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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 Attenuation/Loss In Optical Fibers Bending Loss Bending ... – PowerPoint PPT presentation

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Title: EE 230: Optical Fiber Communication Lecture 5


1
EE 230 Optical Fiber Communication Lecture 5
Attenuation in Optical Fibers
From the movie Warriors of the Net
2
Attenuation/Loss In Optical Fibers
Mechanisms Bending loss Absorption Scattering
loss dBm refers to a ratio with respect to a
signal of 1 mW
3
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
4
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
5
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

6
Graded-index Fiber
  • For r between 0 and a. If a8, the formula is
    that for a step-index fiber
  • Number of modes is

7
Mode number reduction caused by bending
8
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

9
Electron on a Spring Model
Response as a function of Frequency
Mechanical Oscillator Model
10
E-Field of a Dipole
11
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.

12
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

13
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

14
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

15
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

16
Photothermal deflection spectroscopy
17
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

18
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)

19
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

20
Teng immersion technique
21
Intrinsic Material Loss for Silica
Rayleigh Scattering (1/l)4 Due to intrinsic
index variations in amorphous silica
22
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
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