EE 230: Optical Fiber Communication Lecture 5

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EE 230 Optical Fiber Communication Lecture 5
Attenuation in Optical Fibers
From the movie Warriors of the Net
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Attenuation/Loss In Optical Fibers
Mechanisms Bending loss Absorption Scattering
loss dBm refers to a ratio with respect to a
signal of 1 mW
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Bending Loss
Example bending loss 1 turn at 32 mm diameter
causes 0.5 db loss Index profile can be adjusted
to reduce loss but this degrades the fibers other
characteristics Rule of thumb on minimum bending
radius Radiusgt100x Cladding diameter for short
times 13mm for 125mm cladding Radiusgt150x
Cladding diameter for long times 19mm This loss
is mode dependent Can be used in attenuators,
mode filters fiber identifier, fiber tap, fusion
splicing Microbending loss Property of fiber,
under control of fabricator, now very small,
usually included in the total attenuation numbers
Fiber Optics Communication Technology-Mynbaev
Scheiner
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Bending Loss in Single Mode Fiber
Bending loss for lowest order modes
Mode Field distributions in straight and bent
fibers
Microbending Loss Sensitivity vs wavelength
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Bending Loss

• Outside portion of evanescent field has longer
  path length, must go faster to keep up
• Beyond a critical value of r, this portion of the
  field would have to propagate faster than the
  speed of light to stay with the rest of the pulse
• Instead, it radiates out into the cladding and is
  lost
• Higher-order modes affected more than lower-order
  modes bent fiber guides fewer modes

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Graded-index Fiber

• For r between 0 and a. If a8, the formula is
  that for a step-index fiber
• Number of modes is

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Mode number reduction caused by bending
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Absorption

• In the telecom region of the spectrum, caused
  primarily by excitation of chemical bond
  vibrations
• Overtone and combination bands predominate near
  1550 nm
• Low-energy tail of electronic absorptions
  dominate in visible region
• Electronic absorptions by color centers cause
  loss for some metal impurities

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Electron on a Spring Model
Response as a function of Frequency
Mechanical Oscillator Model
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E-Field of a Dipole
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Vibrational absorption

• When a chemical bond is dipolar (one atom more
  electronegative than the other) its vibration is
  an oscillating dipole
• If signal at telecom wavelength is close enough
  in frequency to that of the vibration, the
  oscillating electric field goes into resonance
  with the vibration and loses energy to it
• Vibrational energies are typically measured in
  cm-1 (inverse of wavelength). 1550 nm 6500
  cm-1.

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Overtones and combination bands

• Harmonic oscillator selection rule says that
  vibrational quantum number can change by only 1
• Bonds between light and heavy atoms, or between
  atoms with very different electronegativities,
  tend to be anharmonic
• To the extent that real vibrations are not
  harmonic, overtones and combination bands are
  allowed (weakly)
• Each higher overtone is weaker by about an order
  of magnitude than the one before it

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Overtone absorptions in silica

• Si-O bond fairly polar, but low frequency
• 0?1 at 1100 cm-1 would need six quanta (five
  overtones) to interfere with optical fiber
  wavelengths
• OH bonds very anharmonic, and strong
• 0?1 at 3600 cm-1 0?2 at 7100 cm-1 creates
  absorption peak between windows

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Attenuation in plastic fibers

• C-H bonds are anharmonic and strong, about 3000
  cm-1
• First overtone (0?2) near 6000 cm-1
• Combination bands right in telecom region
• Polymer fiber virtually always more lossy than
  glass fiber

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Absorptive Loss

• Hydrogen impurity leads to OH bonds whose first
  overtone absorption causes a loss peak near 1400
  nm
• Transition metal impurities lead to broad
  absorptions in various places due to d-d
  electronic excitations or color center creation
  (ionization)
• For organic materials, C-H overtone and
  combination bands cause absorptive loss

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Photothermal deflection spectroscopy
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Scattering loss from index discontinuity

• Scatterers are much smaller than the wavelength
  Rayleigh and Raman scattering
• Scatterers are much bigger than the wavelength
  geometric ray optics
• Scatterers are about the same size as the
  wavelength Mie scattering
• Scatterers are sound waves Brillouin scattering

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Raman scattering

• A small fraction of Rayleigh scattered light
  comes off at the difference frequency between the
  applied light and the frequency of a molecular
  vibration (a Stokes line)
• In addition, some scattered light comes off at
  the sum frequency (anti-Stokes)

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Mie scattering from dimensional inhomogeneities

• Similar effect to microbending loss
• Mie scattering depends roughly on ?-2 scattering
  angle also depends upon ?
• In planar waveguide devices, roughness on side
  walls leads to polarization-dependent loss

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Teng immersion technique
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Intrinsic Material Loss for Silica
Rayleigh Scattering (1/l)4 Due to intrinsic
index variations in amorphous silica
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Spectral loss profile of a Single Mode fiber
Spectral loss of single and Multi-mode silica
fiber
Intrinsic and extrinsic loss components for
silica fiber
Fundamentals of Photonics - Saleh and Teich