LASER
LASER is the short form of Light Amplification by Stimulated Emission of Radiation.
Characteristics of a laser beam
1. Directionality: The laser beam is highly directional. It can be focused to a fine point. This property is useful in surgical and industrial applications.
2. Monochromaticity: The laser beam is highly monochromatic. I.e., line width (Dl) associated with laser beams are extremely narrow. For ruby laser, Dl=5×10-4Å.
3. Coherence: Laser beam is highly coherent. It is possible to observe interference effects from two independent laser beams.
4. Brightness: the laser beam is highly intense as compared to ordinary sources of light. This property is used in industry for cutting, drilling and welding operations.
Basic concepts of laser
Interaction of radiation with matter
Consider a system having two energy levels E1 and E2 with E2-E1=hn. When it is exposed to radiation having a stream of photons, each with energy hn, three district processes can take place. They are 1) Absorption 2) Spontaneous emission and 3) Stimulated emission.
Absorption
An atom in the ground state E1 can absorb a photon of energy hn and go to the higher energy state E2. This process is known as absorption and is illustrated in figure.
Rate of absorption R12 is proportional to population (number of available atoms per unit volume) of the lower energy level N1 and u(n), the energy density of radiation u(n).
i.e, R12 µ N1u(n)
R12 = B12N1u(n) ------------(1)
Where B12 is called Einstein coefficient.
Spontaneous Emission
In spontaneous emission, the atoms in the higher energy state E2 eventually return to the ground state by emitting their excess energy spontaneously.
This process is independent of the external radiation. The rate of spontaneous emission R21 is directly proportional to the population of the energy level E2 (N2).
i.e., R21 µ N2
R21 = A21N2 --------------(2)
Where A21 is called Einstein coefficient.
Stimulated emission
In stimulated emission, a photon having energy hn (E2-E1) stimulates an atom in the higher state E2 to make a transition to the lower state E1 with the creation of a second photon.
The rate of stimulated emission R’21 is proportional to population at the energy level E2(N2) and energy density of radiation u(n).
i.e, R’21 µ N2u(n)
R’21 = B21N2u(n) --------------(3)
Einstein’s theory of stimulated emission
Consider a two level energy system E1 and E2. Let N1 and N2 be the number of atoms in the ground state and excited state respectively. Let us assume that only the spontaneous emission is present and there is no stimulated emission of light. At thermal equilibrium,
Rate of absorption = Rate of spontaneous emission
I.e., B12N1u(n) = A21N2
u(n) = (A21/B21)(N2/N1) --------------(4)
By Boltzmann law, N µ e-E/kT
Where ‘k’ is the Boltzmann constant and T, the absolute temperature.
\N2/N1 = e-E2/kT/ e-E1/kT = e- (E2- E1)/kT = e-hn/kT
Substituting this in equation (4),
u(n) = (A21/B21) (1/ehn/kT) --------------(5)
According to the theory of blackbody radiation, the energy density is given by,
Comparing (5) and (6), we observe that equations are not in agreement. To rectify this, Einstein proposed another kind of emission known as stimulated emission. Therefore the total emission is the sum of spontaneous and stimulated emission of radiation.
At thermal equilibrium,
Rate of absorption = Rate of spontaneous emission+Rate of stimulated emission
I.e., B12N1u(n) = A21N2 + B21N2u(n)
u(n)[B12N1-B21N2] = A21N2
u(n) = (A21N2)/[B12N1-B21N2]
Comparing equations (6) and (7),
Equations (8) and (9) represent the relation connecting various Einstein coefficients.
Ratio of rate of spontaneous emission to the rate of stimulated emission,
R = R21/R’21
= [(A21N2] / [B21N2u(n)]
R gives fairly large values at ordinary temperatures. Hence stimulated emission is highly improbable at ordinary temperatures. In order to make stimulated emission dominant over the spontaneous emission, we need,
1. Large radiation density u(n)- for this, some sort of feedback is provided by placing two mirrors. This forms a resonant cavity.
2. A21/B21 small- for this, we choose the excited state a metastable one.
3. N2 > N1- this is called population inversion. This can be accomplished by a pumping mechanism.
Metastable state
Typical life time of an excited state is around 10-8 s. A metastable state is an excited state having a larger life time (~10-3s).
Population inversion and pumping
A system in which N2 > N1 is said to be in a state of population inversion. In general cases, number of atoms in the excited state (N2) is lower than that of the ground state (N1). Therefore, to realize population inversion, atoms in the ground state have to be continuously raised to the higher energy levels by supplying energy continuously. This method is called pumping.
Different pumping mechanisms
1. Optical pumping: Here an external optical source like Xenon flash lamp is employed to produce population inversion. This method is used in Ruby laser and Nd:YAG laser.
2. Direct electron excitation (Electrical pumping): This method is used in gas lasers. In this method, electrons produced during electric discharge directly excite the active atoms to achieve population inversion. This method is used in Argon ion laser.
3. Inelastic atom-atom collisions: In this method, a combination of two types of atoms is used, say A and B, both having same excited state A* and B* that coincide or nearly coincide. In the first step, during electric discharge, A gets excited to A* due to collision with electrons.
A+e ® A*
The excited atom A* now collide with B atom so that B gets excited to B* (metastable).
A*+B ® A+B*
This type of excitation and transition is used in He-Ne laser.
4. Chemical pumping: Here certain suitable exothermic reaction produces active material. For example, hydrogen fluoride chemical laser, in which HF molecules in the excited state result from the following exothermic chemical reaction.
H 2+F2 ® 2HF
5. Heat pumping (Gas dynamic pumping)
Here the active material is heated to a high temperature and rapidly cooled to get necessary population inversion.
Cavity resonator
In the laser, positive feedback may be obtained by placing the active medium between a pair of mirrors which forms an optical cavity. The stimulated signal is amplified as it passes through the medium and fed back by the mirrors. Some commonly used resonators are given below:
Plane-parallel resonator
This consists of two plane mirrors set parallel to one another.
Confocal resonator
This consists of two spherical mirrors of the same radius of curvature R and separated by a distance L such that L=R.
Concentric resonator
This consists of two spherical mirrors having the same radius of curvature R separated by distance L such that L=2R.
In all cases, one mirror will be made 100% reflecting while the other partially reflecting to derive laser output.
LASER SYSTEMS
A laser system generally consists of three components:
1. An active medium with metastable energy levels and having a population inversion between some levels.
2. A pumping mechanism to produce population inversion.
3. A resonant cavity.
He-Ne laser
He-Ne gas laser consists of a fused quartz tube (discharge tube). The tube is filled with a mixture of Helium and Neon gases in the ratio 10:1. Partial pressures of He and Ne in the tube are 1mm of Hg and 0.1mm of Hg respectively.
The ends of the tube have Brewster windows W1 and W2 made of borosilicate glass so that the output is plane polarized. Two mirrors M1 and M2 in which one is fully reflecting and the other one partially reflecting are acting as resonant cavity. Electrodes are connected to a high voltage source. Here population inversion is achieved by direct electron excitation and successive inelastic atom-atom collisions.
The energy level diagram of He-Ne laser is as shown:
The electrons produced during electric discharge interact with the ground state F1 He atoms. As a result, He atoms gets excited to higher energy levels F2 and F3 with low lifetimes.
He + e à He*
The energy levels F2 and F3 of He are very close to E6 and E4 of Ne atom. On collision Ne atom goes to excited states E6 and E4 which are metastable states.
He* + Ne à He + Ne*
Now three types of laser transition are possible.
E6 to E5 (3.39mm)
E4 to E3 (1.15mm)
E6 to E3 (6328Å)
From E3, by spontaneous emission, the atoms comes to the level E2 and thereafter colliding with walls, de excitation takes place and atoms comes to the ground state.
3.39mm and 1.15mm laser beams lie in the infrared region. The popular line of He-Ne laser is 6328Å.
Semiconductor laser (Diode laser/GaAs laser)
GaAs is a direct bandgap semiconductor. Laser transition is possible only in direct bandgap semiconductors. Si and Ge do not give laser transition since they are indirect bandgap semiconductors.
Fermi level (EF) is the highest filled energy level at absolute zero. A semiconductor in which Fermi level lie in the conduction band (in n type) or valence band (in p type) is called a degenerate semiconductor. A p-n junction is used for the fabrication of semiconductor laser. Both p and n regions are made degenerate by heavy doping.
With a forward bias, depletion region (active region) contains a high concentration of electrons in the conduction band and holes in the valence band. Population inversion has occurred in the sense that more states are occupied in the conduction band than in the valence band. At low bias currents, electron-hole recombination takes place spontaneously resulting in a spontaneous emission of photons. This is the principle of a light emitting diode (LED). As the diode current increases, a point is reached, where significant population inversion exist near the junction resulting in a stimulated emission.
One pair of faces perpendicular to the junction is polished so that they act as resonant cavity. The remaining faces are roughened to eliminate laser action in those directions.
In a semiconductor laser, the transitions are associated with the electron states in the conduction band and valence band. The upper and lower energy states are continuous and hence the output is not sharp. Thus coherence and monochromaticity of a GaAs laser are poor. But they have a few advantages. They are
Portable since compact and small.
High efficiency
Highly economical
Can produce both continuous wave and pulsed laser.
Tuning of output is easily possible.
Applications of laser
1. Industrial application: Welding, drilling and cutting.
2. Medical applications: In dermatology, dentistry, ophthalmology, in surgery of tumours, kidney stone and for cancer treatment.
3. For making sensors.
4. In holography.
5. In laser printers.
6. In research.
7. In microelectronics.
8. In accelerating certain chemical reactions.
9. In fibre optic communication.
10. In underwater communication.
11. In military applications.
12. In measuring atmospheric pollutants.
Laser welding, cutting and drilling
Laser welding is better than arc welding and electron beam welding. Here laser beam is focused on to the spot to be welded. Due to the heat generated, the material melts and the impurities in the material such as oxides float up on the surface. Upon cooling the material becomes homogeneous solid structure, which makes it a stronger joint. Nd:YAG laser and CO2 lasers are commonly used for welding.
Advantages of laser welding:
1. Virtually no destruction occurs in the shape of work piece.
2. Can locate welding spot precisely.
3. It’s a non-contact process. Hence no chance for entry of any foreign particle.
4. Ideal in microelectronics where we deal with heat sensitive components.
Laser cutting is generally done assisted by gas blowing. A jet of the oxygen gas is issued through a nozzle at the spot where laser beam is focused. The combustion of the gas burns the metal, thus reducing the laser power requirement for cutting. The blowing action increases the depth and also the speed of cutting.
Advantages of laser cutting
1. The cutting process could be programmed which results in high production rates.
2. The quality of cutting is very high.
3. No thermal damage and chemical change.
4. Cutting a complicated profile even in 3-dimensions is possible.
In laser drilling, powerful laser pulses are used. The intense heat generated over a short duration by the pulses evaporates the material locally, thus leaving a hole. Nd:YAG laser and CO2 lasers are commonly used for drilling.
Advantages of laser drilling
1. No wear and tear.
2. Drilling can be achieved at any oblique angle.
3. Possible to drill very fine holes.
4. Possible to drill very hard and brittle materials.
Measurements of pollutants in the atmosphere
There are various types of pollutants in the atmosphere which includes oxides of nitrogen, carbon monoxide, sulphur oxide, dust, smoke, fly ash etc. In conventional techniques, samples of the atmosphere are collected and then chemical analysis is carried out to find the composition of the pollutants. But this is not a real time data.
In the application of laser for measurement of pollutants, the principle is very much similar to that of RADAR. This technique is called LIDAR which stands for light detection and ranging. Here a pulsed laser is used as the source of light and the light scattered back is detected by a photodetector. The distance to the matter and the concentrations of the matter is obtained by this method.
Absorption technique can also be utilized to study the atmospheric pollutants. Each material absorb light of characteristic wavelength and from the absorption spectrum, the existence of the material can be identified.
We can also use Raman Effect for the study of atmospheric pollutants. The Raman Effect involves scattering of light by gas molecules accompanied by a shift in the wavelength of light. Raman shifts are characteristic of each molecular species.
HOLOGRAPHY
The method of producing the 3-diamensional image of an object due to the interference phenomena of coherent light waves on a photographic plate is known as holography. The idea of holography was first developed by Dennis Gabor in 1948. The invention of laser during 1960 enhanced research in this field.
When an object is photographed by a camera, a 2-dimensional image of 3-dimensional object is obtained. Here only the amplitude of the light wave is recorded on the photographic film. In holography, both the phase and the amplitude of the light waves are recorded in the film. The resulting photograph is called hologram. In Greek, ‘holo’ means whole and ‘graphy’ means writing. So holography stands for whole writing. The recorded hologram has no resemblance to the original object. It has in it a coded form of information of the object. The image is reproduced by a process called reconstruction.
Recording of a hologram
The experimental arrangement for the recording of a hologram using a laser beam is shown below:
A laser beam from a source is made to fall on an optical device called beam splitter. A part of the beam splitter is made to fall on a mirror M2. The beam is reflected waves from the mirror and made to fall on the object. The reflected waves from the surface of the object, called object wave, is made to fall on the photographic plate. The other part of the beam is made to fall on a mirror M1 and then to photographic plate. This beam is called reference wave. The object wave and reference wave interfere and the interference pattern characteristic of the object is recorded on the photographic plate. This recorded interference pattern gives hologram.
Reconstruction of images
In order to view the image, hologram is to be illuminated with the laser having the same wavelength used for recording of the hologram. Illumination of the hologram results in two images - a two dimensional real image and a three dimensional virtual image.
Applications of holography
1) In information storage in computers.
2) In fog droplet camera.
3) In dynamic aerosol camera.
4) In holographic interferometry.
5) In holographic cinema.
6) In acoustical holography.
7) In data processing.
8) Hologram can be used as an optical grating.
9) In information coding.
10) In pattern recognition.
11) In photolithography.
.
LASER is the short form of Light Amplification by Stimulated Emission of Radiation.
Characteristics of a laser beam
1. Directionality: The laser beam is highly directional. It can be focused to a fine point. This property is useful in surgical and industrial applications.
2. Monochromaticity: The laser beam is highly monochromatic. I.e., line width (Dl) associated with laser beams are extremely narrow. For ruby laser, Dl=5×10-4Å.
3. Coherence: Laser beam is highly coherent. It is possible to observe interference effects from two independent laser beams.
4. Brightness: the laser beam is highly intense as compared to ordinary sources of light. This property is used in industry for cutting, drilling and welding operations.
Basic concepts of laser
Interaction of radiation with matter
Consider a system having two energy levels E1 and E2 with E2-E1=hn. When it is exposed to radiation having a stream of photons, each with energy hn, three district processes can take place. They are 1) Absorption 2) Spontaneous emission and 3) Stimulated emission.
Absorption
An atom in the ground state E1 can absorb a photon of energy hn and go to the higher energy state E2. This process is known as absorption and is illustrated in figure.
Rate of absorption R12 is proportional to population (number of available atoms per unit volume) of the lower energy level N1 and u(n), the energy density of radiation u(n).
i.e, R12 µ N1u(n)
R12 = B12N1u(n) ------------(1)
Where B12 is called Einstein coefficient.
Spontaneous Emission
In spontaneous emission, the atoms in the higher energy state E2 eventually return to the ground state by emitting their excess energy spontaneously.
This process is independent of the external radiation. The rate of spontaneous emission R21 is directly proportional to the population of the energy level E2 (N2).
i.e., R21 µ N2
R21 = A21N2 --------------(2)
Where A21 is called Einstein coefficient.
Stimulated emission
In stimulated emission, a photon having energy hn (E2-E1) stimulates an atom in the higher state E2 to make a transition to the lower state E1 with the creation of a second photon.
The rate of stimulated emission R’21 is proportional to population at the energy level E2(N2) and energy density of radiation u(n).
i.e, R’21 µ N2u(n)
R’21 = B21N2u(n) --------------(3)
Einstein’s theory of stimulated emission
Consider a two level energy system E1 and E2. Let N1 and N2 be the number of atoms in the ground state and excited state respectively. Let us assume that only the spontaneous emission is present and there is no stimulated emission of light. At thermal equilibrium,
Rate of absorption = Rate of spontaneous emission
I.e., B12N1u(n) = A21N2
u(n) = (A21/B21)(N2/N1) --------------(4)
By Boltzmann law, N µ e-E/kT
Where ‘k’ is the Boltzmann constant and T, the absolute temperature.
\N2/N1 = e-E2/kT/ e-E1/kT = e- (E2- E1)/kT = e-hn/kT
Substituting this in equation (4),
u(n) = (A21/B21) (1/ehn/kT) --------------(5)
According to the theory of blackbody radiation, the energy density is given by,
Comparing (5) and (6), we observe that equations are not in agreement. To rectify this, Einstein proposed another kind of emission known as stimulated emission. Therefore the total emission is the sum of spontaneous and stimulated emission of radiation.
At thermal equilibrium,
Rate of absorption = Rate of spontaneous emission+Rate of stimulated emission
I.e., B12N1u(n) = A21N2 + B21N2u(n)
u(n)[B12N1-B21N2] = A21N2
u(n) = (A21N2)/[B12N1-B21N2]
Comparing equations (6) and (7),
Equations (8) and (9) represent the relation connecting various Einstein coefficients.
Ratio of rate of spontaneous emission to the rate of stimulated emission,
R = R21/R’21
= [(A21N2] / [B21N2u(n)]
R gives fairly large values at ordinary temperatures. Hence stimulated emission is highly improbable at ordinary temperatures. In order to make stimulated emission dominant over the spontaneous emission, we need,
1. Large radiation density u(n)- for this, some sort of feedback is provided by placing two mirrors. This forms a resonant cavity.
2. A21/B21 small- for this, we choose the excited state a metastable one.
3. N2 > N1- this is called population inversion. This can be accomplished by a pumping mechanism.
Metastable state
Typical life time of an excited state is around 10-8 s. A metastable state is an excited state having a larger life time (~10-3s).
Population inversion and pumping
A system in which N2 > N1 is said to be in a state of population inversion. In general cases, number of atoms in the excited state (N2) is lower than that of the ground state (N1). Therefore, to realize population inversion, atoms in the ground state have to be continuously raised to the higher energy levels by supplying energy continuously. This method is called pumping.
Different pumping mechanisms
1. Optical pumping: Here an external optical source like Xenon flash lamp is employed to produce population inversion. This method is used in Ruby laser and Nd:YAG laser.
2. Direct electron excitation (Electrical pumping): This method is used in gas lasers. In this method, electrons produced during electric discharge directly excite the active atoms to achieve population inversion. This method is used in Argon ion laser.
3. Inelastic atom-atom collisions: In this method, a combination of two types of atoms is used, say A and B, both having same excited state A* and B* that coincide or nearly coincide. In the first step, during electric discharge, A gets excited to A* due to collision with electrons.
A+e ® A*
The excited atom A* now collide with B atom so that B gets excited to B* (metastable).
A*+B ® A+B*
This type of excitation and transition is used in He-Ne laser.
4. Chemical pumping: Here certain suitable exothermic reaction produces active material. For example, hydrogen fluoride chemical laser, in which HF molecules in the excited state result from the following exothermic chemical reaction.
H 2+F2 ® 2HF
5. Heat pumping (Gas dynamic pumping)
Here the active material is heated to a high temperature and rapidly cooled to get necessary population inversion.
Cavity resonator
In the laser, positive feedback may be obtained by placing the active medium between a pair of mirrors which forms an optical cavity. The stimulated signal is amplified as it passes through the medium and fed back by the mirrors. Some commonly used resonators are given below:
Plane-parallel resonator
This consists of two plane mirrors set parallel to one another.
Confocal resonator
This consists of two spherical mirrors of the same radius of curvature R and separated by a distance L such that L=R.
Concentric resonator
This consists of two spherical mirrors having the same radius of curvature R separated by distance L such that L=2R.
In all cases, one mirror will be made 100% reflecting while the other partially reflecting to derive laser output.
LASER SYSTEMS
A laser system generally consists of three components:
1. An active medium with metastable energy levels and having a population inversion between some levels.
2. A pumping mechanism to produce population inversion.
3. A resonant cavity.
He-Ne laser
He-Ne gas laser consists of a fused quartz tube (discharge tube). The tube is filled with a mixture of Helium and Neon gases in the ratio 10:1. Partial pressures of He and Ne in the tube are 1mm of Hg and 0.1mm of Hg respectively.
The ends of the tube have Brewster windows W1 and W2 made of borosilicate glass so that the output is plane polarized. Two mirrors M1 and M2 in which one is fully reflecting and the other one partially reflecting are acting as resonant cavity. Electrodes are connected to a high voltage source. Here population inversion is achieved by direct electron excitation and successive inelastic atom-atom collisions.
The energy level diagram of He-Ne laser is as shown:
The electrons produced during electric discharge interact with the ground state F1 He atoms. As a result, He atoms gets excited to higher energy levels F2 and F3 with low lifetimes.
He + e à He*
The energy levels F2 and F3 of He are very close to E6 and E4 of Ne atom. On collision Ne atom goes to excited states E6 and E4 which are metastable states.
He* + Ne à He + Ne*
Now three types of laser transition are possible.
E6 to E5 (3.39mm)
E4 to E3 (1.15mm)
E6 to E3 (6328Å)
From E3, by spontaneous emission, the atoms comes to the level E2 and thereafter colliding with walls, de excitation takes place and atoms comes to the ground state.
3.39mm and 1.15mm laser beams lie in the infrared region. The popular line of He-Ne laser is 6328Å.
Semiconductor laser (Diode laser/GaAs laser)
GaAs is a direct bandgap semiconductor. Laser transition is possible only in direct bandgap semiconductors. Si and Ge do not give laser transition since they are indirect bandgap semiconductors.
Fermi level (EF) is the highest filled energy level at absolute zero. A semiconductor in which Fermi level lie in the conduction band (in n type) or valence band (in p type) is called a degenerate semiconductor. A p-n junction is used for the fabrication of semiconductor laser. Both p and n regions are made degenerate by heavy doping.
With a forward bias, depletion region (active region) contains a high concentration of electrons in the conduction band and holes in the valence band. Population inversion has occurred in the sense that more states are occupied in the conduction band than in the valence band. At low bias currents, electron-hole recombination takes place spontaneously resulting in a spontaneous emission of photons. This is the principle of a light emitting diode (LED). As the diode current increases, a point is reached, where significant population inversion exist near the junction resulting in a stimulated emission.
One pair of faces perpendicular to the junction is polished so that they act as resonant cavity. The remaining faces are roughened to eliminate laser action in those directions.
In a semiconductor laser, the transitions are associated with the electron states in the conduction band and valence band. The upper and lower energy states are continuous and hence the output is not sharp. Thus coherence and monochromaticity of a GaAs laser are poor. But they have a few advantages. They are
Portable since compact and small.
High efficiency
Highly economical
Can produce both continuous wave and pulsed laser.
Tuning of output is easily possible.
Applications of laser
1. Industrial application: Welding, drilling and cutting.
2. Medical applications: In dermatology, dentistry, ophthalmology, in surgery of tumours, kidney stone and for cancer treatment.
3. For making sensors.
4. In holography.
5. In laser printers.
6. In research.
7. In microelectronics.
8. In accelerating certain chemical reactions.
9. In fibre optic communication.
10. In underwater communication.
11. In military applications.
12. In measuring atmospheric pollutants.
Laser welding, cutting and drilling
Laser welding is better than arc welding and electron beam welding. Here laser beam is focused on to the spot to be welded. Due to the heat generated, the material melts and the impurities in the material such as oxides float up on the surface. Upon cooling the material becomes homogeneous solid structure, which makes it a stronger joint. Nd:YAG laser and CO2 lasers are commonly used for welding.
Advantages of laser welding:
1. Virtually no destruction occurs in the shape of work piece.
2. Can locate welding spot precisely.
3. It’s a non-contact process. Hence no chance for entry of any foreign particle.
4. Ideal in microelectronics where we deal with heat sensitive components.
Laser cutting is generally done assisted by gas blowing. A jet of the oxygen gas is issued through a nozzle at the spot where laser beam is focused. The combustion of the gas burns the metal, thus reducing the laser power requirement for cutting. The blowing action increases the depth and also the speed of cutting.
Advantages of laser cutting
1. The cutting process could be programmed which results in high production rates.
2. The quality of cutting is very high.
3. No thermal damage and chemical change.
4. Cutting a complicated profile even in 3-dimensions is possible.
In laser drilling, powerful laser pulses are used. The intense heat generated over a short duration by the pulses evaporates the material locally, thus leaving a hole. Nd:YAG laser and CO2 lasers are commonly used for drilling.
Advantages of laser drilling
1. No wear and tear.
2. Drilling can be achieved at any oblique angle.
3. Possible to drill very fine holes.
4. Possible to drill very hard and brittle materials.
Measurements of pollutants in the atmosphere
There are various types of pollutants in the atmosphere which includes oxides of nitrogen, carbon monoxide, sulphur oxide, dust, smoke, fly ash etc. In conventional techniques, samples of the atmosphere are collected and then chemical analysis is carried out to find the composition of the pollutants. But this is not a real time data.
In the application of laser for measurement of pollutants, the principle is very much similar to that of RADAR. This technique is called LIDAR which stands for light detection and ranging. Here a pulsed laser is used as the source of light and the light scattered back is detected by a photodetector. The distance to the matter and the concentrations of the matter is obtained by this method.
Absorption technique can also be utilized to study the atmospheric pollutants. Each material absorb light of characteristic wavelength and from the absorption spectrum, the existence of the material can be identified.
We can also use Raman Effect for the study of atmospheric pollutants. The Raman Effect involves scattering of light by gas molecules accompanied by a shift in the wavelength of light. Raman shifts are characteristic of each molecular species.
HOLOGRAPHY
The method of producing the 3-diamensional image of an object due to the interference phenomena of coherent light waves on a photographic plate is known as holography. The idea of holography was first developed by Dennis Gabor in 1948. The invention of laser during 1960 enhanced research in this field.
When an object is photographed by a camera, a 2-dimensional image of 3-dimensional object is obtained. Here only the amplitude of the light wave is recorded on the photographic film. In holography, both the phase and the amplitude of the light waves are recorded in the film. The resulting photograph is called hologram. In Greek, ‘holo’ means whole and ‘graphy’ means writing. So holography stands for whole writing. The recorded hologram has no resemblance to the original object. It has in it a coded form of information of the object. The image is reproduced by a process called reconstruction.
Recording of a hologram
The experimental arrangement for the recording of a hologram using a laser beam is shown below:
A laser beam from a source is made to fall on an optical device called beam splitter. A part of the beam splitter is made to fall on a mirror M2. The beam is reflected waves from the mirror and made to fall on the object. The reflected waves from the surface of the object, called object wave, is made to fall on the photographic plate. The other part of the beam is made to fall on a mirror M1 and then to photographic plate. This beam is called reference wave. The object wave and reference wave interfere and the interference pattern characteristic of the object is recorded on the photographic plate. This recorded interference pattern gives hologram.
Reconstruction of images
In order to view the image, hologram is to be illuminated with the laser having the same wavelength used for recording of the hologram. Illumination of the hologram results in two images - a two dimensional real image and a three dimensional virtual image.
Applications of holography
1) In information storage in computers.
2) In fog droplet camera.
3) In dynamic aerosol camera.
4) In holographic interferometry.
5) In holographic cinema.
6) In acoustical holography.
7) In data processing.
8) Hologram can be used as an optical grating.
9) In information coding.
10) In pattern recognition.
11) In photolithography.
.
