ch 1 optical fiber introduction

Class Note, Optical Fiber Communication (Chapter 1), By: Krishna Prasad Phelu Pg. 1/18

Chapter 1 Optical Fibers: Introduction

Introduction to Optical Fiber Communication

Communication system with light as the carrier and fiber as a communication medium.

‘Light’ for communication has been used for signal fires, reflecting mirrors, signaling lamps like light-

house but limited information transfer.

Light is an EM wave that can be detected.

As carrier frequency increases, bandwidth increase and data rate increases.

Operates at wavelength 800nm to 1700nm

Need for Fiber-Optic Communications

(Ref: Govind P. Agrawal)

The advent of telegraphy in the 1830s replaced the use of light by electricity and began the era of electrical

communications. Telegraphy used essentially a digital scheme through two electrical pulses of different durations

(dots and dashes of the Morse code). The bit rate, B, could be increased to∼10 b/s by the use of new coding

techniques, such as the Morse code. The use of intermediate relay stations allowed communication over long

distances (∼1000 km).

The invention of the telephone in 1876 brought a major change inasmuch as electric signals were transmitted in

analog form through a continuously varying electric current. Analog electrical techniques were to dominate

communication systems for a century or so.

The use of coaxial cables in place of wire pairs increased system capacity considerably. The bandwidth of such

systems is limited by the frequency-dependent cable losses, which increase rapidly for frequencies beyond 10

MHz.

This limitation led to the development of microwave communication systems in which an electromagnetic carrier

wave with frequencies in the range of 1–10 GHz is used to transmit the signal by using suitable modulation

techniques.

The first coaxial-cable system, put into service in 1940, was a 3-MHz system capable of transmitting 300 voice

channels or a single television channel. The first microwave system operating at the carrier frequency of 4 GHz

was put into service in 1948. Since then, both coaxial and microwave systems have evolved considerably.

A commonly used figure of merit for communication

systems is the bit rate–distance product, BL, where B is the

bit rate and L is the repeater spacing.

It was realized during the second half of the twentieth

century that an increase of several orders of magnitude in

the BL product would be possible if optical waves were

used as the carrier.


However, neither a coherent optical source nor a suitable transmission medium was available during the 1950s.

The invention of the laser and its demonstration in 1960 solved the first problem.

It was suggested in 1966 that optical fibers might be the best choice, as they are capable of guiding the

light in a manner similar to the guiding of electrons in copper wires.

The main problem was the high losses of optical fibers - fibers available during the 1960s had losses in excess of

1000 dB/km. A breakthrough occurred in 1970 when fiber losses could be reduced to below 20 dB/km in the

wavelength region near 1µm. The commercial deployment of lightwave systems followed the research and

development phase closely. As a result, the bit rate–distance product of modern lightwave systems can exceed

by a factor of 107 compared with the first-generation lightwave systems.

Optical Fiber Communication system block diagram

(Ref: John M. Senior)

An optical fiber communication system is similar in basic concept to general communication system. The

function of communication system is to convey the signal from the information source over the transmission

medium to the destination. The communication system therefore consists of a transmitter or modulator linked to

the information source, the transmission medium, and a receiver or demodulator at the destination point.

In electrical communications the information source provides an electrical signal, usually derived from a message

signal which is not electrical (e.g. sound), to a transmitter comprising electrical and electronic components which

converts the signal into a suitable form for propagation over the transmission medium. This is often achieved by

modulating a carrier, which, may be an electromagnetic wave. The transmission medium can consist of a pair of

wires, a coaxial cable or a radio link through free space down which the signal is transmitted to the receiver,

where it is transformed into the original electrical information signal (demodulated) before being passed to the

destination.

For optical fiber communications the system, the information source provides signal to a transmitter comprising

an electrical stage which drives an optical source to give modulation of the lightwave carrier. The optical source

which provides the electrical–optical conversion may be either a semiconductor laser or light-emitting diode


(LED). The transmission medium consists of an optical fiber cable and the receiver consists of an optical detector

which drives a further electrical stage and hence provides demodulation of the optical carrier. Photodiodes (p–n,

p–i–n or avalanche) and, in some instances, phototransistors and photoconductors are utilized for the detection

of the optical signal and the optical–electrical conversion.

Thus there is a requirement for electrical interfacing at either end of the optical link and at present the signal

processing is usually performed electrically. But, significant developments have taken place in devices for optical

signal processing which are starting to alter this situation.

The optical carrier may be modulated using either an analog or digital information signal. Analog modulation

involves the variation of the light emitted from the optical source in a continuous manner. With digital

modulation, however, discrete changes in the light intensity are obtained (i.e. on–off pulses). Although often

simpler to implement, analog modulation with an optical fiber communication system is less efficient, requiring

a far higher signal-to-noise ratio at the receiver than digital modulation. Also, the linearity needed for analog

modulation is not always provided by semiconductor optical sources, especially at high modulation frequencies.

For these reasons, analog optical fiber communication links are generally limited to shorter distances and lower

bandwidth operation than digital links.

Following figure shows a block schematic of a typical digital optical fiber link:

Initially, the input digital signal from the information source is suitably encoded for optical transmission. The

laser drive circuit directly modulates the intensity of the semiconductor laser with the encoded digital signal.

Hence a digital optical signal is launched into the optical fiber cable. The avalanche photodiode (APD) detector

is followed by a front-end amplifier and equalizer or filter to provide gain as well as linear signal processing and

noise bandwidth reduction. Finally, the signal obtained is decoded to give the original digital information.

Repeaters can be used for long distance digital communication system. They convert weak and distorted optical

signals to electrical signals and then regenerate original digital pulse train for further transmission. In long

systems numerous amplifiers and repeaters may be used.

Advantages of optical fiber communication

(Ref: John M. Senior)

Communication using an optical carrier wave guided along a glass fiber has a number of extremely attractive

features:

a) Enormous potential bandwidth.

The optical carrier frequency in the range 1013 to 1016 Hz yields a far greater potential transmission bandwidth

than metallic cable systems (i.e. coaxial cable bandwidth typically around 20 MHz over distances up to a

maximum of 10 km) or even millimeter wave radio systems (i.e. systems currently operating with modulation

bandwidths of 700 MHz over a few hundreds of meters).


Indeed, by the year 2000 the typical bandwidth - length product for an optical fiber link incorporating fiber

amplifiers was 5000 GHz km in comparison with the typical bandwidth–length product for coaxial cable of

around 100 MHz km. Hence optical fiber was already demonstrating a factor of 50000 bandwidth – length

improvement over coaxial cable while also providing this superior information-carrying capacity over much

longer transmission distances.

Use of wavelength division multiplexed (WDM) operation can further enhanced bandwidth utilization of an

optical fiber.

b) Small size and weight

Optical fibers have very small diameters which are often no greater than the diameter of a human hair. Hence,

even when such fibers are covered with protective coatings they are far smaller and much lighter than

corresponding copper cables.

Hence optical fibers are portable, easy for ducting and installation as well as suitable for an expansion of signal

transmission within aircraft, satellites and even ships.

c) Electrical isolation

Optical fibers which are fabricated from glass, or sometimes a plastic polymer, are electrical insulators and

therefore there is no arcing, spark or short circuit hazard.

d) Immunity to interference and crosstalk

Optical fibers form a dielectric waveguide and are therefore free from electromagnetic interference (EMI), radio-

frequency interference (RFI), or switching transients giving electromagnetic pulses (EMPs). Hence

The operation of an optical fiber communication system is unaffected by transmission through an

electrically noisy environment

The fiber cable requires no shielding from EMI.

The fiber cable is also not susceptible to lightning strikes if used overhead rather than underground.

No optical interference between fibers and hence, unlike communication using electrical conductors,

crosstalk is negligible, even when many fibers are cabled together.

e) Increased Signal security

The light from optical fibers does not radiate significantly and therefore they provide a high degree of signal

security. Unlike the situation with copper cables, a transmitted optical signal cannot be obtained from a fiber in

a noninvasive manner (i.e. without drawing optical power from the fiber). If the cable is tapped, it’s very easy to

monitor because the cable leaks light, causing the entire system to fail.

Therefore, in theory, any attempt to acquire a message signal transmitted optically may be detected. This feature

is obviously attractive for military, banking and general data transmission (i.e. computer network) applications.

f) Low transmission loss

Optical fiber cables which exhibit very low attenuation or transmission loss in comparison with the copper

conductors. Fibers have been fabricated with losses as low as 0.15 dB km−1. It facilitates the implementation of

communication links with extremely wide optical repeater or amplifier spacings, thus reducing both system cost

and complexity.

g) System reliability and ease of maintenance

Due to low-loss property of optical fiber cables, requirement for intermediate repeaters reduces. Hence with fewer

optical repeaters or amplifiers, system reliability is generally enhanced in comparison with conventional electrical

conductor systems.


h) Potential low cost

The glass which generally provides the optical fiber transmission medium is made from sand – not a scarce

resource. So, in comparison with copper conductors, optical fibers offer the potential for low-cost line

communication for bulk purchases. Overall system costs when utilizing optical fiber communication on long-

haul links are substantially less than those for equivalent electrical line systems because of the low-loss and

wideband properties of the optical transmission medium.

Although optical fiber transmission medium has become competitive with copper wires (i.e. twisted pairs), it has

not yet been achieved in all the other component areas associated with optical fiber communications. For

example, the costs of high-performance semiconductor lasers and detector photodiodes are still relatively high,

as well as some of those concerned with the connection technology (demountable connectors, couplers, etc.).

Basic laws and definitions

(Ref: G. Keiser and John M. Senior)

Refractive index

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. 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. A ray of light travels more slowly in

an optically dense medium than in one that is less dense, and the refractive index gives a measure of this effect.

In free space a light wave travels at a speed c = 3 x 108 m/s. Upon entering a dielectric or non-conducting medium

the wave now travels at a speed v, which is characteristic of the material and is less than c. The ratio of speed of

c to v is the index of refraction n of the material,

Typical values of n are 1.00 for air, 1.33 for water, 1.50 for glass, and 2.42 for diamond.

Reflection and Refraction


When light traveling in a certain medium is reflected off an optically denser material (one with a higher refractive

index), the process is referred to as external reflection. Conversely, the reflection of light off of less optically

dense material (such as light traveling in glass being reflected at a glass-to-air interface) is called internal

reflection.

Total internal reflection

Consider a ray approaching the interface is propagating in a dielectric of refractive index n1 and is at an angle ϕ1

to the normal at the surface of the interface. If the dielectric on the other side of the interface has a refractive

index n2 which is less than n1, then the refraction is such that the ray path in this lower index medium is at an

angle ϕ2 to the normal, where ϕ2 is greater than ϕ1.

From Snell’s law of refraction:

Or:


As n1 is greater than n2, the angle of refraction is always greater than the angle of incidence. As angle of incidence

becomes greater, the angle of refraction also goes on increasing. When 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. The

value of the critical angle is given by1:

At angles of incidence greater than the critical angle on refraction is possible and the light is reflected back into

the originating dielectric medium (total internal reflection) with high efficiency (around 99.9%). Hence, 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.

This is the mechanism by which light propagate down an optical fiber with low loss. Transmission of a light ray

in an optical fiber via a series of total internal reflections at the interface of the silica core and the slightly lower

refractive index silica cladding is shown in figure below. The ray has an angle of incidence ϕ at the interface

which is greater than the critical angle and is reflected at the same angle to the normal.

1 Sin(900) = 1


The light ray shown in above Figure is known as a Meridional ray as it passes through the axis of the fiber core.

This type of ray is the simplest to describe and is generally used when illustrating the fundamental transmission

properties of optical fibers. Another possible ray in optical fiber is Skew ray. A skew ray is a ray that travels in a

non-planar zig-zag helical path which neither pass through the fiber axis nor parallel to the fiber axis.

Any discontinuities or imperfections at the core–cladding interface would probably result in refraction rather than

total internal reflection, with the subsequent loss of the light ray into the cladding.

Acceptance angle

Not all rays entering the fiber core will continue to be propagated down its length. Only rays with a sufficiently

shallow grazing angle (i.e. with an angle to the normal greater than ϕc) at the core–cladding interface are

transmitted by total internal reflection. The geometry concerned with launching a light ray into an optical fiber

is shown in Figure below, which illustrates a meridional ray A at the critical angle ϕc within the fiber at the core–

cladding interface. Ray A enters the fiber core at an angle θa to the fiber axis and is refracted at the air–core

interface before transmission to the core–cladding interface at the critical angle.

Any rays (B) which are incident into the fiber core at an angle greater than θa will be transmitted to the core–

cladding interface at an angle less than ϕc, and will not be totally internally reflected rather than it is refracted

into the cladding and eventually lost by radiation.

Thus for rays to be transmitted by total internal reflection within the fiber core they must be incident on the fiber

core within an acceptance cone defined by the conical half angle θa. Hence θa is the maximum angle to the axis


at which light may enter the fiber in order to be propagated, and is often referred to as the acceptance angle for

the fiber.

Numerical aperture

We can to continue the ray theory analysis to obtain a relationship between the acceptance angle and the

refractive indices of the three media involved, namely the core, cladding and air. This leads to the definition of a

more generally used term, the numerical aperture of the fiber.

Consider light ray incident on the fiber core at an angle θ1 to the fiber axis which is less than the acceptance

angle for the fiber θa.

The ray enters the fiber from a medium (air) of refractive index n0, and the fiber core has a refractive index n1,

which is slightly greater than the cladding refractive index n2. Assuming the entrance face at the fiber core to be

normal to the axis.

Consider the refraction at the air–core interface and using Snell’s law given by:

Considering the right-angled triangle ABC in above figure:

where ϕ is greater than the critical angle at the core–cladding interface. Substituing θ2 in Snell’s law equation2,

Using the trigonometrical relationship sin2X+cos2X=1,

When the limiting case for total internal reflection is considered, ϕ becomes equal to the critical angle, ϕc, for the

core–cladding interface which is sinϕc=n2/n1. Also in this limiting case θ1 becomes the acceptance angle for the

fiber θa. With these substitutions,

Numerical Aperture (NA) is defined as:

2 sin(pi/2 - X) = cosX


Since the NA is often used with the fiber in air where n0 is unity, it is simply equal to sinθa. It may also be noted

that incident meridional rays over the range 0 ≤θ1≤θa will be propagated within the fiber.

Acceptance angle in air for the fiber can be calculated as,

The NA may also be given in terms of the relative refractive index difference Δ between the core and the cladding

which is defined as:

Taking under root,

Hence NA we can also be write as:

for Δ<<1,

The numerical aperture is a very useful measure of the light-collecting ability of a fiber. They are independent of

the fiber core diameter and will hold for diameters as small as 8μm. However, for smaller diameters they break

down as the geometric optics approach is invalid. This is because the ray theory model is only a partial description

of the character of light.

Question 1: A silica optical fiber with a core diameter large enough to be considered by ray theory analysis has

a core refractive index of 1.50 and a cladding refractive index of 1.47. Determine:

(a) the critical angle at the core–cladding interface;

(b) the NA for the fiber;

(c) the acceptance angle in air for the fiber.

Solution:

a) The critical angle ϕc at the core–cladding interface is given by,

b) The NA is:

c) The acceptance angle in air θa is given by:


Question 1: A typical relative refractive index difference for an optical fiber designed for long distance

transmission is 1%. Estimate the NA in air for the fiber when the core index is 1.46. Further, calculate the critical

angle at the core–cladding interface within the fiber. It may be assumed that the concepts of geometric optics

hold for the fiber.

Solution:

Relative refractive index difference, Δ=0.01

So, the NA is

The relative refractive index difference Δ gives:

Hence,

The critical angle at the core–cladding interface is:

Numerical aperture for Skew rays

Skey ray is transmitted without passing through the fiber axis and following a helical path through the fiber.

The helical path traced through the fiber gives a change in direction of 2γ at each reflection, where γ is the angle

between the projection of the ray in two dimensions and the radius of the fiber core at the point of reflection.

In order to calculate the acceptance angle for a skew ray it is necessary to define the direction of the ray in two

perpendicular planes. The geometry of the situation is illustrated in Figure below where a skew ray is shown

incident on the fiber core at the point A, at an angle θs to the normal at the fiber end face. The ray is refracted at

the air–core interface before traveling to the point B in the same plane. The angles of incidence and reflection at

the point B are ϕ, which is greater than the critical angle for the core–cladding interface.


When considering the ray between A and B it is necessary to resolve the direction of the ray path AB to the core

radius at the point B. As the incident and reflected rays at the point B are in the same plane, this is simply cosϕ.

To find another resolution, γ is the angle between the core radius (BR) and the projection of the ray AB onto a

plane BRT normal to the core axis (BT). And θ is the angle between the ray and a line AT drawn parallel to the

core axis. Thus to resolve the ray path AB relative to the radius BR requires multiplication by cos γ and sin θ.

Both of these resolution are that of the same ray AB. So, they should be equal

Using the trigonometrical relationship, sin2X+cos2X=1.

If the limiting case for total internal reflection is now considered, then ϕ becomes equal to the critical angle ϕc

for the core–cladding interface and sinϕc = n2/n1. So above equation can be written as,

Above equation is shown as inequality because here ϕ is specified for limiting case of critical angle ϕc but θ is

not specified for limiting case.

Furthermore, using Snell’s law at the point A,

where θa represents the maximum input axial angle for meridional rays and θ is the internal axial angle.

Substituting for sin θ from above equation gives:

where θas now represents the maximum input angle or acceptance angle for skew rays. It may be noted that the

inequality shown previously is no longer necessary as all the terms are specified for the limiting case.

Thus the acceptance conditions for skew rays are:

and in the case of the fiber in air (n0=1):


Comparisons between meridional ray and skew ray

Comparing NA for meridional rays and skew rays, it may be noted that skew rays are accepted at larger axial

angles in a given fiber than meridional rays, depending upon the value of cos γ.

For cos γ is equal to unity and θas = θa which represents meridional rays. Thus θa is the maximum conical half

angle for the acceptance of meridional rays and it defines the minimum input angle for skew rays.

Skew rays tend to propagate only in the annular region near the outer surface of the core, and do not fully utilize

the core as a transmission medium.

Skew rays increase the light-gathering capacity of the fiber. This increased light-gathering ability may be

significant for large NA fibers, but for most communication design purposes the meridional rays are considered

adequate.

Question 3: An optical fiber in air has an NA of 0.4. Compare the acceptance angle for meridional rays with that

for skew rays which change direction by 100° at each reflection.

Solution:

The acceptance angle for meridional rays with n0=1 is:

The skew rays change direction by 100° at each reflection, therefore γ=50°. Hence the acceptance angle for skew

rays is:

Optical Fiber

(Ref: G. Keiser)

Structure of Optical Fiber

Optical fiber acts as a waveguide or light pipe. It is a single solid dielectric cylinder of radius ‘a’ and index of

refraction n1 as shown in figure below. This cylinder is known as the core of fiber. The core is surrounded by

solid dielectric cladding having a refractive index n2 that is less than n1.

In low-loss and medium-loss fiber core is made from glass and cladding is made from either glass or plastic. But

high loss fiber is made from plastic-core and plastic cladding.

In principle cladding is not necessary for light to propagate along the core of the fiber, it serves several purposes:

Reduces scattering loss of light resulting from dielectric discontinuities from the core into the surrounding

air

Adds mechanical strength

Protects the core from absorbing surface contaminants.

Cladding is further encapsulated in an elastic plastic material called buffer coating. It adds further strength to the

fiber and mechanically isolates or buffers the fibers from small geometrical irregularities, distortions or


roughnesses of adjacent surfaces. These perturbations could otherwise cause scattering losses induced by random

microscopic bends on installation.

Types of Optical fiber

On the basis of core-cladding refractive index profile

a) Step Index

b) Graded Index

On the basis of number of modes supported

a) Single mode

b) Multi Mode

Step Index Fiber

The optical fiber with a core of constant refractive index n1 and a cladding of a slightly lower refractive index n2

is known as step index fiber. This is because the refractive index profile for this type of fiber makes a step

change at the core–cladding interface.

The refractive index profile may be defined as:

Graded index fibers

Graded index fibers do not have a constant refractive index in the core but a decreasing core index n(r) with

radial distance from a maximum value of n1 at the axis to a constant minimum value n2 beyond the core radius

‘a’ in the cladding.

This index variation may be represented as:

Where,


Δ= (n1-n2)/n1, is the relative refractive index difference and

α is the profile parameter which gives the characteristic refractive index profile of the fiber core

Refractive index profile of the fiber core can be expressed as a variation of α

When α=∞ it represents the step index profile

When α=2 it represents a parabolic profile

When α=1 it represents a triangular profile

Single Mode Fiber (SMF)

Single mode fiber sustains only one fundamental mode of light propagation. Single mode the fiber is designed to

allow propagation of only one mode, while all other modes are attenuated by leakage or absorption. Single mode

fiber has small core size and the index of refraction between the core and the cladding changes less than it does

for multimode fibers. Widely deployed single-mode fibers employ a step index profile design.

Advantages of single mode fiber

They exhibit the greatest transmission bandwidths and the lowest losses of the fiber transmission media.

They have a superior transmission quality over other fiber types because of the absence of modal noise.

Suitable for long distance transmission.

Multimode Fiber

Multimode fiber has a larger core diameter than that of single mode fiber, which is large enough to allow the

propagation of many modes within the fiber core simultaneously. When one pulse of signal is generated into

multimode fiber the multiple modes enter the fiber core from different angles and each modes propagates at

different speeds and causes pulse broadening (modal dispersion). Generally it is used for short and medium

distance.

Multimode step index fiber has a core diameter of around 50μm or greater. It allows many different possible

ray paths through the fiber. In multimode step index fiber considerable dispersion may occur due to the differing


group velocities of the propagating modes. This in turn restricts the maximum bandwidth attainable with

multimode step index fibers, especially when compared with single-mode fibers.

Multimode Graded-Index fiber contains a core in which the refractive index diminishes gradually from the

center axis out toward the cladding. The meridional rays appear to follow curved paths through the graded index

multimode fiber core rather than zigzagging off the cladding. Using the concepts of geometric optics, the gradual

decrease in refractive index from the center of the core creates many refractions of the rays as they are effectively

incident on a large number of high to low index interfaces. As a result ray is gradually curved and the ray travels

back towards the core axis, again being continuously refracted.

Multimode graded index fibers exhibit far less intermodal dispersion than multimode step index fibers due to

their refractive index profile. The rays traveling close to the fiber axis have shorter paths when compared with

rays which travel into the outer regions of the core. However, the near axial rays are transmitted through a region

of higher refractive index and therefore travel with a lower velocity than the more extreme rays. This compensates

for the shorter path lengths and reduces dispersion in the fiber.

Hence, multimode graded index fibers with parabolic or near-parabolic index profile cores have transmission

bandwidths which may be orders of magnitude greater than multimode step index fiber bandwidths.

Following table shows the different types of fibers and their dispersive characteristics.

Advantages of Multimode Fiber

The larger core radii of multimode fibers make it easier to launch optical power into the fiber

Lower tolerance required in fiber joints.

Light can be launched into a multimode fiber using a light emitting diode (LED) source, whereas single-

mode fibers must generally be exited with laser diodes. Although LEDs have less optical output power

than laser diodes they are easier to make, are less expensive, require less complex circuitry, and have

longer lifetimes than laser, thus making them more desirable in many applications.


Disadvantage of Multimode Fiber

Multimode fiber suffer from intermodal dispersion where as in single mode fibers intermodal dispersion

effects are not present. When an optical pulse is launched into a fiber, the optical power in the pulse is

distributed over all of the modes of the fiber. Each modes that can propagate in a multimode fiber travels

at a slightly different velocity. This means that the modes in a given optical pulse arrive at the fiber end

at slightly different times, thus causing the pulse to spread out in time as it travels along the fiber.

o This effect can be reduced by using a graded index profile in the fiber core.

Application of OFC:

Fiber optics has tremendous application in every part of life and several such applications have already been

implemented in practice to varying degrees. Some of detailed applications are as follows:

1. Telecommunication/Telephone applications:- The various applications of fiber optics in the

telecommunication area in general could be in voice telephone, video phones, various new services,

messages services and data networks all transmitted over the common carrier link.

2. Military application:- These include communication, command and control links on ships and

aircraft, data links for satellite earth stations and under sea system for which EMI effect, weight and size,

signal leakage and attenuation plays a major role. OFC systems meet this requirement.

3. Space application:- Optical fibers offer the following significant advantages for space environment

;namely high bandwidth( greater than 1 Ghz as compared to 1 Mhz for twisted pair), noise immunity,

inherent radiation, reduce weight (90% weight savings over conventional wire system), low bit error

rate, size and volume reduction, EMI immunity and lower cost.

4. Sensor application:- A fiber optic sensor consists of a length of a fiber that modulates the light passing

though it when exposed to the changing environment one wants to sense. The physical effects

exposed to the environment lead to changes in frequency, intensity and polarization of light and thus

cause variation in the resultant optical signal.

5. Under sea transmission cables:- Coaxial under sea cable systems have been used as one of the major

transmission systems in international telecommunication networks over the past 25 years. But this

system has nearly reached a limit in its ability to increase the capacities. Therefore optical fiber under sea

cable system are considered to be very promising technology to over come these barriers.

6. Broad band applications: Application that are primilarly broadband services include broadcast tv, cable

tv (CTV, community antenna television), remote monitory, and surveillance system.


7. Computer application:- Fiber system are particularly suited for transmission of digital data, such as that

generated by the computers. Interconnections can be made between CPU and memories, CPU and

peripherals and between CPUs.

8. OFC for electric power companies:- Electric power companies are progressively installing the OFC

system for power system protections, supervision and control, measurements etc.

9. Miscellaneous:- These include biomedical applications. General home appliances application. Small

office building and so on.

ch 1 optical fiber introduction

Engineering