The document provides information about lasers and semiconductor lasers, including: 1. It defines lasers and discusses population inversion, which is required for lasing action. Semiconducting lasers use a 3-state system to achieve population inversion. 2. Semiconducting lasers are multilayer semiconductor devices that generate coherent, monochromatic light through stimulated emission. 3. One application is in fibre optic communication, where semiconductor lasers provide a coherent light source that reduces information loss during transmission.

2. 
 Definition of lasers
 Emission and absorption of radiation
 Population Inversion
 Semiconducting lasers
 Materials used for semiconducting laser
 Laser for fibre optics communication
 Quantum Well devices
Lecture Contents

3.  A general reading on lasers:
 A photocopy from a book by Watson p23-64 (easy read)
 Population Inversion and Diode Laser:
 A photocopy from Wilson and Hawkes
 p 169- 182 (more advance reading)
 P 204-223 (more advance reading)
 A general reading + the optical fibre application + on
laser diode
 A photocopy from Kasap
 p.159-166 (optical fibre)
 P.181-196
 EBB 424 Lecture Presentation
 EBB 424 Short Lecture Notes summarising all of the
above.
For the Laser Course You Need:

4. A Test on LED and laser will be conductedA Test on LED and laser will be conducted
on:on:
2626thth
SeptemberSeptember
40 objective questions40 objective questions

5. 
 Group activity 1 (presentation only) = 25% - done
 Group activity 2 (open book test) = 25%
 Test I = 25%
 Test 2 = 25%
Assignments and
Tests

6. Please study the pass year paper and all of thePlease study the pass year paper and all of the
‘typical exam questions’ presented to you in the‘typical exam questions’ presented to you in the
lectures.lectures.
There will be 3.5 questions from OptoelectronicsThere will be 3.5 questions from Optoelectronics
Part.Part.
Compulsory for you to answer 2 questions fromCompulsory for you to answer 2 questions from
both part A and B.both part A and B.
Then choose one question from any parts.Then choose one question from any parts.

7. Objectives (by the end of the lectures on laser studentObjectives (by the end of the lectures on laser student
will be…)will be…)
1.1. Able to state the definition of laserAble to state the definition of laser
2.2. Able to state the principle of population inversionAble to state the principle of population inversion
3.3. Able to explain the principle of semiconducting laserAble to explain the principle of semiconducting laser
4.4. Familiarise with the concept of light simulation andFamiliarise with the concept of light simulation and
polarisationpolarisation
5.5. Able to list down all materials criteria and materialsAble to list down all materials criteria and materials
selection for a given semiconducting laser compound.selection for a given semiconducting laser compound.
6.6. Able to highlight several examples of the applicationAble to highlight several examples of the application
of laser.of laser.

8. 
Diode Laser

9. Typical Application of
LaserThe detection of the binary data stored in the form of pits on the compact disc is
done with the use of a semiconductor laser. The laser is focused to a
diameter of about 0.8 mm at the bottom of the disc, but is further focused to
about 1.7 micrometers as it passes through the clear plastic substrate to strike
the reflective layer. The reflected laser will be detected by a photodiode. Moral
of the story: without optoelectronics there will no CD player!

10. 
 A laser is a device that generates light by a
process called STIMULATED EMISSION.
 The acronym LASER stands for Light
Amplification by Stimulated Emission of
Radiation
 Semiconducting lasers are multilayer
semiconductor devices that generates a
coherent beam of monochromatic light by
laser action. A coherent beam resulted which
all of the photons are in phase.
1. Definition of
laser

11.  An example of application is for the light source for
fibre optics communication.
 Light travels down a fibre optics glass at a speed, =
c/n, where n = refractive index.
 Light carries with it information
 Different wavelength travels at different speed.
 This induce dispersion and at the receiving end the
light is observed to be spread. This is associated
with data or information lost.
 The greater the spread of information, the more loss
 However, if we start with a more coherent beam then
loss can be greatly reduced.
Another Typical Application of
Laser – Fibre Optics

12. Fibre Optics Communication

13. 
1. Absorption
2. Spontaneous Emission
3. Stimulated Emission
1. Absorption
2. Spontaneous Emission
3. Stimulated Emission
3 Mechanisms of Light Emission
For atomic systems in thermal equilibrium with their
surrounding, the emission of light is the result of:
Absorption
And subsequently, spontaneous emission of energy
For atomic systems in thermal equilibrium with their
surrounding, the emission of light is the result of:
Absorption
And subsequently, spontaneous emission of energy
There is another process whereby the atom in an upper energy
level can be triggered or stimulated in phase with the an
incoming photon. This process is:
Stimulated emission
It is an important process for laser action
There is another process whereby the atom in an upper energy
level can be triggered or stimulated in phase with the an
incoming photon. This process is:
Stimulated emission
It is an important process for laser action
Therefore 3 process
of light emission:

14. 
Absorption
E1
E2

15. 
Spontaneous
Emission

16. 
Stimulated
Emission

17.  In 1917 Einstein predicted that:
 under certain circumstances a photon incident
upon a material can generate a second photon of
 Exactly the same energy (frequency)
 Phase
 Polarisation
 Direction of propagation
 In other word, a coherent beam resulted.
Background
Physics

18. 
 Consider the ‘stimulated emission’ as shown
previously.
 Stimulated emission is the basis of the laser
action.
 The two photons that have been produced
can then generate more photons, and the 4
generated can generate 16 etc… etc… which
could result in a cascade of intense
monochromatic radiation.
Background
Physics

19. 
E1
E2
hυ
(a) Absorption
hυ
(b) Spontaneous emission
hυ
(c) Stimulated emission
In
hυ
Out
hυ
E2 E2
E1 E1
Absorption, spontaneous (random photon) emission and stimulated
emission.
© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)

20. 
Stimulated
Emission

21. 
 In a system, all three mechanisms occur.
 However the stimulated emission is very very
sluggish compared to the spontaneous emission
 We need to have a much stimulated emission as
possible for lasing action
 How?
 Refer to the board for the derivation of the
Einstein’s
Background
Physics

22. 
Einstein;s

23. 
 Light or photon must be absorbed in order for us to
have a lasing action
 I(x) = I(o) exp (-αx)
Absorption of Light Through a
Medium
I(o) I(x)

24. 
 Light that falls on a piece of material will decrease
exponentially.
 α = (N1-N2)B21(hf) n/c
 N1 is often more than N2 (N1 < N2)
 Example for tungsten
α is typically 106
m-1
(+ve)
 If we want implication, α must be –ve
 i.e. N2 > N1
Absorption

25.  Therefore we must have a mechanism where N2> N1
 This is called POPULATION INVERSION
 Population inversion can be created by introducing a so call metastable
centre where electrons can piled up to achieve a situation where more N2
than N1
 The process of attaining a population inversion is called pumping and the
objective is to obtain a non-thermal equilibrium.
 It is not possible to achieve population inversion with a 2-state system.
 If the radiation flux is made very large the probability of stimulated emission
and absorption can be made far exceed the rate of spontaneous emission.
 But in 2-state system, the best we can get is N1 = N2.
 To create population inversion, a 3-state system is required.
 The system is pumped with radiation of energy E31 then atoms in state 3 relax
to state 2 non radiatively.
 The electrons from E2 will now jump to E1 to give out radiation.
Population
Inversion

26. 
3 states system

27. 
Population
Inversion
When a sizable population of electrons resides in upper levels,
this condition is called a "population inversion", and it sets the
stage for stimulated emission of multiple photons. This is the
precondition for the light amplification which occurs in a LASER
and since the emitted photons have a definite time and phase
relation to each other, the light has a high degree of coherence.

28. 
 Define the term population inversion for a
semiconducting laser (diode) explain what is the
condition of population inversion.
 Why is population inversion required for a lasing
action?
(40 marks)
Typical Exam
Question…

29. 
 The probability of photon producing a stimulated
emission event can be increased by reflecting back
through the medium several times.
 A device is normally fashioned in such a way that
the 2 ends are made higly reflective
 This is term an oscillator cavity or Fabry Perot cavity
Optical Feedback

30. 
Therefore in a
laser….
Three key elements in a laser
•Pumping process prepares amplifying medium in suitable state
•Optical power increases on each pass through amplifying medium
•If gain exceeds loss, device will oscillate, generating a coherentoutput