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Laser Info's
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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
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:
A Test on LED and laser will be A Test on LED and laser will be conducted on:conducted on:
2626thth September September
40 objective questions40 objective questions
Group activity 1 (presentation only) = 25% -
done Group activity 2 (open book test) = 25% Test I = 25% Test 2 = 25%
Assignments and Tests
Please study the pass year paper and Please study the pass year paper and all of the ‘typical exam questions’ all of the ‘typical exam questions’ presented to you in the lectures. presented to you in the lectures. There will be 3.5 questions from There will be 3.5 questions from
Optoelectronics Part. Optoelectronics Part. Compulsory for you to answer 2 Compulsory for you to answer 2
questions from both part A and B. questions from both part A and B. Then choose one question from any Then choose one question from any
parts. parts.
Objectives (by the end of the lectures on laser Objectives (by the end of the lectures on laser student will be…)student will be…)
1.1. Able to state the definition of laserAble to state the definition of laser2.2. Able to state the principle of population Able to state the principle of population
inversioninversion3.3. Able to explain the principle of Able to explain the principle of
semiconducting lasersemiconducting laser4.4. Familiarise with the concept of light Familiarise with the concept of light
simulation and polarisationsimulation and polarisation5.5. Able to list down all materials criteria and Able to list down all materials criteria and
materials selection for a given materials selection for a given semiconducting laser compound.semiconducting laser compound.
6.6. Able to highlight several examples of the Able to highlight several examples of the application of laser. application of laser.
Objectives (by the end of the lectures on laser Objectives (by the end of the lectures on laser student will be…)student will be…)
1.1. Able to state the definition of laserAble to state the definition of laser2.2. Able to state the principle of population Able to state the principle of population
inversioninversion3.3. Able to explain the principle of Able to explain the principle of
semiconducting lasersemiconducting laser4.4. Familiarise with the concept of light Familiarise with the concept of light
simulation and polarisationsimulation and polarisation5.5. Able to list down all materials criteria and Able to list down all materials criteria and
materials selection for a given materials selection for a given semiconducting laser compound.semiconducting laser compound.
6.6. Able to highlight several examples of the Able to highlight several examples of the application of laser. 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!
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
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
Fibre Optics Communication
1. Absorption2. Spontaneous Emission3. Stimulated Emission
1. Absorption2. Spontaneous Emission3. Stimulated Emission
3 Mechanisms of Light EmissionFor 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:
Absorption
E1
E2
Spontaneous Emission
Stimulated Emission
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
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
E1
E2
h
(a) Absorption
h
(b) Spontaneous emission
h
(c) Stimulated emission
In hOut
h
E2 E2
E1 E1
Absorption, spontaneous (random photon) emission and stimulatedemission.
© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)
Stimulated Emission
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
Einstein;s
Light or photon must be absorbed in order for
us to have a lasing action I(x) = I(o) exp (-x)
Absorption of Light Through a Medium
I(o) I(x)
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
Absorption
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
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.
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…
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
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
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