Fiber Optic Communications.docx

7/28/2019 Fiber Optic Communications.docx

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Dr.Y.Narasimha Murthy ,Ph.D

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FIBER OPTIC COMMUNICATIONS

INTRODUCTION:

Fiber-optic communication systems are light wave systems that employ optical fibers for

information transmission. Such systems have been deployed worldwide since 1980 and haverevolutionized the technology behind tele-communications .Optical communication systems use

high carrier frequencies (~100 THz) in the visible or near-infrared region of the electromagnetic

spectrum. They are sometimes called light wave systems to distinguish them from microwave

systems, whose carrier frequency is typically smaller by five orders of magnitude(~1 GHz).

The development of worldwide telephone networks during the twentieth century led to many

advances in the design of electrical communication systems. The use of coaxial cables in place of

wire pairs increased system capacity considerably. 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 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 110 GHz is used to transmit the signal by using suitable

modulation techniques.

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 and are able to

operate at bit rates ~100 Mb/s. The most advanced coaxial system was put into service in 1975

and operated at a bit rate of 274 Mb/s. A severe drawback of such high-speed coaxial systems is

their small repeater spacing (~1 km), which makes the system relatively expensive to operate.

Microwave communication systems generally allow for a larger repeater spacing, but their bit

rate is also limited by the carrier frequency of such waves.

The idea of using optical fibers for communication was suggested in 1966 , 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. During the 1960s the fiber losses were of the order

of 1000 dB/km. A breakthrough occurred in1970 when fiber losses could be reduced to below

20 dB/km in the wavelength region near 1 m . At about the same time, GaAs semiconductor


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Lasers, operating continuously at room temperature, were demonstrated. The simultaneous

availability of compactoptical sources and a low-lossoptical fibers led to a worldwide effort for

developing fiber-optic communication systems.

Evolution of fiber optic system :

The evolution of Fiber optic system can be divided into five generations in terms of

developments and changes

The first generation of fiber optic systems operated near 0.8 m and used GaAs semiconductor

lasers. After several field trials during the period 197779, such systems became available

commercially in 1980 . They operated at a bit rate of 45 Mb/s and allowed repeater spacings of

up to 10 km. The larger repeater spacing compared with 1-km spacing of coaxial systems was an

important motivation for system designers because it decreased the installation and maintenance

costs associated with each repeater.

The second generation of fiber-optic communication systems became available in the early

1980s, but the bit rate of early systems was limited to below 100 Mb/s because of dispersion in

multimode fibers . This limitation was overcome by the use of single-mode fibers. By 1987,

second-generation lightwave systems, operating at bit rates of up to 1.7 Gb / s with a repeater

spacing of about 50 km, were commercially available.

The introduction of third-generation light wave systems operating at 1.55 m was considerably

delayed by a large fiberdispersion near 1.55 m. Conventional InGaAsP semiconductor lasers

could not be used because of pulse spreading occurring as a result of simultaneous oscillation of

several longitudinal modes. The dispersion problem can be solved either by using dispersion-

shifted fibers designed to have minimum dispersion near 1.55 m or by limiting the laser

spectrum to a single longitudinal mode. Third-generation light wave systems operating at 2.5

Gb/s became available commercially in 1990. Such systems are capable of operating at a bit rate

of up to 10 Gb/s .A drawback of third-generation 1.55-m systems is that the signal is

regenerated periodically by using electronic repeaters spaced apart typically by 6070 km.

The fourth generation of light wave systems makes use of optical amplificationfor increasing

the repeater spacing and of wavelength-division multiplexing(WDM) for increasing the bit rate


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In most WDM systems, fiber losses are compensated periodically using erbium-doped fiber

amplifiers spaced 6080 km apart. Such amplifiers were developed after 1985 and became

available commercially by 1990. The experimental results in 1991 showed the possibility of data

transmission over 21,000 km at 2.5 Gb/s, and over 14,300 km at 5 Gb/s . This performance

indicated that an amplifier-based, all-optical, submarine transmission system was feasible for

intercontinental communication.

The fifth generation of fiber-optic communication systems is concerned with extending the

wavelength range over which a WDM system can operate simultaneously. The conventional

wavelength window, known as the C band, covers the wavelength range 1.531.57m. It is being

extended on both the long- and short-wavelength sides, resulting in the L and S bands,

respectively. The Raman amplification technique can be used for signals in all three wavelength

bands. Moreover, a new kind of fiber, known as the dry fiber has been developed with the

property that fiber losses are small over the entire wavelength region extending from 1.30 to 1.65

m . Availability of such fibers and new amplification schemes may lead to light wave systems

with thousands of WDM channels. The fifth-generation systems also attempt to increase the bit

rate of each channel within the WDM signal. Starting in 2000, many experiments used channels

operating at 40 Gb/s; migration toward 160 Gb/s is also likely in the future. Such systems require

an extremely careful management of fiber dispersion.

Advantages of Optical Fibers :

There are many advantages of optical fibers when compared to other methods.

1. Long distance transmission: Optical fibers have low transmission losses when compared to

copper cables .So,data can be transmitted over longer distances and the number of repeaters

required can be reduced.

2. Information Capacity : Optical fibers have large information capacity ,because of their

longer bandwidths .So,more information or data can be transmitted on a single fiber wire as

compared to a copper wire.This will reduce the cost.

3.Small size and low weight : The dimensions of fiber cabels is relatively small as compared to

copper wires which are very bulky. This is very advantageous in some systems like aircraft,

satellites , ships and in military applications where small light weight cables are preferred when

compared to copper cables.


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4.Immunity to Electrical Interference : As fiber cables are dielectric and non-metallic ,they are

immune to external electric disturbances and also not affected by electromagnetic interference

or electric noise effects due to adjacent channelsor near by electrical equipment. But, this is

very severe in coppercables.

5. Enhanced Safety : As fiber cables do not have the problems of ground loops ,sparks and high

voltages unlike copper cables they offer a high degree of operational safety. The only limitation

is ,care must be taken while handling LASER light to avoid possible damage to eyes.

6. Signal Security : The fiber cable always guides the optical signal and hence there is a high

degree of data security from external disturbances. Where as in copper wires the electrical

signals can be easily tapped off.

Basic optical laws and definitions:

The phenomenon of total internal reflection, is responsible for guiding of light in optical fibers.

A very important optical property associated with the material is its refractive index. The

refractive index of a material is defined as the ratio of velocity of light in free space to that in the

material.

The refractive index n =

The value of n for free space or air is 1.00 and for water 1.33 and for silica glass 1.45-1.55and

for diamond 2.42

Refraction and Reflection :The two important properties of light are Refraction and Reflection. When light travels from one

medium to another medium of different refractive indices, the ray bends at the interface of the

two media. i.e there will be a change in the velocity of the light at the interface .This phenomenais known as Refraction of Light. Some time s depending on the refractive index of the second

medium ,the light will retrace its path and come back into the same path.This phenomena is

called Reflection.

If the angle of incidence is 1 and angle of refraction is 2 and the refractive indices of the two

media are n1 and n2 respectively, the refraction relation is given by

n1.Sin 1 = n2.Sin 2 This law is called Snells law.


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The angle between the incident ray and the normal drawn to the surface is known as angle ofincidence 1 . The angle between the refracted ray and the normal drawn to the interface isknown as angle of refraction 2.

The incident ray , the normal to the interface and the reflected ray all lie in the same plane,which

is perpendicular to the interface plane between the two materials. This plane is called plane of

Incidence.

As the angle of incidence1 in an optically denser medium increases,the refracted angle 2approaches .Beyond this angle there is no refraction possible.Hence the light ray totally

internally reflected into the same medium.The angle incidence for which the angle of refraction

is

is known as the Critical angle (C) .When the incidence angle is higher than critical angle

.the total internal reflection condition is satisfied. In such situation the light is totally reflected

back into the same medium (Glass) with no light escaping (from the glass).

Optical fiber modes and configurations:

An optical fiber consists of a cylindrical core of silica glass surrounded by a solid dielectric

cladding whose refractive index is lower than that of the core. Suppose the refractive index of

core is n1 and that of the clad is n2 , it is remembered that always n2 < n1.The cladding reduces

the scattering losses and also provides mechanical strength to the fiber and also protects the core

from absorbing surface contaminants with which it could come into contact.


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In standard optical fibers the core material is a pure silica glass(SiO2) and is surrounded by a

glass cladding .Higherloss plastic core fibers are also in use.In addition to this most of the

fibers are encapsulated in an elastic ,absorption resistant plastic material.This plastic material

adds further strength to the fiber and mechanically isolates from geometrical irregularities

,distortions or roughness of adjacent surfaces. Otherwise these irregularities cause scattering

losses

So, an optical fiber is a wave guide that works at optical frequencies .This wave guide will be incylindrical form and the light energy propagates parallel to its axis. The propagation of the light

waves through the fibers is decided by the structural characteristics .These structural

characteristics of the fiber decides the information carrying capacity and the response of the

wave guide to the external perturbations.

The propagation of the Light along the fiber cable axis is described in terms of a set of guided

electromagnetic waves called the modes of the wave guide. These guided modes are also termed

as bound or trapped modes of the wave guide. Each mode is a pattern of electric and magnetic

field distributions that is repeated along the fiber at equal intervals. It is found that only certain

discrete number of modes are capable of propagating along the guide.

Types of Fibers

Based on the variations in the material composition(refractive index) of the core there are two

types of Fibers.They are (i) Step Index fiber and (ii) Graded Index fiber .

A step index fiber is one in which the refractive index of the core is uniform throughout and

undergoes an abrupt change (or step) at the cladding boundary.

A graded index fiber is one in which the refractive index of the core varies as a function of the

radial distance from the centre of the core. These two types are explained in the diagram below.


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Both the Step and Grdaed index fibers are classified into single mode and multi mode fibers.A

single mode fiber supports only one mode of propagation and where as multimode fibers

supports many large number of modes as shown in figure below.


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From the above it is clear that the multimode fibers have larger core radii hen compared to

mono mode fibers. So, it is easy to launch optical power into the fiber and also facilitates the

coupling of similar fibers .Another advantage is that light can be launched into a multimode fiber

using a light emitting diode source. whereas the mono-mode fibers are excited using Laser

diodes. The LEDs have longer life than Laser diodes. Hence the multimode fibers have more

applications.

The disadvantage of multimode fibers is they suffer from intermodal dispersion .i.e the pulse that

is launched into the fiber will be distributed overall the modes and each mode may travel with

different velocity and arrive at the fiber end at a slightly different times. This can be reduced by

using a graded index profile in the fiber core.

Step-Index Fibers

Let us consider a step index fiber such that i is the angle of incidence and r is the angle ofrefraction.So,from Snells law

n0Sini = n1Sinr

,

where n1 and n0 are the refractive indices of the fiber core and air, respectively.

Suppose c is the critical angle we can write that

Sin c = n2/n1

where n2 is the cladding index, the ray experiences total internal reflection at the corecladdinginterface.


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From the diagram it is clear that r = (/2 c) .So, we can write thatnoSin i = n1sin r= n1Cos c = (n12n2 2 )1/2

Here noSin i is known as Numerical Aperture(NA) of the fiber.This represents the lightgathering capacity of the optical fiber.

So, NA = n1(2)1/2 Here = (n1 n2)/n1

The is called fractional index change at the core-cladding interface. This should be as large

as possible in order to couple maximum amount of light into the fiber. But this type of fiber has

the limitation with multipath dispersion.

Graded-Index Fibers: The refractive index of the core in graded-index fibers is not constant

but decreases gradually from its maximum value n1 at the core center to its minimum value n2 at

the corecladding interface. Most graded-index fibers are designed to have a nearly quadratic

decrease and are analyzed by using -profile, given by

where a is the core radius. The parameter determines the index profile. A step-index profile isapproached in the limit of large . Aparabolic-index fibercorresponds to = 2.

Similar to the case of step-index fibers, the path is longer for more oblique rays. However, the

ray velocity changes along the path because of variations in the refractive index. More

specifically, the ray propagating along the fiber axis takes the shortest path but travels most


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slowly as the index is largest along this path. Oblique rays have a large part of their path in a

medium of lower refractive index, where they travel faster. It is therefore possible for all rays to

arrive together at the fiber output by a suitable choice of the refractive-index profile. Hence the

graded index fiber will have less multi path dispersion.

Semiconductor Optical Sources :

The major light sources used for fiber optic communication applications are hetero junction

structured semiconductor Laser diodes (Injection Laser Diodes) and Light emitting

Diodes(LEDs).A hetero junction consists of two adjoining semiconductor materials with

different band gap energies. These devices are suitable for fiber transmission systems ,because

they sufficient output power for a wide range of applications. Their optical power output can be

directly modulated by varying the input current to the device. Also they have high efficiency

with compatible dimensional characteristics with those of the optical fiber.

The LEDs and Laser diodes consists of a pn junction constructed by using a direct band gap III-

V semiconducting materials .When this junction is forward biased ,electrons and holes are

injected into the p and n regions respectively .These injected minority charge carriers can

recombine either radiatively ( where a photon of energy h is emitted) or non-radiatively (the

recombination energy is dissipated in the form of heat). So, this pn junction is known as the

active or recombination region.

The difference between LEDs and Laser diodes is that the optical output from an LED is

incoherent , where as the optical output from the Laser diode is coherent. The LED is based on

spontaneous emission and the Laser diode is based on Stimulated emission.

In a coherent source the optical energy is produced in an optical resonant cavity and in an

incoherent LED source, no optical cavity exists for wavelength selectivity and the output

radiation has a broad spectral width. Also the incoherent optical energy is emitted into a

hemisphere according to a cosine power distribution and hence has a large beam divergence. In

general LEDs are used with multimode fibers ,because only the incoherent optical power from an

LED can only be coupled into a multimode fiber .And the Laser diodes are used for single mode

fibers.

The semiconductor material used for the active layer of an optical source must have direct band

gap.Because only direct band gap material has high radiative recombination .In a direct band gap


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semiconductor electrons and holes can recombine directly across the band gap without the need

of a third particle to conserve momentum. The single element semiconductors are not direct band

gap materials. But most of the binary and ternary semiconductors can act as direct band gap

materials.(For example III-V materials like GaP,InP).For operation in the 800900 nm spectralrange the ternary semiconductor material Ga1-x Alx As is used.

LIGHT EMITTING DIODES (LEDs)

The LEDs are used as optical sources where the bit rates less than 100 to 200 Mb/s are required

,with multimode fiber coupled optical power in the tens of microwatts. The LEDs require less

complex drive circuitry than laser diode ,since no thermal or optical stabilization circuits are

needed and LEDs can be fabricated at low costs.

LED Structure : The LEDs used in fiber optic communication applications should have high

radiance output ,fast emission response time and a high quantum efficiency. The emission

response time is the time delay between the application of a current pulse and the onset ofoptical emission. To achieve a high radiance and high quantum efficiency, the LED structure

must provide the stimulated optical emission to the active region of the pn junction where

radiative recombination takes place. Carrier confinement is used to achieve a high quantum

efficiency.

To achieve carrier and optical confinement LED configuration like double hetero structure orhetero junction which consists of two different alloy layers on each side of the active region is

implemented.

The LED structures can be classified as surface-emitting or edge-emitting, depending on

whether the LED emits light from a surface that is parallel to the junction plane or from the edge

of the junction region. Both types can be made using either a pn homo-junction or a

heterostructure design in which the active region is surrounded by p- and n-type cladding layers.

The hetero-structure design leads to superior performance, as it provides a control over the

emissive area and eliminates internal absorption because of the transparent cladding layers. In

the surface emitting configuration a well is etched through the substance of the device ,into

which the fiber is then cemented in order to accept the emitted light.The circular active area is

normally 50m in diameter and up to 2.5m thick. The emission pattern is essentially isotropic

with a 1200

half power beam width. The surface emitter configuration is shown in figure below.


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The edge emitter configuration is shown in figure below.It consists of an active junction region

,which is the source of incoherent light and two guiding layers. Both these guiding layers have a

refractive index which is lower than that of the active region but higher than the index of the

surrounding material. This structure forms a wave guide channel that directs the optical radiation

toward the fiber core. To match the typical fiber core diameters (50 -100 m) the contact stripes

for the edge emitter are 50 to 70 m wide. The edge emitter configuration is shown below.


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The lengths of the active region usually range from 100 to 150 m and the emission pattern of

the edge emitter is more directional than that of the surface emitter.In the plane parallel to the

junction where there is no wave guide effect ,the emitted beam is lambertian(varying as cos)

with a half power width of = 1200 .In the plane perpendicular to the junction the half power

beam width is made as small as 25 to 350by proper choice of waveguide thickness.

SEMICONDUCTOR LASER DIODES :

Semiconductor lasers emit light through stimulated emission. Due to the fundamental

differences between spontaneous and stimulated emission, they are capable of emitting high

powers (~ 100 mW), and also emit coherent light. A relatively narrow angular spread of the

output beam compared with LEDs permits high coupling efficiency (~50%) into single-mode

fibers. A relatively narrow spectral width of emitted light allows operation at high bit rates (~10

Gb/s), since fiber dispersion becomes less critical for such an optical source. Furthermore,

semiconductor lasers can be modulated directly at high frequencies (up to 25 GHz) because of a

short recombination time associated with stimulated emission. Most fiber-optic communication

systems use semiconductor lasers as an optical source because of their superior performance

compared with LEDs.

Laser Diode Structures :

The semiconductor laser diode consists of a thin active layer (thickness ~0.1 m) sandwiched

betweenp-type and n-type cladding layers of another semiconductor with a higher band gap. The

resultingpn hetero-junction is forward-biased through metallic contacts. Such lasers are called

broad-area semiconductor lasers since the current is injected over a relatively broad area

covering the entire width of the laser chip (~100 m).


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The laser light is emitted from the two cleaved facets in the form of an elliptic spot of

dimensions ~1100 m2. In the direction perpendicular to the junction plane, the spot size is ~ 1

m because of the hetero-structure design of the laser. Here the active layer acts as a planar

waveguide because its refractive index is larger than that of the surrounding cladding layers (n

0.3). Similar to the case of optical fibers, it supports a certain number of modes, known as the

transverse modes. In practice, the active layer is thin enough (~ 0.1 m) that the planar

waveguide supports a single transverse mode. However, there is no such light-confinement

mechanism in the lateral direction parallel to the junction plane. Consequently, the light

generated spreads over the entire width of the laser.

In strongly index-guided semiconductor lasers, the active region of dimensions ~0.11 m2 is

buried on all sides by several layers of lower refractive index. For this reason, such lasers are

called buried hetero-structure (BH) lasers.

LASER DIODE MODES AND THRESHOLD CONDITIONS : In a Laser diode ,a Fabry-

Perot resonator cavity is formed with the help of two flat ,partially reflecting mirrors which are

directed to each other . The use of the mirrors is to provide a strong optical feedback in the

longitudinal direction which compensates for optical losses in the cavity. This Laser cavity can

have many resonant frequencies for which the gain is sufficient to overcome the losses. The sides

of the cavity are formed by polishing properly the edges so that unwanted emissions can be

reduced.


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The light radiation within the cavity of the Laser diode sets up a pattern of electric and magnetic

field lines called modes of the cavity.These modes are classified as two independent sets called

Transverse electric(TE) and Transverse Magnetic (TM) modes. Each of these modes can be

described in terms of longitudinal ,lateral and transverse electromagnetic fields along the major

axes of the cavity.The longitudinal modes are related to the length L of the cavity and determine

the principal structure of the frequency spectrum of the emitted optical radiation. As the length L

is very larger than the Lasing wavelength(1m) many longitudinal modes can be formed.

The lateral modes lie in the plane of the pn junction.These modes depend on the width of

thecavity and side wall. It determine the shape of the lateral profile of the Laser beam.The

transverse modes are associated with the electromagnetic field and beam profile in the direction

perpendicular to the plane of the pn junction.These modes are very important as they largely

determine the Laser characteristics like radiation pattern and the threshold current density.

To determine the Lasing condition let us consider the EM wave propagating in the longitudinal

direction

E(z, t) = I(z) ej(wt-z)

Where I(z) is the optical field intensity and w is the optical frequency in radians and is the

propagation constant. The lasing is the condition at which light amplification is possible in the

Laser diode. The basic requirement is the population inversion. The optical amplification of the

selected modes is provided by the feedback mechanism of the optical cavity.In the repeated

passes between the two partially reflecting parallel mirrors ,a portion of the radiation associated

with those modes having the highest optical gain coefficient is retained and further amplified

during each oscillation in the cavity. Lasing occurs when the gain of one or several guided

modes is sufficient to exceed the optical loss during one round trip through the cavity .At the

lasing threshold a steady state oscillation takes place and the magnitude and the phase of the

reflected wave must be equal to the original wave.

The condition for amplitude is I(2L) = I(0) and

for phase e jL = 1

The mode which satisfies the above condition reaches the threshold first.At one set of this

condition all additional energy introduced into the Laser should enhance the growth of this

particular mode.


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Fiber to fiber joints :During the installation of fiber optic communication system ,it is always

important to interconnect the fibers with minimum losses. these interconnections or joints occur

at the optical source ,at the photo detector and at intermediate points within a cable. There are

two types of joints ,namely Splice and connector. The permanent bond between two fibers is

called splice and demountable joint is called connector. The type of technique used for joining

two fibers depends on whether a permanent bond or an easily demountable connection is

required.

The losses due to the joints depend on the parameters like input power distribution to the point

,the length of the fiber between the optical source and the joint,the geometrical and wave guide

characteristics of the two fiber ends at the joint and the fiber end face qualities.

The optical power that can be coupled from one fiber to another is limited by the number of

modes that can be transmitted in each fiber.For example ,if a fiber in which 500 modes can

propagate is connected to a fiber in which only 400 modes can propagate ,then at most 80% of

the optical power from the first fiber can be coupled to the second fiber.

Mechanical alignment is a serious problem while joining two fibers because of their

microscopic size.Radiation losses occur due to the misalignment as the radiation cone of the

emitting fiber does not match with the acceptance of the receiving fiber.The magnitude of the

radiation loss depends on the degree of misalignment.There are three types of misalignments.

Lateral misalignment , longitudinal misalignment and angular misalignment.

Longitudinal separation occurs when the fibers have the same axis but have a gap between their

end faces.Angular misalignment occurs when the two axes form an angle so that the fiber end

faces are no longer parallel.Axial displacement (alos called lateral displacement) occurs when

the axes of the two fibers are separated by a small distance .The most common misalignment that

occurs in practice is axial displacement and it also causes large power loss.

In addition to the mechanical misalignments ,differences in geometrical and waveguide

characteristics of any two fibers being joined can also show effect on fiber couplig.These

include variations in core diameter ,core area ellipticity ,Numerical aperture ,refractive index

profile and core-cladding concentricity of each fiber.

Fiber splicing Techniques: There are various fiber splicing techniques in use .The most

commonly used are Fusion splice , V-groove ,tube mechanical splice ,elastictube splice and

the rotary splice.


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Fusion splices are made by thermally bonding two fiber ends together.In this method ,first the

two fiber ends are pre-aligned and butted together .This is done under a microscope with

micromanupulators.The butt joint is then heated with an electric arc or laser pulse so that the

fiber ends are momentarily melted and hence bounded permanently.This technique produce very

low splice losses of less than 0.06dB.

In the V-groove splice technique ,the two fiber ends are first butted together ina V-shaped groove

and then bonded together with an adhesive or held in place by means of a cover plate.the V-

shaped channel could be either a grooved silicon ,plastic ,ceramic or metal substrate.The splice

loss in this method mainly depends on the fiber size and the eccentricity of the core relative to

the center of the core.

The elastic tube splice is a unique device that automatically performs lateral,longitudinal and

angular alignment.It splices multimode fibers with losses in the same range as fusion splices,with

re;latively less complexity and skill.This splice mechanism basically consists of an elastic tube

with a central hole .The diameter of the hole is slightly less than that of the fiber to be

spliced.When the fiber is inserted ,it expands the hole diameter so that the elastic material exerts

a symmetrical force on the fiber.This symmetric force allows an accurate and automatic

alignment of the axes of the two joined fibers.A wide range of fiber diameters can be inserted

into the elastic tube.So,the fibers to be spliced need not have to be equal in diameter,because

each fiber moves into position independently reltive to the tube axis.


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OPTICAL FIBER CONNECTORS : Connectors are very important to connect two fibers

without loss of the signal.There are different types of connectors available.They are screw-

on,bayonet-mount and push-pull configurations.These include both single channel and

multichannel assemblies ,cable-to cable and cable to circuit card connections . The basic

coupling mechanism used in these connectors will be either the butt-joint or expanded beam

types.But most of the connectors today are butt-joint type .These connectors employ a

metal,ceramic or molded-plastic ferrule for each fiber and precision sleeve into which the ferrule

fits.

A good connector must have the following requirements.

1.Low coupling losses:The connector assembly must maintain correct alignment so that losses

will be minimum.

2.Interchangebility :Connectors from one manufacturer must be compatible with other

manufacturers.

3.Ease of Assembly: The installation of connector must be simple and it should not give trouble

to the technitian.

4.Low Environmental sensitivity: The connectors performance should not be affected by

exrenal conditions like temperature ,dust and moisture etc.

5.Low cost and reliable construction: The connector must be always reliable and must not be

very expensive.

6.Ease of operation: The connection and unmounting must be simple and must be operated with

bare hands with ease.

PHOTO DETECTORS - PRINCIPLE:

A photo detector senses the optical power falling upon it and converts this power into suitable

electric current. The photo detector must have the characteristics of high response or sensitivity

to the incident radiation and sufficient bandwidth to handle desired data rate.The photo detector

should also be insensitive to external temperature variations and other conditions.There are

various types of photo detectors like photo multipliers, photo transistors and photo diodes ,pyro

electric detectors etc. But all these detectors do not meet the fiber optic communication

requirements. Only photo diodes will be alone very useful for such applications


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The photo detectors are used as optical receivers .The role of an optical receiver in a fiber optic

communication system is to convert the optical signal back into electrical form and recover the

data transmitted through the light wave system. Its main component is a photo-detector that

converts light into electricity by using the photoelectric effect. The requirements for a photo-

detector are high sensitivity, fast response, low noise, low cost, and high reliability. Its size

should be compatible with the fiber-core size. These characteristics are best met by photo-

detectors made of semiconductor materials.

Principle :

A reverse-biased pn junction consists of a depletion region, that is essentially devoid of free

charge carriers and where a large built-in electric field opposes flow of electrons from the n-side

to the p-side (and of holes from p to n).When such a pn junction is illuminated with light on one

side, say the p-side , electronhole pairs are created due to absorption. Because of the largebuilt-in electric field, electrons and holes generated inside the depletion region accelerate in

opposite directions and drift to the n- and p-sides, respectively. The resulting flow of current is

proportional to the incident optical power. Thus a reverse-biased pn junction acts as a photo-

detector and is referred to as the pn photodiode.

The electronhole pairs generated inside the depletion region experience a large electric field anddrift rapidly toward the p- or n-side, depending on the electric charge. The resulting current flow

constitutes the photodiode response to the incident optical power. The responsivity of a

photodiode is quite high (R~ 1 A/W) because of a high quantum efficiency.


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p-i-n PHOTO DETECTOR:

The p-i-n photo diode is consists of p and n regions separated by a very lightly n-doped intrinsic

region. It is a very widely used semiconductor photo detector used in fiber optic receivers.The

two important characteristics of the PIN diode are the quantum efficiency and Responsivity.The

PIN detector circuit is shown in the diagram below. In normal operation ,a very large reverse

bias voltage is applied across the diode such that the intrinsic region is fully depleted of charge

carriers.i.e the intrinsic n and p carrier concentrations are negligibly small in comparison with

the impurity concentrations in this region.

When a photon of energy greater than or equal to the band gap energy of the semiconductor

incidents on this,it will 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 as photo

carriers. The design of the photo detector is such that these carriers are generated mainly in the

depletion region where most of the incident light is absorbed. The high electric field present in

the depletion region causes the carriers to separate and move across the reverse bias

junction.This gives rise to a current flow in the external circuit. This current is known as photo

current.

As the charge carriers flow through the material, some electron-hole pairs will recombine and

hence disappear. On average the charge carriers move a distance Ln or Lp for electrons or holes

respectively. This distance is known as diffusion length.The time taken by a hole or electron to

recombine is known as carrier life time and is denoted by tn and tp .The diffusion lengths and

carrier life times are related by


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Ln = (Dn Tn)1/2

and Lp = (DpTp)1/2

where Dn and Dp are the electron and hole diffusion coefficients.

In the photo diode operation it is clear that the optical absorption coefficient strongly depends

on the wavelength for many semiconductor materials. So,a particular semiconductor material can

only be used over a limited wave length range. This is the limitation in the photo diode

operation. Also there is a limitation in the responsivity R of the p-i-n diode.

Avalanche photodiodes - Structure of In GaAs APDs.

To overcome the limitations of p-i-n diodes responsivity and to achieve larger responsivities

this Avalanche Photo diode is used.This diode consists of an additional layer in which secondary

electronhole pairs are generated through impact ionization. So, the APDs multiply thephotocurrent internally before it enters the amplifier circuitry. This carrier multiplication

mechanism is called impact ionization. The newly created carriers also accelerated by the electric

field and gain enough energy to cause further impact ionization. This phenomena is called

avalanche effect.

.

An Avalanche Photodiode (APD) provides higher sensitivity than a standard photodiode. It is

ideal for extreme low-level light (LLL) detection and photon counting. Fabricated using Silicon

or InGaAs materials, these devices provide detectivity from 400 nm - 1100 nm.


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Under reverse bias, a high electric field exists in the p-type layer sandwiched between i-type and

n+- type layers. This layer is referred to as the multiplication layer, since secondary electron

hole pairs are generated here through impact ionization. The i-layer still acts as the depletion

region in which most of the incident photons are absorbed and primary electronhole pairs are

generated. Electrons generated in the i-region cross the gain region and generate secondary

electronhole pairs responsible for the current gain.

The use of APDs instead of PIN photo detectors will result in improved sensitivity in many

applications. In general, APDs are useful in applications where the noise of the amplifier is high

i.e., much higher than the noise in the PIN photo detector. Thus, although an APD is always

noisier than the equivalent PIN, improved signal-to-noise can be achieved in the system for APD

gains up to the point where the noise of the APD is comparable to that of the amplifier.

Structure of In GaAs APDs :

For light wave systems operating in the wavelength range 1.31.6 m, Ge or InGaAs APDs must

be used. The improvement in sensitivity for such APDs is limited to a factor below 10 because

ofa relatively low APD gain (M ~ 10) that must be used to reduce the noise . The performance of

InGaAs APDs can be improved through suitable design modifications to the basic APD

structure. The structure of the In Ga As avalanche Photo Diode is shown in the figure below.


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The main reason for a relatively poor performance of InGaAs APDs is related to the comparable

numerical values of the impact-ionization coefficients eand h . As a result, the band width is

considerably reduced, and the noise is also relatively high. Also, because of a relatively narrow

band gap, InGaAs undergoes tunneling breakdown at electric fields of about 1105 V/cm, a

value that is below the threshold for avalanche multiplication. This problem can be solved in

hetero-structure APDs by using an InP layer for the gain region because quite high electric fields

(> 510 5 V/cm) can exist in InP without tunneling breakdown.

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The above class notes would never been possible with out the help of the following references. I

owe to the following people.

References: 1. Optical fiber communication-G.Keiser.

2.Fiber-Optic Communication SystemsGovind .p Agarwal

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