This document discusses key characteristics of optical fibers that affect their performance as a transmission medium. It describes how wavelength, frequency, reflection, refraction, polarization, and attenuation properties influence fiber optic communication. Specific bands used in optical fibers, including O, C, E, S and L bands, are defined. The document also examines intrinsic and extrinsic factors contributing to fiber attenuation, as well as dispersion which limits bandwidth by spreading out light pulses over time as they travel through the fiber.

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4 CHARATERISTICS OF OPTICAL FIBER
4 CHARATERISTICS OF OPTICAL FIBER
4.1 INTRODUCTION
4.2 OBJECTIVE
4.3 WAVELENGTH
4.4 FREQUENCY
4.5 REFLECTION
4.6 REFRACTION
4.7 POLARIZATION
4.8 ATTENUATION
4.9 BANDS IN OPTICAL FIBER
4.10 USABILITY OF BANDS IN OPTICAL FIBER
4.11 WINDOWS IN FIBER OPTIC
4.12 LOSS CHARACTERISTICS
4.13 DISPERSION
4.14 BANDWIDTH
4.13 SUMMARY
4.14 REFERENCES AND SUGGESTED FURTHER READINGS
4.15 WORKSHEET

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4.1 Introduction
Like any communication system there are some important factors affecting
performance of optical fibers as a transmission medium. The most interest are those
attenuation and bandwidth. Optical-fiber systems have many advantages over metallic-
based communication systems. These advantages include interference, attenuation, and
bandwidth characteristics. Furthermore, the relatively smaller cross section of fiber-optic
cables allows room for substantial growth of the capacity in existing conduits.
4.2 Objective
After reading this unit, you should be able to understand:
 Refraction
 Polarization
 Attenuation
 Dispersion
 Bandwidth
 Optical bands
4.3 WAVELENGTH
It is a characteristic of light that is emitted from the light source and is measures in
nanometers (nm). In the visible spectrum, wavelength can be described as the colour of
the light.
For example, Red Light has longer wavelength than Blue Light, Typical
wavelength for fibre use are 850nm, 1300nm and 1550nm all of which are invisible
(Infrared).

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4.4 FREQUENCY
It is number of pulse per second emitted from a light source. Frequency is
measured in units of hertz (Hz). In terms of optical pulse 1Hz = 1 pulse/ sec.
4.5 Reflection
Reflection is the abrupt change in the direction of propagation of a light ray that
strikes the boundary between two different media. At least some part of the incoming
wave remains in the same medium. Assume the incoming light ray makes an angle θi
with the normal of a plane tangent to the boundary. Then the reflected ray makes an
angle θr with this normal and lies in the same plane as the incident ray and the normal.
Fig : 1Reflection of light
Law of reflection: θi = θr
4.6 Refraction
Refraction is the change in direction of propagation of a wave when the light ray
passes from one medium into another, and changes its speed. Light ray are refracted
when crossing the boundary from one transparent medium into another because the speed
of light is different in different media.
When light passes from one transparent medium to another, the rays are bent
toward the surface normal if the speed of light is smaller in the second medium than in
the first. The rays are bent away from this normal if the speed of light in the second

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medium is greater than in the first. The picture on the right shows a light wave incident
on a slab of glass.
One part of the wave is reflected, and another part is refracted as it passes into the
glass. The rays are bent towards the normal. At the second interface from glass into air
the light passing into the air is refracted again. The rays are now bent away from the
normal.
Fig : 2 Refraction of light
4.7 Polarization
A light wave that is vibrating in more than one plane is referred to as unpolarized
light. Polarized light waves are light waves in which the vibrations occur in a single
plane. The process of transforming unpolarized light into polarized light is known
as polarization.
Fig : 3 Polarization

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4.8 ATTENUATION
Attenuation in optical fiber is caused by intrinsic factors, primarily scattering and
absorption, and by extrinsic factors, including stress from the manufacturing process, the
environment, and physical bending.
4.9 Bands in optical fiber
4.10 Usability of bands in optical fiber
 O Band -Singlemode fiber transmission began in the "O-band" just above the cut-
off wavelength of the SM fiber developed to take advantage of the lower loss of
the glass fiber at longer wavelengths and availability of 1310 nm diode lasers.
 C Band -To take advantage of the lower loss at 1550 nm, fiber was developed for
the C-band. As links became longer and fiber amplifiers began being used instead
of optical-to-electronic-to-optical repeaters, the C-band became more important.
 E Band -The E-band represents the water peak region where a standard fiber is
most affected by attenuation caused by hydroxyl ions present within the glass core

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structure. Today, however, optical fiber manufacturers have dramatically reduced
the losses in the E-band.
 S Band -An S-band fiber laser scheme, which uses multiple fiber Bragg grating
(FBG) elements as feedback elements on each passive branch, is proposed and
described for in-service fault identification in passive optical networks (PONs).
By tuning a wavelength selective filter located within the laser cavity over a gain
bandwidth, the fiber-fault of each branch can be monitored without affecting the
in-service channels
 L Band –L band channels of equal power were launched into the fiber while co-
pumping at 980 nm and counter-pumping at 1480 nm. The 8 channels in the
middle were fixed during the measurements whereas the outer channels were
stepped to lower and higher wavelengths, respectively, to investigate the extend of
the gain band. The total signal input power was approximately -4 dBm for fibers
A and C, and +3 dBm for fiber B and the conventional fiber (the fibers in figure
2). The pump powers were 110 mW at 980 nm for the co-pump and the 1480 nm
counter-pump was varied between 143 and 173 mW to optimize the inversion
level.
4.11 Windows in Fiber Optic
A narrow window is defined as the range of wavelengths at which a fibre best
operates. Typical windows are given below:
Window Operational Wavelength
800nm - 900nm 850nm
1250nm - 1350nm 1300nm
1500nm - 1600nm 1550nm

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Fig : 4 Operating Bands
4.12 Loss characteristics
Attenuation in optical fiber is caused by intrinsic factors, primarily scattering and
absorption, and by extrinsic factors, including stress from the manufacturing process, the
environment, and physical bending.
1. INTRINSIC ATTENUATION
It is loss due to inherent or within the fiber. Intrinsic attenuation may occur as
(I) Absorption - Natural Impurities in the glass absorb light energy.
(II) Scattering - Light rays travelling in the core reflect from small
imperfections into a new pathway that may be lost through the cladding.
The most common form of scattering, Rayleigh scattering, is caused by small
variations in the density of glass as it cools. These variations are smaller than the
wavelengths used and therefore act as scattering objects (see Figure 2).
Fig : 5 Rayleigh scattering

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Scattering affects short wavelengths more than long wavelengths and limits the
use of wavelengths below 800 nm.
Attenuation due to absorption is caused by the intrinsic properties of the material
itself, the impurities in the glass, and any atomic defects in the glass. These impurities
absorb the optical energy, causing the light to become dimmer. While Rayleigh scattering
is important at shorter wavelengths, intrinsic absorption is an issue at longer wavelengths
and increases dramatically above 1700 nm. However, absorption due to water peaks
introduced in the fiber manufacturing process are being eliminated in some new fiber
types.
Fig : 6 Absorption
The primary factors affecting attenuation in optical fibers are the length of the
fiber and the wavelength of the light. Figure shows the loss in decibels per kilometer
(dB/km) by wavelength from Rayleigh scattering, intrinsic absorption, and total
attenuation.

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Fig : 7 Attenuation Vs. Wavelength characteristic
2. EXTRINSIC ATTENUATION
It is loss due to external sources. Extrinsic attenuation may occur as –
(I) Macro bending - The fibre is sharply bent so that the light travelling
down the fibre cannot make the turn & is lost in the cladding.
(II) Micro bending - Microbending or small bends in the fibre caused by
crushing contraction etc. These bends may not be visible with the naked eye.
Attenuation is measured in decibels (dB). A dB represents the comparison between the
transmitted and received power in a system.
Micro bend
Micro bend
Micro bend
Fig : 8 Micro bends

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Fig : 9 Macro bend
4.13 DISPERSION
Dispersion is the spreading of light pulse as its travels down the length of
an optical fibre as shown in figure. The varying delay in arrival time between
different components of a signal "smears out" the signal in time. This causes
energy overlapping and limits information capacity of the fiber.
Dispersion limits the bandwidth or information carrying capacity of a
fibre. The bit-rates must be low enough to ensure that pulses are farther apart and
therefore the greater dispersion can be tolerated.
Dispersion of optical energy within an optical fiber falls into following
categories:
 Intermodal Delay or Modal Delay
 Intramodal Dispersion or Chromatic Dispersion
 Material Dispersion
 Waveguide Dispersion

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 Polarization –Mode Dispersion
Fig : 10 Dispersion
1. INTERMODAL DELAY/ MODAL DELAY
Intermodal distortion or modal delay appears only in multimode fibers. This signal
distortion mechanism is a result of each mode having a different value of the group
velocity at a single frequency.
The amount of spreading that occurs in a fiber is a function of the number of
modes propagated by the fiber and length of the fiber
 Intermodal or modal dispersion causes the input light pulse to spread. The input
light pulse is made up of a group of modes (MULTIMODE). As the modes
propagate along the fiber, light energy distributed among the modes is delayed by
different amounts. Modes travel in different directions, some modes travel longer
distances.
 Modal dispersion occurs because each mode travels a different distance over the
same time span

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 The modes of a light pulse that enter the fiber at one time exit the fiber different
times.
 This condition causes the light pulse to spread.
 As the length of the fiber increases, modal dispersion increases.
Fig : 11 Intermodal Dispersion
2. INTRAMODAL DISPESION
 Pulse spreading that occurs within a single mode
 Intra-modal dispersion occurs because different colors of light travel through
different materials and different waveguide structures at different speeds
 Also called GROUP VELOCITY DISPERSION (GVD)
 Occurs in all types of fibers
 Two main causes : Material dispersion
Waveguide dispersion
A. Material Dispersion
 Arises from variations of the refractive index of the core material as a function of
wavelength
 Different wavelengths travel at different speeds in the fiber material and hence
exit the fiber at different times

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Fig : 12 Material Dispersion
 Material dispersion is a function of the source spectral width.
 The spectral width specifies the range of wavelengths that can propagate in the
fiber.
 Material dispersion is less at longer wavelengths
B. Waveguide Dispersion
 Arises because a Single Mode Fiber confines only 80% of the optical power to the
core
 The other 20% tends to travel through the cladding and hence travels faster
 This results in spreading of the light pulses
 The amount of dispersion depends on the fiber design and the size of the fiber
core relative to the wavelength of operation
 In multimode fibers, waveguide dispersion and material dispersion are basically
separate properties.
 Multimode waveguide dispersion is generally small compared to material
dispersion and is usually neglected.

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 Arises from dependence of waveguide 'size' on wavelength
 Causing light distribution between core and cladding to change with l
 Light distribution and dispersion depend on core-cladding design
 Proportional to source bandwidth and fiber length
 Same dimensions as material dispersion
 Can cancel material dispersion if signs are opposite
4.14 BANDWIDTH
It is defined as the amount of information that a system can carry such that each
pulse of light is distinguishable by the receiver. System bandwidth is measured in MHz or
GHz. In general, when we say that a system has bandwidth of 20 MHz, means that 20
million pulses of light per second will travel down the fibre and each will be
distinguishable by the receiver.
1.13.7 BANDWIDTH-LENGTH PRODUCT
Bandwidth is a length dependent. Longer fibre results in more pulse spreading and
leads to lower BW. As a result, the fibre BW is often given in terms of the BW times
kilometer product. A 1000 MHz x km fibre can usually operate with 100 MHz BW if a 10
km fibre is used or with a 1000 MHz BW if a 1 km fibre is used.
1.13.8 ELECTRICAL AND OPTICAL BANDWIDTH
A distinction must be made between electrical and optical BW. Electrical
bandwidth (BWel) is defined drops to 0.707. The optical bandwidth (BWopt) is defined as
the frequency at which the ratio, PLo/PLi dropped to 1/2. (The ratio Iout/Iin and PLo/PLi have
maximum values of 1). Because PLi and PLo are directly proportional to Iin and Iout
respectively (and not to Iin
2
and Iout
2
as in an all electrical system), the half power point is
equivalent to the half current point. That is the point where Iout/Iin drops to 0.50, not to
0.707. This results in a BWopt that is larger than the BWel.
BWel = 0.707 x BWopt

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It is important to realize that these two parameters represent two ways of
describing the same system. For example, a system can be said to have an optical BW of
10 MHz, which implies that its electrical BW is 7.07 MHz.
Fig : 13 ELECTRICAL AND OPTICAL BANDWIDTH
4.13 Summary
Fiber-optic characteristics can be classified as linear and nonlinear. Nonlinear
characteristics are influenced by parameters, such as bit rates, channel spacing, and power
levels. The loss or attenuation in fibre depends on the wavelength of the light propagating
within it and Dispersion (sometimes called chromatic dispersion) is a limiting factor in
fibre bandwidth.
4.14 References and Suggested Further Readings
 ITU-T manual on OF installation
 EI of BSNL
 EI on underground OF cable laying works by BBNL
 Fiber Optics Technician's Manual
 Understanding optical communication by Dutton
 Planning Fiber Optic Networks by Bob Chomycz
 www.timbercon.com
 http://www.ofsoptics.com

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 http://www.thefoa.org/
 http://www.corning.com
 http://www.fiber-optics.info
 http://www.rp-photonics.com
 http://www.occfiber.com and other websites
4.15 Worksheet
Qu.1 Fill in the Blanks
1. Frequency is measured in units of ………
2. ………… is the change in direction of propagation of a wave when the light ray
passes from one medium into another, and changes its speed.
3. ………… due to absorption is caused by the intrinsic properties of the material
itself, the impurities in the glass, and any atomic defects in the glass.
4. …………. is the spreading of light pulse as its travels down the length of an
optical fibre.
5. ……………… dispersion occurs because different colors of light travel through
different materials.
Qu.2 State True or False
1. Dispersion limits the bandwidth or information carrying capacity of a fibre.
2. The primary factors affecting attenuation in optical fibers are the length of the
fiber and the wavelength of the light.
3. Intermodal distortion or modal delay appears only in single mode fibers.
4. Material dispersion is a function of the source spectral width.
5. Bandwidth is a length dependent.
Qu.3 write down the wavelengths of optical windows?
1.
2.
3.

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Qu.4 What is dispersion?
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Qu.5. Define Bandwidth?
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