1. L A S E R S L LIGHT AAMPLIFICATION by SSTIMULATED EEMISSION of RRADIATION snistforum.com

2. Spontaneous emission snistforum.com

3. Stimulated Emission snistforum.com

4. Three basic processes that may take place between two energy levels E 1 and E 2 Absorption of photon of frequency h = E 1 – E 2, excites the electron from the ground state E 1 to the excited state E 2. The excited state E 2 is not a stable. After a short interval of time, the electron jumps back to the ground state without any external stimulus. In the process it gives out the energy it absorbed in the form of radiation of frequency. This type of emission is called spontaneous emission. It is random and incoherent. Spontaneous and Stimulated Emission snistforum.com

5. The average life time of the electron in the excited state is  10 -6 s. During this short time interval, if a photon of energy h is incident on the atom (i.e., when it is still in the excited state), it is stimulated to make transition to the ground state. Then, it emits a photon of frequency, which is in phase with the incident photon. This process is called stimulated emission. In stimulated emission, the wave emitted by an excited atom is coherent with the stimulating wave i.e., the emitted wave is of the same frequency and with no phase difference. Their superposition increases the amplitude of the stimulating wave i.e., there is amplification. snistforum.com

6. EINSTEIN COEFFICIENTS Transitions between atomic energy levels is a statistical process. We have to use the laws of probability to calculate the rate of transitions between two energy states E 1 and E 2. Let N 1 and N 2 be the no. of atoms per unit volume in states 1 and 2 respectively. An atom in the lower energy state E 1 gets excited to level E 2 by absorbing radiation of frequency (h= Planck’s constant) The rate of the absorption process is proportional to N1N1 N2N2 E1E1 E2E2 Absorption Emission snistforum.com

7. N 1 no. of atoms and the energy density u(  ) of radiation at the frequency. The energy density is defined in such away that u(  ) d  represents radiant energy per unit volume in the frequency interval  and  +d . Hence, the no. of absorptions per unit time per unit volume can be expressed as N 1 B 12 u(  ) Where B 12 is the constant of proportionality; it is a characteristic of the energy levels. In the reverse process i.e. emission, the atom jumps from E 2 to E 1 by emission of radiation. The emission may be spontaneous or stimulated emission. Hence, the rate of spontaneous emission may be represented by N 2 A 21 snistforum.com

8. For stimulated emission, the rate of transition depends on (a) N 2 and (b) u(  ). Thus, the rate of stimulated emission can be expressed as N 2 B 21 u(  ) where B 21 is the proportionality constant. The proportionality constant A 21, B 12 and B 21 are called Einstein coefficients. These constants are determined by the atomic system. Under equilibrium (thermal) conditions, the no. of upward transitions = the number of downward transitions. snistforum.com

9. According to Bolrzmann distribution law, the ratio of N 1 and N 2 is given by Where k = Boltzmann constant snistforum.com

10. According to Planck’s radiation law, the energy density of radiation is given by Where n o = reactive index of the medium comparison and leads to B 12 = B 21 = B The ratio of the number of spontaneous to stimulated emissions is given by snistforum.com

11. Monochromaticity: Properties of LASER light The light waves have the same wavelength (or the same colour). With High intensity. The light waves in a laser beam have very high frequency. Thus, the energy of the laser beam is also very high. snistforum.com

12. Conventional light source Directionality: All the atoms in a laser beam travel in the same direction and have the same plane of polarization. snistforum.com

13. Coherence: Incoherent light wavesCoherent light waves All the light waves in a laser beam are in phase with each other. The word coherence means that the radiations emitted by atoms, molecules, or photons in the source have same phase, same direction, same plane of polarisation, and same wavelength or colour (monochromatic). Phase difference is constant. Two types of Coherences 1. Temporal and 2. Spatial Coherence snistforum.com

14. Temporal Coherence Spatial Coherence It refers to the correlation between the field at a point and the field at the same point at later instant. If the phase difference between E (x, y, z, t 1 ) and E (x, y, z, t 2 ) is constant during the time interval in which observations are made, the wave is said to be temporally coherent. If two fields at two different points of a wave front of a given radiation have constant phase difference, they are said to have spatial coherence. snistforum.com

15. Difference between spontaneous and stimulated emission Spontaneousstimulated 1. Emission of light photon takes place immediately without any inducement. 1. Emission of a light photon is by inducement of a photon having energy equal to the emitted photon energy. 2. Polychromatic radiation2. Monochromatic radiation 3. Incoherent radiation3. Coherent radiation 4. Less directionality4. High directionality 5. Less intense5. High intense snistforum.com

16. POPULATION INVERSION Population inversion is the process in which the population of a particular higher energy state is made more than that of a specified lower energy state. snistforum.com

17. E0E0 E1E1 E2E2 Metastable state Three Level Scheme Example:- Ruby Laser If the collection of atoms is intensely pumped (a large number of atoms are excited) through stimulated absorption to the highest energy level E 2. With intense pumping from E 0 to E 2, because of rapid decay to E 1 (metastable state where has longer life time), it is possible to bring non- equilibrium distribution of atoms where E 1 more populated than E 0 (N 2  N 1 ) and laser transition takes place between E 2 and E 1. snistforum.com

18. Four Level Scheme E0E0 E1E1 E2E2 E3E3 Example:- He – Ne Laser snistforum.com

19. Pumping mechanisms – creation of population inversion:- 1.Optical pumping – Ruby Laser 2.Electric discharge – He – Ne laser 3.Chemical reaction – Atomic Iodine Lasers 4.Injection current – Semiconductor Lasers snistforum.com

20. First laser was demonstrated by T. Maiman in 1960. Ruby: Al 2 O 3 in which some of the Al atoms have been replaced with Cr (0. 5 % by weight). Cr gives its characteristic red color and is responsible for the lasing behavior of the crystal. Cr atoms absorb green and blue light and emit only red light. Ruby Laser : λ = 694.3 nm Optical pumping 4F 1 4F 2 snistforum.com

21. Ruby Rod (Al 2 O 3 +Cr 2 O 3 ) Xenon Flash Lamp Partially reflecting end face Fully coated (reflecting) end face Laser beam snistforum.com

22. Applications of Ruby Lasers 1.Distance measurement using ‘pulse echo’ technique. 2.Holography 3.Atmospheric ranging, scattering studies and lidar measurement. 4.Trimming resistors 5.Drilling high quality holes 6.Target designators and range finders in military applications etc. snistforum.com

23. A helium-neon laser, usually called a He-Ne laser, is a type of small gas laser. Its usual operation wavelength is 632.8 nm, in the red portion of the visible spectrum. Helium-Neon laser :- Electric discharge snistforum.com

24. 1S 3S 2S 3P 2P snistforum.com

25. The gain medium of the laser is a mixture of helium and neon gases, in a 5:1 to 20:1 ratio, contained at low pressure in a glass envelope. The energy or pump source of the laser is provided by an electrical discharge of around 1000 volts through an anode and cathode at each end of the glass tube. The optical cavity of the laser typically consists of a plane, high-reflecting mirror at one end of the laser tube, and a concave output coupler mirror of approximately 1% transmission at the other end. This process is given by the reaction equation: He* + Ne → He + Ne* + ΔE where (*) represents an excited state, and ΔE is the small energy difference between the energy states of the two atoms, of the order of 0.05 eV. snistforum.com

26. The laser process in a He-Ne laser starts with collision of electrons from the electrical discharge with the helium atoms in the gas. This excites helium from the ground state to the long-lived, metastable excited states. Collision of the excited helium atoms with the ground-state neon atoms results in transfer of energy to the neon atoms, exciting neon electrons into the 3s 2 level. This is due to a coincidence of energy levels between the helium and neon atoms. The number of neon atoms entering the excited states builds up as further collisions between helium and neon atoms occur, causing a population inversion. Spontaneous and stimulated emission between the 3s 2 and 2p 4 states results in emission of 632.82 nm wavelength light, the typical operating wavelength of a He-Ne laser. With the correct selection of cavity mirrors, other wavelengths of laser emission of the He-Ne laser are possible. snistforum.com

27. Applications of He – Ne Lasers 1.All interferometric experiments 2.Metrological applications 3.Bar code reading 4.Image processing 5.Holography snistforum.com

28. CARBON DI-OXIDE LASER HeN2N2 CO 2 To Vacuum Pump NaCl Window Mirror snistforum.com

29. Bending Vibrations of the CO 2 molecule The CO 2 molecules vibrational energies are given by E = (m + ½) h where m = 0, 1, 2, …… m is the degree of excitation, which is an integer value Carbon Oxygen Symmetric stretching Asymmetric stretching 100 020 001 snistforum.com

30. Energy transfer through collision E1E1 E2E2 E3E3 E4E4 E5E5 9.6  m 10.6  m Nitrogen Carbon dioxide 001 100 020 010 snistforum.com

31. The discharge tube has 2.5 cm in diameter and 5 m in length and discharge is produced by DC excitations. Sodium chloride Brewster windows are used at the end. Near confocal silicon mirrors coated with aluminum form the resonant cavity. To remove the dissociation products which may contaminate the laser, the continuous flow of the gas mixture is maintained in the tube. The above Fig. shows the schematic diagram of a CO 2 laser. Along with CO 2 there are also nitrogen and helium gases in the apparatus. Nitrogen helps to increase the population of the upper level of CO 2, while helium helps depopulate and the lower helium helps to conduct heat away to the walls of the discharge tube keeping CO 2 cold. snistforum.com

32. Laser action in CO 2 Laser: When a discharge is passed through the tube, the nitrogen molecules are excited and are raised to higher excited state. The excited energy of nitrogen molecules is transferred to carbon-di- oxide molecules through collisions and carbon-di-oxide molecules are raised to their excited vibrational energy level E 5 (001) from their ground state. The energy level ‘E 5 ’ is a metastable state energy level. Hence there is population inversion. Stimulating photons of wavelength 10.6 μm and 9.6 μm induce the CO 2 molecules to undergo stimulated emission by laser transitions from E 5 to E 4 giving laser wavelength of 10.6 μm and from E 5 to E 3 given laser wavelength of 9.6 μm. snistforum.com

33.  Since the laser transition from E 5 to E 4 has higher gain than from E 5 to E 3, the laser usually oscillates at 10.6 μm.  The CO 2 molecules from E 4 and E 3 are returned to their ground through fast decay and diffusion. snistforum.com

34. Semiconductor Laser: Laser beam Depletion region (Active region) N type P Type P-Type: Ga As doped with Germanium. N-Type: Ga As doped with Tellurium snistforum.com

35. EFEF ++ + + + + + + +++ ++++++ Active region Injection of holes Injection Of electrons EFEF C B VB P - Type N - Type Fig. P – N Junction under forward biased resulting injection and recombination Of charge carriers Current Light output Threshold current Spontaneous Stimulated snistforum.com

36. 1.The semiconductor laser is also called as diode laser and these lasers have important application in fiber optics. 2.The wavelength of emitted light depends upon the energy band gap of the material. E g = h = hc/  = hc/E g When we apply biasing voltage to semiconductor diode then the charge carrier recombination takes place in the diode. In such process population inversion is created in a narrow zone called active region. Homo-junction means that a p – n junction is formed by a single crystalline materials such that the basic material has been the same on both sides of the junction. Hetero-junction means that the material on one side of the junction differs from that on the other side of the junction. Ex:- Hetero junction is formed between Ga As and Ga Al As snistforum.com

37. Draw backs of homo – junction lasers: 1.Threshold current density is very large 2.Only pulsed mode output is obtained. 3.Laser output has large beam divergence. 4.Poor coherence and poor stability. 5.Electromagnetic field confinement is poor Advantages of Hetro-junction laser: 1.Low threshold current density. 2.Output is continuous 3.High output power. 4.Narrow beam, high coherence, high monochromocity 5.Long life time of the device. 6.Highly stable. snistforum.com

38. Applications of LASERS: 1.Communication 2.Computers 3.Industry 4.Scientific Research 5.Military operation 6.Medicine Lasers in communication and Atmospheric science: 1.More amount data can be sent because of large band width. 2.More channels 3.Signals cannot be trapped 4.Highly directional, hence greater potential use in space crafts and submarines. 5.Lidars (Light detection and ranging) to study about atmospheric features, i.e. to measure atmospheric pollutants, Ozone concentration, water vapor concentration. snistforum.com

39. Lasers in computers: 1.In LAN, data transfer from one computer to other for short time. 2.During reading and recording the data on CD’s Lasers in Industry: 1.Blast holes in hard materials like diamond, hard stell etc. 2.Source as intense heat 3.To measure distance to making maps by surveyors 4.To cut teeth saws, drill in surgical needle, guide bulldozers 5.In welding: Purity of the material is not altered. Lasers in Scientific Research: 1.To separate isotopes of uranium. 2.To create plasma, this may help the scientists to control nuclear fusion reaction. 3.To create 3D-photography called holography. 4.Recording and reconstruction of hologram to data storage. 5.Holography in optical signal processing. 6.To produce some chemical reactions 7.To produce monomers to polymers 8.Internal structure of the microorganisms and cells are studied accurately. snistforum.com

40. Lasers in Military applications: 1.To target enemy air plane or ship, to determine its distance. 2.To destroy enemy aircraft and missile. 3.As war weapon. 4.To find the velocity of moving object. 5.Target is judged from the strength and spectral distribution of bounced signal. Lasers in Medicine: 1.To remove diseased body tissues. 2.Retinal detachment by eye specialist. 3.To instantly weld injured muscles, ligaments without use of the heat. 4. Argon and CO 2 lasers are used in liver and lungs treatment. 5.To elimination of moles and tumors on skin tissues. 6.In the treatment of Glaucoma. 7.Argon lasers in – Neuro surgery, Ophthalmology, general surgery, dermatology, gynecology. 8.He-Ne Lasers- Diagnostic applications. 9.Ruby lasers – Ophthalmology, dermotology. snistforum.com