1. Laser (Light Amplification by Stimulated Emission of Radiation)
BY SUMITESH MAJUMDER

2. What is Laser? Light Amplification by Stimulated Emission of Radiation
A device produces a coherent beam of optical radiation by stimulating electronic, ionic, or molecular transitions to higher energy levels
Mainly used in Single Mode Systems
Light Emission range: 5 to 10 degrees
Require Higher complex driver circuitry than LEDs
Laser action occurs from three main processes: photon absorption, spontaneous emission, and stimulated emission.

3. Properties of Laser Monochromatic
Concentrate in a narrow range of wavelengths (one specific colour).
Coherent
All the emitted photons bear a constant phase relationship with each other in both time and phase
Directional
A very tight beam which is very strong and concentrated.

4. Basic concepts for a laser
Absorption
Spontaneous Emission
Stimulated Emission
Population inversion

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

6. 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.

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

8. 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

9. Stimulated Emission

10. Laser Diode Optical Cavity
One reflecting mirror is at one end while the other end has a partially reflecting mirror for partial emission
Remaining power reflects through cavity for amplification of certain wavelengths, a process known as optical feedback.
Construction very similar to the ELEDs.

11. Mirror Reflections

12. The operation of the Laser

13. The operation of the Laser
(Pumping the Laser)

14. The operation of the Laser
absorption

15. The operation of the Laser
Spontaneous emission

16. The operation of the Laser
Spontaneous emission
Incoherent light
Accidental direction

17. The operation of the Laser

18. The operation of the Laser
Stimulated emission

19. The operation of the Laser
Light: Coherent, polarized
The stimulating and emitted photons have the same:
frequency
phase
direction

20. How a Laser Works

21. Necessary condition: population inversion
Condition for the laser operation
If n1 > n2
radiation is mostly absorbed absorbowane
spontaneous radiation dominates.
if n2 >> n population inversion
most atoms occupy level E2, weak absorption
stimulated emission prevails
light is amplified
Necessary condition: population inversion

22. Population Inversion
A state in which a substance has been energized, or excited to specific energy levels.
More atoms or molecules are in a higher excited state.
The process of producing a population inversion is called pumping.
Examples:
→by lamps of appropriate intensity
→by electrical discharge

23. n1 - the number of electrons of energy E1
Boltzmann’s equation E1 E2
n1 - the number of electrons of energy E1
n2 - the number of electrons of energy E2
example: T=3000 K E2-E1=2.0 eV

24. E2 E1 Einstein’s relation Probability of stimulated absorption R1-2
Probability of stimulated absorption R1-2
R1-2 = r (n)n1 B where spectral density is r (n)
& Einstein coeff. of absorbtion is B1-2
Probability of stimulated and spontaneous emission :
R2-1 = r (n) n2B2-1 + n2A2-1
 Einstein coeff. of stimulated and spontaneous emission are B2-1& A2-1
Assumption : For a system in thermal equilibrium, the upword and downword
transition rate must be equal :
R1-2 = R n1r (n) B1-2 = n2 (r (n) B2-1 + A2-1)

25. According to Boltzman statistics:
r (n) = =
Planck’s law
B1-2/B2-1 = 1

26. The probability of spontaneous emission A2-1 /the probability of stimulated emission B2-1r(n ):  
Visible photons, energy: 1.6eV – 3.1eV.
kT at 300K ~ 0.025eV.
stimulated emission dominates solely when hn /kT <<1!
(for microwaves: hn <0.0015eV)
The frequency of emission acts to the absorption:
if hn /kT <<1.
x~ n2/n1

27. Two-level Laser System
Unimaginable
as absorption and stimulated processes neutralize one another.
The material becomes transparent.

28. Two level system hn hn =E2-E1 Spontaneous emission E2 E1 absorption
Stimulated emission hn E1 E2
hn =E2-E1

29. Three-level Laser System
Initially excited to a short-lived high-energy state .
Then quickly decay to the intermediate metastable level.
Population inversion is created between lower ground state and a higher-energy metastable state.

31. Three-level Laser System
Nd:YAG laser
He-Ne laser

32. Four-level Laser System
Laser transition takes place between the third and second excited states.
Rapid depopulation of the lower laser level.

33. Four-level Laser System
Ruby laser

34. Multimode Laser Output Spectrum
(Center Wavelength)
Mode
Separation
g(λ)
Longitudinal
Modes

35. Lasing Characteristics
Lasing threshold is minimum current that must occur for stimulated emission
Any current produced below threshold will result in spontaneous emission only
At currents below threshold LDs operate as ELEDs
LDs need more current to operate and more current means more complex drive circuitry with higher heat dissipation
Laser diodes are much more temperature sensitive than LEDs

36. Fabry-Perot Laser (resonator) cavity

37. Modulation of Optical Sources
Optical sources can be modulated either directly or externally.
Direct modulation is done by modulating the driving current according to the message signal (digital or analog)
In external modulation, the laser is emits continuous wave (CW) light and the modulation is done in the fiber

38. Types of Optical Modulation
Direct modulation is done by superimposing the modulating (message) signal on the driving current
External modulation, is done after the light is generated; the laser is driven by a dc current and the modulation is done after that separately
Both these schemes can be done with either digital or analog modulating signals

39. Direct Modulation
The message signal (ac) is superimposed on the bias current (dc) which modulates the laser
Robust and simple, hence widely used
Issues: laser resonance frequency, chirp, turn on delay, clipping and laser nonlinearity

40. Laser Construction A pump source A gain medium or laser medium.
Mirrors forming an optical resonator.

41. Laser Construction

42. Pump Source Provides energy to the laser system
Examples: electrical discharges, flashlamps, arc lamps and chemical reactions.
The type of pump source used depends on the gain medium.
→A helium-neon (HeNe) laser uses an electrical discharge in the helium-neon gas mixture.
→Excimer lasers use a chemical reaction.

43. gain medium
Major determining factor of the wavelength of operation of the laser.
Excited by the pump source to produce a population inversion.
Where spontaneous and stimulated emission of photons takes place.
Example:
solid, liquid, gas and semiconductor.

44. Optical Resonator Two parallel mirrors placed around the gain medium.
Light is reflected by the mirrors back into the medium and is amplified .
The design and alignment of the mirrors with respect to the medium is crucial.
Spinning mirrors, modulators, filters and absorbers may be added to produce a variety of effects on the laser output.

45. Laser Types According to the active material:
solid-state, liquid, gas, excimer or semiconductor lasers.
According to the wavelength:
infra-red, visible, ultra-violet (UV) or x-ray lasers.

46. Solid-state Laser Example: Ruby Laser
Operation wavelength: nm (IR)
3 level system: absorbs green/blue
Gain Medium: crystal of aluminum oxide (Al2O3)
with small part of atoms of aluminum is replaced
with Cr3+ ions.
Pump source: flash lamp
The ends of ruby rod serve as laser mirrors.

47. Ruby laser
Al2O3
Cr+
Energy
rapid decay
LASING

48. How a laser works?

49. Ruby laser
First laser: Ted Maiman
Hughes Research Labs
1960

50. Liquid Laser Example: dye laser
Gain medium: complex organic dyes, such as rhodamine 6G, in liquid solution or suspension.
Pump source: other lasers or flashlamp.
Can be used for a wide range of wavelengths as the tuning range of the laser depends on the exact dye used.
Suitable for tunable lasers.

51. dye laser Schematic diagram of a dye laser
A dye laser can be considered to be basically a four-level system.
The energy absorbed by the dye creates a population inversion, moving the
electrons into an excited state.

52. Gas Laser Example: Helium-neon laser (He-Ne laser)
Operation wavelength: nm
Pump source: electrical discharge
Gain medium : ratio 5:1 mixture of helium and neon gases

53. He-Ne laser

54. Semiconductor laser
Semiconductor laser is a laser in which semiconductor serves as photon source.
Semiconductors (typically direct band-gap semiconductors) can be used as small, highly efficient photon sources.

55. Applications of laser 1. Scientific
a. Spectroscopy b. Lunar laser ranging
c. Photochemistry d. Laser cooling
e. Nuclear fusion

56. Applications of laser 2 Military a. Death ray
b. Defensive applications
c. Strategic defense initiative
d. Laser sight
e. Illuminator
f. Rangefinder
g. Target designator

57. Applications of laser
3. Medical
a. eye surgery
b. cosmetic surgery

58. Applications of laser 4. Industry & Commercial
a. cutting, welding, marking
b. CD player, DVD player
c. Laser printers, laser pointers
d. Photolithography
e. Laser light display