1. Laser

3. LASER L ight A mplification by S timulated E mission of R adiation
C.H. Towns, 1954
T.H. Maiman, 1960
Noble prize, 1964

4. Incandescent vs. Laser Light
Many wavelengths
Multidirectional
Incoherent
Monochromatic
Directional
Coherent

5. Coherence

6. Coherence
Temporal coherence
Coherent length l0 = λ2/Δλ = cT0

7. Spatial Coherence
L should be smaller than λD/a

8. Spatial and Temporal Coherence
Incoherence
Beams can be coherent or only partially coherent (indeed, even incoherent) in both space and time.

9. Monochromaticity Degree of non monochromaticity τ = Δν/ν = c/l0ν=1/νT0
Quality factor = λ/Δλ = l0/λ
Ideally coherent: same energy, momentum,
polarization

10. Absorption and Emission
Excitation potential and critical potential

11. Stimulated Absorption
Energy is absorbed by an atom, the electrons are excited into vacant energy shells.

12. Absorption E1 E2

13. Spontaneous Emission
The atom decays from level 2 to level 1 through the emission of a photon with the energy hv. It is a completely random process.

14. Spontaneous Emission

15. Stimulated Emission atoms in an upper energy level can be triggered or
stimulated in phase by an incoming photon of a specific
energy.

16. Stimulated Emission

17. Stimulated Emission The stimulated photons have unique properties:
In phase with the incident photon
Same wavelength as the incident photon
Travel in same direction as incident photon

18. Electron/Photon Interactions

19. WHY WE NEED META STABLE STATE?
ANSWER IS With having the metastable state above the ground level. Atom reaches the meta stable state (after first stimulated emission) can remain there for longer time period. So the number of atom increases in the meta stable state. And when these atoms come back to the original ground level it emits laser beam.

20. Einstein’s Coefficients

21. Spontaneous emission
A21 :- correspond to spontaneous emission probability per unit time This particular emission can occur without the presence of external field E(v)

22. Stimulated Absorption
B12 :- correspond to stimulated absorption probability per unit time
This type of absorption can occur in presence of external field E(v) only

23. Stimulated Emission
B21 :- correspond to stimulated emission probability per unit time This type of emission can occur in presence of external field E(v) only

24. Total Emission Probability
Spontaneous Emission + Stimulated Emission
A21 + B21 E(v)
Number of atoms that can jump from level E2 to E1 is

25. Total Absorption Probability

26. Equilibrium condition
In case of energy states the number of electron absorbed and emitted should be equal or the rate of change of numbers of atoms in two states should be equal.

27. The rate of change of atoms in E2
It can be given by differentiation
(probability) or

28. At Equilibrium
Then

29. Emission and absorption are same

30. Maxwell Bolzman Distribution
In thermal equilibrium

31. So the equations become
So equation becomes

32. Planck’s Radiation Law
Plank’s gives the formula that how a gas radiate energy.

33. Einstein’s Coefficients
Einstein gives a probability that stimulated emission is same as absorption. Means that if a stimulated absorption can occur then there is same probability that stimulated emission can occur.

34. After comparing with Planks Radiation Law
And

35. Conclusions
Stimulated emission have same probability as stimulated absorption
Ratio between spontaneous and stimulated emission varies with v3
All we need is to calculate one of the probability to find others.

36. BASIC PRINCIPLE NEEDED FOR LASER

37. POPULATION INVERSION
A state of a medium where a higher-lying electronic level has a higher population than a lower-lying level

38. PUMPING
The method particle of raising a particle from lower energy state to higher energy state is called pumping.
TYPES OF PUMPING :
Optical pumping
Electrical pumping
X-ray pumping
Chemical pumping

39. LASER COMPONENTS All lasers have 3 essential components:
A lasing or "gain" medium
A source of energy to excite electrons in the gain medium to high energy states, referred to as "pump" energy
An optical path which allows emitted photons to oscillate and interfere constructively as energy is added or "pumped" into the system, ie, a resonator

41. LASER ACTION

42. Types of Laser According to their sources: Gas Lasers Crystal Lasers
According to their sources:
Gas Lasers
Crystal Lasers
Semiconductors Lasers
Liquid Lasers
According to the nature of emission:
Continuous Wave
Pulsed Laser
According to their wavelength:
Visible Region
Infrared Region
Ultraviolet Region
Microwave Region
X-Ray Region
d. According to different levels
1. 2-level laser
2. 3-level laser
3. 4-level laser
e. According to mode of pumping
1. optical
2. chemical
3. electric discharge
4. electrical

43. 2- Level Laser E2 E2 E1 E1 E2 E1 Spontaneous Absorption Emission
Stimulated
Emission

44. THREE STEP LASER Stimulated absorption
Spontaneous emission to the meta stable state
Stimulated emission from meta stable state to ground state. E2 E1
E2 – E1
META STABLE STATE
E1 – E0 E0

45. 4-Level LASER

46. PRACTICAL LASERS

47. He-Ne LASER
Construction
Helium: Neon= 10:1

48. Energy Level Diagram of He-Ne.
Energy Level Diagram of He-Ne
Power output: mW

49. He: Ne LASER

50. Nd:YAG LASER

51. Diode LASER

52. Emission wavelengths of common lasers

53. APPLICATION S OF LASER

54. Not to be Taken Lightly The Weighty Implications of Laser Technology
Nuclear fusion
Long distance measurement
Holography
Medical
Optical Surgery
General Surgery
Tattoo removal
Entertainment
CD Players
Applications of Laser
Technology
DVD Players
Video Game Systems
Telecommunications
Information tech.
Holograms
Satellites
Military
Weapons
Radar
Industry
2017/9/22

55. Can You See the Light?
Military and Space aircraft are equipped with laser guns
Airplanes are equipped with laser radar
Bad eyesight can be corrected by optical surgery using lasers
Cd-Rom discs are read by lasers
Dentists use
laser drills
Tattoo removal is done using lasers
Laser tech. is used in printers, copiers, and scanners
DVD players read DVD’s using lasers
Laser pointers can enhance presentations
CD-Audio is read by a laser
Bar codes in grocery stores are scanned by lasers
Video game systems such as PlayStation 2 utilize lasers
2017/9/22

56. THANKS SEE YOU IN NEXT LECTURE