This document discusses various sources of attenuation in optical fibers: (1) Attenuation is mainly caused by absorption and scattering. Absorption includes intrinsic absorption which is a natural property of glass, and extrinsic absorption due to impurities in the glass. (2) Scattering includes Rayleigh scattering due to refractive index fluctuations and Mie scattering from fiber imperfections. Both result in loss of power. (3) Other losses come from bending of fibers, connections, and nonlinear effects like stimulated Brillouin and Raman scattering at high powers. Careful fiber design and manufacturing can reduce losses from many of these sources.

1. Taiz University, YEMEN
4 October 2018

2. Taiz University, YEMEN
• Most optical fibers are used for transmitting
information over long distances.
• Two main advantages of fiber: (1) wide bandwidth and
(2) low loss.
• Attenuation caused mainly by absorption and scattering
• Bandwidth is limited by an effect called dispersion.
• Transmission of light along the fiber in different
modes causes modal dispersion while chromatic
dispersion is caused by the light entering the fiber at
different wavelength.
4 October 2018 2

3. Taiz University, YEMEN
• The low attenuation or transmission loss of optical fibers has
proved to be one of the most important factors in bringing about
their wide acceptance in telecommunications.
• As channel attenuation largely determines the maximum
transmission distance prior to signal restoration, optical fiber
communications became especially attractive when the
transmission losses of fibers were reduced below those of the
competing metallic conductors (less than 5 dB km-1).
 Attenuation mainly due to material absorption, material
scattering.
 Others include bending losses, mode coupling losses and losses
due to leaky modes
 There are also losses at connectors and splices.
4 October 2018 3

4. Taiz University, YEMEN
• Attenuation in an optical fiber is measured as the
optical power loss in dB at any point along the fiber
length relative to the input power.
α: attenuation coefficient (dB/km)
4 October 2018 4
Fiber
α
Pi
Po
Length(L)
   
i
o P
P
L
dB
n
Attenuatio /
log
10 10



5. Taiz University, YEMEN
Compute the maximum length of an optical
fiber that exhibits 0.8dB/km attenuation if the
output power is 10 mW and the power launched
at the input is 150 mW.
Answer: 14.7 km
4 October 2018 5
Example (1)

6. Taiz University, YEMEN
When the mean optical power launched into an 8 km length of fiber is 120
μW, the mean optical power at the fiber output is 3 μW.
Determine:
(a) the overall signal attenuation or loss in decibels through the fiber
assuming there are no connectors or splices;
(b) the signal attenuation per kilometer for the fiber.
(c) the overall signal attenuation for a 10 km optical link using the same fiber
with splices at 1 km intervals, each giving an attenuation of 1 dB;
(d) the numerical input/output power ratio in (c).
Answer:
(a) 16.0 dB
(b) 2 dB/km
(c) 29 dB
(d) 794.3
4 October 2018 6
Example (2)

7. Taiz University, YEMEN
4 October 2018 7

8. Taiz University, YEMEN
• Material absorption is a loss mechanism related to
the material composition and the fabrication process
for the fiber, which results in the dissipation of some
of the transmitted optical power as heat in the
waveguide.
• The absorption of the light may be:
4 October 2018 8
Intrinsic absorption
Extrinsic absorption

9. Taiz University, YEMEN
• Intrinsic absorption is a natural property
of glass. It is strong in the ultraviolet (UV)
region and in the infrared (IR) region of
the electromagnetic spectrum.
• However both these considered
insignificant since optical communication
systems are normally operated outside
this region.
4 October 2018 9

10. Taiz University, YEMEN
• In practical optical fibers prepared by conventional
melting techniques, a major source of signal
attenuation is extrinsic absorption from metal element
impurities.
• Some of these impurities namely chromium and
copper can cause attenuation in excess of 1dB/km in
near infrared region.
• Metal element contamination may be reduced to
acceptable levels (i.e. one part in 1010) by glass refilling
techniques such as vapor phase oxidation which largely
eliminates the effects of these metallic impurities.
4 October 2018 10

11. Taiz University, YEMEN
• Another major extrinsic loss mechanism is caused by absorption due to water (as
the hydroxyl or OH ion) dissolved in the glass.
• The absorption occurs almost harmonically at 1.38 µm, 0.95 µm and 0.72 µm as
illustrated in the following Figure.
4 October 2018 11
The absorption spectrum for the hydroxyl (OH) group in silica.

12. Taiz University, YEMEN
• Intrinsic absorption is mostly insignificant in a
wide region where fiber systems operate.
However, these losses prohibit the extension
of fiber system below UV and longer
wavelength.
• Extrinsic losses minimised by reducing
impurities and by avoiding wavelength
corresponding to water absorption.
4 October 2018 12

13. Taiz University, YEMEN
• Linear scattering may be categorized into two
major types:
• Both result from the non-ideal physical
properties of the manufactured fiber which
are difficult and in certain cases, impossible to
eradicate at present.
4 October 2018 13
Rayleigh scattering
Mie scattering

14. Taiz University, YEMEN
• Rayleigh scattering is the dominant intrinsic loss mechanism
in the low absorption window between the ultraviolet and
infrared absorption tails.
• It results from inhomogeneities of a random nature occurring
on a small scale compared with the wavelength of the light.
• These inhomogeneities manifest themselves as refractive
index fluctuations and arise from density and compositional
variations which are frozen into the glass lattice on cooling
• The compositional variations may be reduced by improved
fabrication, but the index fluctuations caused by the freezing-
in of density inhomogeneities are fundamental and cannot be
avoided.
4 October 2018 14

15. Taiz University, YEMEN
• The subsequent scattering due to the density
fluctuations, which is in almost all directions,
produces an attenuation proportional to l/λ4
following the Rayleigh scattering formula.
• The loss (dB/km) can be approximated by the
formula below with λ in μm;
𝛼 = 1.7
0.85
𝜆
4
4 October 2018 15

16. Taiz University, YEMEN
• Linear scattering may also occur at inhomogeneities which
are comparable in size to the guided wavelength.
• These result from the non-perfect cylindrical structure of
the waveguide and may be caused by fiber imperfections
such as irregularities in the core-cladding interface, core-
cladding refractive index differences along the fiber length,
diameter fluctuations, strains and bubbles.
• The inhomogeneities may be reduced by:
 Removing imperfections due to the glass manufacturing
process
 Carefully controlled the coating of fiber
 Increasing the fiber guidance by increase the relative
refractive difference index
4 October 2018 16

17. Taiz University, YEMEN
4 October 2018 17
The measured attenuation spectrum for an ultra-low-loss single-mode fiber (solid line) with the calculated attenuation
spectra for some of the loss mechanisms contributing to the overall fiber attenuation (dashed and dotted lines)

18. Taiz University, YEMEN
• Optical waveguides do not always behave as
completely linear channels whose increase in
output optical power is directly proportional to
the input optical power.
• Several nonlinear effects occur, which in the case
of scattering cause disproportionate attenuation,
usually at high optical power levels.
• This non-linear scattering causes the optical
power from one mode to be transferred in either
the forward or backward direction to the same,
or other modes, at a different frequency.
4 October 2018 18

19. Taiz University, YEMEN
• It depends critically upon the optical power
density within the fiber and hence only
becomes significant above threshold power
levels.
• The most important types of nonlinear
scattering within optical fibers are stimulated
Brillouin and Raman scattering, both of which
are usually only observed at high optical
power densities in long single mode fibers
4 October 2018 19

20. Taiz University, YEMEN
• It occurs when signal power reaches a level sufficient
to generate tiny acoustic vibrations in the glass.
• Acoustic waves change the density of a material and
thus alter its refractive index.
• The resulting refractive index fluctuations can scatter
light.
• This can occur at powers as low as 10 mW in single
mode fibers. The threshold power PB is given by:
4 October 2018 20
watts
v
d
10
4.4
P dB
2
2
-3
B 




21. Taiz University, YEMEN
where d and λ are the fiber core diameter and
the operating wavelength respectively, both
measured in micrometers, αdB is the fiber
attenuation in decibels per kilometer and v is the
source bandwidth (i.e. injection laser) in GHz.
• The equation allows the determination of the
threshold optical power which must be launched
into a single mode optical fiber before Brillouin
scattering occurs.
4 October 2018 21

22. Taiz University, YEMEN
• Stimulated Raman scattering is similar to stimulated
Brillouin scattering except that Raman scattering occurs in
the forward and backward direction and may have an
optical power threshold of up to three orders of magnitude
higher than the Brillouin threshold in a particular fiber.
• Using the same criteria as those specified for the Brillouin
scattering, the threshold optical power for stimulated
Raman scattering PR in a long single mode fiber is given by:
4 October 2018 22
watts
d
10
5.9
P dB
2
-2
R 



23. Taiz University, YEMEN
A long single-mode optical fiber has an
attenuation of 0.5 dB km−1 when operating at a
wavelength of 1.3 μm. The fiber core diameter is
6 μm and the laser source bandwidth is 600
MHz. Compare the threshold optical powers for
stimulated Brillouin and Raman scattering within
the fiber at the wavelength specified.
Answer: 80.3 mW, 1.38 W.
4 October 2018 23
Example

24. Taiz University, YEMEN
• Optical fibers suffer radiation losses at bends or curves on
their paths.
• This is due to the energy in the evanescent field at the bend
exceeding the velocity of light in the cladding and hence the
guidance mechanism is inhibited, which causes tight energy to
be radiated from the fiber.
4 October 2018 24

25. Taiz University, YEMEN
• Large bending losses tend to occur at a critical radius of
curvature Rc, which may be estimated from:
• It may be observed from the expression given that
possible bending losses may be reduced by:
 Designing fibers with large relative refractive index
differences
 Operating at the shortest wavelength possible.
4 October 2018 25
 2
3
2
2
2
1
2
1
4
3
n
n
n
Rc





26. Taiz University, YEMEN
• The above criteria for the reduction of bend
losses also apply to single-mode fibers. One
theory, based on the concept of a single quasi-
guided mode, provides an expression from which
the critical radius of curvature for a single-mode
fiber Rcs can be estimated as:
𝑅𝑐𝑠 ≅
20 𝜆
(𝑛1 − 𝑛2)
3
2
(2.748 − 0.996
𝜆
𝜆𝑐
)−3
where λc is the cutoff wavelength for the single-
mode fiber.
4 October 2018 26

27. Taiz University, YEMEN
Two step index fibers exhibit the following parameters:
(a) a multimode fiber with a core refractive index of
1.500, a relative refractive index difference of 3% and
an operating wavelength of 0.82 μm;
(b) an 8 μm core diameter single-mode fiber with a
core refractive index the same as (a), a relative
refractive index difference of 0.3% and an operating
wavelength of 1.55 μm.
Answer: 9 μm, 34 mm.
4 October 2018 27
Example

28. Taiz University, YEMEN 28
4 October 2018