LASER (semiconducting Lasers)

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LASER(semiconducting Lasers)

• LASER 1 EBB 424E
• Dr Zainovia Lockman

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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

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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.

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Important Announcement 1Test schedule

• A Test on LED and laser will be conducted on
• 26th September
• 40 objective questions

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Assignments and Tests

• Group activity 1 (presentation only) 25 - done
• Group activity 2 (open book test) 25
• Test I 25
• Test 2 25

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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.

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Lecture Laser

• Objectives (by the end of the lectures on laser
  student will be)
• 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.

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Diode Laser
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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!
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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.

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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.

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Fibre Optics Communication
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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

• Absorption
• Spontaneous Emission
• Stimulated Emission

Therefore 3 process of light emission
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Absorption
E1
E2
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Spontaneous Emission
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Stimulated Emission
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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 of propagation
• In other word, a coherent beam resulted.

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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.

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Stimulated Emission
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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
  Einsteins

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Einsteins
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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)
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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 lt N2)
• Example for tungsten
• ? is typically 106m-1 (ve)
• If we want implication, ? must be ve
• i.e. N2 gt N1

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Population Inversion

• Therefore we must have a mechanism where N2 gt 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.

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3 states system
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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.
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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)

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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

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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