is well collimated and

travels long distance with very little spread.• Coherence: it means that all the individual waves of light are

moving preciesly together through time and space.• Brightness: The radiance of laser is an important factor. It is

defined as the power emitted per unit surface area per unit solid angle.

Construction of laser• A laser mainly requires three components for its

operation: (a) an active medium in the form of a laser rod, with energy levels that can be selectively populated; (b) a pumping process to produce population inversion between some of these energy levels; and (c) a resonant cavity containing the active medium which serves to store the emitted radiation and provides feedback to maintain the coherence of the radiation.

CONTD….

• Pump light may be provided by flash lamp or by another laser.

• The most common type of laser uses feedback from an optical cavity which is designed to internally reflect IR or UV waves so they reinforces each other.

• At each end of the cavity, there is a mirror. One mirror is totally reflective and other mirror is partially reflective.

• Depending the design of the cavity the light coming out of the laser may spread out or form a narrow beam.

Principle of laser actionThe principle of a laser is based on four separate

features:

• Absorption

• Spontanious emission

• Stimulated emission

• Population inversion

Contd….Absorption :

• In absorption an incoming photon excites the atomic system from a lower energy state into higher energy state.

• It occurs when a photon strikes an atom with just exactly the proper energy to induce an electronic transition between two energy states.

Contd…. Spontanious emission:• it takes place when an electron in a higher energy

level drops down to a lower energy level and a photon is emitted with an energy equal to the energy difference between the two levels.

Contd….Stimulated emission:• It takes place when a photon with an energy

equal to the energy difference between two levels interacts with the electron in the higher energy level

Population inversion• population inversion is when more atoms are in

an excited state than in their ground state.• It is necessery condition to sustain a laser beam,

so that there are enough excited atoms, that can be stimulated to emit more photons.

• To achieve non-equilibrium conditions, an indirect method of populating the excited state must be used which is done using a three level laser and a four level laser.

Three level & four level laser• Three level laser:

CONTD….• Four level laser:

Types of laserThe most important and known types of laser are

classified as:

• Solid state laser

• Gas laser

• Semiconductor laser

• Dye laser

SOLID STATE LASER• Ruby laser

• Nd – YAG laser

• Nd – Glass laser

Ruby laser: The important type of solid state laser is the ruby

laser.• Ruby laser is historically the first one to be

discovered.• It consist of a ruby rod , xenon flash tube, a

suitable cavity to reflect the light from flash tube to the ruby rod, and high voltage power upply to give electrical energy to the flash tube.

Contd….• The laser is a three level system only three

energy levels are involved in the process of stimulated emission.

• The population inversion is achieved by exciting the atoms with intense light from a xenon flash lamp.

• Thus the atoms are excited from ground state to the upper state by means of absorption.

Nd – YAG laser:

CONTD….• YAG is formed from a mixed oxide system having a

composition of Y3Al5OI2. • Using Czochralski method, the crystal is grown in a

specially designed furnace by dipping a rotating seed into a crucible of molten material and withdrawing it at a constant speed.

• Iridium crucible is used because of high melting point of YAG (1910-1970°C).

• the YAG host has the advantage of having a relatively high thermal conductivity to dissipate the heat generated, thus allowing these crystals to be operated at high repetition rates of the order of many pulses per second. With a continuous source of excitation like tungsten lamp or krypton arc lamp, continuous laser output of about 1 kW power could be obtained.

• Due to these excellent properties, Nd: YAG laser is extensively used in many industrial applications like drilling of holes in solid objects, welding of metals and alloys, etc, and also in medical applications like eye surgery, treatment of cancer, etc.

Nd – Glass laser:• Nd: glass is an important laser material for high

energy applications. • It affords considerable flexibility in size and shape

and can be obtained in large homogeneous spices. Glass is a compound of oxides; the non-metal oxides, such silicon dioxide, phosphorus pent oxide and boron oxides are its main constituents.

• The major disadvantage of glass is that it produces in homogeneously broadened lines which are wider than those found in crystals . This raises the threshold, as a larger Inversion is required for the same gain.

• Another disadvantage of glass is its low thermal conductivity.

GAS LASERS• It has gas or a mixture of gasese as their light

amplifying substance.• It is much cheaper than solid state lasers and

yields highly coherent radiation required for many applications.

• The gas lasers are of continuous type and normally have high coherence.

• He – Ne and carbon dioxide lasers are most widely used gas lasers.

He – Ne laser:• The helium-neon laser is the most widely used of

all lasers mainly because it is much cheaper than the solid-state lasers and yields highly coherent radiation required for many applications.

CONTD….• In this system, an electrical discharge is given in

helium contained in a discharge tube at a pressure of about 1 torr with an admixure of neon at about 0.1torr.

• The discharge excites the He atoms to their first excited level above their ground state.

• Then the excited energy of the helium atom is transferred to the Ne atoms by resonance transfer due to collision between them.

• These excited states radiatively to lower energy neon state giving rise to continuous laser beam

Carbon dioxide laser:• It is a very important laser because of its high

efficiency and high power capacity. • It is a molecular gas discharge with the laser

action taking place between two vibrational level of carbon dioxide gas.

• A suitable mixure of carbon dioxide, nitrogen, and helium gases is taken in a gas discharge tube and excited electrically with the help of a power supply.

• The nitrogen molecules are excited to their first excited the lowervibrational levels giving rise to laser radiation.

• The helium gas helps to populate the upper state but also assists to empty the lower state of carbon dioxide, thus increasing the efficiency of carbon dioxide laser.

CONTD….

• The carbon dioxide laser is used for a wide variety of applications, including eye and tissue surgery, welding, cutting and heat treatment of materials, laser fusion, and beam weapons. Rock sand granites crumble into pieces with a 1.2 kW carbon dioxide laser. One day, such a laser may be used to drill underground tunnels.

Dye laser:

• Liquid lasers are similar to the solid-state lasers.• Dyes exhibit a very high degree of fluorescence.• Different dyes have different emission spectra or

colors. • Dye lasers cover a broad wavelength range from

the ultraviolet at320 nm to the infrared at about 1500 nm.

• The dye laser output can be a very narrow frequency beam.

CONTD….

• important application of dye lasers is for producing ultra short optical pulses.

• The dye lasers are less expensive than the solid-state lasers and are relatively easy to maintain for regular operation.

Semiconductor laser:

• It is based on the principle of electron hole recombination in a direct band gap semiconductor which results in emission of photons.

• It consists of a p-n junction in which p and n regions are heavily doped. Each side of the laser is the order of 1 mm. the width of p-n junction layer is 1 micrometer.

CONTD….

APPLICATION OF LASERS:

• Biomedical applications

• Industrial applications

• Other applications

Biomedical applications:

Flow cytometry:• Flow cytometry is a technique used for measuring

single cells. • Not only is it a key research tool for cancer and

immunoassay disease research, but it is also used in the food industry for monitoring natural beverage drinks for bacterial content or other disease-causing microbes.

• In a basic cytometer, the cells flow, one at a time, through a capillary or flow cell where they are exposed to a focused beam of laser light.

CONTD….• The cell then scatters the light energy onto a

detector or array of detectors. The pattern and intensity of the scattered energy helps to determine the cell size, and shape.

• In many cases the cells are tagged with a variety of fluorochromes designed to selectively adhere to cells or cell components with specific characteristics.

• This is used in many systems to assist with separation or sorting of cells or cellular components.

• The most popular lasers used in flow cytometry are the 488-nm (blue) argon-ion laser and the 632-nm (red) and 594-nm (yellow) He-Ne lasers. However, new violet, blue and red diode lasers and a variety of new DPSS lasers are entering the field.

Surgical Applications:

• Lasers are used in a variety of surgical and dental procedures from cutting tissue, vaporizing tumors, removing tattoos, removing plaque, removing cavities, removing hair and follicles, resurfacing of skin and correcting vision.

• Ultraviolet excimer lasers are used for vision correction because they can removing material from the lens of the eye without causing thermal damage which could blur vision or make the lens opaque.

• Ruby lasers are used for tattoo removal because many of the dyes break down when exposed to 694-nm radiation, yet the skin tissue is left undamaged.

CONTD….• Cosmetic treatment of wrinkles, moles, warts, and

discolorations (birth marks) is often accomplished with near infrared and infrared lasers.

• Lasers are also used to treat macular degeneration, an overgrowth of veins and scar tissue in the retinal region, a condition associated with advancing age. In this procedure, the patient is injected with a selective dye, which enhances the absorption of laser light by the blood in the blood vessels. When the blood vessels absorb laser energy, they wither in size, uncovering the active retina.

• A multi watt green DPSS laser is most commonly used for this application because the green wavelength is not absorbed by the lens or aqueous portion of the eye, which allows the laser to affect only the targeted veins.

Industrial Applications:

• Laser Cutting

• Laser Drilling

• Laser Welding

• Laser glass decoration

• Laser marking

Laser cutting:• Continuous wave lasers like carbon dioxide gas lasers

are extensively used for cutting a wide range of materials, such as graphite, diamond, tungsten, carbide, all metallic foils, ceramics, sapphire, and ferrite.

• In most cases, continuous cutting is carried out with assist gases like oxygen, carbon dioxide, or air, which produces both mechanical and chemical action intensifying the thermal effects.

• This gas-assisted cutting is applicable to the metals of thickness up to 5mmwith cut-widths down to 30 μm.

• The most promising field of laser cutting is the cutting of steels of small thickness (several millimeters) and also of non-metallic materials.

• Use of laser cutters in the garment industry, a new and very useful application of the lasers, has been introduced recently in the developed countries. With the aid of computers, lasers can cut clothing many times faster than the tailors using old techniques.

Laser Drilling:• Laser drilling of metals is based on a face-heating

phenomenon.• Laser enables drilling of a diamond die in a few

minutes as against 20 hours taken by conventional methods.

• Laser light energy is primarily applied in effecting micro openings in rubies and diamonds.

• The plus point about laser drilling is that it does not cause any damage to the diamond or any other processed material.

CONTD….• For laser drilling, usually pulsed carbon dioxide,

Nd:YAG or alexandrite laser is used.

Laser Welding:• This process utilizes mostly the continuous lasers

of the infraredCO2 spectrum and the Nd:YAG lasers, of a wavelength of 10.6 nm and1.06 nm, respectively.

• The advantage of laser welding rests in the absence of physical contact with the electrode, in localized heating and cooling, in welding parts in a protective atmosphere or sealed into optically transparent material .

• Lasers can weld, e.g., air-tight shields of miniature relays, pacemakers, contacts in microelectronics, and metal sheets in car or aircraft industry.

Laser glass decoration:• It is a modification of laser cutting. At the spot

focused laser beam impinges upon the glass surface, the melted glass will evaporate and cracks will appear on its surface. They will diffuse light, producing thus a shiny effect of the lasered ornament. Glass is decorated by lasers whose radiation is easily absorbed by the glass, e.g., by the continousCO2 laser.

Laser Marking:• It is based on local surface evaporation of the

object material. • In this case, the laser beam passes through a

template with the desired pattern.• The depth of the marking usually ranges between

fractions and units of millimeters, the thickness being of the order of micrometers.

• This technique is performed by the powerful pulse laser of pulse energy up to tens of joules, or by the continuous laser, i.e., the Nd:YAG or excimer laser.

• The advantage of laser marking is the non-contact process, eliminating any possible stresses and strains in the lasered material.

Other applications:

• Laser radar in ecology – LIDAR

• Laser Range Finder

• Ring Laser Gyroscope

Laser radar in ecology – LIDAR:• Ground laser radars are used in ecology to

measure air pollution. • In this case it is both reflection and scattering

that are made use of in measurements.• Passing through the atmosphere, the laser pulse

is scattered by the molecules and aerosols present there, causing Mie, Raleigh, or Raman scattering.

• Part of the radiation scattered backwards is concentrated by a telescope, and passing through a filter detected by a photo detector.

• The received signal, whose amplitude at any moment is proportional to the intensity of the scattered radiation is recorded as a function of time, due to which it is possible to obtain also the distance of the scattering body, while the filter width and/or the attached spectrometer determine the spectrum of the received signal.

CONTD….• LIDAR serves to monitor the distribution and

direction of smoke trails; to measure the bottom level and profile of clouds, of atmospheric turbulence, distribution and areas of various emissions in the atmosphere, etc.

Laser Range Finder:• To knock down an enemy tank, it is necessary to

range it very accurately. • Because of its high intensity and very low

divergence even after travelling quite a few kilometers, lasers ideally suited for this purpose.

• The laser range finders using neodymium and carbon dioxide lasers have become a standard item for artillery and tanks.

• These laser rangefinders are light weight and have higher reliability and superior range accuracy as compared to the conventional range finders. The laser range finder works on the principle of radar.

CONTD….• It makes use of the characteristic properties of

the laser beam, namely, monochromaticity, high intensity, coherency, and directionality.

• A collimated pulse of the laser beam is directed towards a target and the reflected 1ight from the target is received by an optical system and detected.

• The time taken by the laser beam for the to and fro travel from the transmitter to the target is measured. When half of the time thus recorded is multiplied by the velocity of light, the product gives the range, i.e., the distance of the target.

CONTD….

• The laser range finder is superior to microwave radar as the former provides better collimation or directivity which makes high angular resolution possible.

• The advantage of greater radiant brightness and the fact that this brightness is highly directional even after travelling long distances, the size of the emitting system is greatly reduced.

• The high monochromaticity permits the use of optical band pass filter in the receiver circuit to discriminate between the signal and the stray light noise.

Ring Laser Gyroscope:• The ring laser gyroscope is an extremely useful

instrument for sensing and measuring very small angles of rotation of the moving objects.

• The main advantages of the ring laser gyroscope are: (I) non-existence of moving parts, (ii) high capability, and (iii) higher reliability as compared to the mechanical gyroscope.

• the laser gyroscope is capable of wide dynamic range and rapid reaction time, the characteristics required for missile guidance.

• The ring laser gyroscope basically consists of a ring cavity around which two laser light beams travel in opposite directions. The operation of the ring laser gyroscopes is based on the so called Signac effect by which rotation of an object is sensed by an interferometric technique.

• In a triangular cavity of a quartz block, laser beam is split into two light beams with the help of suitable mirrors. These two light beam travel in opposite directions in the same path of the cavity, one in the clockwise and the other in the anti-clockwise direction.

• The two light beams then pass through a beam splitter and a beam combiner, behind which a readout detector is placed.

• If the cavity which is acting as an interferometer is stationary, the two light beams travel the same distance in the opposite directions without any path difference and hence no interference takes place.

CONTD….• if the block is rotated clockwise about an axis

through the centre and perpendicular to the plane of the interferometer, the beam travelling in the clockwise direction travels a path length slightly more than the beam travelling in the anti-clockwise direction.

• When these two light beams recombine at the beam combining prism, interference takes place due to the path difference; the interference fringes displaced in the field of view are proportional to the amount of rotation of the block.

• The laser gyroscope uses a helium-neon gas laser to generate monochromatic radiation in the two directions inside the triangular quartz block.

CONTD….• Two photo detectors sense the direction and the

rate of rotation. The output is proportional to the input angle. The whole system is a single plane, rate integrating gyroscope and is capable of measuring rotation rates of the order of 1/10,000 degree/hour.

• The main use of the ring laser gyroscope is for inertial navigation. It is being used in inertial guidance of aircraft, ships, and missiles; flight control; and gun-fire pointing.

CONCLUSION:• A laser is a device that emits electromagnetic

radiation through a process of amplification based on the stimulated emission of photons.

• The wavelength of laser light is extremely pure when compared to other light sources and all of the photons that make up the laser beam have a fixed phase relationship with respect to one another.

• Laser is a powerful source of light having extraordinary properties which are not found in the normal light sources like tungsten lamps, mercury lamps etc.

• The unique property of laser is that its light waves travel very long distances with very little divergence.

CONTD….• More detailed attention has been paid here only

to those fields where lasers are used at present. Based on these laser techniques, some new ones are being developed, as, e.g., for the laser ranging device to be used in cars of the future: the built-in laser radar plus automatic control will keep a safe distance between cars. The spectrally defined laser interaction with matter is made use of in art restorations to remove the dirt from old paintings and statues, as well as in routine maintenance and cleaning of the outer skin of ships and aircraft. It is true that lasers have become irreplaceable and research into their applications still continues.

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