BY: VASHU GUPTA3rd YEAR, ELECTRONICS AND COMMUNICATION ENGINEERING ,JSS ACADEMY OF TECHNICAL EDUCATION, NOIDA PROJECT DONE AT: KDMIPE,ONGC,DEHRADUNUNDER THE GUIDANCE OF Mr. MANJEET SINGH, EE(E&T)

OPTIC FIBRE COMMUNICATION

1.

2012[6 WEEK INDUSTRIAL TRAINING PROJECT REPORT]

11th June – 23rd

July 2012

CERTIFICATE

This is to certify that Mr. VASHU GUPTA, Electronics & Communication 3rd year student of JSS ACADEMY OF TECHNICAL EDUCATION, NOIDA has successfully completed his training from 18th June 2012 to 23rd July 2012 in KDMIPE of “OIL & NATURAL GAS CORP. LTD., DEHRADUN” under the guidance of Mr. Manjeet SINGH (EE E&T). He has worked on the project “OPTIC FIBRE COMMUNICATION”. During the project work he was sincere, dedicated to his work and his performance was very good.

We wish him a bright and successful career in the future.

Mr. S.K. Bose (DGM, E&T)

ONGC, KDMIPE Dehradun

ACKNOWLEDGEMENT

I would like to express my deepest gratitude to Mr. S.K. Bose [D.G.M. (E&T)], KDMIPE, ONGC Dehradun for permitting me to undergo summer training at the organization.

I would like to express my deepest gratitude to Mr. R.B Sharma, Dean, JSS ACADEMY OF TECHNICAL EDUCATION and MR. DINESH CHANDRA, H.O.D, Electronics & communication Engineering Department for allowing me to undergo vocational training at KDMIPE, ONGC Dehradun.

I am thankful to Mr. Manjeet Singh (EE E&T) for his constant guidance and encouragement during my project period.

I would also like to present my sincerest thanks to all the members, staff and officers of the IT division for providing a helping hand at the time of need.

(VASHU GUPTA)

CONTENTS

1. ABOUT THE ORGANISATION

I. OIL AND NATURAL GAS CORP. LTD.

II. INSTITUTES OF ONGC

III. KDMIPE

IV. ROLE OF IT DIVISION

2. INTRODUCTION TO OPTIC FIBRE

3. HISTORY

4. ELEMENTS USED IN OPTIC FIBRE COMMUNICATION

I. TRANSMITTERS

II. RECEIVERS

III. FIBRE CABLE TYPE

IV. AMPLIFIERS

V. WAVELENGTH DIVISION MULTIPLEXING

VI. BANDWIDTH-DISTANCE PRODUCT

VII. DISPERSION

VIII. ATTENUATION

IX. TRANSMISSION WINDOWS

X. REGENERATION

XI. LAST MILE

5. COMPARISON WITH ELECTRICAL TRANSMISSION

6. APPLICATIONS AND USES OF FIBRE OPTICS

I. TELECOMMUNICATION INDUSTRY

II. MEDICINE INDUSTRY

III. OTHER USES

7. WAVELENGTH AND FREQUENCY

8. FIBRE IN LAN

I. GIGABIT ETHERNET

II. 10 GIGABIT ETHERNET

9. SUMMARY

10. BIBLIOGRAPHY

11. LAB VISITS & KEY FEATURES

ABOUT THE ORGANIZATION

OIL AND NATURAL GAS CORP. LTD.

Oil and Natural Gas Corporation Limited (ONGC) (NSE: ONGC, BSE: 500312) is an Indian state-owned oil and gas company headquartered at Dehradun, India. It is a Fortune Global 500 company ranked 413, and contributes 77% of India's crude oil production and 81% of India's natural gas production. It is the highest profit making corporation in India, according to filings with the BSE of latest quarter results External Link. It was set up as a commission on 14 August 1956. Indian government holds 74.14% equity stake in this company.

ONGC is one of the Asia's largest and most active companies involved in exploration and production of oil. It is involved in exploring for and exploiting hydrocarbons in 26 sedimentary basins of India. It produces about 30% of India's crude oil requirement. It owns and operates more than 11,000 kilometers of pipelines in India. It is one of the highest profit making companies in India. In 2010, it stood at 18th position in the Platt’s Top 250 Global Energy Company Rankings.

A turning point in the history of India’s oil sector was in 1994. While the oil sector was on the backburner of India's political realm for some time, is was brought to the forefront by the privatization of India's leading oil E&P organization, the ONGC. Simultaneously, there were steps taken for the enhancement of production on the Bombay High oil fields as the result of a INR 150 billion development investment.

One of Asia's largest oil E&P companies, ONGC became a publicly held company as of February 1994, following the Indian government's decision to privatize. This privatization was conceived and achieved (sweat equity) to a great extent by ONGC’s influential Association of Scientific and Technical Officers (ASTO) – spearheaded by the then Bombay top executives Ganesh P. Shahi and Amarjit S. Jowandha (DGM and Head - Management Services Group too) who worked closely with the then ONGC CMDs S. K. Manglik and S.L. Khosla, IAS and with Dr. Vijay Kelkar, Secretary, Ministry of Petroleum & Natural Gas, Government of India, among others. Amarjit S. Jowandha was able to usher in change from his imbibed learnings, inter alia, from his Alma Mater University of Bombay [Jamnalal Bajaj Institute of Management Studies (also referred to as JBIMS or just Bajaj]. Eighty percent of ONGC assets were subsequently owned

by the government, the other 20% were sold to the public. At this time, ONGC employed 48,000 people and had reserves and surpluses worth INR 104.34 billion, in addition to its intangible assets. The corporation's net worth of INR 107.77 billion was the largest of any Indian company.

After its initial privatization, ONGC had authorized capital of INR 150 billion: it also met its need to raise INR 35 billion to invest in viable oil and gas projects. The Asian Development Bank (ADB) had also set a deadline for privatizing and restructuring at 30 June 1994, if loans were to be granted for development of two ONGC projects. As a consequence of the successful privatization, the loans were granted - US$267 million for development of Gandhar Field, and US$300 million for the gas flaring reduction project in the Bombay Basin. The successfully formulated and implemented privatization strategy put ONGC at par with other large multinational and domestic oil companies.

INSTITUTES OF O.N.G.C

ONGC has institutionalized R&D in the oil and gas , and related sectors and established separate institutions to undertake specific activities in key areas of exploration, drilling, reservoir management, production technology, ocean engineering, safety and environment protection in the form of 9 independently managed R&D centres. Regional laboratories also support these institutes.

LIST OF INSTITUTES:

GEOPIC : GEO data Processing and Interpretation Centre, Dehradun. KDMIPE : Keshava Deva Malviya Institute of Petroleum Exploration,

Dehradun. IDT : Institute of Drilling Technology. IEOT : Institute of Engineering and Ocean Technology. ONGC ACADEMY : Oil and Natural Gas Academy, Dehradun. INBOGS : Institute of Biotechnology and Geotectonic studies, Jorhat. IOGPT : Institute of Oil and Gas Production Technology. IPSHEM : Institute of Petroleum and Safety, Health and Environment

Management, Goa. IRS : Institute of Reservoir Studies, Ahmadabad.

KDMIPE

Keshava Dev Malviya Institute of Petroleum Exploration (KDMIPE). Located in the picturesque Doon Valley of Uttarakhand State was setup as a UNDP-aided institute in 1962 as a Research and Training Institute (R&TI) to catered to all the needs of preliminary R and D, training for developing and generating manpower with the aid of available expertise. In 1970, it was renamed as Institute of Petroleum Exploration (IPE) and on 19th December 1981, it was renamed as Keshav Dev Malaviya Institute of Petroleum Exploration (KDMIPE). Later ten customized R and D institutions such as Reservoir Management, Geophysical Data Processing and Interpretation, Drilling Technology, Production Technology, etc were carved out of KDMIPE. All the ten institutes of ONGC are interconnected under one common umbrella – the COIN and provide a platform for share their knowledge base, identify gaps and overlaps for an effective R&D networking. KDMIPE is the only institution in India which deals with all the aspects of exploration and related activities as also deals exploration of unconventional energy resources like CBM, gas hydrate,shale gas, BCGA, etc.

The Institute comprises 7 Groups with 22 divisions. Its Infrastructure includes relevant, state of the art facilities for Remote sensing, biostratigraphy, high-resolution gravity and magnetic data acquisition and interpretation, Geochemistry, reservoir engineering, petro physical analysis and basin research. Around 25 laboratories covering different type of exploration related activities and researches exist. Stated simply, KDMIPE covers all aspects from outcrops to exploratory tests and all aspects of Geoscientific and Basin Research.

ROLE OF I.T. DIVISION

IT division of KDMIPE provides repair and maintenance services to the various equipments installed in the different labs of KDMIPE through in house expertise.

It also provides repair services and maintenance to through the OEM/OES i.e. outsourcing.

It also caters IT services to the KDMIPE for the complete IT infrastructure such as repair and maintenance of PC’s, Printers, and LAN etc.

It provides various communication services such as EPBAX connections, audio-visuals services in the auditorium of KDMIPE.

INTRODUCTION TO OPTIC FIBRE

Fiber-optic communication is a method of transmitting information from one place to

another by sending pulses of light through an optical fiber. The light forms

an electromagnetic carrier wave that is modulated to carry information. First developed

in the 1970s, fiber-optic communication systems have revolutionized

the telecommunications industry and have played a major role in the advent of

the Information Age. Because of its advantages over electrical transmission, optical

fibers have largely replaced copper wire communications in core networks in

the developed world.

The process of communicating using fiber-optics involves the following basic steps:

Creating the optical signal involving the use of a transmitter, relaying the signal along

the fiber, ensuring that the signal does not become too distorted or weak, receiving the

optical signal, and converting it into an electrical signal.

Fig 1: Collage depicting various optical fibres and their properties.

Optical fiber is used by many telecommunications companies to transmit telephone

signals, Internet communication, and cable television signals. Due to much

lower attenuation and interference, optical fiber has large advantages over existing

copper wire in long-distance and high-demand applications. However, infrastructure

development within cities was relatively difficult and time-consuming, and fiber-optic

systems were complex and expensive to install and operate. Due to these difficulties,

fiber-optic communication systems have primarily been installed in long-distance

applications, where they can be used to their full transmission capacity, offsetting the

increased cost. Since 2000, the prices for fiber-optic communications have dropped

considerably. The price for rolling out fiber to the home has currently become more

cost-effective than that of rolling out a copper based network. Prices have dropped to

$850 per subscriber in the US and lower in countries like The Netherlands, where

digging costs are low.

Since 1990, when optical-amplification systems became commercially available, the

telecommunications industry has laid a vast network of intercity and transoceanic fiber

communication lines. By 2002, an intercontinental network of 250,000 km of submarine

communications cable with a capacity of 2.56 Tb/s was completed, and although

specific network capacities are privileged information, telecommunications investment

reports indicate that network capacity has increased dramatically since 2004.

HISTORY

In 1880 Alexander Graham Bell and his assistant Charles Sumner Tainter created a

very early precursor to fiber-optic communications, the Photo phone, at Bell's newly

established Volta Laboratory in Washington, D.C. Bell considered it his most important

invention. The device allowed for the transmission of sound on a beam of light. On June

3, 1880, Bell conducted the world's first wireless telephone transmission between two

buildings, some 213 meters apart. Due to its use of an atmospheric transmission

medium, the Photophone would not prove practical until advances in laser and optical

fiber technologies permitted the secure transport of light. The Photophone's first

practical use came in military communication systems many decades later.

In 1966 Charles K. Kao and George Hockham proposed optical fibers at STC

Laboratories (STL) at Harlow, England, when they showed that the losses of 1000

dB/km in existing glass (compared to 5-10 dB/km in coaxial cable) was due to

contaminants, which could potentially be removed.

Optical fiber was successfully developed in 1970 by Corning Glass Works, with

attenuation low enough for communication purposes (about 20dB/km), and at the same

time GaAs semiconductor lasers were developed that were compact and therefore

suitable for transmitting light through fiber optic cables for long distances.

After a period of research starting from 1975, the first commercial fiber-optic

communications system was developed, which operated at a wavelength around 0.8 µm

and used GaAs semiconductor lasers. This first-generation system operated at a bit rate

of 45 Mbps with repeater spacing of up to 10 km. Soon on 22 April 1977, General

Telephone and Electronics sent the first live telephone traffic through fiber optics at a 6

Mbit/s throughput in Long Beach, California.

The second generation of fiber-optic communication was developed for commercial use

in the early 1980s, operated at 1.3 µm, and used InGaAsP semiconductor lasers. These

early systems were initially limited by multi mode fiber dispersion, and in 1981

the single-mode fiber was revealed to greatly improve system performance, however

practical connectors capable of working with single mode fiber proved difficult to

develop. By 1987, these systems were operating at bit rates of up to 1.7 Gb/s with

repeater spacing up to 50 km.

The first transatlantic telephone cable to use optical fiber was TAT-8, based on

Desurvire optimized laser amplification technology. It went into operation in 1988.

Third-generation fiber-optic systems operated at 1.55 µm and had losses of about

0.2 dB/km. They achieved this despite earlier difficulties with pulse-spreading at that

wavelength using conventional InGaAsP semiconductor lasers. Scientists overcame this

difficulty by using dispersion-shifted fibers designed to have minimal dispersion at

1.55 µm or by limiting the laser spectrum to a singlelongitudinal mode. These

developments eventually allowed third-generation systems to operate commercially at

2.5 Gbit/s with repeater spacing in excess of 100 km.

The fourth generation of fiber-optic communication systems used optical amplification to

reduce the need for repeaters and wavelength-division multiplexing to increase data

capacity. These two improvements caused a revolution that resulted in the doubling of

system capacity every 6 months starting in 1992 until a bit rate of 10 Tb/s was reached

by 2001. In 2006 a bit-rate of 14 Tbit/s was reached over a single 160 km line using

optical amplifiers.

The focus of development for the fifth generation of fiber-optic communications is on

extending the wavelength range over which a WDM system can operate. The

conventional wavelength window, known as the C band, covers the wavelength range

1.53-1.57 µm, and dry fiber has a low-loss window promising an extension of that range

to 1.30-1.65 µm. Other developments include the concept of "optical solitons " pulses

that preserve their shape by counteracting the effects of dispersion with the nonlinear

effects of the fiber by using pulses of a specific shape.

In the late 1990s through 2000, industry promoters, and research companies such as

KMI, and RHK predicted massive increases in demand for communications bandwidth

due to increased use of the Internet, and commercialization of various bandwidth-

intensive consumer services, such as video on demand. Internet protocol data traffic

was increasing exponentially, at a faster rate than integrated circuit complexity had

increased under Moore's Law. From the bust of the dot-com bubble through 2006,

however, the main trend in the industry has been consolidation of firms and off

shoring of manufacturing to reduce costs. Companies such as Verizon and AT&T have

taken advantage of fiber-optic communications to deliver a variety of high-throughput

data and broadband services to consumers' homes.

ELEMENTS USED IN OPTIC FIBRE

COMMUNICATION

Modern fiber-optic communication systems generally include an optical transmitter to convert an electrical signal into an optical signal to send into the optical fiber, a cable containing bundles of multiple optical fibers that is routed through underground conduits and buildings, multiple kinds of amplifiers, and an optical receiver to recover the signal as an electrical signal. The information transmitted is typically digital information generated by computers, telephone systems, and cable television companies. The devices used by optic fibre communication are as follows or the components through which optic fibre helps in communicating from one place to another.

Transmitters

The most commonly-used optical transmitters are semiconductor devices such as light-

emitting diodes (LEDs) and laser diodes. The difference between LEDs and laser

diodes is that LEDs produce incoherent light, while laser diodes produce coherent light.

For use in optical communications, semiconductor optical transmitters must be

designed to be compact, efficient, and reliable, while operating in an optimal wavelength

range, and directly modulated at high frequencies.

In its simplest form, an LED is a forward-biased p-n junction, emitting light through

spontaneous emission, a phenomenon referred to as electroluminescence. The emitted

light is incoherent with a relatively wide spectral width of 30-60 nm. LED light

transmission is also inefficient, with only about 1 % of input power, or about 100

microwatts, eventually converted into launched power which has been coupled into the

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

low-cost applications.

Communications LEDs are most commonly made from gallium arsenide

phosphide (GaAsP) or gallium arsenide (GaAs). Because GaAsP LEDs operate at a

longer wavelength than GaAs LEDs (1.3 micrometers vs. 0.81-0.87 micrometers), their

output spectrum is wider by a factor of about 1.7. The large spectrum width of LEDs

causes higher fiber dispersion, considerably limiting their bit rate-distance product (a

common measure of usefulness). LEDs are suitable primarily for local-area-

network applications with bit rates of 10-100 Mbit/s and transmission distances of a few

kilometers. LEDs have also been developed that use several quantum wells to emit light

at different wavelengths over a broad spectrum, and are currently in use for local-

area WDM networks.

Today, LEDs have been largely superseded by VCSEL (Vertical Cavity Surface Emitting

Laser) devices, which offer improved speed, power and spectral properties, at a similar

cost. Common VCSEL devices couple well to multi mode fiber.

Fig 2: A GBIC module (shown here with its cover removed), is an optical and electrical transceiver. The electrical

connector is at top right, and the optical connectors are at bottom left.

A semiconductor laser emits light through stimulated emission rather than spontaneous

emission, which results in high output power (~100 mW) as well as other benefits

related to the nature of coherent light. The output of a laser is relatively directional,

allowing high coupling efficiency (~50 %) into single-mode fiber. The narrow spectral

width also allows for high bit rates since it reduces the effect ofchromatic dispersion.

Furthermore, semiconductor lasers can be modulated directly at high frequencies

because of short recombination time.

Commonly used classes of semiconductor laser transmitters used in fiber optics

include VCSEL (Vertical Cavity Surface Emitting Laser), Fabry–

Pérot and DFB (Distributed Feed Back).

Laser diodes are often directly modulated, that is the light output is controlled by a

current applied directly to the device. For very high data rates or very long

distance links, a laser source may be operated continuous wave, and the light

modulated by an external device such as an electro-absorption modulator or Mach–

Zehnder interferometer. External modulation increases the achievable link distance by

eliminating laser chirp, which broadens the linewidth of directly-modulated lasers,

increasing the chromatic dispersion in the fiber.

A transceiver is a device combining a transmitter and a receiver in a single housing.

Receivers

The main component of an optical receiver is a photo detector, which converts light into

electricity using the photoelectric effect. The photo detector is typically a semiconductor-

based photodiode. Several types of photodiodes include p-n photodiodes, p-i-n photodiodes,

and avalanche photodiodes. Metal-semiconductor-metal (MSM) photo detectors are also used

due to their suitability for circuit integration in regenerators and wavelength-division multiplexers.

Optical-electrical converters are typically coupled with a trans impedance amplifier and a limiting

amplifier to produce a digital signal in the electrical domain from the incoming optical signal,

which may be attenuated and distorted while passing through the channel. Further signal

processing such as clock recovery from data (CDR) performed by a phase-locked loop may also

be applied before the data is passed on.

Fig 3: HDMI Fiber Optic Receivers / Transmitters / Transceivers (ZY-HDMI-T/R)

Fiber cable types

An optical fiber consists of a core, cladding, and a buffer (a protective outer coating), in

which the cladding guides the light along the core by using the method of total internal

reflection. The core and the cladding (which has a lower-refractive-index) are usually

made of high-quality silica glass, although they can both be made of plastic as well.

Connecting two optical fibers is done by fusion splicing or mechanical splicing and

requires special skills and interconnection technology due to the microscopic precision

required to align the fiber cores.

Two main types of optical fiber used in optic communications include multi-mode optical

fibers and single-mode optical fibers. A multi-mode optical fiber has a larger core (≥

50micrometers), allowing less precise, cheaper transmitters and receivers to connect to

it as well as cheaper connectors. However, a multi-mode fiber introduces multimode

distortion, which often limits the bandwidth and length of the link. Furthermore, because

of its higher do pant content, multi-mode fibers are usually expensive and exhibit higher

attenuation. The core of a single-mode fiber is smaller (<10 micrometers) and requires

more expensive components and interconnection methods, but allows much longer,

higher-performance links.

Fig 4: (a) A cable reel trailer with conduit that can carry optical fiber.

(b) Single-mode optical fiber in an underground service pit

In order to package fiber into a commercially-viable product, it is typically protectively-

coated by using ultraviolet (UV), light-cured acrylate polymers, then terminated

with optical fiber connectors, and finally assembled into a cable. After that, it can be laid

in the ground and then run through the walls of a building and deployed aerially in a

manner similar to copper cables. These fibers require less maintenance than common

twisted pair wires, once they are deployed.

Specialized cables are used for long distance subsea data transmission,

e.g. transatlantic communications cable. New (2011–2013) cables operated by

commercial enterprises (Emerald Atlantis, Hibernia Atlantic) typically have four strands

of fiber and cross the Atlantic (NYC-London) in 60-70ms. Cost of each such cable was

about $300M in 2011. 

Another common practice is to bundle many fiber optic strands within long-

distance power transmission cable. This exploits power transmission rights of way

effectively, ensures a power company can own and control the fiber required to monitor

its own devices and lines, is effectively immune to tampering, and simplifies the

deployment of smart grid technology.

Amplifiers

The transmission distance of a fiber-optic communication system has traditionally been

limited by fiber attenuation and by fiber distortion. By using opto-electronic repeaters,

these problems have been eliminated. These repeaters convert the signal into an

electrical signal, and then use a transmitter to send the signal again at a higher intensity

than it was before. Because of the high complexity with modern wavelength-division

multiplexed signals (including the fact that they had to be installed about once every

20 km), the cost of these repeaters is very high.

An alternative approach is to use an optical amplifier, which amplifies the optical signal

directly without having to convert the signal into the electrical domain. It is made

by doping a length of fiber with the rare-earth mineral erbium, and pumping it with light

from a laser with a shorter wavelength than the communications signal (typically

980 nm). Amplifiers have largely replaced repeaters in new installations.

Wavelength-division multiplexing

Wavelength-division multiplexing (WDM) is the practice of multiplying the available

capacity of optical fibers through use of parallel channels, each channel on a dedicated

wavelength of light. This requires a wavelength division multiplexer in the transmitting

equipment and a demultiplexer (essentially a spectrometer) in the receiving

equipment. Arrayed waveguide gratings are commonly used for multiplexing and

demultiplexing in WDM. Using WDM technology now commercially available, the

bandwidth of a fiber can be divided into as many as 160 channels to support a

combined bit rate in the range of terabits per second.

Bandwidth–distance product

Because the effect of dispersion increases with the length of the fiber, a fiber

transmission system is often characterized by its bandwidth–distance product, usually

expressed in units of MHz·km. This value is a product of bandwidth and distance

because there is a tradeoff between the bandwidth of the signal and the distance it can

be carried. For example, a common multi-mode fiber with bandwidth–distance product

of 500 MHz·km could carry a 500 MHz signal for 1 km or a 1000 MHz signal for 0.5 km.

Engineers are always looking at current limitations in order to improve fiber-optic

communication, and several of these restrictions are currently being researched. Each

fiber can carry many independent channels, each using a different wavelength of light

(wavelength-division multiplexing (WDM)). The net data rate (data rate without

overhead bytes) per fiber is the per-channel data rate reduced by the FEC overhead,

multiplied by the number of channels (usually up to eighty in commercial dense

WDM systems as of 2008). For instance, NTT was able to achieve 69.1 Tbit/s

transmission by applying wavelength division multiplex (WDM) of 432 wavelengths with

a capacity of 171 Gbit/s over a single 240 km-long optical fiber on March 25, 2010. This

was the highest optical transmission speed recorded at that time.

In intensive development NEC scientists have managed to reach speed of 101 Tbit/s by

multiplexing 370 channels over single fiber, while similar Japanese effort reached 109

terabits per second, but through a difficult production of cable with seven fibers. But this

is barely matching the 50%-per-year exponentially increasing backbone traffic.

Dispersion

For modern glass optical fiber, the maximum transmission distance is limited not by

direct material absorption but by several types of dispersion, or spreading of optical

pulses as they travel along the fiber. Dispersion in optical fibers is caused by a variety of

factors. Intermodal dispersion, caused by the different axial speeds of different

transverse modes, limits the performance of multi-mode fiber. Because single-mode

fiber supports only one transverse mode, intermodal dispersion is eliminated.

In single-mode fiber performance is primarily limited by chromatic dispersion (also

called group velocity dispersion), which occurs because the index of the glass varies

slightly depending on the wavelength of the light, and light from real optical transmitters

necessarily has nonzero spectral width (due to modulation). Polarization mode

dispersion, another source of limitation, occurs because although the single-mode fiber

can sustain only one transverse mode, it can carry this mode with two different

polarizations, and slight imperfections or distortions in a fiber can alter the propagation

velocities for the two polarizations. This phenomenon is called fiber birefringence and

can be counteracted by polarization-maintaining optical fiber. Dispersion limits the

bandwidth of the fiber because the spreading optical pulse limits the rate that pulses

can follow one another on the fiber and still be distinguishable at the receiver.

Some dispersion, notably chromatic dispersion, can be removed by a 'dispersion

compensator'. This works by using a specially prepared length of fiber that has the

opposite dispersion to that induced by the transmission fiber, and this sharpens the

pulse so that it can be correctly decoded by the electronics.

Attenuation

Fiber attenuation, which necessitates the use of amplification systems, is caused by a

combination of material absorption, Rayleigh scattering, Mie scattering, and connection

losses. Although material absorption for pure silica is only around 0.03 dB/km (modern

fiber has attenuation around 0.3 dB/km), impurities in the original optical fibers caused

attenuation of about 1000 dB/km. Other forms of attenuation are caused by physical

stresses to the fiber, microscopic fluctuations in density, and imperfect splicing

techniques.

Transmission windows

Each effect that contributes to attenuation and dispersion depends on the optical wavelength. The

wavelength bands (or windows) that exist where these effects are weakest are the most favorable

for transmission. These windows have been standardized, and the currently defined bands are the

following:

Band Description Wavelength Range

O band Original 1260 to 1360 nm

E band Extended 1360 to 1460 nm

S band short wavelengths 1460 to 1530 nm

C band conventional ("erbium window") 1530 to 1565 nm

L band long wavelengths 1565 to 1625 nm

U band ultra long wavelengths 1625 to 1675 nm

Note: that this table shows that current technology has managed to bridge the second and third windows that were

originally disjoint.

Historically, there was a window used below the O band, called the first window, at 800-

900 nm; however, losses are high in this region so this window is used primarily for

short-distance communications. The current lower windows (O and E) around 1300 nm

have much lower losses. This region has zero dispersion. The middle windows (S and

C) around 1500 nm are the most widely used. This region has the lowest attenuation

losses and achieves the longest range. It does have some dispersion, so dispersion

compensator devices are used to remove this.

Regeneration

When a communications link must span a larger distance than existing fiber-optic

technology is capable of, the signal must be regenerated at intermediate points in the

link by repeaters. Repeaters add substantial cost to a communication system, and so

system designers attempt to minimize their use.

Recent advances in fiber and optical communications technology have reduced signal

degradation so far that regeneration of the optical signal is only needed over distances

of hundreds of kilometers. This has greatly reduced the cost of optical networking,

particularly over undersea spans where the cost and reliability of repeaters is one of the

key factors determining the performance of the whole cable system. The main advances

contributing to these performance improvements are dispersion management, which

seeks to balance the effects of dispersion against non-linearity; and solitons, which use

nonlinear effects in the fiber to enable dispersion-free propagation over long distances.

Last mile

Although fiber-optic systems excel in high-bandwidth applications, optical fiber has been

slow to achieve its goal of fiber to the premises or to solve the last mile problem.

However, as bandwidth demand increases, more and more progress towards this goal

can be observed. In Japan, for instance EPON has largely replaced DSL as a

broadband Internet source. South Korea’s KT also provides a service

called FTTH (Fiber to the Home), which provides fiber-optic connections to the

subscriber’s home. The largest FTTH deployments are in Japan, South Korea, and

China. Singapore started implementation of their all-fiber Next Generation Nationwide

Broadband Network (Next Gen NBN), which is slated for completion in 2012 and is

being installed by Open Net. Since they began rolling out services in September 2010,

Network coverage in Singapore has reached 60% nationwide.

In the US, Verizon Communications provides a FTTH service called FiOS to select high-

ARPU (Average Revenue per User) markets within its existing territory. The other major

surviving ILEC (or Incumbent Local Exchange Carrier), AT&T, uses a FTTN (Fiber to

the Node) service called U-verse with twisted-pair to the home. Their MSO competitors

employ FTTN with coax using HFC. All of the major access networks use fiber for the

bulk of the distance from the service provider's network to the customer.

The globally dominant access network technology is EPON (Ethernet Passive Optical

Network). In Europe, and among Telco’s in the United States, BPON (ATM-based

Broadband PON) and GPON (Gigabit PON) had roots in the FSAN (Full Service Access

Network) and ITU-T standards organizations under their control.

COMPARISON WITH ELECTRICAL TRANSMISSION

The choice between optical fiber and electrical (or copper) transmission for a particular

system is made based on a number of trade-offs. Optical fiber is generally chosen for

systems requiring higher bandwidth or spanning longer distances than electrical cabling

can accommodate.

The main benefits of fiber are its exceptionally low loss (allowing long distances

between amplifiers/repeaters), its absence of ground currents and other parasite signal

and power issues common to long parallel electric conductor runs (due to its reliance on

light rather than electricity for transmission, and the dielectric nature of fiber optic), and

its inherently high data-carrying capacity. Thousands of electrical links would be

required to replace a single high bandwidth fiber cable. Another benefit of fibers is that

even when run alongside each other for long distances, fiber cables experience

effectively no crosstalk, in contrast to some types of electrical transmission lines. Fiber

can be installed in areas with highelectromagnetic interference (EMI), such as alongside

utility lines, power lines, and railroad tracks. Nonmetallic all-dielectric cables are also

ideal for areas of high lightning-strike incidence.

For comparison, while single-line, voice-grade copper systems longer than a couple of

kilometers require in-line signal repeaters for satisfactory performance; it is not unusual

for optical systems to go over 100 kilometers (62 mi), with no active or passive

processing. Single-mode fiber cables are commonly available in 12 km lengths,

minimizing the number of splices required over a long cable run. Multi-mode fiber is

available in lengths up to 4 km, although industrial standards only mandate 2 km

unbroken runs.

In short distance and relatively low bandwidth applications, electrical transmission is

often preferred because of its

Lower material cost, where large quantities are not required

Lower cost of transmitters and receivers

Capability to carry electrical power as well as signals (in specially-designed

cables)

Ease of operating transducers in linear mode.

Optical fibers are more difficult and expensive to splice than electrical conductors. And

at higher powers, optical fibers are susceptible to fiber fuse, resulting in catastrophic

destruction of the fiber core and damage to transmission components.

Because of these benefits of electrical transmission, optical communication is not

common in short box-to-box, backplane, or chip-to-chip applications; however, optical

systems on those scales have been demonstrated in the laboratory.

In certain situations fiber may be used even for short distance or low bandwidth

applications, due to other important features:

Immunity to electromagnetic interference, including nuclear electromagnetic

pulses (although fiber can be damaged by alpha and beta radiation).

High electrical resistance, making it safe to use near high-voltage equipment or

between areas with different earth potentials.

Lighter weight—important, for example, in aircraft.

No sparks—important in flammable or explosive gas environments.

Not electromagnetically radiating, and difficult to tap without disrupting the signal

—important in high-security environments.

Much smaller cable size—important where pathway is limited, such as

networking an existing building, where smaller channels can be drilled and space

can be saved in existing cable ducts and trays.

Resistance to corrosion due to non-metallic transmission medium

Optical fiber cables can be installed in buildings with the same equipment that is used to

install copper and coaxial cables, with some modifications due to the small size and

limited pull tension and bend radius of optical cables. Optical cables can typically be

installed in duct systems in spans of 6000 meters or more depending on the duct's

condition, layout of the duct system, and installation technique. Longer cables can be

coiled at an intermediate point and pulled farther into the duct system as necessary.

APPLICATIONS AND USES OF FIBRE OPTICS

In this section we will show you how optical fibres are used. As you will be able to see when you read further, optical fibres are revolutionising fields like communications and medicine.

Telecommunications Industry

Until the optical fibre network was developed, telephone calls were mainly sent as electrical signals along copper wire cables. As demand for the systems to carry more telephone calls increased, simple copper wires did not have the capacity, known as bandwidth, to carry the amount of information required.

    Systems using coaxial cables like TV aerial leads were used but as the need for more bandwidth grew, these systems became more and more expensive especially over long distances when more signal regenerators were needed. As demand increases and higher frequency signals are carried, eventually the electronic circuits in the regenerators just cannot cope.

     Optical fibres offer huge communication capacity. A single fibre can carry the conversations of every man, woman and child on the face of this planet, at the same time, twice over. The latest generations of optical transmission systems are beginning to exploit a significant part of this huge capacity, to satisfy the rapidly growing demand for data communications and the Internet.  

 The main advantages of using optical fibres in the communications industry are:

 - A much greater amount of information can be carried on an optical fibre compared to a copper cable.

 - In all cables some of the energy is lost as the signal goes along the cable. The signal then needs to be boosted using regenerators. For copper cable systems these are required every 2 to 3km but with optical fibre systems they are only needed every 50km.

- Unlike copper cables, optical fibres do not experience any electrical interference. Neither will they cause sparks so they can be used in explosive environments such as oil refineries or gas pumping stations.

- For equal capacity, optical fibres are cheaper and thinner than copper cables which makes them easier to install and maintain.

 Case Study: British Telecom

     In the UK at the moment there are three million kilometres of optical fibre cable in the BT network. Most of BT's trunk network now uses optical fibre cables. In a recent trial in Bishop's

Stortford optical fibres were actually laid into homes. This allowed customers to receive cable TV and stereo radio as well as phone and information services.

    Optical fibres could be put into all homes but currently the cost of the system including lasers and detectors would be too high for simple telephone calls. Some companies have a direct optical fibre link if they need to send large quantities of information by phone, for example between computers at different business centres.  

    Optical fibre submarine links are in use all around the world. Because of the low loss and high bandwidth of optical fibre systems they are ideal for submarine systems where you want to minimise the amount of complex electronics in regenerators sitting on the sea bed. In fact, the link from the UK to the English Channel Islands is achieved directly without any submerged regenerators.

    The world's first international optical fibre submarine cable was laid by BT in 1986 between the UK and Belgium. It is 112Km in length and has only 3 regenerators. BT was a major partner in the first transatlantic optical fibre cable system - TAT 8 (Transatlantic Telecommunications cable no 8) which was capable of carrying 40,000 telephone calls at once, or the equivalent in data, facsimile, or TV pictures.

    The second transatlantic cable, TAT 9, which came into service in 1992, has twice that capacity and links five separate landing points in the UK the USA, Canada, France and Spain. The transatlantic optical fibre cable network, completed in 1996, spanned 14,000Km and linked the UK, France and the USA . It could handle up to 320,000 phone calls at one time.

    Undersea cables consist of fibres with a copper coated steel conductor which are covered in protective layers of steel and polypropylene. In shallow waters, e.g. the continental shelf, a submersible remote-controlled plough is used to bury the cables one metre below the seabed, to protect them from damage by trawling and ships' anchors.

 Medicine Industry     

   The advent of practicable optical fibres has seen the development of much medical technology. Optical fibres have paved the way for a whole new field of surgery, called laproscopic surgery (or more commonly, keyhole surgery), which is usually used for operations in the stomach area such as appendectomies. Keyhole surgery usually makes use of two or three bundles of optical fibres. A "bundle" can contain thousands of individual fibres". The surgeon makes a number of small incisions in the target area and the area can then be filled with air to provide more room.

    One bundle of optical fibres can be used to illuminate the chosen area, and another bundle can be used to bring information back to the surgeon. Moreover, this can be coupled with laser surgery, by using an optical fibre to carry the laser beam to the relevent spot, which would then be able to be used to cut the tissue or affect it in some other way.

Other Uses

- Optical fibres can be used for the purposes of illumination, often carrying light from outside to rooms in the interiors of large buildings.

 - Another important application of optical fibres is in sensors. If a fibre is stretched or squeezed, heated or cooled or subjected to some other change of environment, there is usually a small but measurable change in light transmission. Hence, a rather cheap sensor can be made   which can be put in a tank of acid, or near an explosion or in a mine and connected back, perhaps through kilometres of fibre, to a central point where the effects can be measured.

    An advantage of fibre-optic sensors is that it is possible to measure the data at different points along the fibre and to know to what points the different measurements relate. These are the so-called distributed sensors.

- Fibre optics are also used to carry high power laser beams from fixed installations within factories to the point of use of the laser light for welding, cutting or drilling. The fibre provides a flexible and safe means of distributing high power laser radiation around a factory so that robots or machine tools can be provided with laser machining          capability.

 - Optical fibres can also be used as simple light guides. At least one fancy modern car has a single high intensity lamp under its bonnet, with optical fibres taking the light to a series of mini-headlamps on the front. Less high tech versions carry light from bulbs to the glove         compartment etc.

- A research group at the Clarendon Laboratory, Oxford, is designing a laser installation at the William Hershel Telescope on La Palma to help astronomers make an 'artificial' star in the layer of atomic sodium which exists at a height of 100km above the Earth's surface.

     The Earth's atmosphere is a big problem for astronomers. It is a gas that is constantly moving which makes the light traveling through it from distant starts flicker. If astronomers could use a reference 'star' whose brightness they knew, then they could allow for this twinkling.

 The telescope will look at how the atmosphere is effecting the artificial star second by second and adjust the telescope's mirror to compensate.   This should allow astronomers to capture pictures of astronomical objects of a quality previously only obtainable from the Hubble Space Telescope. The optical fibre in this case is used to pipe the laser power    needed to create the artificial star from the lasers to the telescope itself.

- As light is not affected noticeably by electromagnetic fields. It also does not interfere with other instruments that do use electricity. For this reason, fibre-optics are also becoming very important for short-range communication and information transfer in applications situations like aircraft. This application is now being extended into motor cars, and plastic optical fibres will soon (say in 5-8 years time) be very common for transmitting information around the car.

 

So we can see that optical fibres are not just passive light pipes. Researchers are finding ways in which they can make the fibres become the active elements of the circuit, e.g. amplifiers or filters. This means that the information could remain in light form from one end of a link to the other, removing the limitations of the electronics in circuits and enabling more of the theoretical information carrying capacity to be used.

 Engineers of the future can look forward to designing and using telecommunications systems that have no loss, infinite bandwidth and high reliability. New services for customers, such as 3D high definition TV and virtual reality information and entertainment systems, could be more easily provided as well as giving them the benefits of lower costs and greater flexibility - an exciting future.

WAVELENGTHS AND FREQUENCY

In this unit you will find frequent references both to the wavelength of light and the frequency of light. It is important to be able to work with both, and convert from one to the other. In the context of optical-fibre communication, the light radiation we are concerned with is usually in the infrared region of the spectrum.

To identify the position in the electromagnetic spectrum it is usual to refer to the wavelength of light, rather than its frequency. Thus I will refer to the ‘1300 nm window’ rather than the 2.3 × 1014 Hz (230 THz) window. T is the SI prefix ‘tera’, the multiplier for 1012.

When talking about the details of the optical spectrum both wavelengths and frequencies are used. In dense wavelength division multiplexing, for example, the carriers are often referred to by their frequencies rather than their wavelengths, and it is important to be able to convert readily from one to the other.

The conversion between wavelength (lambda, λ) and frequency (f) uses the formula:

where c is the speed of light in free space (i.e. in a vacuum) which is

3.00 × 108 m s−1 to three significant figures. This may be rearranged to give:

Note that because the speed used is that of light in free space, references to wavelength are to the wavelength of the light in free space, which is different from (i.e. longer than) the wavelength of the same light in a fibre. When light enters a fibre from free space (or air) it slows down but the frequency remains the same, so the wavelength decreases.

You need to be careful when converting between wavelength and frequency spacings or ranges, rather than individual values. For example, let's suppose you know the output from a light-emitting diode (LED) contains wavelengths between 1300 nm and 1320 nm so that the spectral width is 20 nm and you want to know the frequency range in hertz.

To get the exact frequency range you need to convert each end of the range to frequencies and subtract one from the other. Thus the spectrum of the LED output in this example extends from

to

The spectral range expressed in frequencies is therefore:

Notice that you have to be careful with the accuracy of the intermediate steps in this calculation because it involves subtracting two large and similar numbers.

For small spectral widths there is an approximation formula to convert between spectral width measured in wavelength (Δλ) and spectral width measured in frequency (Δf):

where λ and f are taken at the mid-point of the range. This approximation is valid provided the fractions are small – less than about 1/10, for which we require Δλ < λ/10. In fact, because the approximation avoids the need to subtract two large numbers, it can be a better way of doing the calculation.

Often we may know λ but not f, so we can use f = c/λ to get:

and therefore:

Using the example of the LED again, taking λ in the middle of the range at 1310 nm gives:

FIBRE IN LAN’s

Fibre has been slower to be exploited in LANs than in the core transmission network, for similar reasons to the delay in the use of fibre in the access network, but as the data rate demanded of LANs has increased, the case for using fibre has strengthened.

Although Ethernet specifications (IEEE 802.3 series) have contained standards for the use of fibre backbones for some time, it was with the development of Gigabit Ethernet and 10 Gigabit Ethernet (10 GbE) standards that fibre became the main transmission medium. This section very briefly summarises the use of fibre in these two standards.

In this context 1300 nm is ‘long’ – because it is longer than 800 nm. In other contexts ‘long’ means wavelengths in the 1550 nm window in contrast to the ‘short’ 1300 nm window. And you have already met references to S-band, C-band and L-band, where ‘long’ means 1565–1625 nm and ‘short’ means 1460–1530 nm. It just goes to show that you have to be very careful with terms like long and short.

Gigabit Ethernet:

Gigabit Ethernet specifies four categories:

1000BASE-SX, for transmission over multimode fibre using an LED in the 800 nm window (the wavelength is specified to be between 770 and 860 nm). The specification is for up to 275 m on 62.5/125 mm multimode fibre, or 550 m on 50/125 mm multimode fibre;

1000BASE-LX, for transmission over multimode or single-mode fibre using a laser in the 1300 nm window (specified to be between 1270 and 1355 nm). The distance on both 62.5/125 mm and 50/125 mm multimode fibre is specified as 550 m, and on single-mode fibre as 5 km;

1000BASE-CX specifies transmission on a shielded twisted-pair copper cable up to 25 m;

1000BASE-T specifies transmission on four pairs of unshielded twisted pairs up to 100 m.

In this context 1300 nm is ‘long’ - because it is longer than 800 nm. In other contexts ‘long’ means wavelengths in the 1550 nm window in contrast to the ‘short' 1300 nm window. And you have already met references to S-band, C-band and L-band, where ‘long’ means 1565-1625 nm and ‘short’ means 1460-1530 nm. It just goes to show that you have to be very careful with terms like long and short.

10 Gigabit Ethernet:

The standard for 10 Gigabit Ethernet (IEEE 802.3ae, lOGbE) was approved in July 2002. The main use of lOGbE, initially at least, is for backbone networks which interconnect 10, 100 or 1000 Mbit/s Ethernet hubs. These hubs might be widely separated geographically, so the standard includes physical layer specifications specifically for WAN (wide area network) applications as well as LAN applications. The WAN specification is for operation at slightly under 10 Gbit/s, 9.95328 Gbit/s, so as to be compatible with the SDH.

Both the WAN and LAN physical layers can use optical interfaces specified for use at 850 nm, 1310 nm and 1550 nm as follows (Figure 5):

10GBASE-S operates with a directly modulated 850 nm multimode laser diode and graded-index multimode fibre, at distances up to 300 meters;

10GBASE-L operates with a directly modulated 1310 nm single-mode laser diode and single-mode fibre, at distances up to 10 km;

10GBASE-E operates with an externally modulated 1550 nm single-mode laser diode and single-mode fibre, at distances up to 40 km.

Figure 5: 10 Gigabit Ethernet

A target in the development of lOGbE was for a distance of at least 300 m on multimode fibre, and it is clear from the answer to SAQ 16 that this would not be possible with the 10GBASE-S interface on fibre that just meets the Gigabit Ethernet specification.

There are two solutions to this in 1 OGbE. One is to use new multimode fibre that has been developed with a higher specification, specifically for 1 OGbE. Referred to as ‘10 Gigabit Ethernet fibre’, this has a typical bandwidth-distance product of 2000 MHz·km.

With this fibre the 300 m target can therefore be met (assuming that the power budget can also be balanced at 300 m).

The other solution is based on an additional interface specification, used only for the LAN physical layer (i.e. not the WAN physical layer), the 10GBASE-LX4 interface.

10GBASE-LX4 splits the data into four parallel streams and transmits them on four different wavelengths on a fibre. It therefore uses a type of WDM. The four wavelengths used are 1280 nm, 1300 nm, 1320 run and 1340 nm.

SUMMARY

The building blocks of a basic optical-fibre communications link are the modulated light source, the fibre and the detector. There are choices to be made between different types of light source and fibre, with trade-offs between cost and performance. For example, for high signaling rates over long distances single-mode fibre will be used with a single-mode laser (possibly with external modulation) operating in the 1550 nm window, whereas for short-distance links operating at lower signaling rates multimode fibre will be used with a directly modulated LED operating at around 850 nm.

The maximum transmission distance without regeneration at a given signaling rate over an optical-fibre link is limited by either attenuation or inter-symbol interference. The effects of attenuation are determined by calculating the power budget for the link. The main cause of inter-symbol interference in multimode fibre is multimode distortion,

which is characterized by the bandwidth-distance product of the fibre. Causes of inter-symbol interference in single-mode fibre include dispersion and polarization mode distortion.

Optical amplifiers can overcome the limits due to attenuation, and the effects of dispersion can be mitigated by using dispersion-shifted fibre or through the use of dispersion compensation. Increasing the capacity of a fibre through increased signaling rates becomes more difficult as the signaling rates rise above 40 Gbaud, however, and the alternative of using wavelength division multiplexing (WDM) is preferable.

WDM is usually done in the 1550 nm window because erbium-doped fibre amplifiers can then be used to amplify all channels simultaneously. Fibre non-linearities become important when dense WDM is used, leading to the need for non-zero dispersion-shifted fibre.

Beyond the simple point-to-point link, devices such as fibre splitters and combiners, WDM devices and optical switches allow more sophisticated use of optical-fibre communication, leading ultimately to the concept of an all-optical network.

BIBLIOGRAPHY

http://openlearn.open.ac.uk

www.wikipedia.org Optic fibre communication by GERD KEISER Images From Google pictures search

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