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BE-II SEMESTER ADVANCED PHYSICS

BE-II SEMESTER ADVANCED PHYSICS. UNIT-I LASERS AND WAVE OPTICS DEPARTMENT OF APPLIED PHYSICS. SYLLABUS. Introduction & Quantum Transitions: Absorption, Spontaneous Emission & Stimulated Emission, Metastable State, Population Inversion. Pumping Schemes & LASER Properties,

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BE-II SEMESTER ADVANCED PHYSICS

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  1. BE-II SEMESTER ADVANCED PHYSICS UNIT-I LASERS AND WAVE OPTICS DEPARTMENT OF APPLIED PHYSICS

  2. SYLLABUS • Introduction & Quantum Transitions: Absorption, • Spontaneous Emission & Stimulated Emission, • Metastable State, Population Inversion • Pumping Schemes & LASER Properties, • Spatial and temporal coherence of a light wave • Principle & working of He-Ne LASER

  3. SYLLABUS • Ruby Laser • Semiconductor laser & applications • Introduction, interference in thin film,Wedge • shaped thin film • Newton’s Ring And Antireflection Coatings • Advanced applications of interference in thin film

  4. LEARNING OBJECTIVES • Understand basic laser physics. • Describe the concept of stimulated emission and • what is an active medium. • Learn about the types of laser and their applications. • Know what conditions are necessary for interference • to be observed. • Know how light changes phase when it is reflected • from a surface. • Understand the necessary conditions for interference • to occur in thin films.

  5. LASER L:LIGHT A:AMPLIFICATIONby S:STIMULATED E:EMISSION of R:RADIATION

  6. Difference between Ordinary & laser light Ordinary light Laser light • Incoherent • High Divergence • Low Intensity • Polychromatic • Coherent • Less Divergence • High Intensity • Monochromatic

  7. Characteristics of Laser • Monochromaticity • Coherence • Temporal coherence • Spatial coherence • Intensity • Unidirectionaly • Divergence

  8. Coherence length (lcoh) :The length of the wavetrainupto which it is perfectly sinusoidal . lcoh= c.tcoh= c  • Coherence time(tcoh) : The time for which the wave train is perfectly sinusoidal. • Since  = Δt = 1/ Δטּ lcoh = c  = c. ∆t = c / ∆ν, • We know , ν = c / λ ∴ ∆ν = - c . ∆ λ / λ2 , ignoring minus sign lcoh= λ2 / ∆ λ , Where Δ λ Bandwidth

  9. Three distinct processes can take place. Absorption Spontaneous emission iii) Stimulated emission Quantum processes in Laser:

  10. Absorption and Emission Atom

  11. 1. Absorption • Energy of photon h=E2-E1 ‘ absorb by atoms in the lower energy states and excites to higher energy states. • Einstein Equation for absorption Nab = B12N1ρ(υ)Δt • A + h A*

  12. LASER photon Absorption Energy Excited State Ground State

  13. The process of emission of photons by an excited atoms by its own , without the influence of external agent is called spontaneous emission. A* A + h  Spontaneous Emission Nsp = A21N2Δt , A* A + h (photon) 1. Spontaneous Emission

  14. LASER photon Spontaneous Emission Energy Excited State Ground State

  15. LASER photon photon photon photon photon photon Spontaneous Emission Energy Ground State

  16. 3. Stimulated Emission • The process of emission of photons by an excited • atom through a forced or triggered transition • A* + h = A + 2 h • Nst = B21 N2 ρ(υ) Δt

  17. LASER photon photon photon photon photon photon Stimulated Emission Energy Metastable State Ground State

  18. Comparison between Spontaneous and Stimulated Emission

  19. Condition for Light Amplification • Condition for Stimulated emission to dominate • Spontaneous Emission • 2) Condition for stimulated emission to dominate • absorption transitions This condition indicates that Stimulated transition will overwhelm the absorption process if N2 is greater than N1. The system must achieve the state of population inversion.

  20. N 2 Population Inversion • In thermal equilibrium state, N 1 >> N2 which is governed by Boltzman’s equation N2 /N1=e- ΔE/kT • Population inversion is a condition in which population of upper energy level N2 far exceeds the population of lower energy level N1 i.e. N2 >> N1. N2 N1 N1 Normal State N 2 << N1 Thermal Equilibrium State Inverted StateN 2 >> N1 Population inversion State

  21. Metastable State • Metastable state can be defined as a state where excited atom can remain for longer time than the normal excited state. • Atoms stay in metastable states for about 10-6 to 10-3s. This is 103 to 106 times longer than the time of stay of atom at excited levels. • If the metastable states do not exist, there could be no population inversion, no stimulated emission and hence no laser operation.

  22. Components of Laser The essential components of Laser are • An active medium • A pumping agent • Optical Resonator

  23. An active medium • A medium in which light gets amplified is called an active medium. • The medium may be solid , liquid or gas. • Active centres are those atoms which are responsible for stimulated emission.

  24. Pumping • The process of supplying energy to the medium with a view to transfer it into the state of population inversion is known as pumping. • Techniques to achieve the state of population inversion: • Optical pumping (used in Ruby Laser) • Electric discharge (used in He-Ne Laser) • Direct Conversion (used in Semiconductor Laser)

  25. Fabry - Perot optical resonator (Resonant cavity) Optical resonator consist of two opposing plane parallel mirrors, with an active material placed in between them. One of the mirror is semitransparent while the other is 100 % reflecting. The mirrors are set normal to optic axis of the material. 100 % reflecting mirror Semi-transparent mirror Active medium Optic axis

  26. Action of Optical Resonator

  27. Condition for Steady State Oscillation • For waves making a complete round trip inside the resonator, phase delay must be some multiple of 2. 2L = m  ; ( m = 1,2,3,…) • Length L of the optical resonator should • accommodate an integral number of standing half • waves.

  28. Pumping Schemes Pumping Schemes are Classified as: Two-level Three-level and Four –level schemes. Two-level scheme will not lead to laser action. Three-level and four-level schemes are important and widely employed.

  29. Two Level Pumping Scheme • Pumping radiation excites the ground state atoms. • Induces transitions from the upper level to the • lower level. • Hence, population inversion cannot be attained in • a two-level pumping scheme. E2 Energy E1

  30. Three Level Pumping Scheme Absorption band E Non-radiative transition Metastable state E3 Laser transition E2 Ground state E1 E • Major disadvantage of a three level scheme  • Efficiency is less • Output  Pulsating beam

  31. Four Level Pumping Scheme Stimulated Emission of Radiation E4 Non radiative transition E3 Pumping E2 Natural depletion E1 • Four level lasers are more efficient. • Four level lasers can operate in a Continuous Wave mode. Ground State

  32. Types of LASER Solid-state lasers – Ruby laser, “Nd:YAG“, Nd:Glass, lasers etc Gas lasers - He-Ne, He-Cd, CO2, N2 lasers etc Semiconductor lasers ( diode lasers)-These electronic devices are generally very small and use low power. GaAs, GaAsP

  33. Ruby LASER Fully Silvered Mirror Partially silvered Mirror A helical Photographic flash lamp filled with Xenon 0.5 cm 4 cm Coolant Outlet Inlet H. V. Power Supply • Ruby Laser is a first solid state laser developed • in 1960 by T.H. Maiman • Ruby laser rod; a synthetic Ruby crystal of • Al2O3 doped with 0.05% Cr3+ ions.

  34. Energy level Diagram (Ruby Laser) • Cr3+ atom absorb green and blue bands of wave length from xenon flash lamp & excited to E3 & E3’ respectively • Radiative transitions from E2 to E1 emits  Red photon with peak near 6943 A0. E3’ Non radiative transition E3 Metastable state E2 Stimulated emission Green Energy (ev) Blue Pumping E1 Ground state

  35. He-Ne Laser • First gas laser was developed in 1961 by Ali Javan and his coworkers

  36. LASER Structure of He Ne Laser He-Ne Mixture Partially Reflecting Mirror 100% Reflecting Mirror - + Electrodes

  37. Construction: • Discharge tube of about 50 cm long, 1 cm in diameter, filled with a mixture of He & Ne gases in the ratio of 10:1 which is active medium. • Ne-atoms are active centers- have energy levels suitable for laser transitions • He-atoms is efficient to excite the Ne-atoms. • Energy transfer between He and Ne-atom takes place through collision and the Ne atoms get excited.

  38. Energy Transfer Through Atomic Collisions : Laser transition Helium Neon E6 F3 3.39 µm 20.66eV 20.61eV E5 E4 F2 6328Å 1.15 µm E3 Energy Spontaneous emission (6000Å) E2 Excitation by collision with electrons De-excitation by collision withwalls E1 F1 Energy levels of Helium and Neon atoms and transitions between the levels.

  39. Working: • He atoms are excited to levels F2 & F3 – metastable levels. • E4 & E6 levels in Ne are metastable states  accumulation of atoms takes place in level E6 and E4 • Population inversion can be achieved between:E6 a nd E5, E6 and E3 levels E4 and E3 levels • E6 E3 transitions; laser beam of red colour at 632.8 nm (6328 A) • E4 E3 transitions; laser beam at wavelength of 1150 nm(11500 A ) • E6 E5transitions; laser beam in IR region at 3390nm(33900 A)

  40. Semiconductor Laser R.N. Hall and his coworkers made the first semiconductor laser in 1962. A semiconductor diode laser is a specially fabricated p-n junction device that emits coherent light when it is in forward biased. Current flow Active region Roughened surface Optically flat and parallel faces P type N type Laser output Optically flat and parallel faces

  41. Working: . • Semiconductor laser is heavily doped PN junction diode • When diode is forward biased electron from C.B. recombine with holes in V. B. • During recombination it emits energy in the form of light and junction acts as laser which emites coherence beam of laser light • At low FB. Junction acts as a LED which emits incoherent light • A GaAsP laser emits light of wavelength 9000 Ao in IR region (red)

  42. Energy band structure of a semiconductor diode • When the junction is F.B, electrons and holes are injected across the junction to cause population inversion. • Population inversion is created in a very narrow zone called the active region

  43. Applications of Laser Industrial applications Applications in the field of medical science Astronomical and geophysical applications Metrology applications Applications in communication Defence application Environmental monitoring and Scientific Research

  44. ENTERTAINMENT APPLICATION Laser show

  45. DEFENCE APPLICATION Finger print detection Detection of submarine and mines Laser at war time

  46. OPTICAL COMMUNICATION Frequency in the visible region ~ 1014 cycle/sec Frequency in the microwave region ~ 109 cycle/sec i.e. communication capacity: light wave 105 > microwave

  47. SCIENCE AND TECHNOLOGY Laser fusion Laser Eraser Compact Disk (CD)

  48. HOLOGRAPHY DISPLAY HOLOGRAM: EXHIBIT

  49. SECURITY HOLOGRAM An Embedded Hologram™ cannot be removed, erased, duplicated or simulated by photocopying, photography or scanning.

  50. MEDICAL APPLICATION Brain tumor surgery Eye surgery

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