This narrated power point presentation attempts to analyse the reasons for attenuation in optical fibers due to linear effects such as absorption, scattering and fiber bend. The material will be useful for KTU final year B Tech students who prepare for the subject EC 405, Optical Communications.

1. 1
Linear Effects in Optical
Fibers
MEC

2. 2
Linear Effects
• Attenuation
• Absorption
- Intrinsic Absorption
- Extrinsic Absorption
• Linear Scattering
- Rayleigh Scattering
- Mie Scattering
• Bending Losses
- Micro bends
- Macro bends

3. 3
Fiber Losses

4. 4
Attenuation
• Loss of signal strength in dB/Km
• Signal strength reduces with distance.

5. 5
Attenuation
Ps - input source optical power
Po - received output
αdB - signal attenuation per unit length (dB)
L - fiber length, αP – attenuation coefficient.
pL
o sP P e 


6. 6
Absorption
• Portion of attenuation resulting from the
conversion of optical power into another energy
form, such as heat.
• Due to presence of impurities (metal particles,
moisture etc.) in the fiber.
• Light of a particular wavelength is absorbed and
dissipated as heat.
• Influenced by defects in atomic structure and
impurities (eg: diffusion of hydrogen).
• Intrinsic and Extrinsic.

7. 7
Material absorption losses
• Related to material composition and
fabrication process.
• Dissipation of some transmitted
optical power as heat in the waveguide.
• Intrinsic - interaction with major glass
components.
• Extrinsic - impurities within the glass.

8. 8
Intrinsic absorption
• Pure silicate glass has little intrinsic absorption.
• Two major intrinsic absorption mechanisms at
optical wavelengths.
• Low intrinsic absorption window over 0.8 to 1.7
μm.
• A fundamental absorption edge with peaks
centered in the ultraviolet region due to
stimulation of electron transitions within the
glass by higher energy excitations.

9. 9
Intrinsic absorption
• At wavelengths above 7 μm fundamentals of
absorption bands from the interaction of photons
with molecular vibrations within the glass.
• Strong absorption bands due to oscillations of
structural units such as Si–O (9.2 μm), P–O (8.1
μm), B–O (7.2 μm) and Ge–O (11.0 μm) within
the glass.
• Effects minimized by suitable choice of both core
and cladding compositions.
• In non-oxide glasses such as fluorides and
chlorides, infrared absorption peaks occur at
much longer wavelengths (50 μm) .

10. 10
Intrinsic Absorption
interaction of photons with the molecular vibrations

11. 11
Extrinsic absorption
• Caused by metallic impurities such as iron,
nickel, and chromium.
• Chromium and copper, in their worst valence
state cause attenuation in excess of 1 dB km−1
in the near-infrared region.
• Water forms silicon-hydroxyl(Si-OH) bonds,
which are bonded into the glass structure,
stretching vibrations between 2700 and 4200
nm.
• Harmonics or overtones of fundamental
absorption occur at 1.38, 0.95, and 0.72 μm.

12. 12
OH- absorption
• Harmonic overtones of fundamental
absorption occur at 1.38, 0.95, and 0.72
μm.

13. 13
Absorption losses due to metallic
ion impurities in glasses

14. 14
Linear scattering losses
• Transfers optical power from one mode to
another.
• Signal may get attenuated if transfer of
power to a leaky mode.
• Rayleigh and Mie scattering result from
the non-ideal physical properties.

15. 15
Rayleigh Scattering
• Occur due to material inhomogeneties.
• Inhomogeneities manifest as refractive index
fluctuations, arise from density & compositional
variations frozen into the glass lattice on cooling.
• Index fluctuations cannot be avoided.
• Compositional variations reduced by improved
fabrication.
• Inversely proportional to the fourth power of the
wavelength(1/λ4)

16. 16
Rayleigh Scattering Coefficient
• Rayleigh Scattering Coefficient
• λ - optical wavelength, n – refractive index of the
medium, p - average photoelastic coefficient, βc
- isothermal compressibility at a fictive
temperature ToF, and K - Boltzmann’s constant
(1.381 x 10−21 J / K) .
• Fictive temperature - temperature at which the
glass can reach thermal equilibrium.

17. 17
Transmission Loss Factor
• Transmission Loss Factor
L is the length of the fiber.
• Fundamental component of Rayleigh
scattering strongly reduced by operating at
the longest possible wavelength.

18. 18
Mie scattering
• Occurs when the size of inhomogeneties
is comparable to the guided wavelength.
• eg : non-perfect cylindrical structure, core-
cladding refractive index difference,
irregularities in core-cladding interface,
change in fiber diameter with length,
presence of air bubbles etc.
• Scattering mainly in the forward direction.

19. 19
Mie Scattering
• Inhomogenities (Mie scattering) reduced by:
(a) removing imperfections due to the glass
manufacturing process.
(b) carefully controlled extrusion and coating
of the fiber.
(c) increasing the fiber guidance by increasing the
relative refractive index difference.

20. 20
Fiber Bends
• Microbend and macrobend.

21. 21
Fiber bend loss
• Radiation losses at bends or curves on light
paths.
• Losses due to microbends and macrobends.
• Part of light wave in cladding is called
evanescent field.
• Light traveling inside the fiber slower than
evanescent field outside the fiber.
• Energy in the evanescent field at the bend
exceeds the velocity of light in the cladding and
hence guidance mechanism inhibited, causes
light to be radiated from the fiber.

22. 22
Radiation Loss at fiber bend
• Part of the mode in the
cladding outside the
dashed arrowed line
required to travel
faster than the velocity
of light to maintain
plane wavefront. Since
it cannot do this, the
energy contained in
this part of the mode is
radiated away.

23. 23
Radiation Loss at the fiber bend
• Loss represented as radiation attenuation
coefficient:
R - radius of curvature of the fiber bend
c1, c2 - constants independent of R.
• Large bending losses occur in multimode
fibers at critical radius of curvature Rc

24. 24
Macrobending Loss
• Macrobending losses reduced by:
(a) designing fibers with large relative
refractive index differences;
(b) operating at the shortest wavelength
possible.
• Sharp bends with critical radius of curvature
avoided.
• For cut off wavelength λc, critical radius of
curvature for a single-mode fiber Rcs

25. 25
Macrobending Loss
• α - profile parameter,
a - core radius, R -
bend radius, ∆ - index
difference.
• Macrobend Loss

26. 26
Microbending Losses
• Radius of curvature a few micrometers.
• Can occur due to:
1. Non-Uniformities in fiber manufacturing.
2. Non-uniform mechanical tensile forces –
fiber pressed against a rough surface.
3. Non-uniform lateral pressure created
during fiber cabling.
• Cause mode coupling b/w adjacent modes.
• Minimized by extruding a compressible jacket
over fiber.

27. 27
Microbending Losses
• Multimode fiber:
• N - number of bumps, h – bump height per unit
length, b - fiber diameter, a - core radius, Δ -
relative refractive index difference, E - elastic
modulus of surrounding medium, EF - elastic
modulus of the fiber.

28. 28
Microbending Losses
• Single-mode fiber:
αm - attenuation constant, K = , wave
vector, a - core radius, and Fd - half of
mode field diameter.

29. 29
Microbending Loss

30. 30
End