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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. Mechanisms: Bending loss Absorption Scattering loss dBm refers to a ratio with respect to a signal of 1 mW. Bending Loss.

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ee 230 optical fiber communication lecture 5
EE 230: Optical Fiber Communication Lecture 5

Attenuation in Optical Fibers

From the movie

Warriors of the Net

attenuation loss in optical fibers
Attenuation/Loss In Optical Fibers
  • Mechanisms:
    • Bending loss
    • Absorption
    • Scattering loss
    • dBm refers to a ratio
    • with respect to a
    • signal of 1 mW
bending loss
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:
    • Radius>100x Cladding diameter for short times
    • 13mm for 125mm cladding
    • Radius>150x 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

bending loss in single mode fiber
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

bending loss1
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
graded index fiber
Graded-index Fiber

For r between 0 and a. If α=∞, the formula is that for a step-index fiber

Number of modes is

absorption
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
electron on a spring model
Electron on a Spring Model

Response as a function of Frequency

Mechanical Oscillator Model

vibrational absorption
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.
overtones and combination bands
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
overtone absorptions in silica
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
attenuation in plastic fibers
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
absorptive loss
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
photothermal deflection spectroscopy

Arc lamp

Lock-in amplifier

Chopper

Lens

HeNe

Detector

Sample cuvette

Photothermal deflection spectroscopy
scattering loss from index discontinuity
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
raman scattering
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)
mie scattering from dimensional inhomogeneities
Mie scattering from dimensional inhomogeneities
  • Similar effect to microbending loss
  • Mie scattering depends roughly on λ-2; scattering angle also depends upon λ
  • In planar waveguide devices, roughness on side walls leads to polarization-dependent loss
teng immersion technique

Tunable IR laser

Chopper

Lock-in Amplifier

Detector

Motor stage

Teng immersion technique
intrinsic material loss for silica
Intrinsic Material Loss for Silica

Rayleigh Scattering ~ (1/l)4

Due to intrinsic index variations in amorphous silica

spectral loss profile of a single mode fiber
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