FIBER OPTIC CONCEPTS
LEARNING OBJECTIVES
Upon completion of this chapter, you
should be able to do the following:
Understand the nature
of light propagation. |
Discuss the
electromagnetic theory of light. |
Describe the
properties of light reflection, refraction, diffusion, and absorption. |
Explain how optical
fibers transmit light. |
Identify the basic
optical fiber material properties. |
Describe the ray and
mode theories of light propagation along an optical fiber. |
State the difference
between multimode and single mode optical fibers. |
Explain how optical
fibers attenuate and distort light signals as they travel along the optical
fiber. |
Understand the
processes of light attenuation and dispersion. |
Propagation of light
Properties of light
Reflection of light
Refraction of light
Diffusion of light
Absorption of light
Basic optical-material
properties
Basic structure of an optical
fiber
Propagation of light along a
fiber
Mode Theory
Optical fiber types
Multimode Fibers
Properties of optical
transmission
Attenuation
Dispersion
Summary
Answers
FIBER OPTIC LIGHT TRANSMISSION
Fiber optics deals with the transmission
of light energy through transparent fibers. How an optical fiber guides light
depends on the nature of the light and the structure of the optical fiber. A
light wave is a form of energy that is moved by wave motion. Wave motion can be
defined as a recurring disturbance advancing through space with or without the
use of a physical medium. In fiber optics, wave motion is the movement of light
energy through an optical fiber.
To fully understand the concept of wave
motion, refer to NEETS Module 10 - Introduction
to Wave Propagation, Transmission Lines, and Antennas. Before we
introduce the subject of light transmission through optical fibers, you must
first understand the nature of light and the properties of light
PROPAGATION OF LIGHT
The exact nature of light is not fully
understood, although people have been studying the subject for many centuries.
In the 1700s and before, experiments seemed to indicate that light was composed
of particles. In the early 1800s, a physicist Thomas Young showed that light
exhibited wave characteristics.
Further experiments by other physicists
culminated in James Clerk (pronounced
The advent of quantum physics successfully
explained the photoelectric effect in terms of fundamental particles of energy
called quanta. Quanta are known as photons when referring to
light energy.
Today, when studying light that consists
of many photons, as in propagation, that light behaves as a continuum - an
electromagnetic wave. On the other hand, when studying the interaction of light
with semiconductors, as in sources and detectors, the quantum physics approach
is taken. The wave versus particle dilemma can be addressed in a more formal
way, but that is beyond the scope of this text. It suffices to say that much
has been reconciled between the two using quantum physics. In this manual, we
use both the electromagnetic wave and photon concepts, each in the places where
it best matches the phenomenon we are studying.
The electromagnetic energy of light is a
form of electromagnetic radiation.
Light and similar forms of radiation are
made up of moving electric and magnetic forces. A simple example of motion
similar to these radiation waves can be made by dropping a pebble into a pool
of water. In this example, the water is not actually being moved by the outward
motion of the wave, but rather by the up-and-down motion of the water. The
up-and-down motion is transverse, or at right angles, to the outward motion of
the waves. This type of wave motion is called transverse-wave motion.
The transverse waves spread out in expanding circles until they reach the edge
of the pool, in much the same manner as the transverse waves of light spread
from the sun. However, the waves in the pool are very slow and clumsy in
comparison with light, which travels approximately 186,000 miles per second.
Light radiates from its source in all
directions until it is absorbed or diverted by some substance (fig. 2-1). The
lines drawn from the light source (a light bulb in this instance) to any point
on one of the transverse waves indicate the direction that the wavefronts are
moving. These lines, are called light rays.
Figure 2-1. - Light rays and wavefronts
from a nearby light source.
Although single rays of light typically do
not exist, light rays shown in illustrations are a convenient method used to
show the direction in which light is traveling at any point. A ray of light can
be illustrated as a straight line.
Q.1 Quantum physics successfully explained
the photoelectric effect in terms of fundamental particles of energy called
quanta. What are the fundamental particles of energy (quanta) known as when
referring to light energy?
Q.2 What type of wave motion is represented by the motion of water?
PROPERTIES OF LIGHT
When light waves, which travel in straight
lines, encounter any substance, they are either reflected, absorbed,
transmitted, or refracted. This is illustrated in figure 2-2. Those substances
that transmit almost all the light waves falling upon them are said to be transparent.
A transparent substance is one through which you can see clearly.
Clear glass is transparent because it
transmits light rays without diffusing them (view A of figure 2-3). There is no
substance known that is perfectly transparent, but many substances are nearly
so. Substances through which some light rays can pass, but through which
objects cannot be seen clearly because the rays are diffused, are called translucent
(view B of figure 2-3). The frosted glass of a light bulb and a piece of oiled
paper are examples of translucent materials. Those substances that are unable
to transmit any light rays are called opaque
(view C of figure 2-3). Opaque substances either reflect or absorb all the
light rays that fall upon them.
Figure 2-2. - Light waves reflected,
absorbed, and transmitted.
Figure 2-3. - Substances: A. Transparent;
B. Translucent; and C. Opaque.
All substances that are not light sources
are visible only because they reflect all or some part of the light reaching
them from some luminous source.
Examples of luminous sources include the
sun, a gas flame, and an electric light filament, because they are sources of
light energy. If light is neither transmitted nor reflected, it is absorbed or
taken up by the medium. When light strikes a substance, some absorption and
some reflection always take place. No substance completely transmits, reflects,
or absorbs all the light rays that reach its surface.
Q.3 When light waves encounter any
substance, what four things can happen?
Q.4 A substance that transmits almost all of the light waves falling upon it is
known as what type of substance?
Q.5 A substance that is unable to transmit any light waves is known as what
type of substance?
REFLECTION OF LIGHT
Reflected waves are simply those waves that are neither
transmitted nor absorbed, but are reflected from the surface of the medium they
encounter. When a wave approaches a reflecting surface, such as a mirror, the
wave that strikes the surface is called the incident wave, and the one that bounces back is called
the reflected wave (refer to figure 2-4). An imaginary line
perpendicular to the point at which the incident wave strikes the reflecting
surface is called the normal, or the perpendicular. The angle between
the incident wave and the normal is called the angle of incidence.
The angle between the reflected wave and
the normal is called the angle of
reflection.
Figure 2-4. - Reflection of a wave.
If the surface of the medium contacted by
the incident wave is smooth and polished, each reflected wave will be reflected
back at the same angle as the incident wave. The path of the wave reflected
from the surface forms an angle equal to the one formed by its path in reaching
the medium.
This conforms to the law of reflection
which states: The angle of incidence is equal to the angle of reflection.
The amount of incident-wave energy that is
reflected from a surface depends on the nature of the surface and the angle at
which the wave strikes the surface. The amount of wave energy reflected
increases as the angle of incidence increases. The reflection of energy is the
greatest when the wave is nearly parallel to the reflecting surface. When the
incidence wave is perpendicular to the surface, more of the energy is
transmitted into the substance and reflection of energy is at its least. At any
incident angle, a mirror reflects almost all of the wave energy, while a dull,
black surface reflects very little.
Light waves obey the law of reflection.
Light travels in a straight line through a substance of uniform density. For
example, you can see the straight path of light rays admitted through a narrow
slit into a darkened room. The straight path of the beam is made visible by
illuminated dust particles suspended in the air. If the light is made to fall
onto the surface of a mirror or other reflecting surface, however, the
direction of the beam changes sharply.
The light can be reflected in almost any
direction, depending on the angle with which the mirror is held.
Q.6 What is the law of reflection?
Q.7 When a wave is reflected from a surface, energy is reflected. When is the
reflection of energy the greatest?
Q.8 When is the reflection energy the least?
Q.9 Light waves obey what law?
REFRACTION OF LIGHT
When a light wave passes from one medium
into a medium having a different velocity of propagation (the speed waves can
travel through a medium), a change in the direction of the wave will occur.
This change of direction as the wave enters the second medium is called refraction.
As in the discussion of reflection, the wave striking the boundary
(surface) is called the incident wave,
and the imaginary line perpendicular to the boundary is called the normal.
The angle between the incident wave and the normal is called the angle of
incidence. As the wave passes through the boundary, it is bent either
toward or away from the normal. The angle between the normal and the path of
the wave through the second medium is the angle of refraction.
A light wave passing through a block of
glass is shown in figure 2-5. The wave moves from point A to point B at a
constant speed. This is the incident wave. As the wave penetrates the glass
boundary at point B, the velocity of the wave is slowed down. This causes the
wave to bend toward the normal. The wave then takes the path from point B to
point C through the glass and becomes both the refracted wave from the
top surface and the incident wave to the lower surface. As the wave
passes from the glass to the air (the second boundary), it is again refracted,
this time away from the normal, and takes the path from point C to point D.
After passing through the last boundary, the velocity increases to the original
velocity of the wave. As illustrated, refracted waves can bend toward or away
from the normal.
This bending depends on the velocity of
the wave through different mediums.
The broken line between points B and E is
the path that the wave would travel if the two mediums (air and glass) had the
same density.
Figure 2-5. - Refraction of a wave.
Another interesting condition can be shown
using figure 2-5. If the wave passes from a less dense to a more dense medium,
it is bent toward the normal, and the angle of refraction (r) is less than the
angle of incidence (i). Likewise, if the wave passes from a more dense to a
less dense medium, it is bent away from the normal, and the angle of refraction
(r1) is greater than the angle of incidence (i1).
An example of refraction is the apparent
bending of a spoon when it is immersed in a cup of water. The bending seems to
take place at the surface of the water, or exactly at the point where there is
a change of density.
Obviously, the spoon does not bend from
the pressure of the water. The light forming the image of the spoon is bent as
it passes from the water (a medium of high density) to the air (a medium of
comparatively low density).
Without refraction, light waves would pass
in straight lines through transparent substances without any change of
direction. Figure 2-5 shows that rays striking the glass at any angle other
than perpendicular are refracted. However, perpendicular rays, which enter the glass
normal to the surface, continue through the glass and into the air in a
straight line - no refraction takes place.
Q.10 A refracted wave occurs when a wave
passes from one medium into another medium. What determines the angle of
refraction?
Q.11 A light wave enters a sheet of glass at a perfect right angle to the
surface. Is the majority of the wave reflected, refracted, transmitted, or
absorbed?
DIFFUSION OF LIGHT
When light is reflected from a mirror, the
angle of reflection equals the angle of incidence. When light is reflected from
a piece of plain white paper; however, the reflected beam is scattered, or diffused,
as shown in figure 2-6. Because the surface of the paper is not smooth, the
reflected light is broken up into many light beams that are reflected in all
directions.
Figure 2-6. - Diffusion of light.
Q.12 When light strikes a piece of white
paper, the light is reflected in all directions. What do we call this
scattering of light?
ABSORPTION OF LIGHT
You have just seen that a light beam is
reflected and diffused when it falls onto a piece of white paper. If the light
beam falls onto a piece of black paper, the black paper absorbs most of
the light rays and very little light is reflected from the paper.
If the surface upon which the light beam
falls is perfectly black, there is no reflection; that is, the light is totally
absorbed. No matter what kind of surface light falls upon, some of the light is
absorbed.
TRANSMISSION OF LIGHT THROUGH OPTICAL
FIBERS
The transmission of light along optical
fibers depends not only on the nature of light, but also on the structure of
the optical fiber. Two methods are used to describe how light is transmitted
along the optical fiber. The first method, ray theory, uses the concepts
of light reflection and refraction. The second method, mode theory,
treats light as electromagnetic waves. You must first understand the basic
optical properties of the materials used to make optical fibers. These
properties affect how light is transmitted through the fiber.
Q.13 Two methods describe how light
propagates along an optical fiber. These methods define two theories of light
propagation. What do we call these two theories?
BASIC OPTICAL-MATERIAL PROPERTIES
The basic optical property of a material,
relevant to optical fibers, is the index of refraction. The index of
refraction (n) measures the speed of light in an optical medium. The index of
refraction of a material is the ratio of the speed of light in a vacuum to the
speed of light in the material itself. The speed of light (c) in free space
(vacuum) is 3 X 108 meters per second (m/s). The speed of light is
the frequency (f) of light multiplied by the wavelength of light
(λ). When light enters the fiber material (an optically dense
medium), the light travels slower at a speed (v). Light will always travel
slower in the fiber material than in air. The index of refraction is given by:
A light ray is reflected and refracted
when it encounters the boundary between two different transparent mediums. For
example, figure 2-7 shows what happens to the light ray when it encounters the
interface between glass and air. The index of refraction for glass (n1)
is 1.50. The index of refraction for air (n2) is 1.00.
Figure 2-7. - Light reflection and
refraction at a glass-air boundary.
Let's assume the light ray or incident ray
is traveling through the glass. When the light ray encounters the glass-air
boundary, there are two results. The first result is that part of the ray is
reflected back into the glass. The second result is that part of the ray is
refracted (bent) as it enters the air. The bending of the light at the
glass-air interface is the result of the difference between the index of
refractions. Since n1 is greater than n2, the angle of
refraction (Θ2) will be greater than the angle of
incidence (Θ1). Snell's law of refraction is used to
describe the relationship between the incident and the refracted rays at the
boundary. Snell's Law
is given by:
As the angle of incidence (Θ1)
becomes larger, the angle of refraction (Θ2) approaches 90
degrees. At this point, no refraction is possible. The light ray is totally
reflected back into the glass medium. No light escapes into the air. This
condition is called total internal reflection.
The angle at which total internal
reflection occurs is called the critical
angle of incidence. The critical angle of incidence
(Θc) is shown in figure 2-8. At any angle of incidence
(Θ1) greater than the critical angle, light is totally
reflected back into the glass medium. The critical angle of incidence is
determined by using Snell's Law. The critical angle is given by:
Figure 2-8. - Critical angle of incidence.
The condition of total internal reflection
is an ideal situation.
However, in reality, there is always some
light energy that penetrates the boundary. This situation is explained by the
mode theory, or the electromagnetic wave theory, of light.
Q.14 What is the basic optical-material
property relevant to optical fiber light transmission?
Q.15 The index of refraction measures the speed of light in an optical fiber.
Will light travel faster in an optically dense material or in one that is less
dense?
Q.16 Assume light is traveling through glass, what happens when this light
strikes the glass-air boundary?
Q.17 What condition causes a light ray to be totally reflected back into its
medium of propagation?
Q.18 What name is given to the angle where total internal reflection occurs?
BASIC STRUCTURE OF AN OPTICAL FIBER
The basic structure of an optical fiber
consists of three parts; the core, the cladding, and the coating
or buffer. The basic structure of an optical fiber is shown in figure
2-9. The core is a cylindrical rod of dielectric material. Dielectric
material conducts no electricity. Light propagates mainly along the core of the
fiber. The core is generally made of glass. The core is described as
having a radius of (a) and an index of refraction n1. The core is
surrounded by a layer of material called the cladding. Even though light
will propagate along the fiber core without the layer of cladding material, the
cladding does perform some necessary functions.
Figure 2-9. - Basic structure of an
optical fiber.
The cladding layer is made of a
dielectric material with an index of refraction n2. The index of
refraction of the cladding material is less than that of the core material. The
cladding is generally made of glass or plastic. The cladding performs the following
functions:
For extra protection, the cladding is
enclosed in an additional layer called the coating or buffer. The
coating or buffer is a layer of material used to protect an
optical fiber from physical damage. The material used for a buffer is a type of
plastic.
The buffer is elastic in nature and
prevents abrasions. The buffer also prevents the optical fiber from scattering
losses caused by microbends. Microbends occur when an optical fiber is placed
on a rough and distorted surface. Microbends are discussed later in this
chapter.
Q.19 List the three parts of an optical
fiber.
Q.20 Which fiber material, core or cladding, has a higher index of refraction
PROPAGATION OF LIGHT ALONG A FIBER
The concept of light propagation, the
transmission of light along an optical fiber, can be described by two theories.
According to the first theory, light is described as a simple ray. This theory
is the ray theory, or geometrical optics, approach. The advantage of the ray
approach is that you get a clearer picture of the propagation of light along a
fiber. The ray theory is used to approximate the light acceptance and guiding
properties of optical fibers. According to the second theory, light is
described as an electromagnetic wave. This theory is the mode theory, or wave
representation, approach. The mode theory describes the behavior of light
within an optical fiber. The mode theory is useful in describing the optical
fiber properties of absorption, attenuation, and dispersion. These fiber
properties are discussed later in this chapter.
Q.21 Light transmission along an optical
fiber is described by two theories. Which theory is used to approximate the
light acceptance and guiding properties of an optical fiber?
Ray Theory
Two types of rays can propagate along an
optical fiber. The first type is called meridional rays. Meridional rays
are rays that pass through the axis of the optical fiber. Meridional rays are
used to illustrate the basic transmission properties of optical fibers.
The second type is called skew rays. Skew
rays are rays that travel through an optical fiber without passing through
its axis.
MERIDIONAL RAYS. - Meridional rays can be classified as bound or
unbound rays. Bound rays remain in the core and propagate along the axis of the
fiber. Bound rays propagate through the fiber by total internal
reflection. Unbound rays are refracted out of the fiber core. Figure
2-10 shows a possible path taken by bound and unbound rays in a step-index
fiber. The core of the step-index fiber has an index of refraction n1.
The cladding of a step-index has an index of refraction n2, that is
lower than n1. Figure 2-10 assumes the core-cladding interface is
perfect. However, imperfections at the core-cladding interface will cause part
of the bound rays to be refracted out of the core into the cladding. The light
rays refracted into the cladding will eventually escape from the fiber. In
general, meridional rays follow the laws of reflection and refraction.
Figure 2-10. - Bound and unbound rays in a
step-index fiber.
It is known that bound rays propagate in
fibers due to total internal reflection, but how do these light rays enter the
fiber? Rays that enter the fiber must intersect the core-cladding interface at
an angle greater than the critical angle (Θc). Only those
rays that enter the fiber and strike the interface at these angles will
propagate along the fiber.
How a light ray is launched into a fiber
is shown in figure 2-11. The incident ray I1 enters the fiber at the
angle Θa. I1 is refracted upon entering the
fiber and is transmitted to the core-cladding interface. The ray then strikes
the core-cladding interface at the critical angle (Θ c). I1
is totally reflected back into the core and continues to propagate along the fiber.
The incident ray I2 enters the fiber at an angle greater than
Θa. Again, I2 is refracted upon entering the
fiber and is transmitted to the core-cladding interface. I2 strikes
the core-cladding interface at an angle less than the critical angle (Θc).
I2 is refracted into the cladding and is eventually lost. The light
ray incident on the fiber core must be within the acceptance cone
defined by the angle Θa shown in figure 2-12.
Angle Θa is defined
as the acceptance angle. The acceptance
angle (Θa) is the maximum angle to the
axis of the fiber that light entering the fiber is propagated. The value of the
angle of acceptance (Θa) depends on fiber properties and
transmission conditions.
Figure 2-11. - How a light ray enters an
optical fiber.
Figure 2-12. - Fiber acceptance angle.
The acceptance angle is related to the
refractive indices of the core, cladding, and medium surrounding the fiber.
This relationship is called the numerical aperture of the fiber. The numerical
aperture (NA) is a measurement of the ability of an optical fiber to capture
light. The NA is also used to define the acceptance cone of an optical fiber.
Figure 2-12 illustrates the relationship
between the acceptance angle and the refractive indices. The index of
refraction of the fiber core is n1. The index of refraction of the
fiber cladding is n2. The index of refraction of the surrounding
medium is n0. By using Snell's law and basic trigonometric
relationships, the NA of the fiber is given by:
Since the medium next to the fiber at the
launching point is normally air, n0 is equal to 1.00. The NA is then
simply equal to sin Θa.
The NA is a convenient way to measure the
light-gathering ability of an optical fiber. It is used to measure source-to-fiber
power-coupling efficiencies. A high NA indicates a high source-to-fiber
coupling efficiency.
Source-to-fiber coupling efficiency is
described in chapter 6. Typical values of NA range from 0.20 to 0.29 for glass
fibers. Plastic fibers generally have a higher NA. An NA for plastic fibers can
be higher than 0.50.
In addition, the NA is commonly used to
specify multimode fibers.
However, for small core diameters, such as
in single mode fibers, the ray theory breaks down. Ray theory describes only
the direction a plane wave takes in a fiber. Ray theory eliminates any
properties of the plane wave that interfere with the transmission of light
along a fiber. In reality, plane waves interfere with each other. Therefore,
only certain types of rays are able to propagate in an optical fiber. Optical
fibers can support only a specific number of guided modes. In small core
fibers, the number of modes supported is one or only a few modes. Mode theory
is used to describe the types of plane waves able to propagate along an optical
fiber.
SKEW RAYS. - A possible path of propagation of skew rays is
shown in figure 2-13.
Figure 2-13, view A, provides an angled
view and view B provides a front view.
Skew rays propagate without passing
through the center axis of the fiber.
The acceptance angle for skew rays is
larger than the acceptance angle of meridional rays. This condition explains
why skew rays outnumber meridional rays. Skew rays are often used in the
calculation of light acceptance in an optical fiber. The addition of skew rays
increases the amount of light capacity of a fiber. In large NA fibers, the
increase may be significant.
Figure 2-13. - Skew ray propagation: A.
Angled view; B. Front view.
The addition of skew rays also increases
the amount of loss in a fiber. Skew rays tend to propagate near the edge of the
fiber core. A large portion of the number of skew rays that are trapped in the
fiber core are considered to be leaky rays. Leaky rays are predicted to
be totally reflected at the core-cladding boundary. However, these rays are
partially refracted because of the curved nature of the fiber boundary. Mode
theory is also used to describe this type of leaky ray loss.
Q.22 Meridional rays are classified as
either bound or unbound rays. Bound rays propagate through the fiber according
to what property?
Q.23 A light ray incident on the optical fiber core is propagated along the
fiber. Is the angle of incidence of the light ray entering the fiber larger or
smaller than the acceptance angle (Θa)
Q.24 What fiber property does numerical aperture (NA) measure?
Q.25 Skew rays and meridional rays define different acceptance angles. Which
acceptance angle is larger, the skew ray angle or the meridional ray angle?
Mode Theory
The mode theory, along with the ray
theory, is used to describe the propagation of light along an optical fiber.
The mode theory is used to describe the properties of light that ray theory is
unable to explain. The mode theory uses electromagnetic wave behavior to
describe the propagation of light along a fiber. A set of guided
electromagnetic waves is called the modes
of the fiber.
Q.26 The mode theory uses electromagnetic
wave behavior to describe the propagation of the light along the fiber. What is
a set of guided electromagnetic waves called?
PLANE WAVES. - The mode theory suggests that a light wave can
be represented as a plane wave. A plane
wave is described by its direction, amplitude, and wavelength
of propagation. A plane wave is a wave whose surfaces of constant phase are
infinite parallel planes normal to the direction of propagation.
The planes having the same phase are
called the wavefronts. The wavelength (λ) of the plane wave is given
by: where c is the speed of light in a
vacuum, f is the frequency of the light, and n is the index of
refraction of the plane-wave medium.
Figure 2-14 shows the direction and wavefronts
of plane-wave propagation. Plane waves, or wavefronts, propagate along the
fiber similar to light rays. However, not all wavefronts incident on the fiber
at angles less than or equal to the critical angle of light acceptance
propagate along the fiber. Wavefronts may undergo a change in phase that
prevents the successful transfer of light along the fiber.
Figure 2-14. - Plane-wave propagation.
Wavefronts are required to remain in phase
for light to be transmitted along the fiber. Consider the wavefront incident on
the core of an optical fiber as shown in figure 2-15. Only those wavefronts
incident on the fiber at angles less than or equal to the critical angle may
propagate along the fiber. The wavefront undergoes a gradual phase change as it
travels down the fiber. Phase changes also occur when the wavefront is
reflected. The wavefront must remain in phase after the wavefront transverses
the fiber twice and is reflected twice. The distance transversed is shown
between point A and point B on figure 2-15. The reflected waves at point A and
point B are in phase if the total amount of phase collected is an integer
multiple of 2π radian. If propagating wavefronts are not in phase, they
eventually disappear. Wavefronts disappear because of destructive
interference. The wavefronts that are in phase interfere with the
wavefronts that are out of phase. This interference is the reason why only a
finite number of modes can propagate along the fiber.
Figure 2-15. - Wavefront propagation along
an optical fiber.
The plane waves repeat as they travel
along the fiber axis. The direction the plane waves travel is assumed to be the
z direction as shown in figure 2-15. The plane waves repeat at a distance equal
to λ/sinΘ. Plane waves also repeat at a periodic frequency
β = 2π sin Θ/λ. The quantity β is
defined as the propagation constant
along the fiber axis. As the wavelength (λ) changes, the value of
the propagation constant must also change.
For a given mode, a change in wavelength
can prevent the mode from propagating along the fiber. The mode is no longer
bound to the fiber. The mode is said to be cut off. Modes that are bound at one
wavelength may not exist at longer wavelengths. The wavelength at which a mode
ceases to be bound is called the cutoff
wavelength for that mode. However, an optical fiber is always
able to propagate at least one mode. This mode is referred to as the
fundamental mode of the fiber. The fundamental mode can never be cut off.
The wavelength that prevents the next
higher mode from propagating is called the cutoff wavelength of the fiber. An
optical fiber that operates above the cutoff wavelength (at a longer
wavelength) is called a single mode fiber. An optical fiber that operates below
the cutoff wavelength is called a multimode fiber. Single mode and multimode
optical fibers are discussed later in this chapter.
In a fiber, the propagation constant of a
plane wave is a function of the wave's wavelength and mode. The change in the
propagation constant for different waves is called dispersion. The
change in the propagation constant for different wavelengths is called chromatic dispersion. The change in
propagation constant for different modes is called modal dispersion.
These dispersions cause the light pulse to
spread as it goes down the fiber (fig. 2-16). Some dispersion occurs in all
types of fibers. Dispersion is discussed later in this chapter.
Figure 2-16. - The spreading of a light
pulse.
MODES. - A set of guided electromagnetic waves is called the modes
of an optical fiber.
Maxwell's equations describe
electromagnetic waves or modes as having two components. The two components are
the electric field, E(x, y, z), and the magnetic field, H(x, y, z). The
electric field, E, and the magnetic field, H, are at right angles to each
other. Modes traveling in an optical fiber are said to be transverse. The
transverse modes, shown in figure 2-17, propagate along the axis of the fiber.
The mode field patterns shown in figure 2-17 are said to be transverse electric
(TE). In TE modes, the electric field is perpendicular to the direction of
propagation.
The magnetic field is in the direction of
propagation. Another type of transverse mode is the transverse magnetic (TM)
mode. TM modes are opposite to TE modes. In TM modes, the magnetic field is
perpendicular to the direction of propagation. The electric field is in the
direction of propagation. Figure 2-17 shows only TE modes.
Figure 2-17. - Transverse electric (TE)
mode field patterns.
The TE mode field patterns shown in figure
2-17 indicate the order of each mode. The order of each mode is
indicated by the number of field maxima within the core of the fiber. For example,
TE0 has one field maxima. The electric field is a maximum at the
center of the waveguide and decays toward the core-cladding boundary. TE0
is considered the fundamental mode or the lowest order standing wave. As the
number of field maxima increases, the order of the mode is higher. Generally,
modes with more than a few (5-10) field maxima are referred to as high-order
modes.
The order of the mode is also determined
by the angle the wavefront makes with the axis of the fiber. Figure 2-18
illustrates light rays as they travel down the fiber. These light rays indicate
the direction of the wavefronts. High-order modes cross the axis of the fiber
at steeper angles. Low-order and high-order modes are shown in figure 2-18.
Figure 2-18. - Low-order and high-order
modes.
Before we progress, let us refer back to
figure 2-17.
Notice that the modes are not confined to
the core of the fiber. The modes extend partially into the cladding material.
Low-order modes penetrate the cladding only slightly. In low-order modes, the
electric and magnetic fields are concentrated near the center of the fiber.
However, high-order modes penetrate further into the cladding material. In
high-order modes, the electrical and magnetic fields are distributed more
toward the outer edges of the fiber.
This penetration of low-order and
high-order modes into the cladding region indicates that some portion is
refracted out of the core. The refracted modes may become trapped in the
cladding due to the dimension of the cladding region. The modes trapped in the
cladding region are called cladding modes. As the core and the cladding
modes travel along the fiber, mode coupling occurs. Mode coupling is the
exchange of power between two modes. Mode coupling to the cladding results in
the loss of power from the core modes.
In addition to bound and refracted modes,
there are leaky modes.
Leaky modes are similar to leaky rays. Leaky
modes lose power as they propagate along the fiber. For a mode to remain
within the core, the mode must meet certain boundary conditions. A mode remains
bound if the propagation constant β meets the following boundary
condition:
where n1 and n2 are
the index of refraction for the core and the cladding, respectively. When the
propagation constant becomes smaller than 2πn2/λ,
power leaks out of the core and into the cladding. Generally, modes leaked into
the cladding are lost in a few centimeters. However, leaky modes can carry a
large amount of power in short fibers.
NORMALIZED FREQUENCY. - Electromagnetic waves bound to an optical fiber
are described by the fiber's normalized frequency.
The normalized frequency determines
how many modes a fiber can support. Normalized frequency is a dimensionless
quantity.
Normalized frequency is also related to
the fiber's cutoff wavelength. Normalized frequency (V) is defined as:
where n1 is the core index of
refraction, n2 is the cladding index of refraction, a is the
core diameter, and λ is the wavelength of light in air.
The number of modes that can exist in a
fiber is a function of V. As the value of V increases, the number of modes
supported by the fiber increases. Optical fibers, single mode and multimode,
can support a different number of modes. The number of modes supported by
single mode and multimode fiber types is discussed later in this chapter.
Q.27 A light wave can be represented as a
plane wave. What three properties of light propagation describe a plane wave?
Q.28 A wavefront undergoes a phase change as it travels along the fiber. If the
wavefront transverses the fiber twice and is reflected twice and the total
phase change is equal to 1/2π, will the wavefront disappear? If yes,
why?
Q.29 Modes that are bound at one wavelength may not exist at longer
wavelengths. What is the wavelength at which a mode ceases to be bound called?
Q.30 What type of optical fiber operates below the cutoff wavelength?
Q.31 Low-order and high-order modes propagate along an optical fiber. How are
modes determined to be low-order or high-order modes?
Q.32 As the core and cladding modes travel along the fiber, mode coupling
occurs. What is mode coupling?
Q.33 The fiber's normalized frequency (V) determines how many modes a fiber can
support. As the value of V increases, will the number of modes supported by the
fiber increase or decrease?
OPTICAL FIBER TYPES
Optical fibers are characterized by their
structure and by their properties of transmission. Basically, optical fibers
are classified into two types. The first type is single mode fibers. The second
type is multimode fibers. As each name implies, optical fibers are classified
by the number of modes that propagate along the fiber. As previously explained,
the structure of the fiber can permit or restrict modes from propagating in a
fiber. The basic structural difference is the core size. Single mode fibers are
manufactured with the same materials as multimode fibers. Single mode fibers
are also manufactured by following the same fabrication process as multimode
fibers.
Single Mode Fibers
The core size of single mode fibers is
small. The core size (diameter) is typically around 8 to 10 micrometers
(μm). A fiber core of this size allows only the fundamental or lowest
order mode to propagate around a 1300 nanometer (nm) wavelength. Single mode
fibers propagate only one mode, because the core size approaches the
operational wavelength (λ). The value of the normalized frequency
parameter (V) relates core size with mode propagation.
In single mode fibers, V is less than or
equal to 2.405. When V ≤ 2.405, single mode fibers propagate the
fundamental mode down the fiber core, while high-order modes are lost in the
cladding. For low V values (≤1.0), most of the power is propagated in
the cladding material. Power transmitted by the cladding is easily lost at fiber
bends. The value of V should remain near the 2.405 level.
Single mode fibers have a lower signal
loss and a higher information capacity (bandwidth) than multimode fibers.
Single mode fibers are capable of transferring higher amounts of data due to low
fiber dispersion. Basically, dispersion is the spreading of light as light
propagates along a fiber. Dispersion mechanisms in single mode fibers are
discussed in more detail later in this chapter. Signal loss depends on the
operational wavelength (λ). In single mode fibers, the wavelength
can increase or decrease the losses caused by fiber bending. Single mode fibers
operating at wavelengths larger than the cutoff wavelength lose more power at
fiber bends. They lose power because light radiates into the cladding, which is
lost at fiber bends. In general, single mode fibers are considered to be
low-loss fibers, which increase system bandwidth and length.
Q.34 The value of the normalized frequency
parameter (V) relates the core size with mode propagation. When single mode
fibers propagate only the fundamental mode, what is the value of V?
Multimode Fibers
As their name implies, multimode fibers
propagate more than one mode. Multimode fibers can propagate over 100 modes.
The number of modes propagated depends on the core size and numerical aperture
(NA). As the core size and
NA increase, the number of modes
increases. Typical values of fiber core size and NA are 50 to 100 μm and
0.20 to 0.29, respectively.
A large core size and a higher NA have several
advantages. Light is launched into a multimode fiber with more ease. The higher
NA and the larger core size make it easier to make fiber connections. During
fiber splicing, core-to-core alignment becomes less critical. Another advantage
is that multimode fibers permit the use of light-emitting diodes (LEDs). Single
mode fibers typically must use laser diodes. LEDs are cheaper, less complex,
and last longer. LEDs are preferred for most applications.
Multimode fibers also have some
disadvantages. As the number of modes increases, the effect of modal dispersion
increases. Modal dispersion (intermodal dispersion) means that modes arrive at
the fiber end at slightly different times. This time difference causes the
light pulse to spread. Modal dispersion affects system bandwidth. Fiber
manufacturers adjust the core diameter, NA, and index profile properties of
multimode fibers to maximize system bandwidth.
Q.35 The number of modes propagated in a
multimode fiber depends on core size and numerical aperture (NA). If the core
size and the NA decrease, will the number of modes propagated increase or
decrease?
Q.36 Modal dispersion affects the bandwidth of multimode systems. It is
essential to adjust what three fiber properties to maximize system bandwidth?
PROPERTIES OF OPTICAL FIBER TRANSMISSION
The principles behind the transfer of
light along an optical fiber were discussed earlier in this chapter. You
learned that propagation of light depended on the nature of light and the
structure of the optical fiber. However, our discussion did not describe how
optical fibers affect system performance.
In this case, system performance deals
with signal loss and bandwidth.
Signal loss and system bandwidth describe
the amount of data transmitted over a specified length of fiber. Many optical
fiber properties increase signal loss and reduce system bandwidth. The most
important properties that affect system performance are fiber attenuation and
dispersion.
Attenuation reduces the amount of optical
power transmitted by the fiber. Attenuation controls the distance an optical
signal (pulse) can travel as shown in figure 2-19. Once the power of an optical
pulse is reduced to a point where the receiver is unable to detect the pulse,
an error occurs. Attenuation is mainly a result of light absorption, scattering,
and bending losses.
Dispersion spreads the optical pulse as it travels along the fiber. This
spreading of the signal pulse reduces the system bandwidth or the
information-carrying capacity of the fiber. Dispersion limits how fast
information is transferred as shown in figure 2-19. An error occurs when the
receiver is unable to distinguish between input pulses caused by the spreading
of each pulse. The effects of attenuation and dispersion increase as the pulse
travels the length of the fiber as shown in figure 2-20.
Figure 2-19. - Fiber transmission
properties.
Figure 2-20. - Pulse spreading and power
loss along an optical fiber.
In addition to fiber attenuation and
dispersion, other optical fiber properties affect system performance. Fiber
properties, such as modal noise, pulse broadening, and polarization, can reduce
system performance.
Modal noise, pulse broadening, and
polarization are too complex to discuss as introductory level material.
However, you should be aware that attenuation and dispersion are not the only
fiber properties that affect performance.
Q.37 Attenuation is mainly a result of
what three properties?
Attenuation
Attenuation in an optical fiber is caused
by absorption, scattering, and bending losses. Attenuation is the loss
of optical power as light travels along the fiber. Signal attenuation is
defined as the ratio of optical input power (Pi) to the optical
output power (Po). Optical input power is the power injected into
the fiber from an optical source. Optical output power is the power received at
the fiber end or optical detector. The following equation defines signal
attenuation as a unit of length:
Signal attenuation is a log relationship.
Length (L) is expressed in kilometers. Therefore, the unit of attenuation is
decibels/kilometer (dB/km). As previously stated, attenuation is caused by
absorption, scattering, and bending losses. Each mechanism of loss is
influenced by fiber-material properties and fiber structure. However, loss is
also present at fiber connections. Fiber connector, splice, and coupler losses
are discussed in chapter 4. The present discussion remains relative to optical
fiber attenuation properties.
Q.38 Define attenuation.
ABSORPTION. - Absorption is a major cause of signal loss in
an optical fiber. Absorption is defined as the portion of attenuation
resulting from the conversion of optical power into another energy form, such
as heat. Absorption in optical fibers is explained by three factors:
Imperfections in the atomic structure
induce absorption by the presence of missing molecules or oxygen defects.
Absorption is also induced by the diffusion of hydrogen molecules into the
glass fiber. Since intrinsic and extrinsic material properties are the main
cause of absorption, they are discussed further.
Intrinsic Absorption. - Intrinsic absorption is caused by basic
fiber-material properties. If an optical fiber were absolutely pure, with no
imperfections or impurities, then all absorption would be intrinsic. Intrinsic
absorption sets the minimal level of absorption.
In fiber optics, silica (pure glass) fibers
are used predominately. Silica fibers are used because of their low intrinsic
material absorption at the wavelengths of operation.
In silica glass, the wavelengths of
operation range from 700 nanometers (nm) to 1600 nm. Figure 2-21 shows the
level of attenuation at the wavelengths of operation. This wavelength of
operation is between two intrinsic absorption regions. The first region is the ultraviolet
region (below 400-nm wavelength). The second region is the infrared
region (above 2000-nm wavelength).
Figure 2-21. - Fiber losses.
Intrinsic absorption in the ultraviolet
region is caused by electronic absorption bands. Basically, absorption occurs
when a light particle (photon) interacts with an electron and excites it to a
higher energy level. The tail of the ultraviolet absorption band is shown in
figure 2-21.
The main cause of intrinsic absorption
in the infrared region is the characteristic vibration frequency of atomic
bonds. In silica glass, absorption is caused by the vibration of silicon-oxygen
(Si-O) bonds. The interaction between the vibrating bond and the
electromagnetic field of the optical signal causes intrinsic absorption. Light
energy is transferred from the electromagnetic field to the bond. The tail of
the infrared absorption band is shown in figure 2-21.
Extrinsic Absorption. - Extrinsic absorption is caused by impurities
introduced into the fiber material. Trace metal impurities, such as iron, nickel,
and chromium, are introduced into the fiber during fabrication. Extrinsic
absorption is caused by the electronic transition of these metal ions from
one energy level to another.
Extrinsic absorption also occurs when
hydroxyl ions (OH-) are introduced into the fiber. Water in silica
glass forms a silicon-hydroxyl (Si-OH) bond. This bond has a fundamental
absorption at 2700 nm. However, the harmonics or overtones of the fundamental
absorption occur in the region of operation. These harmonics increase extrinsic
absorption at 1383 nm, 1250 nm, and 950 nm. Figure 2-21 shows the presence of
the three OH- harmonics. The level of the OH- harmonic
absorption is also indicated.
These absorption peaks define three
regions or windows of preferred operation. The first window is centered at 850
nm. The second window is centered at 1300 nm. The third window is
centered at 1550 nm. Fiber optic systems operate at wavelengths defined
by one of these windows.
The amount of water (OH-)
impurities present in a fiber should be less than a few parts per billion.
Fiber attenuation caused by extrinsic absorption is affected by the level of
impurities (OH-) present in the fiber. If the amount of impurities
in a fiber is reduced, then fiber attenuation is reduced.
Q.39 What are the main causes of
absorption in optical fiber?
Q.40 Silica (pure glass) fibers are used because of their low intrinsic
material absorption at the wavelengths of operation. This wavelength of
operation is between two intrinsic absorption regions. What are these two
regions called? What are the wavelengths of operation for these two regions?
Q.41 Extrinsic (OH-) absorption peaks define three regions or
windows of preferred operation. List the three windows of operation.
SCATTERING. - Basically, scattering losses are caused by the
interaction of light with density fluctuations within a fiber. Density changes
are produced when optical fibers are manufactured.
During manufacturing, regions of higher
and lower molecular density areas, relative to the average density of the
fiber, are created. Light traveling through the fiber interacts with the
density areas as shown in figure 2-22. Light is then partially scattered in all
directions.
Figure 2-22. - Light scattering.
In commercial fibers operating between
700-nm and 1600-nm wavelength, the main source of loss is called Rayleigh
scattering. Rayleigh scattering is the main loss mechanism between the
ultraviolet and infrared regions as shown in figure 2-21. Rayleigh
scattering occurs when the size of the density fluctuation (fiber defect)
is less than one-tenth of the operating wavelength of light. Loss caused by
Rayleigh scattering is proportional to the fourth power of the wavelength
(1/λ4). As the wavelength increases, the loss caused by
Rayleigh scattering decreases.
If the size of the defect is greater than
one-tenth of the wavelength of light, the scattering mechanism is called Mie
scattering. Mie scattering, caused by these large defects in the fiber
core, scatters light out of the fiber core. However, in commercial fibers, the
effects of Mie scattering are insignificant. Optical fibers are manufactured
with very few large defects.
Q.42 What is the main loss mechanism
between the ultraviolet and infrared absorption regions?
Q.43 Scattering losses are caused by the interaction of light with density
fluctuations within a fiber. What are the two scattering mechanisms called when
the size of the density fluctuations is (a) greater than and (b) less than
one-tenth of the operating wavelength?
BENDING LOSS. - Bending the fiber also causes attenuation.
Bending loss is classified according to the bend radius of curvature: microbend
loss or macrobend loss.
Microbends are small microscopic bends of the fiber axis that
occur mainly when a fiber is cabled. Macrobends are bends having a large
radius of curvature relative to the fiber diameter. Microbend and macrobend
losses are very important loss mechanisms. Fiber loss caused by microbending
can still occur even if the fiber is cabled correctly. During installation, if
fibers are bent too sharply, macrobend losses will occur.
Microbend losses are caused by small discontinuities or
imperfections in the fiber. Uneven coating applications and improper cabling
procedures increase microbend loss. External forces are also a source of
microbends. An external force deforms the cabled jacket surrounding the fiber
but causes only a small bend in the fiber. Microbends change the path that propagating
modes take, as shown in figure 2-23. Microbend
loss increases attenuation because low-order modes become
coupled with high-order modes that are naturally lossy.
Figure 2-23. - Microbend loss.
Macrobend losses are observed when a fiber bend's radius of
curvature is large compared to the fiber diameter.
These bends become a great source of loss
when the radius of curvature is less than several centimeters. Light
propagating at the inner side of the bend travels a shorter distance than that
on the outer side. To maintain the phase of the light wave, the mode phase
velocity must increase. When the fiber bend is less than some critical radius,
the mode phase velocity must increase to a speed greater than the speed of
light. However, it is impossible to exceed the speed of light. This condition
causes some of the light within the fiber to be converted to high-order modes.
These high-order modes are then lost or radiated out of the fiber.
Fiber sensitivity to bending losses can be
reduced. If the refractive index of the core is increased, then fiber
sensitivity decreases. Sensitivity also decreases as the diameter of the
overall fiber increases. However, increases in the fiber core diameter increase
fiber sensitivity. Fibers with larger core size propagate more modes. These
additional modes tend to be more lossy.
Q.44 Microbend loss is caused by
microscopic bends of the fiber axis. List three sources of microbend loss.
Q.45 How is fiber sensitivity to bending losses reduced?
DISPERSION
There are two different types of
dispersion in optical fibers.
The types are intramodal and intermodal
dispersion. Intramodal, or chromatic, dispersion occurs in all types of fibers.
Intermodal, or modal, dispersion occurs only in multimode fibers. Each type of
dispersion mechanism leads to pulse spreading. As a pulse spreads, energy is
overlapped. This condition is shown in figure 2-24. The spreading of the
optical pulse as it travels along the fiber limits the information capacity of
the fiber.
Figure 2-24. - Pulse overlap.
Intramodal Dispersion
Intramodal, or chromatic, dispersion
depends primarily on fiber materials. There are two types of intramodal
dispersion. The first type is material dispersion. The second type is waveguide
dispersion.
Intramodal dispersion occurs because different colors of light travel
through different materials and different waveguide structures at different
speeds.
Material dispersion occurs because the spreading of a light pulse is
dependent on the wavelengths' interaction with the refractive index of the
fiber core. Different wavelengths travel at different speeds in the fiber
material. Different wavelengths of a light pulse that enter a fiber at one time
exit the fiber at different times. 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.
Waveguide dispersion occurs because the mode propagation constant
(β) is a function of the size of the fiber's core relative to the
wavelength of operation. Waveguide dispersion also occurs because light
propagates differently in the core than in the cladding.
In multimode fibers, waveguide dispersion
and material dispersion are basically separate properties. Multimode waveguide
dispersion is generally small compared to material dispersion. Waveguide
dispersion is usually neglected.
However, in single mode fibers, material
and waveguide dispersion are interrelated.
The total dispersion present in single
mode fibers may be minimized by trading material and waveguide properties depending
on the wavelength of operation.
Q.46 Name the two types of intramodal, or
chromatic, dispersion.
Q.47 Which dispersion mechanism (material or waveguide) is a function of the
size of the fiber's core relative to the wavelength of operation?
Intermodal Dispersion
Intermodal or modal dispersion causes the
input light pulse to spread. The input light pulse is made up of a group of
modes. As the modes propagate along the fiber, light energy distributed among
the modes is delayed by different amounts. The pulse spreads because each mode
propagates along the fiber at different speeds. Since 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, as
shown in figure 2-25. The modes of a light pulse that enter the fiber at one
time exit the fiber a different times. This condition causes the light pulse to
spread. As the length of the fiber increases, modal dispersion increases.
Figure 2-25. - Distance traveled by each
mode over the same time span.
Modal dispersion is the dominant source of
dispersion in multimode fibers. Modal dispersion does not exist in single mode
fibers. Single mode fibers propagate only the fundamental mode. Therefore,
single mode fibers exhibit the lowest amount of total dispersion. Single mode
fibers also exhibit the highest possible bandwidth.
Q.48 Modes of a light pulse that enter the
fiber at one time exit the fiber at different times. This condition causes the
light pulse to spread. What is this condition called?
SUMMARY
Now that you have completed this chapter,
let's review some of the new terms, concepts, and ideas that you have learned.
You should have a thorough understanding of these principles before moving on
to chapter 3.
A LIGHT WAVE is a form of energy
that is moved by wave motion.
WAVE MOTION is defined as a recurring disturbance advancing
through space with or without the use of a physical medium.
SCIENTIFIC EXPERIMENTS seem to show that light is composed of tiny
particles, while other experiments indicate that light is made up of waves.
Today, physicists have come to accept a theory concerning light that is a
combination of particle (ray) theory and wave (mode) theory.
TRANSVERSE WAVE MOTION describes the up and down wave motion that is at
right angle (transverse) to the outward motion of the waves.
LIGHT RAYS, when they encounter any substance, are either
transmitted, refracted, reflected, or absorbed.
REFLECTION occurs when a wave strikes an object and bounces
back (toward the source). The wave that moves from the source to the object is
called the incident wave , and the wave that moves away from the object
is called the reflected wave.
The LAW OF REFLECTION states that
the angle of incidence is equal to the angle of reflection.
REFRACTION occurs when a wave traveling through two different
mediums passes through the boundary of the mediums and bends toward or
away from the normal.
The RAY THEORY and the MODE
THEORY describe how light energy is transmitted along an optical fiber.
The INDEX OF REFRACTION is the
basic optical material property that measures the speed of light in an optical
medium.
SNELL'S LAW OF REFRACTION describes the relationship between the incident
and the refracted rays when light rays encounter the boundary between two
different transparent materials.
TOTAL INTERNAL REFLECTION occurs when light rays are totally reflected at
the boundary between two different transparent materials. The angle at which
total internal reflection occurs is called the critical angle of incidence.
The CORE, CLADDING, and COATING
or BUFFER are the three basic parts of an optical fiber.
The RAY THEORY describes how light
rays propagate along an optical fiber. MERIDIONAL RAYS pass through the
axis of the optical fiber. SKEW RAYS propagate through an optical fiber
without passing through its axis.
BOUND RAYS propagate through an optical fiber core by total
internal reflection. UNBOUND RAYS refract out of the fiber core into the
cladding and are eventually lost.
The ACCEPTANCE ANGLE is the maximum
angle to the axis of the fiber that light entering the fiber is bound or
propagated.
The light ray incident on the fiber core
must be within the acceptance cone defined by the acceptance angle to be
propagated along an optical fiber.
NUMERICAL APERTURE (NA) is a measurement of the ability of an optical
fiber to capture light.
The MODE THEORY uses
electromagnetic wave behavior to describe the propagation of light along an
optical fiber. A set of guided electromagnetic waves are called the modes
of the fiber.
MODES traveling in an optical fiber are said to be transverse. Modes are
described by their electric, E(x,y,z), and magnetic, H(x,y,z), fields. The
electric field and magnetic field are at right angles to each other.
NORMALIZED FREQUENCY determines how many modes a fiber can support. The
number of modes is represented by the normalized frequency constant.
SINGLE MODE and MULTIMODE
FIBERS are classified by the number of modes that
propagate along the optical fiber. Single mode fibers propagate only one mode
because the core size approaches the operational wavelength. Multimode fibers
can propagate over 100 modes depending on the core size and numerical aperture.
ATTENUATION is the loss of optical power as light travels
along an optical fiber. Attenuation in an optical fiber is caused by
absorption, scattering, and bending losses.
DISPERSION spreads the optical pulse as it travels along the
fiber. Dispersion limits how fast information is transferred.
ABSORPTION is the conversion of optical power into another
energy form, such as heat. INTRINSIC
ABSORPTION is caused by basic fiber-material properties. EXTRINSIC ABSORPTION is caused by
impurities introduced into the fiber material.
SILICA FIBERS are predominately used in fiber optic
communications. They have low intrinsic material absorption at the wavelengths
of operation.
The WAVELENGTH OF OPERATION in
fiber optics is between 700 nm and 1600 nm. The wavelength of operation is
between the ultraviolet (below 400 nm) and infrared (above 2000 nm) intrinsic
absorption regions.
EXTRINSIC ABSORPTION occurs when impurities, such as hydroxyl ions (OH-),
are introduced into the fiber. OH- absorption peaks define three
regions or windows of preferred operation. The first window is centered at 850
nm. The second window is centered at 1300 nm. The third window is centered at
1550 nm.
SCATTERING losses are caused by the interaction of light with
density fluctuations within a fiber. Rayleigh scattering is the main
source of loss in commercial fibers operating between 700 nm and 1600 nm.
MICROBENDS are small microscopic bends of the fiber axis that
occur mainly when a fiber is cabled. MACROBENDS are bends having a large
radius of curvature relative to the fiber diameter.
INTRAMODAL, or CHROMATIC, DISPERSION occurs because light
travels through different materials and different waveguide structures at
different speeds.
MATERIAL DISPERSION is dependent on the light wavelengths interaction
with the refractive index of the core. WAVEGUIDE DISPERSION is a
function of the size of the fiber's core relative to the wavelength of
operation.
INTERMODAL, or MODAL, DISPERSION occurs because each mode
travels a different distance over the same time span.