TCOM 503 Fiber Optic Networks

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TCOM 503Fiber Optic Networks

• Spring, 2006
• Thomas B. Fowler, Sc.D.
• Senior Principal Engineer
• Mitretek Systems

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Topics for TCOM 503

• Week 1 Overview of fiber optic communications
• Week 2 Brief discussion of physics behind fiber
  optics
• Week 3 Light sources for fiber optic networks
• Week 4 Fiber optic components fabrication and
  use
• Week 5 Modulation of light, its use to transmit
  information
• Week 6 Noise and detection
• Week 7 Optical fiber fabrication and testing of
  components

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Week 3 Light sources and propagation of light in
fibers

• Overview of light production
• Physical background for lasers and LEDs
• Construction and operation of LEDs
• Characteristics and uses of LEDs in fiber optics
• Principle of the laser
• Types of lasers and their characteristics and
  uses
• Semiconductor lasers for fiber optics

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Light production

• Basic principle Rapid change of state of an
  electron
• Must go from high energy state to lower energy
  state
• Loss of energy must appear somewhere
• Emitted in form of light
• Lattice vibrations
• Electron can be bound to a particular atom or
  molecule or be free (part of electron gas), as
  in most conductors
• Two types of emission of light
• Spontaneous
• Stimulated

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Light production spontaneous

• Normal case
• Electron in high energy state is unstable
• Spontaneously returns to lower state
• Occurs in a few picoseconds
• Photon emitted in process
• Direction, phase random
• Energy of photon determined by transition
  undergone by electron

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Energy bands and light production

3p
Energy levels split into bands
overlap
3s
2p
Electrons dropping and giving off light
2s
1s
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Light production stimulated

• Electron in high energy metastable state
• Can remain there for a long time
  (microseconds)
• Can fall back spontaneously, or be stimulated
  to emit its energy by another photon
• Incident photon must have correct energy
• Newly emitted photon will have same wavelength,
  phase, and direction as incident (stimulating)
  photon

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Sources of energy for photon emission

• Heat
• Electrical discharge
• Electric current
• Chemical reaction
• Biochemical reaction
• Absorption of light
• Nuclear radiation

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Sources of energy heat

• Most common way of providing energy to boost
  electrons into higher energy states
• Electrons may go to multiple states and then fall
  back
• Depending on nature of heating process, result
  may be either black body radiation, or emission
  of energy at specific wavelengths characteristic
  of the material
• Black body radiation is at all wavelengths, with
  distribution a function of temperature but not
  material
• Characteristic of incandescent light bulbs

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Sources of energy electrical discharge

• Electric current passed through gas
• Energy from current ionizes gas
• Breaks chemical bonds
• Energizes electrons in gas
• When they rejoin molecules, energy emitted in
  form of light
• Called florescence
• Principle of florescent lights
• Photons emitted at specific wavelengths, but at
  random times, with random phases
• Specific wavelengths easily seen with spectroscope

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Sources of energy electric current

• Electric current applied to semiconductor p-n
  junction
• Requires combination of electrons and holes for
  current to flow
• Electrons go from high energy conduction band to
  lower energy valence band
• Yields either spontaneous emission or stimulated
  (laser) emission, depending on device fabrication

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Sources of energy chemical reactions

• Some chemical reactions give off light
• Not always associated with heat, as in burning or
  explosions
• Atoms, molecules restructured
• Some electrons may be left in high energy states
• Energy may be given off as light
• At present, not a source of light for lasers

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Sources of energy biochemical reactions

• Known as bioluminescence
• Special type of chemical reactions which occur in
  living things
• Operate by chemical reactions which leave
  electrons in high energy state
• Result is spontaneous emission
• No lasing action from animals known at present

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Sources of energy absorption of light

• Substance absorbs light of one wavelength
• Moves to higher energy (excited) state
• Gives off light at different wavelength
• Spontaneous
• Florescent light bulbs
• Light produced by discharge (in Ar gas) is UV
• UV light absorbed by coating (phosphor) and
  reemitted as visible light
• Stimulated
• Ruby laser
• Tube on outside provides energy to ruby crystal

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Sources of energy nuclear radiation

• Ionization caused by a particles (Helium nuclei)
• Electrons scattered, moved to high energy levels
• Fall back may result in emission of light
• Similar effect can be caused by high energy b
  particles (electrons)
• g rays (high energy photons) can also scatter
  electrons and result in ionization

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Types of spontaneous emission

• Incandescent light light produced through
  heating of material
• Black body radiation
• Fluorescent light light produced through energy
  source other than heat
• Ceases after energy source removed
• Phosphorescent light light produced through
  energy source other than heat
• Continues after energy source removed

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Energy sources and stimulated emission

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Construction and functioning of LEDs

• LEDs Light Emitting Diodes
• Free electrons in conduction band recombine with
  holes
• Enter lower energy valence band
• Energy emitted in form of photon
• Easy to calculate l of emitted light
• hf ephoton
• lf c
• f c/l
• hc/l ephoton
• l hc/ephoton
• h Plancks constant, 4.14 x 10-15 eV/s
• c speed of light, 3 x 108 m/sec

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Practical considerations

• Some energy possessed by electrons is in form of
  lattice momentum
• Must be same in both conduction and valence bands
• If not, some energy must be given up as a
  phonon, a quantum of energy in form of lattice
  vibration
• Makes emission of photon unlikely
• Impurity sites allow electrons to jump between
  bands without emitting photon
• Common semiconductor materials such as silicon
  and germanium have this problem

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Practical considerations (continued)

• Common materials that can be used
• Silicon
• Germanium

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Practical considerations (continued)

• Easy to get light emitted, but hard to get it out
  of p-n junction and into a fiber
• Must concentrate light production in a small
  (active) region
• Must get enough power to active region in order
  to produce desired amount of light
• Must be able to dissipate power

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Double heterojunction

Source Dutton
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Schematic of double heterojunction

Source Dutton
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Actual devices

Edge emitting LED
Surface emitting LED
Source Dutton
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Coupling to fiber

Source Dutton
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Characteristics of LEDs

• Low cost (compared to some lasers)
• Use of lasers of type developed for CD players
  began in 1996 because cost is 1/10 that of LEDs
• Low power (100 microwatts)
• Some high power LEDs are now available (75 mw)
• Relatively wide spectral width 5 of
  wavelength, or 50-100 nm
• Will cause problems with chromatic dispersion
• Incoherent light
• Not suitable for single mode fiber
• Physics of device prevent extremely rapid
  modulation (switching on and off)
• Limited to about 300 Mbps

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Lasers overview

• LASER Light Amplification by Stimulated
  Emission of Radiation
• Best type of light for optical communications
• Single (or rather narrow) wavelength
• Coherent
• Can be modulated precisely (0.5 fsec or 2,000
  THz)
• Can produce relatively high power, 20 mw for
  communications lasers
• Easily coupled into fiber (50-80)

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Lasers overview (continued)

• Disadvantages
• Cost
• Until recently, much more expensive than LEDs
• Extremely high performance applications require
  extremely high stability
• Thermal regulation to assure wavelength stability
• P-n diode to measure power and adjust bias
  current
• Most lasers are single frequency, and only
  available in a limited number of frequencies
• Tunable lasers have recently become available
• Range narrow
• Slow to tune
• Analog modulation not generally done but is
  feasible

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Lasers basic principles

• Material capable of high energy metastable state
  and correct bandgaps
• Stimulated emission
• Mirrors to confine light
• Population inversion

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Stimulated emission

Source Dutton
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Difference between stimulated and spontaneous
emission

• Emitted photons do not have same wavelength,
  phase, direction as exciting photons or other
  energy source
• Direction, time of emission random
• Many different wavelengthsblack body radiation
• No metastable statesphoton emitted
  instantaneously

Spontaneous emission
Source Dutton
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Bandgaps and energy levels

• Only certain materials are capable of lasing
  action
• Electrons can only orbit in certain positions
• Correspond to discrete energy levels
• Can only change by gain or loss of fixed quanta
  of energy (multiples of Plancks constant, k)

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Bandgaps and energy levels (continued)

• Atom has ground state and excited states,
  which refer to energy of electrons
• In ground state, electrons are at their lowest
  permissible energy levels
• Occur in what are often referred to as shells
• Innermost shells have electrons tightly bound
• Low energy states
• Unavailable for bonding or conduction
• Outermost shell called valence shell or band
• Higher energy (more loosely bound)
• Electrons can easily absorb energy and move to
  still higher energy bands
• Chemical reactions involve valence band

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Bandgaps and energy levels (continued)

• In a solid, where atoms are bound together,
  situation is more complicated
• Pauli exclusion principle prevents any two
  electrons from having exactly the same energy
• Leads to bands rather than single values for
  energy
• Structure of the bands determines nature of the
  material

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Bandgaps

• Conduction band

Valence band
Conductor
Insulator
Semiconductor
Large gap
Overlap no gap
Small gap
Source Redrawn from Dutton
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Bandgaps (continued)

• Conductors valence band and conduction band
  overlap
• Situation in metals, where electrons exist as a
  gas
• Large number of electrons available to conduct
• Small applied voltage moves them
• Impulses transmitted at about 2/3 c
• Individual electrons travel very slowly (drift
  velocity)

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Bandgaps (continued)

• Insulators large energy gap between conduction
  and valence bands
• Large amount of applied energy needed to move an
  electron into conduction band
• Can happen occasionally through thermal effects,
  etc.
• Few electrons available unless applied electric
  field extremely large 1000s of volts (insulator
  breaks down)
• Typical of non-metals

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Bandgaps (continued)

• Semiconductors Valence and conduction bands very
  close
• Only small amount of energy needed to promote
  electron to conduction band
• Between metals and non-metals

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Semiconductors - elements
Source Lawrence Livermore Labs
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Semiconductors silicon

• Lattice structure
• Pure crystal near absolute 0 (-273.15o C) has no
  free electrons, cannot conduct
• At room temperature, some electrons in conduction
  band due to thermal energy
• As group IV element, silicon has 4 valence
  electrons

Source Dutton
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Semiconductorsdoping

• To control properties of semiconductors, and get
  useful materials, necessary to add impurities
• Called dopants
• Added in very controlled manner to extremely pure
  silicon (or other material)
• Typically 1 atom of dopant per 108 atoms of Si
• Boron atom substituted for silicon atom in
  lattice
• 3 electrons in valence band, leaves deficit in
  lattice, known as hole
• Called p-type, where p comes from positive
• Phosphorus atom substituted for silicon atom in
  lattice
• 5 electrons in valence band, surplus in lattice,
  known as free electron
• Called n-type, where n comes from negative

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Semiconductorsp-type, n-type

Source Dutton
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Properties of p-type, n-type semiconductors

• Both conduct electricity, but not as well as
  conductors
• N-type contains free and mobile electrons
• P-type holes act as positive particles
• Move much slower than electrons
• In reality, bound electrons migrate in opposite
  direction, but visualization of holes moving is
  accurate
• Electricity travels much slower in p-type
  material
• Lattice with holes can exist next to lattice with
  free electrons
• Little interaction
• Holes, electrons combine at boundary
• Release of energy can yield photon

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p-n junctions no applied voltage

• Electrons diffuse from n-type region, holes from
  p-type region
• Combine in middle to form depletion zone
• Leaves net negative charge in p-type region and
  net positive charge in n-type region
• Inhibits further crossing and recombining

Source Dutton