Optical Fiber Communication 121102

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Optical Fiber Communication(121102) Chris
Roeloffzen
Chair Telecommunication engineering (EWI) Floor
8 HOGEKAMP EL/TN building (north) Telephone 489
2804 E-mail c.g.h.roeloffzen_at_el.utwente.nl
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Light sources
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Light source
Block diagram of an optical communication system

• Light source modulation ? spectrum of the RF
  signal will shift to the optical frequencies
• Intensity modulation is often used
• Used wavelength is determined by
• Transmission parameters
• attenuation
• dispersion
• Availability, reliability, cost of
• light sources
• detectors

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Transmission parameters
Attenuation of a glass optical fiber
Material dispersion of glass
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Light-emitting diode (LED)
Based on spontaneous emission of photons in the
p-n junction of semiconductors Conduction band
populated by electrons Valence band populated by
holes A current generates electrons. Spontaneous
emission due to recombination of electrons and
holes (recombination radiation) Eg bandgap
energy between valence band and conduction
band The wavelength of the emitted radiation
is where h is Plancks constant internal
quantum efficiency ?i number of photons /
injected carriers ?i lt 1 non-radiative decay
processes External quantum efficiency even lower
due to losses en reflections
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Bandgap structure of a semiconductor

• Electron energy versus momentum
• Indirect bandgap semiconductor ? recombination
  momentum change ? difference in momentum creates
  phonon ? low probability ? low ?i
• Direct-bandgap semiconductor ? only recombination
  ? low ?I (50-80)

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Design of a LED

• High efficiency
• Correct beam geometry ? large coupling efficiency
  with the fiber
• Direct modulation at high rates
• rapid discharge of heat (junction temperature ? ?
  light output ?)

• Surface emitter ? light is emitted perpendicular
  to the junction
• Edge emitter ? light is emitted in the plane of
  the junction

LED surface emitter with a homojunction
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Design of a LED

• Fiber coupling
• emitting area lt area of the fiber core coupling
  with lens for high efficiency (multimode fiber)
• emitting area gt area of the fiber core direct
  coupling (always for single mode fiber)

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Double heterostructure LED
Electron energy diagram for the various layers

• Advantages of a DH LED
• Electrons and holes are confined to the active
  layer ? increasing the density of both type of
  carriers ? higher efficiency
• Less absorption of radiation through the
  transparent layers at both sides (larger bandgap)
• Wider range of wavelengths by varying the
  composition of the material of the active layer

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Double heterostructure LED
DH LED (surface emitter) with fiber pigtail
attached
DH LED (edge emitter) with carrier confinement
and optical guiding layers
Advantages of an edge emitter improved coupling
into the fiber because of well directed radiation
(optical guidance of the DH structure)
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Materials for LEDs

• 0.8 0.9 ?m GaAs or AlxGa1-xAs
• Al, Ga group III As group V ? III-V compound
• ? 1.3-1.6 ?m GaxIn1-xPyAs1-y
• (also III-V semiconductor)
• Both materials are direct-bandgap semiconductors
  (for a wide range of x and y)

Emitted wavelength and refractive index of
AlxGa1-xAs as a function of the mole fraction x.
(Both carrier confinement and optical guiding
when less Al in active layer)
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Autocorrelation functions and spectra
Time dependent signal Autocorrelation
function Power spectral density
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Optical spectrum and coherence function

• Time dependent signal
• Spectra resemble closely the Gaussian bell shape
• Energy of the electrons display a Fermi statistic
• Energy distribution of the emitted photons shows
  a gamma statistic, with a standard deviation
• k Boltzmanns constant
• absolute temperature
• due to impurities
• ?? 25-30 nm (?0.8 0.9 nm)
• ?? 50-100 nm (?1 1.3 nm)

Typical LED spectra
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Optical spectrum and coherence function
The electromagnetic field of a LED can be
described as stochastic band-pass process where
?c 2??, ?c/? x(t) and y(t) are real
independent bandpass processes (phase of w(t) is
random) Expectation Autocorrelation In
optics Rww(?) is the coherence function of the
source
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Principle of a laser
This configuration is a Fabry-Pérot laser Lasing
requires population inversion N2 gt N1 Roundtrip
gain 1 (stationary situation) The wavelength
in the material with a refractive index n is ?/n
(for AlGaAs n?3.6). A standing wave occurs in the
cavity if twice the length L corresponds to an
integer multiple of the wavelength
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Principle of a laser
The laser spectrum can consist of several lines
the so called longitudinal modes (multimode
laser) where N is the group index for large
values of m the distance between the longitudinal
modes becomes
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The semiconductor laser diode

• By putting the LED in an optically resonant
  cavity the device can act as a laser
• Lasing occurs if the forward current is so large
  that population inversion takes place
• number of electrons in conduction band x number
  of holes in valence band
• gt
• number of electrons in valence band x number of
  holes in conduction band
• The current at which the laser action starts is
  called the threshold currentFor currents below
  the threshold the device behaves like a LED

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Stripe geometry of a DH laser
construction
Far-field emission pattern
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Stripe geometry of a DH laser

• Important is the confinement of the current to
  the 13 ?m wide strippassivation can be done by-
  proton bombardment ? high resistance- SiO2
  (quartz) layer ? high resistance
• the active layer is still protected from the
  environment
• Lasers with this property are called gain-guided
  lasers. The refractive index depends on the
  current (though the carrier density). This
  results in a rather instable beam

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Buried-heterostructure laser

• Planar waveguide structure
• photolithography and etching so that only a small
  stripe active layer stack remains
• refill with semiconductor material with lower
  refractive index ? optical guiding in the plane
  of the junction
• Lasers with this property are called index-guided
  lasers.- symmetric and stable far filed
  pattern- small threshold current (10-15 mA)

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Single-mode laser
Distributed feedback (DFB) laser diode

• In order to select one longitudinal mode for the
  cavity an additional wavelength selective element
  is required, for example a grating

Distributed Bragg Reflector (DBR) laser diode
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Coherence function and spectrum of a
semiconductor laser
The field component of the light emitted of an
unmodulated laser is written as where the
amplitude E0 is supposed to be constant and the
phase ?(t) changes slowly wrt ?ct (random walk
stochastic process) For a single-mode laser (one
longitudinal mode) the autocorrelation assumes
the form where ?c is the coherence time Lc ?cc
is the coherence length The spectrum
is Spectra of this shape are said to have a
Lorentz profile centered around fc?c/2?. Typical
line width is 50 MHz
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Coherence function and spectrum of a
semiconductor laser

• Temperature effects
• ? ? ? of the bandgap and ? refractive index ?
  spectral shift to lower ?, about 0.2 0.7 nm/K
  (dependent of ?)
• For a multimode laser
• The lines in the spectrum have Lorenz
• shape and the envelope
• (distance approx 0.5 nm)
• has a Gaussian profile
• (width approx 5 nm).
• Spectrum of the envelope is
• smaller than a LED

Spectrum Coherence function
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Lateral modes
Most lasers diodes radiate only the fundamental
transversal mode Gaussian intensity profile
Near- and far-field pattern
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Semicondictor laser vs LED

• LD and LED have a different L-I curve
• Linearity of the source is important for analog
  systems
• The L-I characteristic of an LD depends greatly
  on temperature not for LED
• Power supplied by both devices is similar (10-200
  mW)
• The coupling efficiency of a fiber is approx 90
  for LD and lt10 for LED

Optical power vs current of an ILD and LED
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Semiconductor laser vs LED

• Modulation speed
• LED limited due to the spontaneous recombination
  time of the carriers and large capacitance
• LD very fast due to extremely short stay of
  electron in the conduction band, due to
  stimulated recombination
• Optical spectrum
• LDs have narrower spectrum that LEDs single mode
  lasers even 104 smaller. Less dispersion with LDs
• Temperature effects
• - Peltier element is required for LD to stabilize
  the temperature and thus the optical power. Nor
  required for LEDs
• Lifetime
• - Both LEDs and LD have expected lifetime of 107
  hours.
• Costs
• - LEDs are less expensive than LDs