Lasers - An Engineering Introduction

www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS | NEWS | RESULTS

Notes Compiled by: Dr. Santhosh D Shenoy, M.Sc., Ph.D. www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS | NEWS | RESULTS

UNIT -5 LASERS

LASER is the acronym of Light Amplification by Stimulated Emission of Radiation. It is a process by which a coherent, highly monochromatic and perfectly parallel beam of light called laser light is obtained. 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 (∆λ) associated with laser beams are extremely narrow. For ruby laser, ∆λ=5×10-4A0. 3. Coherence: Laser beam is highly coherent. It is possible to observe interference effects from two independent laser beams. 4. High Intensity: 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 such that E1> E2, with E2-E1=hν. When it is exposed to radiation having a stream of photons, each with energy hν, three distinct processes can take place. They are 1) Induced Absorption 2) Spontaneous emission and 3) Stimulated emission. 1) Induced Absorption:

An atom in the ground state E1 can absorb a photon of energy hν and go to the higher energy state E2. This process is known as Induced absorption. Rate of absorption R12 is proportional to population (number of available atoms per unit volume) of the lower energy level N1 and U(ν), the energy density of radiation. i.e, R12 ∝ N1U(ν) R12 = B12N1U(ν) ------------(1) where B12 is a constant called Einstein coefficient of induced absorption. 2) 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 ∝ N2 R21 = A21N2 --------------(2) where A21 is called Einstein coefficient of spontaneous emission. 3) Stimulated emission:



In stimulated emission, a photon having energy hν (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 R121 is proportional to population at

the energy level E2(N2) and energy density of radiation u(ν). i.e, R1

21 ∝ N2U(ν) R1

21 = B21N2U(ν) --------------(3) Einstein’s theory of stimulated emission:

Consider a two level energy system E1 and E2 such that E2 > E1. 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(ν) = A21N2

or U(ν) = �A21B12� ( N2

N1) --------------(4)

But by Boltzmann law, N ∝ e−EkT

where ‘k’ is the Boltzmann constant and ‘T’, the absolute temperature.

∴( N2N1

) = 𝑒−𝐸2𝑘𝑇

𝑒−𝐸1𝑘𝑇

= e-(E2-E1)

kT = e−hυkT

Substituting this in equation (4), U(ν) = �A21

B12� 1

ehυkT

--------------(5)

According to the theory of blackbody radiation, the energy density is given by, U(υ) = 8πhυ3

c3 1

ehυkT−1

-------------- (6)

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(ν) = A21N2 + B21N2U(ν) i.e., U(ν) [B12N1-B21N2] = A21N2 i.e., U(ν) = A21N2

(B12N1-B21N2)

= A21B12

N1N2

− B21B12

But ( N1N2

) = ehνkT

Thus, U(ν) = A21B12

ehνkT − B21

B12

----------------- (7)

Comparing equations (6) and (7), we get, 𝐴21

𝐵12= 8πhν

3

c3 -------------------- (8)

and B21B12

= 1 => B21= B12 --------------(9) Equations (8) and (9) represent the relations connecting various Einstein coefficients. Ratio of rate of spontaneous emission to the rate of stimulated emission,



R = R21R21

1

= A21N2B21N2U(ν)

R = 8πhν3

c3 c3

8πhν3 e

hνkT − 1

R = ehνkT − 1

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(ν)- 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 of the order 10-3s. Population inversion:

It is a state in which number of atoms in an excited state is more than that of in ground state. Let N1 be the number of atoms in the ground state and N2 be the number of atom in the excited state Thus a system in which N2 > N1 is said to be in a state of population inversion. Pumping:

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):

In this method, electrons produced during electric discharge directly excite the active atoms to achieve population inversion. This method is used in gas lasers like 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: 1) Plane-parallel resonator: This consists of two plane mirrors set parallel to one another.

2) 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.

3) 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. Cavity resonator amplifies light of certain frequencies which is given by, fn = 𝐧𝐜

𝟐µ𝐋

where ‘n’ represent number of modes, ‘c’ the velocity of light, ‘L’ the length of the cavity and ‘µ’ the refractive index of the active medium. Since c = f λ, we can write λ = c

f = 𝟐µ𝐋

𝐧

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.



Three level laser systems:

Population inversion is usually achieved using either a three level or a four level laser system. In a three level laser system, three energy levels are involved as shown in figure:

E1 is the ground state. E2 and E3 are excited states. E2 is a metastable state and E3 with short life time. Transition probability from level E1 to E3 is very high. From E3, atoms undergo rapid decay to level E2. Thus population inversion is achieved between E2 and E1. Very high pump power required here because the terminal level of the laser transition is the ground state. Thus efficiency is lower. Eg:- Ruby laser. Four level laser systems:

In a four level laser system, four energy levels are involved as shown in figure:

E1 is the ground state and E2, E3 and E4 are excited states. E3 is metastable while E2 and E4 have short life times. Hence population inversion is achieved between E2 and E3. Here population inversion can easily achieved with lower pumping requirements. Thus efficiency is greater. Eg:- He-Ne laser. He-Ne laser: The He-Ne laser was constructed in 1960, by Ali Javan, W.R. Bennett and D.R. Herriott, at Bell Laboratories, USA.



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 weight 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:

He Ne

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 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 He + Ne* Now three types of laser transition are possible from E6 to E5 (3.39µm), E4 to E3 (1.15µm) and E6 to E3 (6328Å). The atoms 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. The 3.39µm and 1.15µm 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. The doping concentration is of the order of 1017 to 1019 dopant atoms /cm3. 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. Since the energy gap of GaAs is 1.4 eV, the wavelength of the emitted light is,

λ = hcEg

= 𝟔.𝟔𝟑×𝟏𝟎−𝟑𝟒×𝟑×𝟏𝟎𝟖

𝟏.𝟒×𝟏.𝟔×𝟏𝟎−𝟏𝟗 = 8879 Å

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

1. Portable since compact and small. 2. High efficiency. 3. Highly economical. 4. Can produce both continuous wave and pulsed laser. 5. 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:

By virtue of unique characteristics like directionality, high intensity and coherence of laser, they are widely used in industry for welding, cutting and drilling operations. 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. Measurement 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. Interference is the phenomenon of superposition of two or more light waves and redistribution of their energies.

In Greek, ‘Holos’ means complete and ‘Graphos’ means writing. So holography stands for complete writing. The idea of holography was first developed by Dennis Gabor in 1948. For this he was awarded the Nobel prize in Physics. Because of the non availability of good coherent sources, this idea was not a success during the initial stages. The invention of laser during 1960 enhanced research in this field and its tremendous potential for applications in diversified fields was realized. 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. 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 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.

***************** VTU Model Question paper

5.a) 1) Emission of a photon by an excited atom due to interaction of external energy is called i) Spontaneous emission ii) Stimulated emission iii) Induced absorption iv) Light amplification 2) In He-Ne laser, the ratio of He-Ne gas molecules in the order i) 1:10 ii) 10:1 iii) 1:1 iv) 1:2 3) The life time of an atom in a metastable state is of the order of i) a few seconds ii) unlimited iii) a nanosecond iv) few milliseconds 4) Pumping process used in diode laser is i) Optical pumping ii) Forward bias iii) electrical discharge iv) none of these b) With help of energy level diagram, describe the construction and working of He-Ne laser. c) Explain the principle of holography and mention its applications. d) A laser beam with power 1 mW lost for 10 ns. If the number of photons emitted per second is 3.941 × 107, calculate the wavelength of laser. (4+6+6+4)

December 08 / January 09

5 a. 1) The emission of photon without aided by any external agency is called i) Light amplification ii) Induced absorption iii) Stimulated emission iv) Spontaneous emission. 2) Let n1 be the number density of lower energy E1 and n2 be the number density of higher energy E2, if n2>n1 is called



i) Thick population ii) Inverted population iii) Normal population iv) No population. 3) Supply of energy to atoms for excitation is called i) Glowing ii) Bombarding iii) Incidenting iv) Pumping. 4) Important characteristic of laser beam is i) Interference ii) Diffraction iii) Dispersion iv) Coherence (04 Marks) b) Obtain an expression for energy density of radiation under equilibrium condition in terms of Einstein coefficient. (08 Marks) c) Describe the construction and working of Semiconductor laser. (08 Marks)

June-July 2009 5 a. 1) Pumping process used in diode laser is i) Optical pumping ii) Forward bias iii) Electric discharge iv) None of these 2) The life time of an atom in a metastable state is of the order of i) a few seconds ii ) unlimited iii) a nanosecond iv) few milliseconds 3) The purpose of the optical resonator in a laser is i) To provide cover to the active medium ii) To provide path for atoms iii) To provide selectivity of photons iv) To send laser in specified direction. 4) In He-Ne laser, the ratio of He-Ne is in the order i) 1:10 ii) 1:1 iii) 10:1 iv) 100:1 (04 Marks) b. With the help of energy level diagram, describe the construction and working of He-Ne laser. (08 Marks) c. Write a note on measurement of pollutants in a atmosphere using laser. (04 Marks) d. A laser beam with power per pulse is mw lasts 10 ns, if the number of photons emitted per pulse is 3.941 ×107, calculate the wavelength of laser. (04 Marks)

December 09 / January 10

5 a. 1) The life time of an atom at the ordinary excited state is of the order of i) few milli second ii) few nano second iii) few micro second iv) Unlimited 2) The relation between Einstein’s coefficient’s A and B is i) 8πhλ

3

c3 ii) 8πh2υ3

c3 iii) 8πhυ3

c3 iv) 8πhλ

3

c2

3) The number of modes of standing waves in the resonant cavity of length 1m, if He-Ne laser operating at wavelength 6328 Å is i) 3.14 × 106 ii) 1.58 × 106 iii) 3.16 × 106 iv) None of these 4) From a broken hologram which is 10% of the original, if reconstruction of image is being done, then i) Only 10% of information of the object can be obtained ii) Complete information of the object is obtained iii) No information of the object can be obtained iv) None of these (04 Marks) b. Obtain an expression for energy density of radiation under equilibrium condition in terms of Einstein coefficient. (07 Marks) c. Describe the recording and reconstruction processes in holography, with the help of suitable diagrams. (05 Marks) d. A ruby laser emits pulse of 20 ns duration with average power per pulse being 100 kW. If the number of photons in each pulse is 6.981 × 1015, calculate the wavelength of photons. (04 Marks)

May/June 2010

5 a.1) Which of the following is not a laser property? i) Highly monochromatic ii) High directionality iii) Very narrow bandwidth iv) Highly divergent 2) The life time of an atom in the excited state is of the order of i) Millisecond ii) Few seconds iii) Nano seconds iv) Unlimited



3) Pumping technique used in He-Ne gas laser is i) Forward bias ii) Optical pumping iii) Electrical discharge iv) High injection current 4) 3D image of an object constructed by hologram is the process of i) Intensity recording ii) Phase information recording iii) Both phase and intensity information recording iv) Transmission and reflection recording (04Marks) b. Discuss the possible ways through which radiation and matter interaction takes place. (06 Marks) c. Describe the construction and working of semiconductor laser. (06 Marks) d. Calculate on the basis of Einstein’s theory the number of photons emitted per second by He-Ne laser source emitting light of wavelength 6328Å with an optical power 10 mW. (04 Marks)

January 2011

5 a 1) Rate of induced absorption depends on i) number of atoms in the lower energy state ii) the energy density iii) number of atoms in the higher energy state iv) Both i and ii 2) In semiconductor laser, the material used is i) any semiconductor ii) direct band gap semiconductor iii) indirect band gap semiconductor iv) not a semiconductor 3) The required condition to achieve laser action in a system is i) state of population inversion ii) existence of metastable state iii) a resonant cavity iv) all the three 4) In recording the image on the photographic plate the reference beam and the object beam undergo ______________________ at the photographic plate i) diffraction ii) reflection iii) interference iv) polarization (04 Marks) b. Explain the construction and working of He-Ne laser, with the help of suitable diagrams. (08 Marks) c. Mention the applications of holography. (04 Marks) d. The average output power of laser source emitting a laser beam of wavelength 633nm is 5 mW. Find the number of photons emitted per second by the laser source. (04 Marks)

June/July 2011 5 a. Choose your answers for the following: i) Wavelength of a laser beam can be used as a standard of A) time B) temperature C) angle D) length ii) Image is stored on a hologram in the form of

A) interference pattern B) diffraction pattern C) Photograph D) none of these iii) Which event is likely to take place, when a photon of energy equal to difference in energy

between two levels is incident in a system A) Absorption B) emission C) absorption and emission D) none of these

iv) Quartz plates are fixed at the ends of the discharge tube in a He-Ne laser so that A) there won’t be leakage of gas B) the tube can withstand high electric voltage C) the loses light can pass out without any loss D) the emergency light is polarized

(04 Marks) b. Explain the requisites and conditions of a laser system. (05 Marks) c. Describe the principle and working of LIDAR used to measure pollutant in atmosphere.

(06 Marks) d. Find the member of mode of standing waves and their frequency separation in the resonant cavity

of 1 m length of He-Ne operating at a wavelength of 632.8 nm. (05 Marks)



December 2011 5 a. Choose your answers for the following: i)Emission of a photon by an excited atom due to interaction of external energy is called

A) Spontaneous emission B) stimulated emission C) induced absorption D) Light amplification ii) Pumping process used in diode laser is

A) Optical pumping B) forward bias C) electrical discharge D) none of these iii) Image stored in a hologram in the form of

A) Interference pattern B) diffraction pattern C) photography D) none of these iv) Important characteristic of laser beam is A) interference B) diffraction C) dispersion D) coherence (04 Marks) b. Describe the construction of He-Ne laser and explain its working, with the help of energy level

diagram. (06 Marks) c. Describe the recording and reconstruction process in holography, with the help of suitable

diagrams. (06 Marks) d. A He-Ne gas laser is emitting a laser beam with an average power of 4.5 mW. Find the number of

photons emitted per second by the laser. The wavelength of the emitted radiation is 6328Å. (04 Marks)

***************

Lasers - An Engineering Introduction

Documents

Transcript of Lasers - An Engineering Introduction