UNIT 2

FIBER OPTICS AND OPTICAL COMMUNICATION

OPTICAL FIBERA bundle of optical fibers

A fiber optic audio cable being illuminated on one end

CONTENTSCONTENTS

• INTRODUCTION• BASIC STRUCTURE OF OPTICAL FIBER• LIGHT PROPOGATION THROUGH FIBER• PARAMETERS RELATED TO AN OPTICAL

FIBER• TYPES OF FIBERS• ATTENUATION • DISPERSION• APPLICATIONS

BASIC STRUCTURE OF FIBER

• An optical fiber is a cylindrical structure• An optical fiber is essentially a wave guide for light• Its consists of three layers: 1. Core 2. Cladding 3. Jacket or coating• The refractive index of the cladding is less than

that of the core.

BASIC STRUCTURE ( Cont…)

• An optical fiber contains three layers:

1.Core :It carries the light signals.

2.Cladding:It keeps the light in the core.

3.Coating: It protects the cladding from damage.

BASIC STRUCTURE ( Cont…)• An optical fiber consists of a cylindrical centre core of refractive

index n1,cladding of slightly lower refractive index n2 as shown in fig.

• The refractive index distribution is given by

n( r ) = n1 0< r < a

= n2 r > a where a is the radius of the core

n1 core

n2 ----------------cladding

Air

r

Refractive index distribution

CORE CHARACTERISTICS• The diameter of light The diameter of light

carrying region of the carrying region of the fiber is the core diameter.fiber is the core diameter.

• The larger the core the The larger the core the more rays of light that more rays of light that travel in the core.travel in the core.

• The larger the core the The larger the core the more optical power that more optical power that can be transmitted.can be transmitted.

• The core has a higher The core has a higher refractive index than the refractive index than the cladding.cladding.

CLADDINGCLADDING

• It protects the core It protects the core from absorbing from absorbing surface contaminants.surface contaminants.

• It adds mechanical It adds mechanical strength to the fiber.strength to the fiber.

• It reduces the It reduces the scattering losses.scattering losses.

BUFFER /COATINGBUFFER /COATING

• Adds further strength to Adds further strength to the fiber.the fiber.

• Mechanically isolates the Mechanically isolates the fiber from small fiber from small geometrical irregularities.geometrical irregularities.

• Coating provides an outer Coating provides an outer envelope and protection envelope and protection to other layers in optical to other layers in optical fiber.fiber.

LIGHT PROPOGATION THROUGH FIBER

• Optical fibers work on the principle of Total Internal Reflection

• n1 and n2 be the refractive index of the core and cladding of the fiber respectively

• If angle of incidence and refraction are Φ1 & Φ2 then by Snell’s law of refraction

sin Φ1 n2

=

sin Φ2 n1

• The critical angle is given by sin Φc = (n2 / n1)

• At the angles of incidence greater than critical angle, the light is reflected back into the originating dielectric medium .This phenomenon is known as “ Total Internal Reflection”

Refraction and Total Internal Reflection

LIGHT PROPOGATION THROUGH FIBER(Cont….)

• Because refractive index of the core is greater then that of the cladding , light traveling in the core will remain in it due to total internal reflection as long as the light strikes the core-cladding interface at an angle greater than the critical angle.

• The transmission of a light ray in an optical fiber is shown in fig.It is guided by the series of total internal reflection at the core cladding interface.this ray has an angle of incidence Φ at the interface,which is greater than critical angle and reflected at the same angle to the normal.ΦΦ

ΦΦ Φ Φ

Low index cladding

Low index cladding

Core of high index……………………………………….……………………………………..

Fig. The transmission of a light ray in a perfect optical fiber

PARAMETERS RELATED TO AN OPTICAL FIBER

• Cone of Acceptance: The cone of acceptance is the area in front of the fiber face that determines the angle of light waves that will be accepted in to a fiber.A fiber may accept and propagate many of light which are incident at an angle less than the angle presented by the cone of acceptance. The half-angle of this

cone is called the Aacceptance angle

Numerical Aperture• The numerical aperture (NA) of an optical system is a dimensionless

number that characterizes the range of angles over which the system can accept or emit light.

• In optical fiber the numerical aperture measures the acceptance angle of a fiber ,which is the maximum angle at which the core of the fiber will take in light that will be contained within the core.

Air

……………………… ………………………………………………………………..Core of refractive index n1Φ

i

Cladding of refractive index n2

Cladding of refractive index n2

n0

Fig. Guidance of light in an optical fiber

Consider a ray that is incident on the entrance aperture of the fiber at an angle iFrom fig

sin i n1

= ………………….(1)sin n0

If this ray has to suffer total internal reflection at the core cladding interface,

sin Φ = sin ( 90 - ) = cos > n2 / n1

Thus sin = ( 1- cos2 ) 1/2 < [ 1 – (n2 / n1 )2 ] 1/2 ………………(2)

form eq.1 sin i = n1 / n0 sin Using eq 2 sin i < n1 / n0 [ 1 – (n2 / n1 )2 ] 1/2

or sin i < [(n1

2 – n22) / n0

2]1/2 If (n1

2 – n22) n0

2 then for all values of i, total internal reflection will occurs. Assuming n0 = 1 the maximum values of sin i for a ray to be guided is given by Sin im = (n1

2 – n22) 1/2 when n1

2 < n22 + 1

= 1 when n12 > n2

2 + 1

Thus if a cone of light is incident on one end of the fiber ,it will be guided through it provided the semi angle of the cone is less than im.This angle is the measure of the light gathering power of the fiber and as such defines the numerical aperture of the fiber by the following equation

N.A = (n12 – n2

2) 1/2 = n1( 2 ∆ )1/2

Where ∆ = n12 – n2

2 / 2 n1

2 ……………………………..(3)

V- Number The V- Number or normalized frequency can be given as

V = 2 a ( NA) / or V = 2 a n1( 2 ∆ )1/2 / where ∆ is given by equation (3) V - number is a dimension less parameter It give the information about the core radius a, the refractive

index difference ∆ and operating wavelength

V number give the information about the number of modes in the fiber.

For single mode fiber V should be less than 2.405 For multimode mode fibers must have relatively larger V

number. When V >> 1the total number of modes is given by N = V2 / 2

TYPES OF FIBERSTYPES OF FIBERSOptical fiber may be classified in terms of

(1) Refractive index profile of the core

(2) Numbers f modes propagating in the fiber

There are two types of fiber on the basis of Refractive index profile of the core

(1) Step index fiber

(2)Graded index fiber

1. Step index fiber: the refractive index of the core is uniform throughout and undergoes an abrupt change at the cladding boundary.

2. Graded index fiber: the core refractive index is made to vary as the function of radial distance from the center of the fiber

or

If the core has a non uniform refractive index that gradually decreases from center toward the core cladding interface the fiber is called a Graded index fiber

Both of these fibers can be further divided into single-mode and multi-mode

TYPES OF FIBERS( Cont………)TYPES OF FIBERS( Cont………)1.1. Single-mode fiberSingle-mode fiber: It sustains only one mode of propagation.: It sustains only one mode of propagation.2.2. Multi-mode fiberMulti-mode fiber: It contains many hundred of modes or it supported a : It contains many hundred of modes or it supported a

no. of modesno. of modes

TYPES OF FIBERS( Cont………)TYPES OF FIBERS( Cont………)

TYPES OF FIBERS (Cont…….)TYPES OF FIBERS (Cont…….)

ADVANTAGES OF MULTI-MODE OVER SINGLE-ADVANTAGES OF MULTI-MODE OVER SINGLE-MODE FIBER:MODE FIBER:

1)1) Larger the core radii of multi mode fiber makes it easier to launch Larger the core radii of multi mode fiber makes it easier to launch optical power into fiber & facilitate the connecting together of similar optical power into fiber & facilitate the connecting together of similar fibers.fibers.

2)2) Light can be launched into a multimode fiber using a LED whereas Light can be launched into a multimode fiber using a LED whereas single mode fiber must generally be excited with laser diodes.single mode fiber must generally be excited with laser diodes.

Disadvantages:Disadvantages:1)1) Multimode fiber suffers from inter modal dispersion , Multimode fiber suffers from inter modal dispersion , which results in

the spreading of pulses and limits the usable bandwidth

TYPES OF FIBERS (Cont…….)TYPES OF FIBERS (Cont…….)

STEP INDEX SINGLEMODE FIBERBANDWIDTH-5 GHZ*KMCORE DIAMETER –5-7 MICRONS

STEP INDEX MULTI MODE FIBERBANDWIDTH-20 MHZ*KMCORE DIAMETER –100-250 MICRONS

GRADED INDEX MULTIMODE FIBERBANDWIDTH-800 MHZ*KMCORE DIAMETER-50-100 MICRONS

Attenuation• Attenuation is the loss of optical power as light travels along the fiber.

• Attenuation in fiber optics, also known as transmission loss, is the reduction in intensity of the light beam (or signal) with respect to distance travelled through a transmission medium.

Or

It is the reduction of light power over the length of the fiber.

– It’s mainly caused by scattering.

– It depends on the transmission frequency.

– It’s measured in dB/km

– Attenuation is an important factor limiting the transmission of a digital signal across large distances.

)(log10 10 inout PPdB

Attenuation (Cont….) There are three mechanisms responsible for attenuation in optical fiber

Absorption by materials Scattering Bending losses

(1) Absorption by materials

The absorption of light by the core and cladding material of a fiber during propagation is the main source of attenuation. Absorption in optical fibers is explained by three factors:

• Imperfections in the atomic structure of the fiber material • The intrinsic or basic fiber-material properties • The extrinsic (presence of impurities) fiber-material properties

Imperfections in the atomic structure induce absorption by the presence of missing molecules or oxygen defects. Absorption is also induced by the diffusion of hydrogen molecules into the glass fiber.

Intrinsic Absorption. - Intrinsic absorption is caused by basic fiber-material properties. If an optical fiber absolutely pure, with no imperfections or impurities, then all absorption would be intrinsic. Intrinsic absorption sets the minimal level of absorption.

In silica glass, the wavelengths of operation range from 700 nanometers (nm) to 1600 nm. Fig.1 shows the level of attenuation at the wavelengths of operation. This wavelength of operation is between two intrinsic absorption regions. The first region is the ultraviolet region (below 400-nm wavelength). The second region is the infrared region (above 2000-nm wavelength).

Intrinsic absorption in the ultraviolet region is caused by electronic absorption bands. Basically, absorption occurs when a light particle (photon) interacts with an electron and excites it to a higher energy level.

The main cause of intrinsic absorption in the infrared region is the characteristic vibration frequency of atomic bonds. In silica glass, absorption is caused by the vibration of silicon-oxygen (Si-O) bonds. The interaction between the vibrating bond and the electromagnetic field of the optical signal causes intrinsic absorption. Light energy is transferred from the electromagnetic field to the bond.

Extrinsic Absorption. - Extrinsic absorption is caused by impurities introduced into the fiber material. Metal impurities, such as iron, nickel, and chromium, are introduced into the fiber during fabrication. Extrinsic absorption is caused by the electronic transition of these metal ions from one energy level to another.

Scattering When light is scattered by an obstruction ,the result is power loss. The local

microscopic density variation in glass cause local variation in refractive index. These variation ,which are inherent in the manufacturing process and cannot be eliminated act as obstruction and scattered light in all directions. This is known as Rayleigh scattering .The Rayleigh scattering depends upon wavelength and it varies as 1/ and becomes important at lower wavelengths.

During manufacturing, regions of higher and lower molecular density areas, relative to the average density of the fiber, are created. Light traveling through the fiber interacts with the density areas as shown in fig.2 Light is then partially scattered in all directions.

Scattering depends not on the specific type of material but on the size of the particles relative to the wavelength of light. The closer the wavelength is to the particle size, the more scattering.

Bending losses• These losses occurs due to imperfections and deformations present in

the fiber structure.Micro bending and macro bending losses are two types of bending losses.

• Microbend losses occurs when the core surface has small variation in shape.These variation change the angle at which light strikes the core cladding interface and can cause the light to refract into the cladding rather than refract into core. Microbend loss shown in fig.

• Macrobend losses: Excessive bending of the cable or fiber may result in loss known as macroband loss. The fiber is sharply bent so the light traveling through the fiber cannot make the turn and is lost in the cladding as shown in fig.

Macrobend

Fig. Macrobend losses in a fiber

Dispersion• Dispersion is the spreading out of a light pulse in time as it propagates inside the

fiber.

• Dispersion is measured in units of time and is defined as

t = (t2P2

- t2P1 )

where tP1 - width of input pulse

where tP2 - width of output pulse

• Total dispersion of a fiber depends on its length• It determines the bandwidth and channel carrying capacity of the fiber. • There are two types of dispersion

Dispersion(Cont…..)• Inter-modal dispersion • Intra-modal dispersion

1. Wave guide dispersion

2. Material dispersion

Inter-modal dispersion : Inter-modal dispersion occurs because of the different

modes in which the light propagates in the fiber travel different paths. This causes differences in the arrival time of the rays at the receiver and hence a distortion of the

signal.

Fig. For Multimode fiber

Intermodal DispersionLow order modeHigh order mode

Cladding

Core

Light pulse

t0 t

Spread,

Broadenedlight pulse

IntensityIntensity

Axial

Schematic illustration of light propagation in a slab dielectric waveguide. Light pulseentering the waveguide breaks up into various modes which then propagate at differentgroup velocities down the guide. At the end of the guide, the modes combine toconstitute the output light pulse which is broader than the input light pulse.

© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)

• Inter-modal dispersion is less in fibers which have a parabolic refractive index parabolic profile in the core region i.e graded-index fiber

• With a graded-index fiber, the light follows a more curved path. The high-order modes spend most of the time traveling in the lower-index cladding layers near the outside of the fiber. These lower-index core layers allow the light to travel faster than in the higher-index center layers. Therefore, their higher velocity compensates for the longer paths of these high-order modes.

•Intermodal dispersion can be completely eliminated by using a single-mode fiber.

Intra-modal dispersion • Intramodal dispersion is pulse spreading that occurs with in a single mode.

• This is due to the fact that group velocity of guided mode is a function of the wavelength.

• Intramodal dispersion is also known as Chromatic dispersion.

• Intramodal dispersion has the two regions:

1. Material dispersion

2. Waveguide dispersion

Material dispersion: Material dispersion is caused by the wavelength dependence of the refractive index on the fiber core material.

or

The refractive index of the core material depends upon the wavelength of the guided mode. As the group velocity of the given mode depends upon the refractive index of the core material of fiber , group velocity of an given mode depends upon the wavelength.

Fig1. For Intermodal dispersionFig 2. For Material dispersionFig 3 .For waveguide dispersion

Wave guide dispersion Waveguide dispersion is only important in single mode fibers. It is caused by the fact that

some light travels in the fiber cladding compared to most light travels in the fiber core. It is shown as the 3rd illustration in the first picture.

Since fiber cladding has lower refractive index than fiber core, light ray that travels in the cladding travels faster than that in the core. Waveguide dispersion is also a type of chromatic dispersion.  It is a function of fiber core size, V-number, wavelength and light source line width.

Fiber optic cables have a much greater bandwidth than metal cables.

Ability to carry much information

Fiber optic cables weight less than a copper wire cable

Lower-power transmitters can be used instead of the high-voltage electrical transmitters used for copper wires.

Support higher data rates,and at greater distances.

Immune to all kind of interference including lightning.

Unaffected by most chemicals.

Unaffected by outdoor atmospheric condition.

Smaller and lighter.

Ideal for secure communication systems because it is difficult to tap.

Not susceptible to noise from electronic instruments

Cost of initialization and installation is high.

Require specialized skills and knowledge much different from those required for installation of electrical cables.

There is possibility of hazard when working with optical fiber

like glass shards and optical radiation.

Fiber is delicate so has to be handled carefully.

Communication is not totally in optical domain, so repeated electric –optical – electrical conversion is needed.

The splicing and testing equipments are very expensive as compared to copper equipments.

Telecommunications

Local Area Networks

Cable TV

CCTV

Optical Fiber Sensors

LONG DISTANCE COMMUNICATION BACKBONES

INTER-EXCHANGE JUNCTIONS

VIDEO TRANSMISSION

BROADBAND SERVICES

COMPUTER DATA COMMUNICATION (LAN, WAN etc..)

MILITARY APPLICATION

NON-COMMUNICATION APPLICATIONS (sensors etc…)

Fiber optic communication system

Fiber optic communication system

Transducer Modulator Carrier Channel

DetectorSignal ProcessorTransducer

Transmitter Optical fiber

Receiver

Fig : Block diagram of communication system

First analog signal is converted to its digital form (usually 0 or 1 )

It is then fed into electric to optical converter (LEDs or Laser) which generates the flashes of light or pulse.

This pulse of light is then incident in optical fiber keeping in mind its critical angle.

Light is then reached to its receiving end by total internal reflection.

At the receiving end sensors like photodiode is used to convert light flash back into digital signal.

Optical Sources Optical Sources

Definition: A device that converts electrical signal into optical signal

Optical sources or emitters operate on the idea that electromagnetic energy can only appear in a discrete amount known as a quanta. These quanta are called photons.

Energy in one photon varies directly with the frequency Optical sources or emitters are :

Light-Emitting Diodes ( surface emitting and edge emitting Led’s)

Laser Diodes

Light-Emitting Diodes LEDs produce incoherent light by spontaneous emission LEDs are incoherent sources of light Output power has a large spectral width The output beam is extremely divergent LEDs output can be launched only into large dimension multimode fiber Only about 1 % of input power, or about 100 microwatts can be launched into the

optical fiber. However, due to their relatively simple design, LEDs are very useful for low-cost

applications.

Laser Diodes Laser laser diodes produce coherent light by stimulated emission Laser diodes are coherent sources of light Output power has a narrow spectral width The output beam is highly monochromatic and very directional Laser laser diodes output can be launched into either single mode or multimode fiber Large fraction of input power can be launched into the optical fiber.

Light-Emitting Diodes

An LED is form of junction diode that is operated with forward bias Instead of generating heat at the PN junction, light is generated and passes through

an opening LEDs can be visible spectrum or infrared

Low cost

Low power

Broad spectral width

Can be modulated to several hundred MHz

Light-Emitting Diodes

Fig. Surface emitting LED

Light-Emitting Diodes

Fig .Edge emitting LED

Optical Detectors or Optical Receivers

Definition: convert optical signal into electrical signal

Types: p-i-n photo detector: photon-electron converter Avalance photo detector (APD): more sensitive for high speed systems

Photodetector parameters: Responsivity: the amount of current produced per unit of input optical power High sensitivity at the operating wavelength Short response time to obtain a suitable bandwidth Small size Low bias voltage Large electrical response to the received optical signal High reliability Stability of performance

Principle of optical detector• Photo detector is reverse biased p-n junction device.

• Semiconductor photodiode detector operate on the basis of the internal photoeffect.

• It is known as the photogeneration of an electron – hole pair.

+

-Electron

Hole

hPhoton

Eg

Fig. Photogenerationin of an Electron –hole in semiconductor

Conduction band

Valence band

Principle of optical detector (Cont…..)

A photodiode is a p-n junction whose reverse current increases when it absorb photons. The drift and diffusion regions are indicated by 1 and 2 respectively

12 23 3

p n

Photon

+ +- -

+-+

-

Electric field E

- +

iP

Fig . Photons illuminating an idealized reverse biased p-n photodiode detector

Depletion layer

p-i-n Photodiode

The most common optical detector used with fiber-optic systems is the p-i-n diode

The p-i-n diode is operated in the reverse-bias mode As a photo detector, the p-i-n diode takes advantage of its wide depletion

region, in which electrons can create electron-hole pairs The low junction capacitance of the p-i-n diode allows for very fast switching

p i n

Fig. p-i-n photodiode structure

p-i-n Photodiode

Fig .p-i-n photodiode

Fiber optic communication system