Semiconductor laser by Kunsa Haho of ethiopia

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

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

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

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.

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.

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.

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.

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

F.g schematical description of basic components of laser

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.

List of solid state lasers
•
•

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 ……

• 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

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.

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

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

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

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

Fig. Fundamental construction of a laser

There are three basic processes through which EM
radiation can interact with matter.
•

Fig.excitation and emission of particles

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

f.g.Families of semiconductor lasers

•
Fig.Schematic diagram of a double-heterostructure where the active medium
(hatched area) consists of GaAs,(a), and InGaAsP, (b).

Spontaneous and stimulated emission in
semiconductor laser
•

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

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.

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

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

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

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

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.

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.

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

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.

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)

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

Referecnces:
1.Prasanta Kumar Basu,Bratati Mukhopadhyayand Rikmantra Basu,Semiconductor Laser
Theory
2.Orazio Svelto , Principles of Lasers-Springer US (2010)
3.(Springer Series in Optical Sciences) Takahiro Numai - Fundamentals of Semiconductor
Lasers-Springer (2004)
4.W. Koechner - Solid-State Lasers - A Graduate Text ( 2003)
5.Anil,Kumar Maini,Lasers and optoelectronics
6.Hans Joachim Eichler Jürgen Eichler Oliver Lux,Lasers Basics, Advances and
Applications
7.Manijeh Razeghi,Fundamentals of Solid State Engineering,3rd Edition

Semiconductor laser by Kunsa Haho of ethiopia

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