Optical communication unit 1

EC8751
OPTICAL COMMUNICATION
By
T.SELVALAKSHMI,B.E.,

UNIT-1
INTRODUCTION TO OPTICAL FIBERS

INTRODUCTION TO COMMUNICATION SYSTEM
• Communication is broadly defined as the transfer of information from one point to another.
• In a communication system the information is transferred by modulating the information signal onto an
electromagnetic wave.
• This modulated carrier is then transmitted to the destination where it is received and the original information
signal is obtained by demodulation.
• Sophisticated techniques have been developed for this process using electromagnetic carrier waves
operating at radio frequencies as well as microwave and millimeter wave frequencies.
• Communication may also be achieved using an electromagnetic carrier which is selected from the optical
range of frequencies.

ELEMENT OF AN OPTICAL FIBER TRANSMISSION
LINK

ELEMENT OF AN OPTICAL FIBER TRANSMISSION
LINK
• A fiber optic data link sends input data through fiber optic
components and provides this data as output information.
• It has the following three basic functions
1. To convert an electrical input signal to an optical signal
2. To send the optical signal over an optical fiber
3. To convert the optical signal back to an electrical signal

PARTS OF A FIBER OPTIC DATA LINK
• A fiber optic data link consists of three parts
1. Transmitter,
2. Optical fiber, and
3. Receiver .

PARTS OF A FIBER OPTIC DATA LINK

TRANSMITTER
• The transmitter converts the input signal to an optical signal suitable for
transmission.
• It consists of two parts,
1. An interface circuit and
2. A source drive circuit.
• The transmitter's drive circuit converts the electrical signals to an optical signal.
• The two types of optical sources are LEDs and laser diodes.
• The optical source launches the optical signal into the fiber.

RECEIVER
• The receiver converts the optical signal exiting the fiber back into an electrical
signal.
• The receiver consists of two parts,
1. The optical detector and
2. The signal-conditioning circuits
• The receiver should amplify and process the optical signal without introducing
noise or signal distortion.
• An optical detector can be either a PIN diode or an avalanche photodiode.

OPTICAL FIBER LINK
• A fiber optic data link also includes passive components other than an optical
fiber.
• Passive components used to make fiber connections affect the performance of the
data link.
• Fiber optic components used to make the optical connections include optical
splices, connectors, and couplers.

MAJOR ELEMENTS OF AN OPTICAL FIBER
LINK

BASIC OPTICAL LAWS AND DEFINITIONS

LAW OF REFLECTION
• This conforms to the law of
reflection states that “the angle of
incidence is equal to the angle of
reflection”.

REFRACTION OF LIGHT
• When a light wave passes from one
medium into a medium having a
different velocity of propagation, a
change in the direction of the wave
will occur.
• This change of direction as the wave
enters the second medium is called
refraction.

RAY THEORY TRANSMISSION
To consider the propagation of light within an optical fiber utilizing the ray theory model it is
necessary to take account of the refractive index of the dielectric medium.
Refractive index:
• The refractive index of a medium is defined as “ the ratio of the velocity of light in a vacuum to
the velocity of light in the medium. Its is referred as n ”.
n=c/v
Where, c- Velocity of light in free space
v- Velocity of light in medium

1. SNELL'S LAW:
• Snell's law predicts how the ray will change direction as it passes from one medium into another,
or it is reflected from the interface between two media.
Where, n1- of core medium
n2- cladding
• As n1 is greater than n2, the angle of refraction is always greater than the angle of incidence.

LIGHT RAYS INCIDENT ON A HIGH TO LOW
REFRACTIVE INDEX INTERFACE

2. CRITICAL ANGLE:
• The angle of refraction is 90° and the
refracted ray emerges parallel to the
interface between the dielectrics, the angle
of incidence must be less than 90°. This is
the limiting case of refraction and the angle
of incidence is now known as the critical
angle φc.

3. TOTAL INTERNAL REFLECTION:
• At angles of incidence greater than the critical angle the light is reflected back
into the originating dielectric medium with high efficiency.
• Total internal reflection occurs at the interface between two dielectrics of
differing refractive indices when light is incident on the dielectric of lower index
from the dielectric of higher index, and the angle of incidence of the ray exceeds
the critical value.

CONDITION FOR TOTAL INTERNAL REFLECTION
1. The angle of incidence is equal
to the angle of reflection
2. The angle of incidence must be
greater than the critical angle
φc.

4. ACCEPTANCE ANGLE
• The maximum angle of
incidence that a ray can make
that will result in TIR is called
acceptance angle.

5. NUMERICAL APERTURE
• The Numerical aperture of a fiber is defined as the sine of the largest angle an
incident ray can have for TIR in the core Rays launched outside the angle
specified by fibers NA will excite radiation modes of fiber.
• A higher core index, with respect to the cladding, means larger NA.

5. NUMERICALAPERTURE
• The relative refractive index difference Δ between the core and the cladding
which is defined as:
• NA in terms of Δ,

TRANSMISSION OF LIGHT THROUGH
OPTICAL FIBERS

TRANSMISSION OF LIGHT THROUGH OPTICAL
FIBERS
• Two methods are used to describe how light is transmitted along the optical fiber.
• The first method is ray theory- it uses the concepts of light reflection and
refraction.
• The second method is mode theory- it treats light as electromagnetic waves.

RAY OPTICS
• Two types of rays can propagate along an optical fiber.
• They are,
1. Meridional rays
2. Skew rays
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.

MERIDIONAL RAYS:
• Meridional rays can be classified as bound or
unbound rays.
• Bound rays propagate through the fiber by
total internal reflection.
• Unbound rays are refracted out of the fiber
core. .

SKEW RAYS
• Skew rays are rays that travel through
an optical fiber without passing
through its axis.
• Figure: The helical path taken by a
skew ray in an optical fiber.

OPTICAL MODES AND CONFIGURATIONS

OPTICAL FIBER MODES AND CONFIGURATIONS
Basic structure of an optical fiber:
• The basic structure of an optical fiber consists of three parts,
1. The core,
2. The cladding, and
3. The coating or buffer.

BASIC STRUCTURE OF AN OPTICAL FIBER

1. CORE:
• 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 a radius of (a) and an index of refraction n1.

2. CLADDING:
• The core is surrounded by a layer of material called the cladding.
• 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:
1. Reduces loss of light from the core into the surrounding air
2. Reduces scattering loss at the surface of the core
3. Protects the fiber from absorbing surface contaminants
4. Adds mechanical strength

3. BUFFER
• 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 micro
bends.

ELECTROMAGNETIC MODE THEORY FOR OPTICAL
PROPAGATION

1. ELECTROMAGNETIC WAVES
• The basis for the study of electromagnetic wave propagation is provided by
Maxwell’s equations.
• For a medium with zero conductivity these vector relationships may be written in
terms of curl equations:

OPTICAL FIBER TYPES
• Optical fibers are characterized by their structure and by their properties of
transmission.
• Optical fibers are classified into two types.
1. Single mode fibers
2. Multimode fibers.

1. SINGLE MODE FIBERS
• The core size of single mode fibers is small.
• The core size of single mode fibers is small.
• The core size is around 8 to 10 micrometers.
• A fiber core allows only the fundamental or lowest order mode to propagate around a 1300 nm
wavelength.
• Single mode fibers propagate only one mode.
• In single mode fibers, normalized frequency parameter (V) is less than or equal to 2.405.

• When V is 2.405, single mode fibers propagate the fundamental mode down the fiber core.
• For low V is 1.0, most of the power is propagated in the cladding material.
• Power transmitted by the cladding is easily lost at fiber bends.
• Single mode fibers are capable of transferring higher amounts of data due to low fiber dispersion.
• Dispersion mechanisms in single mode fibers are discussed in more detail later in this chapter.

• Signal loss depends on the operational wavelength.
• 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.

2. MULTIMODE FIBERS
• 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).
• NA increase, the number of modes increases.
• Typical values of fiber core size and NA are 50 to 100 mu and 0.20 to 0.29, respectively.
• A large core size and a higher NA have several advantages.

ADVANTAGES OF MULTIMODE FIBERS:
1. Light is launched into a multimode fiber with more ease.
2. The higher NA and the larger core size make it easier to make fiber connections.
3. During fiber splicing, core-to-core alignment becomes less critical.
4. Multimode fibers permit the use of light-emitting diodes (LEDs).
5. Single mode fibers typically must use laser diodes.
6. LEDs are cheaper, less complex, and last longer.
7. LEDs are preferred for most applications.

DISADVANTAGES OF MULTIMODE FIBERS
1. As the number of modes increases, the effect of modal dispersion increases.
2. Modal dispersion means that modes arrive at the fiber end at slightly different times.
3. This time difference causes the light pulse to spread.
4. Modal dispersion affects system bandwidth.
5. Fiber manufacturers adjust the core diameter, NA, and index profile properties of multimode
fibers to maximize system bandwidth.

OPTICAL FIBERS
• Fibers are classified according to the number of modes that they can propagate.
• An optical fiber's refractive index profile and core size further distinguish single
mode and multimode fibers.
• The refractive index profile describes the value of refractive index as a function
of radial distance at any fiber diameter.

OPTICAL FIBERS
• Fiber refractive index profiles classify single mode and multimode fibers as follows:
1. Multimode step-index fibers
2. Multimode graded-index fibers
3. Single mode step-index fibers
4. Single mode graded-index fibers
• The below figure shows the refractive index profiles and light propagation in multimode step-index,
multimode graded-index, and single mode step-index fibers.

OPTICAL FIBERS
• In a step-index fiber, the refractive index of the core is uniform and undergoes an abrupt change
at the core-cladding boundary.
• Step-index fibers obtain their name from this abrupt change called the step change in refractive
index.
• In graded-index fibers, the refractive index of the core varies gradually as a function of radial
distance from the fiber center.
• Single mode and multimode fibers can have a step-index or graded-index refractive index profile.

1. MULTIMODE STEP-INDEX FIBERS
• A multimode step-index fiber has a core of radius (a) and a constant refractive index n1.
• A cladding of slightly lower refractive index n2 surrounds the core.
• The refractive index profile is n(r) for this type of fiber.
• n(r) is equal to n1 at radial distances r < a (core).
• n(r) is equal to n2 at radial distances r ≥ a (cladding).
• Notice the step decrease in the value of refractive index at the core-cladding interface.
• This step decrease occurs at a radius equal to distance (a).

• The difference in the core and cladding refractive index is the parameter &Delta:
• The number of modes that fibers propagate depends on &Delta, core radius (a) of the fiber.
• The number of propagating modes also depends on the wavelength of the transmitted light.
• It has hundreds of propagating modes.
• Most modes in multimode step-index fibers propagate far from cutoff.
• Modes that are cut off cease to be bound to the core of the fiber.

• It have large core diameters and large numerical apertures.
• A large core size and a large NA make it easier to couple light from a light-emitting diode (LED)
into the fiber.
• It's core size is typically 50 &mu or 100 &mu.
• It have limited bandwidth capabilities.
• Dispersion, mainly modal dispersion, limits the bandwidth or information carrying capacity of the
fiber.
• Short-haul, limited bandwidth, low-cost applications typically use multimode step-index fibers.

FIGURE: THE REFRACTIVE INDEX PROFILE
FOR MULTIMODE STEP-INDEX FIBERS.

2. MULTIMODE GRADED-INDEX FIBERS
• A multimode graded-index fiber has a core of radius (a).
• The value of the refractive index of the core (n1) varies according to the radial distance (r).
• The value of n1 decreases as the distance (r) from the center of the fiber increases.
• The value of n1 decreases until it approaches the value of the refractive index of the cladding (n2).
• The value of n1 must be higher than the value of n2 to allow for proper mode propagation.
• The value of n2 is constant and has a slightly lower value than the maximum value of n1.
• The relative refractive index difference is determined using the maximum value of n1 and the value of n2.
• The profile parameter alpha determines the shape of the core's profile.
• As the value of alpha increases, the shape of the core's profile changes from a triangular shape to step.

• Most multimode graded-index fibers have a parabolic refractive index profile.
• Multimode fibers with near parabolic graded-index profiles provide the best performance.
• In multimode graded-index fibers, assume that the core's refractive index profile is parabolic (α=2).
• Light propagates in multimode graded-index fibers according to refraction and total internal reflection.
• The gradual decrease in the core's refractive index from the center of the fiber causes the light rays to be
refracted many times.
• The light rays become refracted or curved, which increases the angle of incidence at the next point of
refraction.
• Light rays may be reflected to the axis of the fiber before reaching the core-cladding interface.

FIGURE: THE REFRACTIVE INDEX PROFILE
FOR MULTIMODE GRADED-INDEX FIBERS

THE REFRACTIVE INDEX PROFILES FOR
DIFFERENT VALUES OF ALPHA

REFRACTIVE INDEX GRADING AND LIGHT
PROPAGATION IN MULTIMODE GRADED-
INDEX FIBERS

• The NA of a multimode graded-index fiber is at its maximum value at the fiber axis.
• This NA is the axial numerical aperture [NA(0)].
• NA(0) is approximately equal to
• However, the NA for graded-index fibers varies as a function of the radial distance (r).
• NA varies because of the refractive index grading in the fiber's core.
• The NA decreases from the maximum, NA(0), to zero at distances greater than the core-cladding boundary distance (r>a).
• Multimode graded-index fibers typically have over one-hundred propagating modes.

• In most applications, a multimode graded-index fiber with a core and cladding size of
• 62.5/125 mu offers the best combination of the following properties:
1. Relatively high source-to-fiber coupling efficiency
2. Low loss
3. Low sensitivity to micro bending and macro bending
4. High bandwidth
5. Expansion capability
• For example, local area network (LAN) and shipboard applications use multimode graded-index fibers with
a core and cladding size of 62.5/125 mu.

3. SINGLE MODE STEP-INDEX FIBERS
• There are two basic types of single mode step-index fibers:
1. Matched clad and
2. Depressed clad.
• Matched cladding means that the fiber cladding consists of a single homogeneous layer of dielectric
material.
• Depressed cladding means that the fiber cladding consists of two regions:
1. The inner and
2. Outer cladding regions.

• Matched-clad and depressed-clad single mode step-index fibers have unique refractive index profiles.
• A matched-clad single mode step-index fiber has a core of radius (a) and a constant refractive index n1.
• A cladding of slightly lower refractive index surrounds the core. The cladding has a refractive index n2.
• A depressed-clad single mode step-index fiber has a core of radius (a) with a constant refractive index n1.
• A cladding, made of two regions, surrounds the core.
• Single mode step-index fibers propagate only one mode, called the fundamental mode.

• Single mode operation occurs when the value of the fiber's normalized frequency is between 0 and 2.405
• When the value of V is less than 1, single mode fibers carry a majority of the light power in the cladding
material.
• Single mode fiber cutoff wavelength is the smallest operating wavelength when single mode fibers
propagate only the fundamental mode.
• The higher the operating wavelength is above the cutoff wavelength, the more power is transmitted through
the fiber cladding.

• A single mode step-index fiber has low attenuation and high bandwidth
properties.
• Present applications includes long-haul, high-speed telecommunication systems.
• Future applications include single mode fibers for sensor systems.
• Short cable runs, low to moderate bandwidth requirements, and high component
cost make installation of single mode fiber.

FIGURE: DEPRESSED-CLAD REFRACTIVE INDEX
PROFILE

4. SINGLE MODE GRADED-INDEX FIBERS
• There are several types of single mode graded-index fibers.
• These fibers are not standard fibers and are typically only used in
specialty applications.

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.

1. 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 wave-fronts. The wavelength of the plane wave
is given by:

Wave-front propagation along an
optical fiber
Plane-wave propagation

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
1. The electric field, E(x, y, z), and
2. 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.

MODES
• 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 : TRANSVERSE ELECTRIC (TE) MODE FIELD
PATTERNS

FIGURE: LOW-ORDER AND HIGH-ORDER
MODES

CUTOFF WAVELENGTH
• It may be noted that single-mode operation only occurs above a theoretical cutoff
wavelength λc given by:
• where Vc is the cutoff normalized frequency. Hence λc is the wavelength above
which a particular fiber becomes single-moded

CUTOFF WAVELENGTH
• Thus for step index fiber where Vc = 2.405, the cutoff wavelength is given by:

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 – core index of refraction, n2– is the cladding index of refraction, a - core diameter,
and lambda - wavelength of light in air.

MODE-FIELD DIAMETER AND SPOT SIZE
• For a Gaussian power distribution in a single mode
optical fiber, the mode field diameter (MFD) is
defined as the point at which the electric and magnetic
field strengths are reduced to 1/e of their maximum
values, i.e., the diameter at which power is reduced to
1/e2 (0.135) of the peak power
• . For single mode fibers, the peak power is at the center
of the core.

PHASE VELOCITY
• The envelope of the wave package or group of waves travels at a group velocity υg With in all
electromagnetic waves, whether plane or otherwise, there are points of constant phase. For plane
waves these constant phase points form a surface which is referred to as a wave-front. As a
monochromatic light wave propagates along a waveguide in the z direction these points of
constant phase travel at a phase velocity υp given by

FIGURE: THE FORMATION OF A WAVE PACKET FROM THE
COMBINATION OF TWO WAVES WITH NEARLY EQUAL
FREQUENCIES.

GROUP VELOCITY
• However, it is impossible in practice to produce perfectly monochromatic light waves, and light energy is
generally composed of a sum of plane wave components of different frequencies.
• Often the situation exists where a group of waves with closely similar frequencies propagate so that their resultant
forms a packet of waves.
• The formation of such a wave packet resulting from the combination of two waves of slightly different frequency
propagating together.
• This wave packet does not travel at the phase velocity of the individual waves but is observed to move at a group
velocity υg given by

IMPORTANT FORMULAS
1. Refractive index n = c/v
2. Critical angle
3. Numerical aperture
4. Relative refractive index difference Δ,

IMPORTANT FORMULAS
6. NA in terms of Δ,
7. Wavelength of the plane wave,
8. Cutoff Wavelength,
9. Cutoff Wavelength for SI fiber,

IMPORTANT FORMULAS
10. Normalized frequency (V),
11. Phase Velocity,
12. Group Velocity,

Optical communication unit 1

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