UNIT-III

FIBER OPTICS

3.1 Introduction

Fiber optics is a technology in which electrical signals are

converted into optical signals and transmitted through a thin glass

fiber and reconverted into electrical signals.

3.2 Structure of optical fiber

A practical optical fiber has three coaxial regions. The

innermost light guiding region is known as the Core. It is

surrounded by a coaxial middle region known as the cladding.

This layer serves to confine the light to the core. The refractive

index of cladding is always lower than that of the core. The

outermost region is called as sheath or buffer. This protects the

cladding and core from abrasions, contamination and moisture.

The sheath also increases the mechanical strength of the fiber.

3.3 Principle or light propagation in Fiber

Light propagates from one end to other end in a fiber due to

total internal reflection at the core and cladding interface.

For the phenomenon of total internal reflection to take place

in a fiber the following conditions are to be satisfied.

1. The refractive index of the core (n1) must be greater than

the refractive index of the cladding (n2).

2. At the core-cladding interface the angle of incidence i

between the ray and the normal to the interface must be

grater than the critical angle.

Note : Critical angle of a medium is the value of the angle of

incidence at which the angle of refraction is 90.

3.4 Propagation of light in optical fiber

Consider an optical fiber into which light enters form one

end as shown in Fig. 3.3. Let n1 be the refractive index of the core

and n2 be the refractive index of the cladding. i.e., n1>n2. Let n0 be

the refractive index of the medium from which light is launched

into the fiber. Let the ray enter the fiber at an angle i to the axis

of fiber. This ray refracts at an angle r and strikes the core-

cladding interface at an angle .

For total internal reflection to occur > c and n1 > n2

Applying Snell’s law at the entrance of the fiber we get

sin i / sin r = n1 / n0

The largest value of i occurs when = c

But we know for total internal reflection to occur

Substituting (4) in (3) we get

If the fiber is kept in air than n0 = 1

The angle imax is called as the acceptance angle of the fiber.

Acceptance angle is the maximum angle with which a light

ray can enter into the fiber and still total internally reflect.

The main function of an optical fiber is to accept and

transmit light. The light gathering ability of the fiber is called as

numerical aperture. It is a measure of the amount of light that can

be accepted by a fiber.

The numerical aperture is also defined as the sine of the

acceptance angle.

Substituting (10) in (8)

3.5 Types of optical fibers

Optical fibers are classified into various categories depending

upon

1. the material used

2. the mode of propagation and

3. the refractive index profile

3.5.1Classification base on material

Based on the materials used for the optical fiber, they are

classified into

Glass fiber and

Plastic fiber

Glass fiber

When the optical fiber is made by fusing mixtures of metal

oxides and silica glasses, then they are known as glass fibers.

The difference of refractive index between core and cladding is

achieved in glass fibers by doping them with suitable materials.

Plastic fiber

There fibers are made of plastics and are of low cost. They

exhibit greater signal attenuation. Plastic fibers can be handled easily

due to its toughness and durability.

Examples

Polystyrene core and methylmethacrylate cladding.

Polymethylmethacrylate core and a cladding made of its

co-polymer.

3.5.2Classification base on mode of propagation

Base on the mode of propagation optical fiber are

classified as

Single mode fiber and

Multimode fiber

Single mode fiber

If only one mode is transmitted through the fiber, then it is said

to be single mode fiber. These fibers have smaller core diameter

(around 10 m) and the difference between the refractive index of

core and cladding is very small.

Multimode fiber

If more than one mode is transmitted through the fiber, then it

is said to be multimode fiber. These fibers have longer core diameter

and the difference between the refractive index of core and cladding

is large.

3.5.3Classification based or refractive index profile

Based on the refractive index profile optical fiber are classified

as

Step index fiber and

Graded index fiber

Step index fiber

The refractive indices of the core and cladding very like a step

and so it called as step index fiber. Both single mode and multimode

signals can be propagated through this fiber. For easy handling and

to reduce the susceptibility to microbending, the thickness of the

cladding is made 10 times as that of the core radius.

Graded index fiber

In this type of fiber, the refractive index of the core varies

radially from the fiber axis. The refractive index is maximum at the

centre and gradually decreases so that it is minimum at the core-

cladding interface. The variation of the refractive index of the core

(n) with radius (x), measured from the centre of the core is given by

When n1 refractive index at the centre of the core

a radius of the core

= n1-n2 / n1

p grading profile index number.

3.5.4 Differences between single mode and multimode

fiber

Single mode fiber Multimode fiber

1. If only one mode can

propagate through the fiber,

then it is said to be single

mode fiber.

If large number of paths or

modes can propagate through

the fiber, then it is said to be

multimode fiber.

2. The single mode fiber has

smaller core diameter (10m)

and the difference between

Generally in multimode fiber

the code diameter and the

relative refractive index

the refractive indices of the

core and the cladding is very

small.

difference are large than the

single mode fiber.

3. In practice there is no

dispersion (i.e no degradation

of signal during traveling

through the fiber).

Even though there is self

focusing effect there is signal

degradation due to multimode

dispersion and material

dispersion.

4. Launching of light into single

mode fibers and joining of two

fibers are very difficult.

Launching of light into fiber

and joining of two fibers are

easy in these fibers.

5. Fabrication is very difficult and

hence the fibers is so costly.

Fabrication is easy and so fiber

is cheap.

6. Laser diode is used for

launching light into the fiber.

Semiconductor laser is used.

3.5.5 Difference between step index fiber and graded

index fiber.

Step index fiber Graded index fiber

1. The refractive indices of air,

cladding, and core very step by

step.

In this fiber, the refractive

index is high at the centre of

the core and it gradually

decreases form centre towards

the core-cladding interface

2. The diameter of the core is

about 50-200m in the case of

multimode fiber and 10 m in

the case of single mode fiber.

The diameter of the core is

about 50 m in the case of

multimode fiber.

3. The light rays propagating

through it are in the form of

The light rays propagating

through it are in the form of

meridional rays which will

cross the fiber axis during

every reflection at the core-

cladding boundary and are

propagating in a zig-zag

manner

skew rays (or) helical rays

which will not cross the fiber

axis at any time and are

propagating around the fiber

axis in a helical (or) spiral

manner.

4. Bandwidth of fiber is about 50

MHz km for multimode step

index fibers. For single mode

index fibers. For single mode

index fibers, the bandwidth is

more than 1000 MHz.

Bandwidth of fiber is form 200

MHz km to 600 MHz km even

though theoretically it has

infinite bandwidth.

5. Attenuation is more for

multimode step index fibers.

For single mode step index

fiber it is very less.

Attenuation is less.

6. Numerical Aperture is more for

multimode step index fibers.

For single mode step index

fiber it is very less.

Numerical aperture is less.

3.6 Fabrication of optical fiber

Optical fibers are normally prepared from glass and plastic

materials. In the case of glass fibers silica is the major raw material.

Among the common plastic fibers polysterene is used as core

material.

The various optical fiber fabrication techniques are

1. Modified chemical vapour deposition

2. Preform technique of fiber drawing and

3. Double crucible technique of fiber drawing.

3.6.1Double crucible technique

This method is also know as direct-melt technique. The

traditional glass making procedure is adopted here. The schematic

diagram of double crucible technique is shown in Fig 3.8.

In double crucible method the fiber is drawn from the molten

state of the purified component material. The glass rods for the core

and cladding materials are made separately by melting mixtures of

purified powders to make the appropriate glass composition.

These core and cladding rods which are in rod form are fed into

the two concentric platinum crucibles. The platinum crucible avoid

contamination. The core is in the inner crucible while the cladding in

the outer crucible. The crucibles are kept in a muffle furnace capable

of melting the glass rods to a temperature between 800 and 1200°C.

The fibers are drawn from the molten state through the orifices

in the bottom of the two concentric crucibles in a continuation

production process. This method is more suitable for step - index of

all plastic fiber. The index grading is achieved through the diffusion

of mobile ions across the core-cladding interface within the molten

glass.

Advantages

1. This techniques enables continuous drawing of the fiber and

so the cost is also reduced.

2. Graded and step index fibers with attenuation as low as 3.4

dB km-1 can be produced by this technique.

Limitation

Fibers with same characteristics are not produced always.

3.7. Joining of Fibers

Increasing the length of the fiber or joining a broken fiber is an

essential requirement. The fiber optic system uses many means of

inter connecting or joining lengths of fibers with low insertion loss,

high strength and simplicity. The various ways of connecting fibers

are

Splice

Connector

Coupler

3.7.1Splicing

Splicing is nothing but a permanent joint made between two

optical fibers. The two types of splicing techniques available are

1. Fusion splicing and

2. Mechanical splicing

Before splicing, the surface has to be prepared. A clean

cross section perpendicular to the fiber axis can be obtained by

scribing the surface with a diamond scribe and cleaning it at the

scribe. The surface should be polished then with fine powder.

Fusion splicing

The two fiber ends are viewed through a microscope and

butted together using micropositioners. When the two ends of the

fiber are alligned an electric arc is struck across the joint. This

causes the two fiber ends to melt. As they come to contact,

surface tension forces help the two fibers to self-align.

An average loss per splice of 0.1 dB or less can be obtained

for well-matched fibers. Fusion eliminates reflection at the

interface and if done properly will result in a strong permanent

connection.

Mechanical splicing provides greater flexibility and is used

for short and medium haul routes. The various types of aligning

configuration are

(i) V-grove

(ii) Precision sleeve

(iii) Loose tube and

(iv) Rod sleeve

V- Grove

Figure 3.10 shows the V-grove. The fibers to be joined are

placed in the groove. In this method the alignment of the fiber can be

well controlled. The two fibers are made to slide in the groove and

epoxied when they touch. In this method the errors are very minimal.

Precision sleeve

In this method the fibers are inserted through the central hole

of the sleeve. The epoxy applied at the fiber ends enhance the

joining.

Loose tube splice

Fibers whose ends are epoxied are inserted into loose-tube

splice. When the fiber is bend it causes the tube to rotate and the

fibers align at one of the corner. The epoxy in the ends enhance the

joint.

Rod splice

Fibers are aligned inside the metal rods as in figure. Epoxy is

applied to the fiber ends. The heat shrinkable sleeve is placed over

the assembly. The heat applied secures the rods and squeezes them

against the fiber.

3.8 Loss in optical fibers

When a light signal is transmitted through an optical fiber, it

undergoes loss of signal. The two factors which affect the

transmission of light waves in optical fibers are

1. Attenuation and

2. Dispersion

3.8.1Attenuation

If an input power Pin results in an output power Pout then the loss

in decibel. The ratio of power depends on particular wavelength of

optical source. The basic attenuation mechanism in a fiber are

absorption, scattering and radiative losses of the optical energy.

Where dB - is the signal attenuation per unit length in

decibel

L - is the length of an optical fiber

Material Absorption Losses

Material absorption is a loss mechanism related to the

molecules of the basic fiber material. It greatly depends on

wavelength of light used. The absorption of light may be of three

categories namely,

a. Intrinsic absorption

b. Atomic defect

c. Extrinsic absorption

a. Intrinsic absorption

An absolute pure optical fiber has little intrinsic absorption

due to its basic material structure. This absorption cannot be

avoided.

b. Absorption by atomic defect

This absorption is caused due to inhomogenity of materials

like missing molecule, high density clusters of atom group or

oxygen, defects. This absorption is neligible.

C. Extrinsic absorption losses

The presence of impurities like iron, copper, chromium etc is

the major problem in signal attenuation. These contaminate the

fiber. Also the propagation of light in the fiber are greatly affected

due to interaction of impurity electron and photon (1ight ).

Extrinsic loss mechanism is also caused by the presence of

hydroxyl or OH- ions in the fiber. At 950, 1250,1380 nm the

absorption increases drastically. Hydroxyl ions absorb the light in

these wavelengths.

Scattering Losses

Scattering losses occur when a wave interacts with a

particle on its way and removes energy in the direction of

propagation and transverse it in the other direction. The

scattering losses are classified into two types namely,

a. Linear scattering losses and

b. Non-linear scattering losses.

a. Linear scattering losses

In this scattering the amount of light power that is

transferred from a wave is proportional to the power of the

wavelength. The two types of linear scattering losses are

i. Rayleigh scattering and

ii. Mie scattering

i. Rayleigh Scattering

The inhomogenity of the material like refractive index

fluctuation and compositional variation will cause an attenuation

called Rayliegh scattering. According to Rayliegh scattering, the

loss is inversely proportional to the fourth power of the

wavelength and is given by

Attenuation 1 / 4

This scattering loss can be reduced by operating at the

longest possible wavelength.

ii. Mie scattering

The imperfections of fiber such as irregularities in the core-

cladding interface, core-cladding refractive index difference,

strains and bubbles will cause Mie scattering. This scattering can

cause significant loss.

b. Non-linear scattering

High values of electric field within the fiber creates non-linear

scattering. The scattering is accompanied by a frequency shift of

the scattered lights. The two types of non-linear scattering are

Brillouin scattering

Raman scattering

Brillouin scattering

The Brillouin scattering may be regarded as the modulation

of light through thermal molecular vibrations within fiber. In this

scattering process, the incident photon produces a photon of

frequency 'f' called acoustic frequency. This produces an optical

frequency shift causing loss in transmission.

c. Stimulated Raman Scattering

Unlike Brillouin scattering, stimulated Raman scattering

produces high frequency optical phonon rather than an acoustic

phonon. The magnitude of loss is greater than Stimulated

Brilliouin scattering.

Non-linear scattering losses can be reduced to minimum by

using an optical signal level less than threshold optical power.

Bending losses or Radiation losses

Macro and micro bendings

Optical fibers suffer radiation losses when there are small

bend in the fiber or variations in the surface of the core of the

fiber by environment. These variations are known as micro bends.

A small change (bend) in the shape of the fiber core causes micro

bend losses. The light ray incident on the core cladding interface

will change its angle due to micro bends and refracts through the

cladding surface rather than to reflect into the core.

Micro bend losses can also be caused when the fiber is

subjected to strain. Micro bends in the core-cladding interface is

shown in Fig. 3.12.

Macro bend losses occur when the fiber bend is larger than

its diameter. Bending losses can be minimized by keeping the

fiber straight at length.

3.8.2Dispersion Losses

When an optical signal is sent into the fiber the pulase

spreads / broaden as it propagates through the fiber. This is know

as dispersion. The dispersion losses are of three types namely,

i. Modal dispersion

ii. Material dispersion and

iii. Wave guide dispersion

Modal Dispersion

Modal dispersion occurs in fibers that have more than one

mode of propagation. r When many modes of light ray travels

through the fiber, they differ in the path line (i.e) some rays will

reach the core end before the another. The information

transmitted is stretched out. If the pulse stretch is much, they

begin to overlap each other and may reach a point with delay

between them.

Material Dispersion

Material dispersion occurs due to the variation of refractive

index of the material and wave length of light used.

The change in size of the signal is because, light of different

wave length travels with different speeds. This is due to the fact

that the speed of light depends on the refractive index of the

material through which it is transmitted.

Wave Guide Dispersion

Wave guide dispersion occurs only in the fibers with single

mode. This is caused due to the shape of the fiber core and its

chemical composition. As frequency is a function of wavelength

the group velocity of light in a fiber varies with frequency. Since

the cladding has lower refractive index than the core, the light

will reach the end of the fiber fast than the light travels in the

core.

Among the three dispersions

Intermodal dispersion > Material dispersion> Wave guide

dispersion

3.9 Fiber Optic Communication System

The fiber optic communication system can be classified into

three major units.

1. A transmitter which converts electrical signal into light

signal.

2. An optical fiber which transmits the light signal by

total internal reflection.

3. A receiver that captures the signal from the fiber and

converts them into electrical signal.

The transmitter consists of a light source and necessary drive

circuits. The transducer converts the non-electrical message into

an electric signal and this is given to the light source. The light

source can be either a LED or a semiconductor laser. The light

waves get modulated with the signal and by flashing the LED,

digital modulated signal are obtained. This signal is transmitted

through the optic fiber.

At the receiver end, the light from the fiber is coupled to a

semiconductor photodiode. This converts light signal into

electrical signal. The electrical signals are amplified and decoded

to obtain the message. This output is fed to the transducer to get

the original audio or video signal.

Advantages

1. This has enormous bandwidth. So large number of

signals can be passed.

2. Light weight when compared to conventional cables.

3. Optical fibers are cheap.

4. They are not hazardous as there is no passage of

electric current.

5. The transmission through optic fiber is more secure and

private.

6. External electrical signals or sources cannot interface

with the information passed through the optical fibers.

7. The transmission loss per unit length of an optical fiber

is low i.e., 4dB/km.

3.10 Light Sources for Fiber Optics

Light sources for fiber optics act as signal emitters and

must meet certain requirement.

1. The light emitted must be monochromatic.

2. The light emitted must be intense enough so that

transmission over relatively longer distances is possible

inspite of the intrinsic losses.

3. The light sources must be capable of being easily

modulated.

4. The light sources must be small and compact so that the

output can be easily and effective coupled to the fibers.

5. The light sources must be durable and inexpensive.

Light emitting diodes and semiconductor laser diodes

satisfy almost all the above requirements. Hence they are

extensively used as light sources for fiber optic communication.

3.10.1 LED

The two main types of LED most often used in fiber optical

systems are the surface etched well emitter (S-LED) and the

edge emitter (E-LED). They are designed for more efficient

coupling into optical fibers. In the S-LED, a well is etched into the

top of a planar LED structure. This enables the fiber end to be

kept as close as possible to the light emitting region. In S-LED

the current is confined to a smaller circular region of the surface.

This area is typically 20 to 50 m in diameter. The fiber is held in

position by the use of a transparent epoxy resin as shown in Fig

3.16.

In E-LED, the radiation is confined to a narrow light structure

very similar to that of double hetro structure laser as shown in

Fig.3.17. Total optical powers from edge emitters are typically

several times smaller than from surface emitters. Narrow beam

divergence gives rise to more coupled power.

In general, E-LEDs are prepared to use with small NA fibers

whereas S-LEDs, have an almost linear relation ship between

drive current and light output. This makes the LED more suitable

for amplitude modulation.

3.10.2 Laser source

Lasers are highly monochromatic and intense than LED.

Semiconductor lasers exhibit much higher powers, narrow beam

divergence and a small emitting area. Hence laser couple

significantly give more power into fibers than LED. They are

useful for small diameter low numerical aperture fibers, in

particular single mode fibers. Most commonly used laser is Ga-As

laser.

The homojunction Ga-As semiconductor is dealt already in

previous chapter.

3.11 Photodetectors

At the output end of an optical transmission line there must

be a receiving device which interprets the information contained

in the optical signal. The first element of this receiver is a

photodetector which senses the luminescent power falling upon it

and converts the variation of this optical power into a

correspondingly varying electric current. Since the optical signal

is generally weakend and distorted when it emerges from the end

of the fiber, the photodetector must meet very high performance

requirement.

The photodetector should also be insensitive to variations in

temperature, be compatible with the physical dimensions of the

optical fiber, have a reasonable cost in relation to the other

components of the system, and have a long operating life.

Several types of photodetectors are in existence. Among

these are photomultipliers, pyroelectric detectors, and

semiconductor-based photoconductors, phototransistors and

photodiodes.

Of the semiconductor-based photodetectors, the photo

diode is used almost exclusively for fiber optic systems because

of its small size, high sensitivity, and fast response time. The two

types of photodiodes used are the pin photo detector and the

avalanche photodiode (APD).

3.11.1 The Pin Photo detector

The most commonly used semiconductor photo detector is

the pin photo diode shown schematically in Fig. 3.18. The device

structure consists of p and n regions separated by a very lightly

n-dope1intrinsic (i) region. In normal operation a sufficiently large

reverse bias voltage is applied across the device so that, the

intrinsic region is fully depleted of carriers. That is, the intrinsic n and

p carrier concentrations are negligibly small in comparison with the

impurity concentration in this region.

When an incident photon has an energy greater than or equal

to the band gap energy of the semi conductor material, the photon

can give up its energy and excite an electron from the valence band

to the conduction band. This process generates free electron-hole

pairs which are known photocarriers. The photo detector is normally

designed, so that, these carriers are generated mainly in the

depletion region (the depleted intrinsic region) where most of the

incident light is absorbed.

The high electric field present in the depletion region causes

the carriers to separate and to be collected across the reverse-biased

junction. This gives rise to a current flow in an external circuit, with

one electron flowing for every carrier pair generated. This current

flow is known as photocurrent.As the charge carriers flow through

the material, some electron-hole pair will recombine and hence

disappear. On an average, the charge carriers move a distance Ln or

Lp for electrons and holes, respectively. This distance is known as the

diffusion length.

3.11.2 Avalanche Photodiodes

Avalanche photodiodes (APDs) internally multiply the

primary signal photocurrent before it enters the input circuitary of

the following amplifier. This increases the receiver sensitivity

since the photocurrent is a amplified before encountering the

thermal noise associated with the receiver circuit.

For carrier multiplication to take place, the photogenerated

carriers must traverse a region where a very high electric field is

present. In this high-field region a photogenerator electron or hole

can gain enough energy so that it ionizes bound electrons in the

valence band upon colliding with them. This carrier multiplication

mechanism is known as impact ionization.

The newly created carriers are also accelerated by the high

electric field, thus gaining enough energy to cause further impact

ionization. This phenomenon is the avalanche effect. Below the

diode breakdown voltage a finite total number of carriers are

created. Whereas, above breakdown the number can be infinte.

A commonly used structure for achieving carrier

multiplication with very little excess noise is the reach-through

avalanche photo diode (RAPD) shown in Fig.3.20. RAPD is

composed of a high-resistivity p-type material deposited as an

epitaxial layer on a p+ (heavily doped p-type) substrate. A p-type

diffusion or ion implant is then made in the high-resistivity

material followed by the construction of an n+ (heavily doped n-

type) layer. For silicon the dopants normally used to form these

layers are boron and phosphorus, respectively. This configuration

is referred to as a n+ p p+ structure. The layer is basically an

intrinsic material that has some p doping because of imperfect

purification.

The term "reach-through" arises from the photo diode

operation.

When a low reverse- bias voltage is applied, most of the

potential drop is across the pn+ junction.

The depletion layer widens with increasing bias until a

certain voltage is reached at which the peak electric field at the

pn+ junction is about 5 to 10 percent below the needed to cause

avalanche breakdown. At this point the depletion layer just

"reaches through" to the nearly intrinsic region.

In normal usage the RAPD is operated in fully depleted

mode. Light enters the device through the p+ region and it is

absorbed. The photon gives up its energy, thereby creating

electron-hole pairs, which are then separated by the electric field

in the region. The photogenerated electrons drift through the 1f

region to the pn + junction where a high electric field exists. It is in

this high-field region that carrier multiplication takes place.

3.12 Fiber Optic Sensors

Fiber optic sensor is transducer optical which can convert

various input (physical quantity) into an electrical signal in a

measurable form.

The application of fiber optic sensors are in increase due to

less cost and improved quality than the traditional sensors. The

fiber optic sensors are used for measuring and sensing various

parameters like temperature, electric field, magnetic field,

current, humidity, and acoustic vibrations.

The optical sensors are non contact and generally high

accuracy devices and systems. In fiber sensors, the optical wave

is the information carrier and sensor.

Whenever, optic sensors are used for measuring a physical

parameter, anyone of the characteristics like amplitude, intensity,

phase, polarization, frequency and direction of propagation of the

wave gets modulated by the measured quantity.

3.12.1 Advantages of fiber optic sensors

1. They are light in weight and small in size.

2. They have good geometrical flexibility.

3. Fiber optic sensors are free from the risk of sparks

since they are made of silica.

4. They are electrically passive, i.e., they are immune to

electromagnetic interference and also do not distort

the surrounding electrical and magnetic fields.

5. the chemical and environmental ruggedness is more.

6. These sensors have large bandwidth and are high

sensitive.

3.12.2 Types of Optic Sensors

Fiber optic sensors are of two types

1. Extrinsic or hybrid fiber optic sensors (also called

passive sensors).

2. Intrinsic or all fiber sensors (also called active

sensors).

Extrinsic Sensors

In extrinsic sensors, the interaction between the light and

the measuring parameter (the quantity under measurement)

takes place outside the fiber.

In this type of sensors, the fiber acts merely as a wave

guide. This type of sensors has a sensor head and the sensed

optical signal will be transferred to the measurement point with

low attenuation and incresed mechanical stability for signal

processing.

These sensors can be used for the measurement of

voltages, current, temperature, pressure, force, displacement etc.

Intrinsic Sensors

In intrinsic sensors, the interaction between light and the

measuring parameter takes place directly on the fiber itself and

produces a change in the transmission characteristics.

Liquid level sensor and Faraday sensors or gyroscope are of

this type.

3.12.3 Displacement Sensor

The displacement sensor is used to measure the

displacement of moving targets or objects. This employs a pair of

optic fiber in which one carry light from a remote source to the

object and the other receive the light reflected from the object

and carry it to a photodetector.

Finely focused laser beam coming from an optical fiber is

allowed to fall on the object and the intensity of the light reflected

from the moving target is measured using a detector. By proper

calibration, we can obtain the displacement of the object in terms of

the strength of the output signal of the photo detector.

3.12.4 Pressure Sensor

The pressure sensor works by using the concept of

microbending. Fig. 3.22 shows the pressure sensor. When the fiber is

pressed by a spacer, bents are produced and some amount of light is

lost through the wall of the fiber. As the pressure applied changes,

the intensity of light collected by the detector also changes.

A callibration graph can be drawn initially by apllying different

loads on the spacer. Using the graph the pressure corresponding to

various objects can be found out.

3.13 Endoscope

In medicine, optical fibers are used to study ~he interior of

the lungs, liver and other parts of the human body by using

endoscope. The fiber endoscope uses a bundle of flexible fibers

Types of endoscope: Important types of endoscope

used in medicine to study various parts of the human body are as

follows.

1. Bronchoscope : This is used to check the presence of

foreign bodies and infections in trachea and larger

airways.

2. Cystoscope : This is used to check the presence of

tumours, inflation and to stones in urinary bladder.

3. Laparoscope : It is used to detect the presence of

tumours and to perform family planning operation in

abdominal cavity.

4. Gastroscope : It is used to detect the presence of

tumours and gastric ulcer in stomach.

5. Opthalmoscope : It is used to study the state of blood

vassels in high blood pressure and retinal detachement

in eyes.

Construction and working

Fig 3.24 shows the structure of an endoscope. Usually in an

endoscope there are two fibers namely inner fiber and outer

fiber. The outer fiber is used to illuminate the inner structure of

the body under study. The inner fiber is used to collect the

reflected light from that area under study.

An optical light source attached at the entrance of the fiber

illuminates the part of the body under study. The light collections

equipment and image viewing arrangement are placed at the

ends of the inner fiber. With this arrangement the internal

structure of the body can be examined.

Endoscope are also used for treatment of diseases and

surgery. For example Laparoscope is used for family planning

operations and removal of tumors in the abdominal cavity.

3.14 Comparison between laser diodes and Light emmiting

diodes

S.No Laser diodes LED

1. Laser diodes are more

suitable as light sources in

optical fiber communication

due to their longer lifetime

and high modulation rates

(0r) high bandwidth length

LEDs are not suitable as light

sources in the long distance

optical fiber communication

due to their higher threshold

current level and non linearity

with respect to drive current

product. and power output.

2. They have narrow spectral

width, high optical power

output, efficient waveguide

structure and high

directional coherent

radiation.

They have low optical power

output and incoherent

radiation.

3. The carrier life time and the

output pulse rise time are

small due to stimulated light

emission and so the

radiative recombination rate

is increased. This lead to

increase of bandwidth or bit

rate and output power

The carrier life time and

output pulse rise time are

more and so the radiative

recombination rate is

decreased. Thus lead to

decrease of bandwidth or bit

rate and output power.

4. The design of laser diode

with low threshold current

and high coherence is so

complicated and hence the

cost of the laser diode is

more

The design of LED is simple

and hence the cost of diode is

cheap.

5. This is mostly suitable as

light source in the long

distance optical fiber

communication system due

to its high optical power

output.

This is more suitable for

display of alphabets and

numbers in the dot matrix or

7 segment systems and not

used in the long distance

optical fiber communication

system due to its low optical

power output.