Phd student presentation assignment at Adds Ababa university,physics department

1. Course Title: Interaction of Light With Matter
[Phys7132]
Assignment‐3
Title of Presentation: Semiconductor Lasers
By: Kunsa Haho ID: pgr/01947/12
Addis Ababa, Ethiopia
November , 2012 E.C

2. Goal of presentation
 Under standing the principle of semiconductor
lasers
 To discuss different application of semiconductor
lasers
 To distinguish between semiconductor lasers and
other type of lasers

3. Outline of Presentation
1.Introduction to lasers
 Definition laser
 Properties of Laser Light
 Basic components of lasers
 Classification of lasers
 working principle of lasers
2.Semiconductor lasers
 Classification of semiconductor lasers
 Emission of radiation in semiconductor laser
 Special features of semiconductor lasers
 Dis‐advantage of semiconductor lasers
 Materials of semiconductor lasers
 Main applications of Semiconductor Lasers

4. 1.Introduction to lasers
i. Definition
LASER is an acronym for Light Amplification by Stimulated
Emission of Radiation.
• Laser is a device that amplifies or increases the
intensity of light and produces a highly directional,
monochromatic coherent light which is used as a light
source for various optical and electronic devices.
• Laser light has extra‐ordinary properties which are not
present in the ordinary light sources like sun and
incandescent lamp.

5. Properties of Laser Light
I. Directionality:‐ lasers emit radiation in a highly
directional, collimated beam with a low angle of
divergence i.e. energy carried by laser beam can
be collected easily and focused onto a small area.
II. Monochromaticity:‐This property is due to the
following two circumstances:
i.Only an e.m. wave of frequency (E2‐E1) can be
amplified and
ii.Since the two‐mirror arrangement forms a
resonant cavity, oscillation can occur only at the
resonance frequencies of this cavity.

6. III. Coherence:‐ It means that the spatial and
temporal variation of the electric field of the two
waves for every point on the beam cross‐section
is the same(i.e stimulated emission).
• Brightness:‐ is define as the power emitted per
unit area per unit solid angle. Thus, lasers have a
higher brightness than any other light source.

7. Basic components of laser
A laser consists of three important components:
1. Laser medium : The laser medium is a medium
where spontaneous and stimulated emission of
radiation takes place
2. A pump source or energy source is the part of a
laser system that provides energy to the laser
medium.
Pumping is to achieve population inversion.

8. Most commonly used pump sources are as follows:
• Optical pumping
• Electric discharge or excitation by electrons
• Inelastic atom‐atom collisions
• Thermal pumping
• Chemical reactions
3.Optical resonator: The laser medium is
surrounded by two parallel mirrors which provides
feedback of the light. One mirror is fully reflective
whereas another one is partially reflective
These two mirrors as a whole is called optical
resonator or optical cavity or resonating cavity

9. F.g schematical description of basic components of laser

10. Classification of lasers
The lasers can be classified in a number of ways, as
given below.
A.On the basis of state of the laser medium
A Solid state lasers: in this type of lasers
Laser medium is solid i.e. glass or crystalline materials
are used
Pumping Source is light energy . Light sources such as
flashtube, flash lamps, arc lamps, or laser diodes are
used
solid lasers use the energy levels of atoms embedded in
a host material.

11. List of solid state lasers
i.Ruby laser (i.e first solid‐state laser ),
ii. Titanium‐sapphire Laser( )
iii.Neodymium‐doped solid state lasers
• Nd: YAG (neodymium doped with yttrium
aluminium garnet i.e. ) laser,
• Nd : Glass
• Nd : YLF (Nd‐doped lithium yttrium fluoride laser
i.e. YLiF4).
• Nd :YVO4

12. cont”ed‐‐‐‐
iv.YAG‐lasers
• Yb :YAG (Ytterbium‐doped YAG laser)
• Er:YAG(Erbium‐doped YAG lasers)
• Pr:YAG(Praseodymium‐doped YAG laser)
v.Tunable solid state lasers
• Alexandrite =Cr:LiSAlF laser (chromium‐doped lithium
strontium aluminum fluoride laser)
• Cr:LiCaF laser (chromium‐doped lithium calcium fluoride
laser)
vi.Fiber laser
• fiber laser
• fiber laser
• fiber laser
• etc

13. Semiconductor lasers: are different from solid‐state
lasers.
Pump source :electrical energy is used
And there is no host medium for semiconductor
lasers
Semiconductor lasers are also known as laser diodes
• e.g. (GaAs) laser
We will see this type of laser in detail later ……

14. • A liquid laser is a laser that uses the liquid as laser
medium. Liquid lasers use the energy levels of
atoms or molecules dissolved in a liquid.
Pump source : light energy
A dye laser is an example of the liquid laser. A dye
laser is a laser that uses an organic dye (liquid
solution) as the laser medium
Example of dye lasers such as 7‐hydroxycoumarin

15. A gas laser is a laser in which an electric current is
discharged through a gas inside the laser medium
to produce laser light. In gas lasers, the laser
medium is in the gaseous state.
Stimulated transitions occur in atoms between
electronic states and in molecules between
rotational, vibrational, or electronic states.

16. List of gas lasers
 Helium– Neon (He‐Ne) lasers is the first gas laser
 Argon ion lasers,
 Carbon dioxide lasers (CO2 lasers is source
of infrared radiation)
 Carbon monoxide lasers (CO lasers),
 Nitrogen lasers,
 Hydrogen lasers,
 Excimer laser(generates intense UV radiation) like
KrF excimer laser and others

17. B.On the basis of manner of pumping
• Flash light lasers: e.g solid state lasers
• Chemical action lasers:‐ e.g.dye lasers
• Electric discharge lasers: semiconductor lasers

18. C.On the basis of nature of output
• Pulsed wave lasers
• Continuous (or direct) wave lasers
D.On the basis of spectral region of light
• Ultraviolet lasers
• Visible lasers: such as GaAsP laser
• Infrared lasers: such as GaAs laser

19. ii.working principle of lasers
A number of conditions must be satisfied to achieve
lasing action. They are listed below:
1. Population inversion
2. Optical resonator
3. Lasing medium
4. Means of excitation
5. Host medium

20. Fig. Fundamental construction of a
laser

21. There are three basic processes through
which EM radiation can interact with matter.
1. Spontaneous Emission: the process by which
electrons in the excited state return to the
ground state by emitting photons.
The corresponding rate equation is
Where, is called the Einstein A‐coefcient and is
related to the spontaneous emission lifetime by
, .

22. 2. Absorption: is the process by which electrons in
the ground state absorbs energy from photons
to jump into the higher energy level.
The rate of absorption is given by
where:
• B12 is the Einstein B‐coeffcient for absorption
• is the energy density of the incident
photon Flux

23. 3.Stimulated Emission: is the process by which
incident photon interacts with the excited
electron and forces it to return to the ground
state.
The corresponding rate equation is
Where, is the Einstein B‐coefcient for
stimulated emission.

24. Fig.excitation and emission of particles

25. In thermal equilibrium. In the time average, the ratio
N2/N1 is a constant. Therefore, the absorption rate
has to be equal to the emission rate,
= +

26. 2.Semiconductor lasers
• In semiconductor lasers, the light emission is
obtained through a p‐n junction as a result of
recombination of electrons and holes.
F.g.Typical structure of semiconductor laser

27. f.g.Families of semiconductor lasers

28. Homostructure junction laser (2 m ‐30 m):‐
In this type of laser, the pumping process is
achieved in a p‐n junction where both p‐type and n‐
type regions, being of the same.
The active medium is the junction region between
an n‐doped and a p‐doped part of a crystal.
This was the first semiconductor laser type.
Example: GaAs junction laser, containing an n‐
GaAs/p‐ GaAs junction.

29. Double‐heterostructure laser (2 m ‐30 m):‐ : The
active medium is an undoped film embedded in n‐
and p‐doped materials
Example: GaAs/GaAlAs laser, containing the layers
n GaAlAs/GaAs/p GaAlAs.
Fig.Schematic diagram of a double-heterostructure where the active medium (hatched
area) consists of GaAs,(a), and InGaAsP, (b).

30. Quantum well laser (0.3 m ‐2 m):‐ : The active
medium is an undoped quantum film. Adjacent to
the quantum film, there is on one side n‐doped
material and on the other side p‐doped material.
The materials act as injectors of electrons and
holes,respectively.
Quantum well lasers are available for
Visible
 near infrared, and
near UV spectral range
And they are dominate presently the
semiconductor laser field with respect to
applications.

31. Quantum wire laser:‐ Quantum wire lasers, with
quantum wires embedded in injector material, are
in the first stage of realization.
Quantum dot laser: This type of a bipolar
semiconductor laser is being developed.
Quantum cascade laser (QCL): This laser type is
presently in a very active state of development.
Frequency range: (2 m ‐28 m) ,as cooled (70 m
300 m):‐

32. • Superlattice Bloch laser (=Bloch laser =Bloch
oscillator): This type of laser exists only as an idea
on the basis of theoretical studies. The active
element is a doped semiconductor superlattice.
The superlattice is composed of two different
semiconductor materials, for instance, GaAs and
AlAs
• Wave length range: (100 m ‐1mm) it is
hyphotetical

33. Spontaneous and stimulated emission
in semiconductor laser
Spontaneous emission:‐when the external voltage is
applied ,the semiconductor material allows the carrier
recombination with the emission of light in the
depletion region this leads to light emitted diode and
semiconductor laser.
Spontaneous emission give rise to light emitted diode
(LED) i.e. energy released is equal to band gap energy

34. F.g.Recombnation of carriers under
forward bias in P‐N junction

35. Stimulated emission
 In semiconductor laser both n+ and p+ type
materials are heavly doped.Due to that the
population inversion is already achieved.
 Heavy p‐doping causes the ferim level to enter in
to the V.B in p‐side and whereas heavy n‐doping
causes ferim level to enter in to the C.B in n side.
 These are called quasi‐ ferm level.

36. Fig.Carrier combination under forward bias which give
rise to stimulated emission and acts as semiconductor
laser

37. The condition for stimulating emission in a
semiconductor is depends on both quasi‐ferim
level separation energy and bandgap energy
Thus any radiation energy more than confined
to active region will be amplified.
So, heavy doping (generative doping) of P‐N
junction provides stimulation and acts as a
semiconductor laser.

38. Fig.above the threshold current P‐N junction starts lasing
action and become semiconductor laser

39. Difference between LED and
semiconductor laser
LED Semiconductor laser
Operation current Below threshold(low) Above threshold(high)
Phase of emitted photon Random Coherent
Spectral Line width Wide Very narrow
Optical mode Multimode source Monomode source
Internal quantum efficiency low high
Brightness Increases linearly Increases suddenly
Light emission Weak Intense
Internal quantum efficiency=

40. consider two discrete energy levels and let us see
Radiative transitions three processes: absorption,
stimulated and spontaneous emission.

41. The transition rate of absorption (number of
transitions per m3) is equal to
Whrere,
= spectral energy density of the radiation in
energy scale
=Einstein coefficient of absorption (in units of
3 −1) in energy scale
(E)=probability that level 1 is occupied;
=probability that level 2 is empty.
(E)=probability that level 2 is occupied.

42. The rate of stimulated emission processes is
equal to
Where,
=h =Einstein coefficient of stimulated
emission in energy scale.
=probability that level 1 is empty.
The rate of spontaneous emission processes is
equal to
,

43. The occupation probability of level 2 is given by the
Fermi–Dirac distribution
= 1
Where, is the quasi‐Fermi energy of the electrons
in the conduction band and T is the lattice temperature.
The occupation probability of level 1 is
= 1
is the quasi‐Fermi energy of the electrons in the
valence band.

44. At thermal equilibrium, the transition rates of
upward and downward transitions are equal
,
and the Fermi energies coincide,
= =
It follows that,
This must be equal to the expression given by
Planck’s radiation law i.e

45. The comparison yields
and
We find again the Einstein relations.

46. Special features of semiconductor
lasers
Semiconductor lasers exhibit many features and
advantages over other forms of lasers:
1.compactness:almost all laser are tiny with size
below 1mm3 and with light weight.
2.Excitation by bias: lasers are pumped by
electrically pumped(with bias voltage few volts and
drive current is few milampers).in contrast other
lasers need optical power or electrical discharge.

47. 3.Room temperature operation: the device
operate at room temperature and emit continuous
waves
4.Wide wave length coverage:‐ semiconductor
lasers can cover a very wide range of wavelength
from ultra violate to far infrared.
5.Wide gain band width:‐ it shows a high gain over
a wide wavelength range.single semiconductor
laser can be tuned within the gain band width
range.
6.Direct modulation:the intensity or frequency
can be modulated directly by changing the bias
current.

48. 7. High coherence
8. Generation of ultrashort optical pulses: It is
possible to generate ultrashort optical pulses of
subnanosecond to picosecond width by means of
gain switching and mode locking
9. Mass producibility
10.High reliability: have a long lifetime
11.Monolithic integration: many lasers on a
substrate

49. Dis‐advantage of semiconductor lasers
• Temperature characteristics: output power
change sensitively with change in ambient
temperature.
• Noise characteristics
• Divergent output beam:An external lens is
required to obtain a collimated beam.

50. Semiconductor laser materials
Common materials for semiconductor lasers are
• GaAs (gallium arsenide)
• AlGaAs (aluminum gallium arsenide)
• GaP (gallium phosphide)
• InGaP (indium gallium phosphide)
• GaN (gallium nitride)
• InGaAs (indium gallium arsenide)
• GaInNAs (indium gallium arsenide nitride)
• InP (indium phosphide)
• GaInP (gallium indium phosphide)

51. Application of semiconductor lasers
1. Optical Communication e.g.fiber optical
communication
2. Medicine :for bloodless surgery,removal of
tumors,infected cells
3. Measurements of long distances
4. Nuclear fusion
5. Scientific research
6. National defence
7. Weather forcasting
8. Industry
9. etc

52. Thank you for
attention!