Fiber optic communications

Dr.Y.Narasimha Murthy ,Ph.D
yayavaram@yahoo.com

FIBER OPTIC COMMUNICATIONS
INTRODUCTION:
Fiber-optic communication systems are lightwave systems that employ optical fibers for
information transmission. Such systems have been deployed worldwide since 1980 and have
revolutionized 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 1–10 GHz is used to transmit the signal by using suitable
modulation technique.
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 fibre 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 μm . At about the same time, GaAs semiconductor

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Lasers, operating continuously at room temperature, were demonstrated. The simultaneous
availability of compact optical sources and a low-loss optical fibers led to a worldwide effort for
developing fiber-optic communication systems.

Fig:Electromagnetic Spectrum
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 μm and used GaAs semiconductor
lasers. After several field trials during the period 1977–79, such systems became available

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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 light-wave 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 μm was considerably
delayed by a large fiber dispersion near 1.55 μm. 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 μm 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-μm systems is that the signal is
regenerated periodically by using electronic repeaters spaced apart typically by 60–70 km.
The fourth generation of light wave systems makes use of optical amplification for increasing
the repeater spacing and of wavelength-division multiplexing (WDM) for increasing the bit rate
In most WDM systems, fiber losses are compensated periodically using Erbium-doped fiber
amplifiers spaced 60–80 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 1.53–1.57μm. 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

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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
μm . 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. Infomation 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.
4.Immunity to Electrical Interference : As fiber cables are dielectric and non-matallic ,they are
immune to external electric disturbances and also not affected by electromagnetic interference
or electric noise effects due to adjacent channels or near by electrical equipment. But, this is
very severe in copper cables.
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.

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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 phenomena
is 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 Ф1 and angle of refraction is Ф2 and the refractive indices of the two
media are n1 and n2 respectively ,the refraction relation is given by

n1.Sin Ф1 = n2.Sin Ф2 . This law is called Snell’s law.

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The angle between the incident ray and the normal drawn to the surface is known as angle of
incidence Ф1 . The angle between the refracted ray and the normal drawn to the interface is
known as angle of refraction Ф2.

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 incidence Ф1 in an optically denser medium increases ,the refracted angle Ф2
approaches .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 .Higher –loss 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 in
cylindrical 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 as 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 that 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 θi is the angle of incidence and θr is the angle of
refraction.So,from Snell’s law

n0 Sinθi = n1 Sinθr

where n1 and n0 are the refractive indices of the fiber core and air, respectively.Suppose θc is the
critical angle we can write that

Sin θc = n2/n1

where n2 is the cladding index, the ray experiences total internal reflection at the core–cladding
interface.

From the diagram it is clear that θr = (π/2 – θc) .So, we can write that
noSin θi = n1sin θr = n1Cos θc = (n12 –n2 2 )1/2

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Here noSin θi is known as Numerical Aperture (NA) of the fiber. This represents the light
gathering capacity of the optical fiber.

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

The Δ is called fractional index change at the core-cladding interface.This Δ 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 core–cladding interface. Most graded-index fibers are designed to have a nearly quadratic
decrease and are analyzed by using α-profile, given by

where a is the core radius. The parameter α determines the index profile. A step-index profile is
approached in the limit of large α. A parabolic-index fiber corresponds to α = 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
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

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

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of a third particle to conserve momentum.The single element semiconductors are not direct band
gap materials. But most of the binary and terinary semiconductors can act as direct band gap
materials.(For example III-V materials like GaP,InP).For operation in the 800 – 900 nm spectral
range 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 of
optical 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 or
hetero 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 p–n homojunction or a
heterostructure design in which the active region is surrounded by p- and n-type cladding layers.
The heterostructure 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
50µm in diameter and up to 2.5µm 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 um) the contact stripes
for the edge emitter are 50 to 70 um wide. The edge emitter configuration is shown below.

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The lengths of the active region usually range from 100 to 150 um 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 cosθ)
with a half power width of θ║ = 1200 .In the plane perpendicular to the junction the half power
beam width is made as small as 25 to 350 by 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
(upto 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 μm) sandwiched
between p-type and n-type cladding layers of another semiconductor with a higher band gap. The
resulting p–n 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 μm).

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The laser light is emitted from the two cleaved facets in the form of an elliptic spot of
dimensions ~ 1×100 μm2. In the direction perpendicular to the junction plane, the spot size is ~ 1
μm because of the heterostructure 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 (Δn
≈ 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 μm) 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 ~0.1×1 μm2 is
buried on all sides by several layers of lower refractive index. For this reason, such lasers are
called buried heterostructure (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.
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 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(1um) many longitudinal modes can be
formed.
The lateral modes lie in the plane of the pn junction. These modes depend on the width of the
cavity and side wall. It determines 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

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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) e j(wt-βz)
Where I(z) is the optical field intensity and w is the optical frequency in radians and β.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 –jβL = 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.
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

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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 ,elastic –tube splice and
the rotary splice.
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 in a 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.

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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.
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.

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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

The photo detectors are used as optical receivers .The role of an optical reciver 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 p–n 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 p–n junction is illuminated with light on one
side, say the p-side , electron–hole pairs are created due to absorption. Because of the large
built-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

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proportional to the incident optical power. Thus a reverse-biased p–n junction acts as a photo-
detector and is referred to as the p–n photodiode.

The electron–hole pairs generated inside the depletion region experience a large electric field and
drift 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.

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

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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
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 diode’s
responsivity and to achieve larger responsivities this Avalanche Photo diode is used. This diode
consists of an additional layer in which secondary electron–hole pairs are generated through
impact ionization. So, the APDs multiply the photocurrent internally before it enters the
amplifier circuitry.This carrier multiplication mechanism is called impact ionization. The newly

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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.

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 electron–hole pairs are
generated. Electrons generated in the i-region cross the gain region and generate secondary
electron–hole 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 :

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For light wave systems operating in the wavelength range 1.3–1.6 μm, Ge or InGaAs APDs must
be used. The improvement in sensitivity for such APDs is limited to a factor below 10 because of
a 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.

The main reason for a relatively poor performance of InGaAs APDs is related to the comparable
numerical values of the impact-ionization coefficients αe and αh . 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 1×105 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
(> 5×10 5 V/cm) can exist in InP without tunneling breakdown.
--------------xxxxxxxxxxxxxxx----------------
The above class notes would have not been possible with out the help of the following
references. I owe very much to the following people.
References: 1. Optical fiber communication-G.Keiser.
2. Fiber-Optic Communication Systems – Govind .p Agarwal

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