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Light sources of optical fiber

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					Light sources of optical fiber
             Group Members
     Hafsa Ahmed Roll No.2k7/TC/27
      Aisha Aziz Roll No.2k7/TC/12
      Qurat-ul-ain Roll No.2k7/TC/64
              Light sources

Light sources used for optical systems
 must be at wavelenghts efficiently
 propagated by the optical fiber.
Light sources produces sufficient power to
 allow light to propagate through the fiber
 without causing distortion in the cable or in
 the reciever.
               Light sources

LAMP (Tungsten)
Spectral width > 1000 nm
LED
Spectral width 30 – 50 nm
ILD
  Spectral width 1 – 3 nm
Light sources
   List of Light sources in optical fiber

LEDs (Light Emitting Diodes)
ILD (Injection Laser Diodes)
Light Detectors
Lasers
                 LEDs

An LED is a p-n junction diode, usually
 made from a semiconductor material such
 as aluminum gallium-arsenide (AlGaAs) or
 gallium-arsenide-phosphide (GaAsP).
LEDs emit light by spontaneous emission
 –light is emitted as a result of the
 recombination of electron and holes.
            Types of LEDs

Homojunction LEDs
Heterojunction LEDs
Burrus Etched-well Surface Emitting LED
Edge Emitting LED
          Homojunction LEDs

A p-n junction made from two different
 mixtures of the same types of atoms is
 called homojunction structure
Two types of Homojunction LED
Silicon doped gallium arsenide
Planer diffused
    Homojunction LEDs

Silicon doped gallium arsenide
Typical output power aprox. 2mW
Wavelength 940nm
Planner diffused homojunction LEDs
Output power approx. 500micro W.
Wavelength 900nm.
       Homojunction LEDs
silicon-doped gallium
                        planar diffused
    arsenide
         Heterojunction LEDs

Heterojunction LEDs are made from a p
 type semiconductor mateial of one set of
 atoms and an n type semiconductor
 material from another set.
Light emitted from edge of the material
 and are therefore often called edge
 emitters.
Heterojunction LED
Heterojunction LED

Advantages:
 Increase in current density generates
 more brilliant spot
 Smaller emitting area  easier to couple
 light into fiber
 Small effective area has small
 capacitance  higher speeds
Burrus Etched Well Surface Emitting LED

Mostly used in Telecommunication
 application, data rates in excess of
 100Mbps are required.
It was developed in Bell Laboratories.
It emits light in all directions.
This device is more efficient than standard
 surface emitters, and allow more power to
 be coupled into the optical fiber.
Burrus etched-well surface-emitting LED
           Edge-emitting LED

It is Developed by RCA
They emits more directional light patterns
 than surface emitting LEDs.
Edge emitting LEDs has narrower
 bandwidths.
Its construction is similar to planner and
 Burrus diodes.
Edge-emitting LED
            Injection laser diode
 Below a certain threshold current, an ILD acts as
  similarly to an LED.
 Above the threshold current, an ILD oscillates:
  lasing occurs.
 Construction of ILD is similar to LED except that
  the ends are highly polished.
 Mirror like ends of ILD traps the photons in
  active region and recombine with holes at higher
  energy level this process is called lasing.
Injection laser diode
                ILD vs LED
 Coherent light
 Higher radiant output power
 Higher bitrates
 Monochromatic light: less  wavelength
  dispersion

 10 times more expensive than LEDs
 shorter lifetime
 more temperature dependent
LED and ILD radiation patterns

             Broad beam

             Less directed radiation pattern



             Narrow concentrated beam

             More direct radiation
             pattern
            Light Detectors

There are two devices commonly used to
 detect light energy in fiber optic
 communication receivers.
PIN diodes (positive intrinsic negative)
APDs        (Avalanche photo diode)
                  PIN Diode
 A pin diode is a depletion layer photodiode
 Most commonly used as a light detector in fiber
  optic communication systems
 PIN diode operates opposite to LED
 Most photons are absorbed by electrons in the
  valence band of intrinsic material
 When electrons are absorbed, they add
  sufficient energy to generate carriers in the
  depletion region and allow current to flow
  through the device.
   Light detectors: PIN photo diode
Light entering in PIN diode is absorbed by
 the intrinsic material and adds energy to
 moves electrons from valance band to
 conduction band
To cause current to flow in a photodiode,
 light of sufficient energy must be absorbed
 to give valance electron enough energy to
 jump the energy gap.
Energy gap for silicon is 1.12eV.
      Light detectors: PIN photo diode




  Energy gap for silicon:
   Eg  1.12 eV  1.79 1019 J
    Energy of a photon
                                            1109 nm
         E p  hf

h = Planck’s constant = 6.63 10-34 J/Hz
  Avalanche photo diode




Avalanche effect

APD is more sensitive than PIN diode
        Avalanche photo diode

APD is a pipn structure.
Light enter in diode is absorbed by heavily
 doped n-layer.
Ionization occur When ipn junction is
 reverse bias.
Ionized carriers cause more ionization to
 occur. This process increase internal gain
 or carrier multiplication.
        Avalanche photo diode

APDs are more sensitive than PIN diodes
 and require less additional amplification.
Disadvantages are:
Long transit time
Additional internally generated noise due
 to the avalanche multiplication factor.
  Characteristics of Light Detectors
Responsitivity: Is a measure of the
conversion efficiency of a photodetector.
                eG
           R       [A/W]
                 h

     e = electron charge (1.6 10-19 coulomb)
     v = frequency of the light
      = quantum efficiency
     G = internal gain (>1 for APD)
   Characteristics of Light Detectors
 Dark current: The leakage current that flows through a
  photodiode with no light input. Thermally generated.

 Transit time: The time it takes a light-induced carrier to
  travel across the depletion region of a semiconductor.
  This parameter determines the maximum bit rate
  possible with a particular photodiode.

 Light sensitivity: The minimum optical power a light
  detector can receive and still produce a usable electrical
  output signal.
  Characteristics of Light Detectors
Spectral response: The range of wavelength
values that a given photodiode will respond.
                 LASER

Laser is acronym for light amplification
 stimulated by the emission of radiation.
Laser technology deals with the
 concentration of light into very small,
 powerful beam.
The radiation of a laser is extremely
 intense and directional.
                 Laser

First laser was developed by Theodore H.
 Maiman, a scientist who worked for
 Hughes Aircraft Company in Califonia.
Uranium lasers were developed in 1960
Helium lasers were developed in 1960
Semiconductors lasers were developed in
 1962
            Types of Lasers

There are four types of lasers:
Gas
Liquid
Solid
Semiconductor
             Types of Laser

Gas lasers: Gas lasers use a mixture of
 helium and neon enclose in a glass tube.
Liquid Lasers: Liquid lasers use organic
 dyes enclosed in a glass tube for an active
 medium.
Solid Lasers: Solid lasers use a solid,
 cylindrical crystal, such as ruby,for the
 active medium.
            Types of Lasers
Semiconducor Lasers: Semiconductor
 lasers are made from semiconductor p-n
 junctions and are commonly called ILDs.
The excitation mechanism is a dc power
 supply that controls the amount of current
 to the active medium.
The output light from an ILD is easily
 modulated, making it very useful in many
 electronic communications applications.
         Laser Characteristics
 All types of lasers have several common
   characteristics.
 They all use
1. An active material to convert energy into
   laser light.
2. A pumping source to provide power or
   energy
3. Optics to direct the beam through the
   active material to be amplified
         Laser Characteristics

4.Optics to direct the beam into narrow
    powerful cone of divergence
5.A feedback mechanism to provide
    continuous operation
6. An output coupler to transmit power out of
    the laser.
            Laser Construction
 Figure below will show the basic construction
  of laser.
 Power source is connected to a flashtube that
  is coiled around a glass tube that holds the
  active medium.
 One end of the glass tube is a polished mirror
  face 100% internal reflection.
 The flashtube is energized by a trigger pulse
  and produce a high level burst of light.
LASER
      Laser construction Cont…
The process of pumping raises the level of
 the chromium atoms from ground state to
 an excited energy state.
The ions than decay, falling to an
 intermediate energy level.
When the population of ions in the
 intermediate level is greater than the
 ground state, a population inversion
 occurs.
      Laser construction Cont…

The population inversion causes laser
 action to occur.
The frequency of the energy determines
 the strength of the photons, higher
 frequencies cause greater-strength
 photons

				
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posted:9/14/2010
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