 Attenuation in a fiber optic signal is the loss
of optical power as the signal travels through the
fiber.
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Attenuation is caused by the fact that no
manufacturing process can produce a perfectly
pure fiber.
Either by accident or by design, the fiber will
always
have
some
characteristic
that
attenuates the signal passing through it.
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The wavelength of the light passing through the
fiber also affects attenuation.
Attenuation decreases as wavelength increases
There are certain wavelengths that are more
easily absorbed in plastic and silica fibers than
others such 730, 950, 1250, and 1380 nm.
These range are considered high-loss
regions.
So, 850 nm, 1300 nm, and 1550 nm in silica
fiber and in the visible range of 650 nm for
plastic are establishing standard operating
wavelengths.
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The effect on a signal due to attenuation
increases with the length of the fiber.
Attenuation behaves differently from
Dispersion
Where, its effects accumulate
Whereas, dispersion is determined by factors
within the fiber and the signal’s wavelength and
spectral width
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There are two types of attenuation:
Absorption and Scattering
 Absorption
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The amount of absorption depends on the type of material
and the wavelength of the light passing through it.
The wavelengths that do not pass through are mostly
absorbed by impurities that have been placed in, or coated
on, the lens material (such as sunglasses).
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In an optical fiber,
Absorption occurs when impurities such as
water or ions of materials such as copper or
chromium absorb certain wavelengths
By keeping these impurities as low as possible,
manufacturers can produce fibers with a
minimum of attenuation
Scattering
 Scattering is caused by atomic structures and
particles in the fiber redirecting light that hits
them.
 This phenomenon is described by the British
physicist Lord Rayleigh.
 Rayleigh scattering is also the answer to the
age-old question “Why is the sky blue?”
 The blue that we see is actually the more
prevalent blue wavelengths of light from the
sun being scattered by particles in the
atmosphere.
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As the sun moves toward the horizon and the light
must pass through more of the atmosphere, the
scattering increases to the point where the blue light
is almost completely attenuated, leaving the red
wavelengths, which are less affected by the
scattering.
Rayleigh scattering depends on the relationship
between wavelength and the size of the structures
in the fiber.
Scattering increases as the wavelength of the light
approaches the size of the structures, which means
that as the wavelength decreases, it is more likely to
be scattered.
This is one of the main reasons that
infrared wavelengths are used in fiber optics

The wavelengths of infrared are less subject to
scattering than visible wavelengths.
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Total Attenuation
Total attenuation is the combination of the
effects of absorption and scattering in a fiber.
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Many of a fiber’s characteristics are determined
by the relationship between the core and the
cladding.
The numerical aperture (NA) expresses the lightgathering ability of the fiber.
The NA is a dimensionless number, meaning that it
is to be used as a variable in determining other
characteristics of the fiber, or as a means of
comparing two fibers.
[(𝒏𝟏 )𝟐 − (𝒏𝟐 )𝟐 ]

𝑵𝑨 =

where 𝒏𝟏 is the refractive index of the core and 𝒏𝟐 is
the refractive index of the cladding
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In order for light to be contained within a
multimode fiber it must stay above the critical
angle, or the angle at which it reflects off of the
boundary between the core and the cladding,
rather than penetrating the boundary and
refracting through the cladding.
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In order to maintain the critical angle, light must
enter within a specified range called the cone of
acceptance also known as the acceptance
angle
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This region is defined by a cone extending
outside the fiber core. Light entering the core
from outside of the cone will either miss the core
or enter at an angle that will allow it to pass
through the boundary with the cladding and be
lost.
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The cone of acceptance is determined using the
numerical aperture:
NA = sin where  is 1/2 of the angle measuring
the cone of acceptance.
Another useful term is the maximum coupling
angle.
The acceptance angle is also used to determine
how light emerges from a fiber. The light that comes
out of a fiber end is the light that has not been
absorbed or lost in the cladding
The exception of a small percentage of light that has
propagated in the boundary between the core and
the cladding, what emerges is coming out at an
angle equal to or greater than the critical
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The types of bending we will look at are
microbends and macrobends
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Microbend
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Microbends are small distortions of the boundary
layer between the core and cladding caused by
crushing or pressure.
Microbends change the angle of incidence within
the fiber
Changing the angle of incidence forces high-order
light rays to reflect at angles that prevent further
reflection, causing them to be lost in the cladding
and absorbed
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Macrobends
Macrobends occur when the fiber is bent around a
radius that can be measured in centimeters.
These tight radii change the
angle of incidence within the
fiber, causing some of the
light rays to reflect outside of
the fiber and, as with
microbending, be lost in the
cladding and absorbed.