Transcript LASER - Universiti Sains Malaysia

(semiconducting Lasers)
LASER 1 EBB 424E
Dr Zainovia Lockman
LASER
Lecture Contents
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
For the Laser Course You
Need:
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.
Important
Announcement 1:
Test schedule
A Test on LED and laser will be
conducted on:
26th September
40 objective questions
Assignments and Tests
Group activity 1 (presentation only) =
25% - done
Group activity 2 (open book test) =
25%
Test I = 25%
Test 2 = 25%
Information
about the exam
Please study the pass year paper and all of the
‘typical exam questions’ presented to you in the
lectures.
There
will
be
3.5
questions
from
Optoelectronics Part.
Compulsory for you to answer 2 questions
from both part A and B.
Then choose one question from any parts.
Lecture: Laser
Objectives (by the end of the lectures on
laser student will be…)
1.
2.
3.
4.
5.
6.
Able to state the definition of laser
Able to state the principle of population inversion
Able to explain the principle of semiconducting
laser
Familiarise with the concept of light simulation and
polarisation
Able to list down all materials criteria and materials
selection for a given semiconducting laser
compound.
Able to highlight several examples of the
application of laser.
Diode Laser
Typical Application of Laser
The 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!
1. Definition of laser
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.
Another Typical Application
of Laser – Fibre Optics
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.
Fibre Optics Communication
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
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:
1. Absorption
2. Spontaneous Emission
3. Stimulated Emission
Absorption
E1
E2
Spontaneous Emission
Stimulated Emission
Background Physics
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
 In
of propagation
other word, a coherent beam
resulted.
Background Physics
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.
E2
h
h
E2
E2
h
In
h
Out
h
E1
E1
(a) Absorption
(b) Spontaneous emission
E1
(c) Stimulated emission
Absorption, spontaneous (random photon) emission and stimulated
emission.
© 1999 S.O. Kasap,
Optoelectronics
(Prentice Hall)
Stimulated Emission
Background Physics
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
Einstein;s
Absorption of Light Through
a Medium
Light or photon must be absorbed in
order for us to have a lasing action
I(x) = I(o) exp (-x)
I(o)
I(x)
Absorption
Light that falls on a piece of material
will decrease exponentially.
 = (N1-N2)B21(hf) n/c
N1 is often more than N2 (N1 < N2)
Example for tungsten
 is typically 106m-1 (+ve)
If we want implication,  must be –ve
i.e. N2 > N1
Population Inversion
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.
3 states system
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.
Typical Exam Question…
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)
Optical Feedback
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
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