ECE 4105 Optical Fiber Communications. Introduction and... · First Generation of Optical Fiber...

ECE 4105

Optical Fiber Communications

Prof. Dr. Monir Hossen ECE, KUET

Department of Electronics and Communication Engineering, KUET

Department of Electronics and Communication Engineering, KUET 2

Communication may be broadly defined as the transfer of

information from one point to another.

When the information is to be conveyed over any distance a

communication system is usually required.

Electromagnetic spectrum of communication

Definition


Why Fiber Optics?

Fiber can support huge bandwidth demand, because:

At implementation level:

Present capacity is 100 Gbps (single fiber network).

IEEE currently investigating for 400 Gbps or 1 Tbps

(due after 2017).

At research level:

Technical University of Denmark (DTU)

investigated 1 Tbps in 2009.

Karlsruhe Institute of Technology (KIT) investigated

26 Tbps in 2011.

Recently, the DTU investigated 43 Tbps.


Development of Communications 1. Visual optical communications:

The mainstream of communications was the visual optical

communication using visible optical carrier waves or light

for thousands years.

Simple system such as beacon-fires in ancient Egypt,

ancient China, old Greek and Rome was developed to

color-combination signal fires with binoculars.

Other techniques were semaphore flags, reflecting mirrors

and signaling lamps.

They have been disappearing since the Mores telegraph

invention (1837).


2. Electric communications

Electric communication started from the invention of

telegraph.

First transatlantic cable (between London and New York) in

1866: Queen Victoria’s message to President Buchanan.

It took the Queen 1.5 hours to receive a reply from President

Buchanan.

Chief engineer of the project: Lord Kelvin.

1940 - first coaxial cable system, 3MHz system with 300 voice

channels or a single television channel, the bandwidth is

limited by the frequency dependent cable losses which

increases rapidly beyond 10MHz.

Development of communications Contd.


1975 - the most advanced coaxial cable system, 274Mb/s with

repeater spacing of 1km. Very expensive to operate.

Development of Communications Contd.


3. Communication using high-frequency electromagnetic

waves: Inventions of Bell’s telephone (1876) and Marconi’s wireless

communication (1896) were two epoch-making events.

Information was carried and transmitted by electromagnetic

(EM) carrier wave.

Since then the frequency of EM carrier wave has been rising.

Depending on their wavelengths these EM carriers can be

transmitted over considerable distances but are limited in the

amount of information they can convey by their frequencies.

The information-carrying capacity is directly related to the

bandwidth or frequency extent of the modulated carrier,

which is generally limited to a fixed fraction of the carrier

frequency.



The greater the carrier frequency is, the larger the available transmission bandwidth. Thus the system has information-carrying capacity of the communication.

So, radio communication was developed to higher frequencies, i.e., VHF and UHF, leading to the introduction of the even higher frequency microwave and latterly, millimeter wave transmission.



Until 1960, the coherent wave generation together with the

communication technology increases the used frequency by

one order of magnitude every six years in average.

However, they got stagnant for about twenty years.

There seemed to be an invisible wall at around 1 mm

wavelength.

Photoelectronic era started by a leap from 300 GHz (mm

wave) to 300 THz (μm wave).

The leap A in the fundamental technology occurs with the

aid of the invention of laser and the leap B in the

communication technology with the practicability of optical

fibers.



The communication at optical frequencies offers an increase

in the potential usable bandwidth by a factor of around 104

over high frequency microwave transmission.

An additional benefit of the use of high carrier frequencies is

the general ability of the communication system to

concentrate the available power within the transmitted

electromagnetic wave, thus gives an improved system

performance.

Development of communications Cond.


4. Optical communication

Using the light as the carrier signal in communication

system is nothing new.

In 1880 Alexander Graham Bell invented the photophone

just fourth year after his invention of telephone.

He demonstrated that speech could be transmitted on a

beam of light.

Bell focused a narrow beam of sunlight onto a thin mirror

on a diaphragm.

When the sound waves of human speech caused the

diaphragm with mirror to vibrate, the beam of sunlight was

modulated and the amount of energy transmitted to the

light detector varied correspondingly.



The light reaching the selenium detector caused its resistance,

therefore the intensity of the current in a telephone receiver,

to vary, setting up speech waves at the receiver end.

Bell managed to give speech transmission over a distance of

200 m by using his ingenious invention.

Problems for optical communication were: (a) the lack of

suitable light source; (b) the severe influence of disturbances

such as rain, snow, fog, dust and atmospheric turbulence.

From 1930 to 1940s, research on optical communication

between warships was carried out in some countries.

The signal source was a high-power bulb with current

modulation. The receiver was a paraboloidal mirror and the

signal was demodulated by photoelectric valve.



It did not reach practicability.

What is new today are the techniques available for generating

a light beam that can be modulated at extremely high rates

and, equally important transmitted through a low-loss optical

fiber several miles long with acceptable loss of energy.

Modern light-wave communication had its birth in the 1960s.

Laser provided a powerful coherent light source, together with

the possibility of modulation at high frequency.

In addition, the low beam divergence of the laser made

enhanced free space optical transmission a practical possibility,

which was still restricted by the atmospheric conditions and

made limited success.



Some links have been implemented for applications such as

the linking of TV camera to a base vehicle and for data links

of a few hundred meters between buildings.

Similar techniques could also be used for inter-satellite

communication in outer space.

For optical communications, because light gets absorbed on

foggy or rainy days, early work was aimed at beams of light

propagating inside tubes running in similar ducts to the

proposed millimetric waveguide system.



5. Light transmission path

(a) Optical fiber

The essence of optical fiber is a homogeneous dielectric rod

although a communication optical fiber with good light

transmission characteristics possesses complex internal

distribution of refractive indices.

Light transmits in the fiber through multi-total internal

reflection.

British physicist John Tyndall demonstrated light transmission

in 1870 in water flow from an opening under a big tank.

Such structure of light waveguide was used in stomach

endoscope in fifties but few considered the possibility to apply

optical fiber for telecommunication.



There were probably two reasons:

i. The loss factor was too large (10-8). The attenuation owing to dielectric itself reaches 500 dB/km.

ii. Large dispersion causes distortion of transmission along optical fibers, e.g. it broadens transmitted pulse and finally overlaps it with its neighbors.

(b) Dielectric thin-film waveguide supported by a frame (1965)

(c) Lens array waveguide

(1962-64) and gas lens

waveguide (1964-65)



6. Optical fiber came forth

Reliable information transfer using light wave can be achieved

via dielectric waveguides or glass optical fibers to avoid

degradation of the optical signal by the atmosphere.

Proposal for telecommunication with optical fibers were made

in 1966 by Kao and Hockham.

They analyzed various causes of light loss in glass and showed

that low-loss optical fibers for telecommunications could be

fabricated through improving technologies.

They also discussed a structure of core-cladding with minute

difference in refractive index (named weakly guiding fiber in

1970 by Gloge).



Technologies developed rapidly.

In 1970 Corning Glass Works, produced a silica fiber with

a signal-power transmission of better than 1% over a

distance of 1 km (i.e., an attenuation of 20 dB/km), which

was comparable to existing copper electrical system.

During the next two decades, the transmission rose to

about 96% over 1 km (i.e., an attenuation of only 0.16

dB/km).

Bell Laboratory developed a MCVD (chemical vapor

deposition) technology, which became a standard in optical

fiber production.

Development of communications Cond.


Applications of Optical Fibers

o Image Transport

o Optical Communications

o Optical Fiber Sensors


Advantages of Optical Communications Comprehensive economic aspects

• Enormous potential bandwidth: frequency (~ 1014 Hz) much

higher than micro-wave so that the available transmission

bandwidth could be 104 larger (~ 3 GHz);

• Low loss transmission: Extremely low attenuation, minimum

attenuation ~ 0.2 dB/km c.f. 5-10 dB/km of the coaxial cables;

• Small size and weight: Extremely small fiber size (diameter ~

100-150 μm) and ~ 30 g/km;

• Ruggedness and flexibility: Good flexibility (bendable to mm

curvature radius);

• Cheap and abundant material: Made of quartz (SiO2, sand)

instead of Cu.


Security

• Electrical isolation (non-conductive, non-radiative and non-

inductive): no earth loop and interface problem; no effects of

lightening, surge current, or power cables on transmitting

signals of light.

• Immunity to interference and crosstalk: free from

electromagnetic interference, crossing, disturbance fire (endure

1000°C high temperature), no spark.

• Unparallel signal security: The light from optical fibers does

not radiate significantly.

Ultrapure glass fibers have become the premier

communication medium.

Advantages of Optical Communications Contd.


Transmission Characteristics of Different Communication Medias


Basics of the Optical Fiber Communication

System


First Generation of Optical Fiber Communication System

In 1977, field trails on optical fiber communications were held,

and proved to be very successful! Operating wavelength was ~

0.8 μm, 50 - 100 Mb/s, repeater spacing 10 km.

Around this time another major development: Material

dispersion in silica fiber is zero at 1.3 μm, with OH- free glass,

attenuation ~ 0.5 to 1 dB/km at 1.3 μm, and 0.2 db/km at 1.55

μm.

Initially no laser and detector

work at such wavelengths!


2nd Generation of Optical Fiber Communication System

In 1980, operating at 1.3 μm started appearing with multimode

fiber the bit rate is limited to 100 Mb/s and the repeater spacing

is ~ 20km.

In 1981, laboratory demonstration of 2 Gb/s over 44 km of

single mode fiber, and it is clear that single mode fiber offers

much wider bandwidth.

There was a worldwide rush to develop single mode fiber optic

systems, with monthly report of advances. Repeater spacing

stretched from 30 km to 200 km, and bit rate increased from

140 Mb/s to Gb/s.

In 1987, second generation of 1.3 μm fiber optic systems

employing single mode fibers, with bit rate up to 1.7 Gb/s and

repeater spacing of 50 km.


3rd Generation of Optical Fiber Communication System

Repeater spacing is limited by the fiber loss (0.5 dB/km at 1.3

μm). Since loss in silica fiber is 0.2 dB/km at 1.55 μm, there is

an increased interest to operate at this wavelength.

System development was delayed due to the large material

dispersion in single mode fiber; the solution is to use either

dispersion shifted fiber or single longitudinal mode laser.

In 1985, laboratory demonstration - 4Gb/s over 100 km.

In 1990, 2.4 Gb/s system is commercially available.

Development of 10 Gb/s commercial system is underway.

Design and implementation of inter-city and undersea systems

of up to 200 km long, trans-oceanic fiber optic systems as well

as massive reduction in the cost of traffic.


4th Generation of Optical Fiber Communication System

Increases the bit rate using wavelength division multiplexing (WDM)

or frequency division multiplexing (FDM).

Increase in repeater spacing using optical amplification.

Coherent communication using homodyne or heterodyne detection

system: first demonstrated in 1981/82, showed that one could exploit

the optical spectrum in an analogous manner to that of the radio

frequency. At 1.55μm, a spectral width of 20000 GHz is available!

In 1990, 2.5 Gb/s over 2223 km without repeaters, loss over fiber is

compensated by using optical fiber amplifiers in every 80 km.

In 1991, 2.5 Gb/s over 4500 km and 10 Gb/s over 1500 km

demonstrated. By using a recirculating loop - 2.4 Gb/s over 21000

km and 5 Gb/s over 14300 km.

Optical fiber amplifier provides trans-oceanic communication

system of 9000-10,000 km without repeaters!


Soliton transmission - optical pulses that preserve their shapes during

propagation in a lossless fiber by counteracting the effect of dispersion

through the fiber nonlinearity.

Basic idea proposed in 1973. Laboratory demonstration in 1988 over

4000 km of fiber by compensating the fiber loss through Raman

scattering.

Since 1989, all experiments use optical fiber amplifiers. 10 Gb/s over

1000 km and 20 Gb/s over 350 km. By using recirculating loop, 2.4

Gb/s over 12,000 km.

Optical fiber communication systems: long term cost trends will favor

the use of fiber to the home for videophone, HDTV etc.

Development of Optoelectronic integrated circuits (OEIC's) will

further reduces the cost and makes deployment more cost effective.

Optical interconnects - optics is already tackling the backplane wiring

problem.

5th Generation of Optical Fiber Communication System


Ray Theory of Transmission A transparent dielectric rod, typically of silica glass with a

refractive index of around 1.5, surrounded by air, proved to be an impractical waveguide due to its unsupported structure, especially when very thin waveguides were considered in order to limit the number of optical modes propagated, and the excessive losses at any discontinuities of the glass-air interface.

It was required that the smooth surface of a single fiber kept clean of moisture, dust, oil, etc. to prevent leakage.

Light also might leak from one fiber to another (cross-talk) as large numbers of fibers are packed in close proximity.

Applications of glass optical waveguides as optical imaging and medical diagnosis led to proposals for a clad dielectric rod in mid-1950s in order to overcome these problems.


This structure is illustrated in Fig. 2.1, which shows a

transparent core with a refractive index n1 surrounded by a

transparent cladding of slightly lower refractive index n2.

The cladding supports the waveguide structure whilst also, when

sufficiently thick, substantially reducing the radiation loss into

the surrounding air.

At the cladding-air interface, the light field decays to negligibly

small.

Figure 2.1 Optical fiber waveguide with n2 ≤ n1.

Ray Theory of Transmission Cntd.


Total Internal Reflection

,

The refractive index of a dielectric medium is defined as the

ratio of the velocity of light in a vacuum to the velocity of light in

the medium.

When a ray is incident on the interface between two dielectrics

of differing refractive indices, e.g. glass-air as shown in Fig. 2.2,

it travels more slowly in the optically dense glass than in the less

dense air, and the refractive angle is larger than the incident

angle. Total internal reflection (TIR)

occurs provided that the incident

angle is greater than the critical angle ,

, i.e., the glancing

angle is sufficiently small.

12

1 /sin nnc


Total Internal Reflection Cntd.. A meridional ray (passes through the axis of an optical fiber)

passes through the axis of the fiber core.

This type of ray is the simplest to describe and is generally

used when illustrating the fundamental transmission

properties of optical fiber. A meridional ray might undergo

several thousand reflections per meter, .

Typically, D ~ 10μm, (seldom used in size much smaller than

this, available as small as 2μm), the glass core has an index n1

of 1.62 and the cladding n2 ~ 1.52.

tan/ DLN


Acceptance Angle We now concern about light rays entering the fiber from air.

Only rays with a sufficiently shallow grazing angle at the core-

cladding interface are transmitted by TIF.

It is clear that not all rays entering the fiber core will continue

to be propagated down its length.

There is a maximum θa (a for air), for which the internal ray

will impinge at the critical angle ɸc.

Any rays B incident on the input face at an angle to the fiber

axis larger than θa will strike the internal core-cladding

interface at angle less than and will not have TIF.

B is refracted into the cladding

and eventually lost by

radiation. This situation is

shown by Fig.2.4.


Acceptance Angle Contd.

The maximum angle θa to the fiber axis at which light may

enter the fiber in order to be propagated is often referred to as

the (total) acceptance angle for the fiber.

Start with (θf is the refractive

angle, f for fiber)

i.e.,

and .

This is a relationship of the acceptance angle and the refractive

indices of the three media involve, namely the core, cladding

and air.

ffc nn 2

12 sin1)90sin(/sin

an

nnn 2

2

1

2

012 sin1/

2

2

2

1

0

1sin nn

na


Numerical Aperture

The quantity n0sinθa is defined as the numerical aperture, or

NA

NA = n0sinθa .

The NA is independent of the fiber core diameter and will

hold for diameters as small as 8 μm.

When interference phenomena are considered it is found that

only rays with certain discrete characteristics propagate in

the fiber core.

Thus the fiber will only support a discrete number of guided

modes.

This becomes critical in small core diameter fibers which

only support one of a few modes.

Hence, electromagnetic mode theory must be applied in these

cases.


Numerical Aperture Contd..

The index difference of a waveguide is usually characterized by a

normalized parameter

The numerical aperture can be expressed as

For typical multimode fibers, we have Δ ~ 1 – 3 %, NA ~ 0.2 – 0.4

It has been defined the relative aperture as D/f, the ratio between

the aperture diameter D and the focal length f.

Its inverse is the focal ratio or f-number, often written f/ as a

single symbol, that is

The NA should clearly relate to the f-number of the system, and,

in fact,

In the air ni = 1 and the largest value of NA is 1, NA ≤ 1. Fibers

with a wide variety of numerical apertures, from about 0.2 up to

and including 1.0, are commercially obtainable.

Dff //#

)(2

1/#

NAf


Skew Rays Besides the meridional rays in the

optical waveguide, there is

another category of ray exists

which is transmitted without

passing through the fiber axis.

These rays, which greatly

outnumber the meridional rays,

follow a helical path through the fiber, as illustrated in Fig. 2.6,

and are called skew rays.

When projecting into the cross-section, each reflection gives a

change in the direction of 2γ as shown in Fig.2.6(b).

When the light input to the fiber is nonuniform, skew rays will

therefore tend to have a smoothing effect on the distribution of

the light as it is transmitted, giving a more uniform output.

More reflec-tions, better the smoothing.


Skew Rays Contd… To calculate the acceptance angle for

a skew ray it is necessary to define

the direction of the ray in two

perpendicular planes.

The geometry of the situation is

shown in Fig.2.7.

A skew ray is incident on the end face of a fiber core at the

point A at an incident angle θs.

The normal at the point A is AT parallel to the core axis SR.

The ray is refracted at the air-core inter-face before traveling

to the point B in the same incident plane ABT.

However, the normal at the point B is BR along the core radius

and the incident plane is ABR which makes an angle γ at the

cross-section RBT.


Skew Rays Contd… The incidence (and reflection) angle at the point B is , and >

C .

The new incident angle satisfies

For the critical angle c,

The maximum input axial angle for skew rays is

The acceptance condition for skew rays are:

If cosγ = 1, θas reduces to θa like meridional rays. Thus θa defines

the maximum conical half angle for the acceptance of meridional

rays but it defines the minimum input angle for skew rays.

cossincos

2/12

12 )/(1coscossin nnc

2/12

12

0

1

0

1

0

1 )/(1coscos

cossinsin nn

n

n

n

n

n

n cas

cossin2

2

2

10 nnnNA as


Skew Rays Contd… As shown in Fig.2.6(b), skew rays tend to propagate only in

the annular region near the outer surface of the core, and do

not fully utilize the core as a transmission medium.

However, they are complementary to meridional light-

gathering capacity of the fiber.

This increased light-gathering ability may be significant for

large NA fibers, but for most communication design purposes

meridional rays are considered adequate.

Self Study: All related Examples of Chapter 2

Assignment: A typical relative refractive index difference for an optical fiber designed for long

distance transmission is 1%. Estimate the NA and the solid acceptance angle in air

for the fiber when the core index is 1.46. Further, calculate the critical angle at the

core-cladding interface within the fiber. It may be assumed that the concepts of

geometric optics hold for the fiber.

Thanks for Your Kind

Attention

Department of Electronics and Communication Engineering, KUET

ECE 4105 Optical Fiber Communications. Introduction and... · First Generation of Optical Fiber...

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