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CHAPTER 1

INTRODUCTION

You have undoubtedly heard about the wonders of fiber optics. AT & T, Sprint and other

large telecommunication companies have saturated the airwaves and print media with

advertisements heralding this bright new technology. Futurists talk about the marvels of light

wave communications and photonic technology. Omni magazine writes about Fiberopolis.Long-distance telephone calls travel through optical fiber crossing the United States and

spanning the oceans to connect the continents.

The enthusiasm is not mere hype; fiber optic technology is real and important. From coast to

coast, phone companies are laying fiber in the ground, pulling cable through manholes and

stringing it between poles. The military is buying fiber for portable battlefield

communications systems, due to its superior performance. Medical fiber optic systems allow

physicians to peer inside the human body with out surgery. Very few technologies ever

realize the fantastic growth rates predicted for them by market analyst. Fiber optics, however,

has exceeded predictions.

As we near the year 2000, fiber optics will become common in your everyday life. It will

enter the office environment. In our home, it will provide services that would have been

impractical without it: high-definition TV, secondary education classes in the comfort of your

home, a paper-less, environmentally clean newspaper.

1.1 What is Fiber Optics

In its simplest terms, fiber optics is a medium for carrying information from one point to

another in the form of light. Unlike the copper form of transmission, fiber optics is not

electrical in nature.

A basic fiber optic system consists of a transmitting device, which generates the light signal;

an optical fiber cable, which carries the light; and a receiver, which accepts the light signal

transmitted. The fiber itself is passive and does not contain any active, generative properties.

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Corning Cable Systems manufactures and sells those components considered to be part of the

passive fiber transmission subsystem; i.e., not active electronic components.

1.2 Basic Principles of Fiber Optics

Since its invention in the early 1970s, the use and demand of optical fiber has grown

tremendously. The uses of optical fiber today are quite numerous. The most common are

telecommunications, medicine, military, automotive, and industrial.

Telecommunications applications are widespread, ranging from global networks to local

telephone exchanges to subscribers' homes to desktop computers. These involve the

transmission of voice, data, or video over distances of less than a meter to hundreds of

kilometers, using one of a few standard fiber designs in one of several cable designs.

Companies such as AT&T, MCI, and U.S. Sprint use optical fiber cable to carry plain old

telephone service (POTS) across their nationwide networks. Local telephone service

providers use fiber to carry this same service between central office switches at more local

levels, and sometimes as far as the neighborhood or individual home.

Optical fiber is also used extensively for transmission of data signals. Private networks areowned by firms such as IBM, Rockwell, Honeywell, banks, universities, Wall Street firms,

and more. These firms have a need for secure, reliable systems to transfer computer and

monetary information between buildings to the desktop terminal or computer, and around the

world. The security inherent in optical fiber systems is a major benefit.

Cable television or community antenna television (CATV) companies also find fiber useful

for video services. The high information-carrying capacity, or bandwidth, of fiber makes it

the perfect choice for transmitting signals to subscribers.

Finally, one of the fastest growing markets for fiber optics is intelligent transportationsystems, smart highways with intelligent traffic lights, automated toll booths, and changeable

message signs to give motorists information about delays and emergencies.

These are only a few of the many applications possible with the use of optical fiber. We focus

primarily on telecommunications uses of optical fiber.

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1.3 Evolution of Optical Fiber Communication

During the last twenty years the staggering success of optical fibre communications has

enabled people to communicate widely across the world as never before. In spite of the recent

economic difficulties the sector in experiencing the underlying growth in optical fibre

communications is strong and will surely stabilize.

When fibre optical communications were introduced the network was based on copper

transmission systems for both long haul (coaxial) and access (twisted pairs) and the fibers

were effectively grafted on the existing network infrastructure. This approach did not take

into account the salient features of optical transmission except capacity and link length. As

the state of the art in fibre communications advanced in terms of components and systems it

became clear that they could change completely the fabric of the network and provide amultiplicity of services to satisfy the demand of the broad band multi-media environment of

the future. The purpose of this talk is to outline the current trends in optical networks and

introduce the technologies that will enable the implementation of future optical networks.

The talk begins with the current network trends that are shaped in a multimedia environment

with broadband requirements and the expectations it imposes on future technologies in the

context of an all optical network. The future enabling technologies will be outlined with

emphasis on optical technology and optical and electronic signal processing.

1.3.1 Optical Communication History

The transmission of information by means of light has a much longer history than electrical

communication. The first written evidence is at the end of the sixt h century BC AeschylusOresteia where he mentioned passing the news on of Troys downfall by fire signals via a long

chain of relay stations from Asia Minor to Argos. Three centuries after another Greek,

Polybius described an arrangement by which the whole Greek alphabet could be transmitted

by fire signals using a two-digit, five level codes. This was the first optical communication

link that allowed the transmission of messages not previously agreed upon.

At the end of the eighteen century AD Claude Chappes optical telegraph allowed the

transmission of a signal over the 423 km distance from Paris to Strasbourg within a time of

six minutes. In the middle of the nineteenth century, optical telegraphy was replaced by

electrical telegraphy, which at this time allowed a faster signal transmission.

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However, although optical communication exhibited low practical importance in the next

decades, its development proceeded. An example is the report of Graham Bell in 1880 about

the transmission of speech over a beam of light.

The invention of laser boosted the development of optical communication and opened newsections of research. The main disadvantage of optical communication was the need for a

high transmitter power and close repeater spacing because of the atmospheric phenomenon

like fog, precipitation and turbulence in the air.

The solution of this main disadvantage was the use of optical waveguides that not only gave a

solution to the atmospheric disturbances but moreover force the laser beam to follow a certain

path. A series of different ways, like continuously and discontinuous lightwave guidance, as

well as different materials, like metallic and glass waveguides, has been used. In 1970

Kapron, Keck and Maurer achieved an attenuation value of only 20 dB/km with silica fibres.This event marked the first generation of OFCS.

First Generation_In this first generation the systems worked at 0.85 m wavelength because

the semiconductor optical sources were light emitting diodes (LEDs) which emitted light at

0.85 m. At this wavelength the attenuation was high and has motivated farther investigation

on attenuation values of silica-based fibers. At the beginning of 1978 the best measured

attenuation values for silica-based monomode fibres where under 2 dB/km at 0.85 m

wavelength, that has been already used, and under 0.5 dB/km at 1.3 m wavelength. Figure

1.2 shows the loss spectrum of a silica fibre.

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Figure 1.1 Loss spectrum of a single-mode silica fibre

Second Generation_Therefore, optical communication proceeded to its next generation. The

second generations systems worked at 1.3 m wavelength after Lasers has been developed

to emit at that wavelength, where there is a minimum in the silica fibre attenuation curve.

Third Generation_The need for even bigger optical links by achieving an even lower

minimum in the silica fibre attenuation curve leads us in the next generation. In the third

generation the systems worked at 1.5 m wavelength. However, although the attenuation is

low a new problem exists called chromatic dispersion. To overcome this, dispersion shifted

fibres have been developed.

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Figure 1.2 A modern repeater

Even with the small attenuation of the third generation the needs for optical link length was

big therefore, other ways of extending the length of an optical length have been used. The

solution was to use, an already used solution in electrical communications, repeaters. Optical

repeater is just an electric filter and an amplifier that deducts the noise out of the received

signal and then amplifies it between a receiver that receives the weak signal and a transmitter

that transmits the amplified signal.

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1.4 Block Diagram

mmMIC

OPTICAL FIBER

Figure 1.3 Block Diagram

MICROPHONE

AMPLIFIER SECTIONSUPPLY SECTION

AMPLIFIER SECTION

SPEAKER

SUPPLY SECTION

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1.5 Circuit Diagram

Figure 1.4 Circuit Diagram

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1.6 Component List

Item Quantity Reference Part1 1 C1 C CER 104 PF

2 1 C2 C EL10 100M/35V

3 4 D1,D2,D3,D4 IN4007

4 1 D5 LED

5 2 J1,J2 RLMT(02M)

6 1 J3 RLMT(03M)

7 1 Q1 BC547

8 1 Q2 BC558

9 2 R1,R5 R 4K710 2 R2,R3 R 10K

11 1 R4 R 1OOE

12 1 R6 R1K

13 1 U1 741

14 1 VR1 POT10TV IM

1.7 Circuit Operation

Most fiber optic transmitters typically have and amplifier or buffer, driver, optical source, and

sometimes an optical connector or interface. The transmitter in this circuit has an acoustic

microphone for converting sound waves to an electrical signal, and requires a nine-volt

battery with holder to provide electrical power.

Pushing the momentary-close switch, SW1, activates the optical voice link by applying 9 volt

battery power to the indicator light, microphone, audio circuits and fiber optic LED. The

switch must be closed for the transmitter to operate by generating light to carry audio signals.

In the diagram are the receiver circuit functions: photo detector, amplifier, adjustable volume

control and miscellaneous electronics. Following a signal as it exits from the optical fiber, in

the form of light. The circuit is basically an amplifier. The IR signals are picked up by the

photodiode and converted into electrical variation which are amplified by the op-amp

(operational Amplifier) IC-741 used in the inverting mode with a single supply using divider

network of resistors. The gain of IC can be set by varying the feed back through VRresistance (can place a 2.2. M variable). Here the output of IC is further amplified by the push

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pull amplifier using transistors BEL368/69 pair. The output of the amplifier is taken from

emitter of two transistors, with a filter C from speaker.

1.8 Comparison Between Fiber Optic Cable and Copper Cable

FIBRE

1. Transmission with Photons

2. Does not radiate Radio Frequency

interference (RFI) nor is susceptible to

interference.

3. Fibre Optics Networks operate at

speeds upto 2.5 GB per sec..

4. Fibers have much higher bandwidth.

Single mode > 100 GHz for 100m.

5. Signals can be transmitted over much

longer distances > 40,000m for single

mode.

6. Cost is up to 100 times more

however, maintenance is very cheap.

COPPER

1. Transmission with Electrons.

2. Radiates signals capable of

interfering with other electronic

equipment.

3. Copper wire Networks operate at

speeds up to 155 MB per sec.

4. Bandwidth is lower for 100m

10MHZ.

5. Signal attenuation and distortion is

much higher hence distances are

smaller about 100m.

6. Cost is low, but maintenance is

expensive.

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1.9 Other Applications of Optical Fiber

1.9.1 In Medicine

Recent growth in the use of optics technology for biomedical research and health care has

been explosive. Using fibre optics in medical procedures allows things like MRI (Magnetic

Resonance Imaging) and PET (Position Emission Tomography) images to be transmitted to

anyone, anywhere. Using fibre optics equipment during surgery means using less invasive

procedures. This reduces recovery time, is less traumatic on the body, and costs less for the

procedure.

1.9.2 In Architecture

Fibre does not use electric current or have interference. Therefore, they can be exposed towater condition. ex. Lighting a pond or waterfall. Fibre optics lighting can also replace neon

lights. It uses less energy, maintenance costs are lower and are easier to install.

1.9.3 In Airlines:

Fibre Optics cable is light, so there is no need for heavier cable to be on board an airline.

Airplanes can have internal communication systems which are not exposed to outside

elements. This enables pilots to see damage to certain parts of the plane without waiting to

land.

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1.9.4 In Automobiles

Automobiles today have bulky lighting systems. With fibre optics cable being small enough

to maboeuvre around, automobile designers can have more freedom while designing theirvehicle. Optical systems used in cars can control things like stereo, heat and air-conditioners.

CATV is also adopting fibre optics, using a unique analogue transmission scheme, but the

move to compressed digital video is already planned. Only fibre has the necessary band with

for carrying voice, data and video simultaneously.

The LAN has also become fibre based. Mainframes have discovered fibres. Only the desktop

is currently a battlefield between copper and fibre.

The Rs. 3500 Crore telephone cable industry in India has notched up a 30 % growth in 1997-

98. The major constituents of this sector are the jelly filled telephone cables and optical fibre

cables. Optical Fibre cables have only about six operators and another 14 more are expected

to join in. As per the department of telecommunications (DOT) estimates, the total demand

for other optical fibre cables was at 2, 19,700 fibre-km in 1997-98. The demand is stated to

increase to 2, 85,600 fibre-km by 1997-98, and then go up to 8, 15,700 fibre-km by the end of

the century.

One of the main manufacturers in India is AT&T finolex fibre Optic Cables Ltd. which is a

joint venture between Lecent Technologies, USA (erstwhile System and Technologies

division of AT&T) and Finolex Cables Ltd.

The future of fibre optics is the future of communications. With fibres, you have high

bandwidth and lots of upgradeability. Applications like multimedia and video conferencing

make high bandwidth very essential. Over wide area networks, the installed fibre optic

infrastructure can be expanded to accommodate almost unlimited traffic. CATV operators are

installing fibres very fast, since advanced digital TV will thrive in a fibre based environment.

Even wireless communications need fibre, connecting local low powder cellular or PCStransceivers to the switching matrix.

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CHAPTER 2

PRINTED CIRCUIT BOARDS

In electronics, printed circuit boards, or PCBs, are used to mechanically support and

electrically connect electronic components using conductivepathways, ortraces, etched from

copper sheets laminated onto a non-conductive substrate. Alternative names are printed

wiring board (PWB), and etched wiring board. Populating the board with electronic

components forms a printed circuit assembly (PCA), also known as a printed circuit board

assembly (PCBA). PCBs are rugged, inexpensive, and can be highly reliable. They requiremuch more layout effort and higher initial cost than either wire-wrapped orpoint-to-point

constructed circuits, but are much cheaper, faster, and consistent in high volume production.

2.1 Categories of PCB

2.1.1 Motherboard

The principal board that has connectors for attaching devices to the bus. Typically, the

mother board contains the CPU,memory, and basic controllers for the system. On PCs, the

motherboard is often called the system board or mainboard.

2.1.2 Expansion board

Any board that plugs into one of the computer's expansion slots. Expansion boards include

controller boards, LAN cards, and video adapters.

http://en.wikipedia.org/wiki/Electronicshttp://en.wikipedia.org/wiki/Electronic_componenthttp://en.wikipedia.org/wiki/Conductor_%28material%29http://en.wikipedia.org/wiki/Signal_tracehttp://en.wikipedia.org/wiki/Industrial_etchinghttp://en.wikipedia.org/wiki/Laminatedhttp://en.wikipedia.org/wiki/Printed_Circuit_Board_Assemblyhttp://en.wikipedia.org/wiki/Wire_wraphttp://en.wikipedia.org/wiki/Point-to-point_constructionhttp://en.wikipedia.org/wiki/Point-to-point_constructionhttp://www.webopedia.com/TERM/P/device.htmlhttp://www.webopedia.com/TERM/P/bus.htmlhttp://www.webopedia.com/TERM/P/CPU.htmlhttp://www.webopedia.com/TERM/P/memory.htmlhttp://www.webopedia.com/TERM/P/controller.htmlhttp://www.webopedia.com/TERM/P/system.htmlhttp://www.webopedia.com/TERM/P/PC.htmlhttp://www.webopedia.com/TERM/P/expansion_board.htmlhttp://www.webopedia.com/TERM/P/expansion_board.htmlhttp://www.webopedia.com/TERM/P/expansion_slot.htmlhttp://www.webopedia.com/TERM/P/local_area_network_LAN.htmlhttp://www.webopedia.com/TERM/P/video_adapter.htmlhttp://www.webopedia.com/TERM/P/video_adapter.htmlhttp://www.webopedia.com/TERM/P/local_area_network_LAN.htmlhttp://www.webopedia.com/TERM/P/expansion_slot.htmlhttp://www.webopedia.com/TERM/P/expansion_board.htmlhttp://www.webopedia.com/TERM/P/PC.htmlhttp://www.webopedia.com/TERM/P/system.htmlhttp://www.webopedia.com/TERM/P/controller.htmlhttp://www.webopedia.com/TERM/P/memory.htmlhttp://www.webopedia.com/TERM/P/CPU.htmlhttp://www.webopedia.com/TERM/P/bus.htmlhttp://www.webopedia.com/TERM/P/device.htmlhttp://en.wikipedia.org/wiki/Point-to-point_constructionhttp://en.wikipedia.org/wiki/Point-to-point_constructionhttp://en.wikipedia.org/wiki/Wire_wraphttp://en.wikipedia.org/wiki/Printed_Circuit_Board_Assemblyhttp://en.wikipedia.org/wiki/Laminatedhttp://en.wikipedia.org/wiki/Industrial_etchinghttp://en.wikipedia.org/wiki/Signal_tracehttp://en.wikipedia.org/wiki/Conductor_%28material%29http://en.wikipedia.org/wiki/Electronic_componenthttp://en.wikipedia.org/wiki/Electronics

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Controller board_ A special type of expansion board that contains a controller for a

peripheral device. When you attach new devices, such as a disk drive or graphics monitor, to

a computer, you often need to add a controller board.

Network Interface Card (NIC)_An expansion board that enables a PC to be connected to a

local-area network (LAN).

Video adapter_ An expansion board that contains a controller for a graphics monitor.

2.1.3 Daughter card

Any board that attaches directly to another board. Printed circuit boards are also called cards.

2.2 Manufacturing

2.2.1 Patterning (etching)

The vast majority of printed circuit boards are made by adhering a layer of copper over the

entire substrate, sometimes on both sides, (creating a "blank PCB") then removing unwanted

copper after applying a temporary mask (eg. by etching), leaving only the desired copper

traces. A few PCBs are made by adding traces to the bare substrate (or a substrate with a very

thin layer of copper) usually by a complex process of multiple electroplating steps.

There are three common "subtractive" methods (methods that remove copper) used for the

production of printed circuit boards:

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1. Silk Screen Printing uses etch-resistant inks to protect the copper foil. Subsequentetching removes the unwanted copper. Alternatively, the ink may be conductive,

printed on a blank (non-conductive) board. The latter technique is also used in the

manufacture ofhybrid circuits.

2. Photoengraving uses a photomask and chemical etching to remove the copper foilfrom the substrate. The photomask is usually prepared with a photoplotter from data

produced by a technician using CAM, or computer-aided manufacturing software.

Laser-printed transparencies are typically employed for phototools; however, direct

laser imaging techniques are being employed to replace phototools for high-resolution

requirements.

3. PCB Milling uses a two or three-axis mechanical milling system to mill away thecopper foil from the substrate. A PCB milling machine (referred to as a 'PCB

Prototyper') operates in a similar way to aplotter, receiving commands from the host

software that control the position of the milling head in the x, y, and (if relevant) z

axis. Data to drive the Prototyper is extracted from files generated in PCB design

software and stored in HPGL orGerberfile format.

"Additive" processes also exist. The most common is the "semi-additive process. In this

version, the unpatterned board has a thin layer of copper already on it. A reverse mask is then

applied. (Unlike a subtractive process mask, this mask exposes those parts of the substrate

that will eventually become the traces.) Additional copper is then plated onto the board in the

unmasked areas; copper may be plated to any desired weight. Tin-lead or other surface

platings are then applied. The mask is stripped away and a brief etching step removes the

now-exposed original copper laminate from the board, isolating the individual traces.The

additive process is commonly used for multi-layer boards as it facilitates the plating-through

of the holes (vias) in the circuit board.

http://en.wikipedia.org/wiki/Silk_screenhttp://en.wikipedia.org/wiki/Silk_screenhttp://en.wikipedia.org/wiki/Hybrid_circuithttp://en.wikipedia.org/wiki/Photoengravinghttp://en.wikipedia.org/wiki/Photoengravinghttp://en.wikipedia.org/wiki/Photoplotterhttp://en.wikipedia.org/wiki/Computer-aided_manufacturinghttp://en.wikipedia.org/w/index.php?title=PCB_milling&action=edithttp://en.wikipedia.org/w/index.php?title=PCB_milling&action=edithttp://en.wikipedia.org/wiki/Plotterhttp://en.wikipedia.org/wiki/HPGLhttp://en.wikipedia.org/wiki/Gerber_Filehttp://en.wikipedia.org/wiki/Gerber_Filehttp://en.wikipedia.org/wiki/HPGLhttp://en.wikipedia.org/wiki/Plotterhttp://en.wikipedia.org/w/index.php?title=PCB_milling&action=edithttp://en.wikipedia.org/wiki/Computer-aided_manufacturinghttp://en.wikipedia.org/wiki/Photoplotterhttp://en.wikipedia.org/wiki/Photoengravinghttp://en.wikipedia.org/wiki/Hybrid_circuithttp://en.wikipedia.org/wiki/Silk_screen

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2.2.2 Lamination

Some PCBs have trace layers inside the PCB and are called multi-layer PCBs. These are

formed by bonding together separately etched thin boards.

2.2.3 Drilling

Holes, or vias, through a PCB are typically drilled with tiny drill bits made of solid tungsten

carbide. The drilling is performed by automated drilling machines with placement controlled

by a drill tape or drill file. These computer-generated files are also called numerically

controlled drill (NCD) files or"Excellon files". The drill file describes the location and size

of each drilled hole. When very small vias are required, drilling with mechanical bits is costly

because of high rates of wear and breakage. In this case, the vias may be evaporated by

lasers. Laser-drilled vias typically have an inferior surface finish inside the hole. These holes

are called micro vias. It is also possible with controlled-depth drilling, laser drilling, or by

pre-drilling the individual sheets of the PCB before lamination, to produce holes that connect

only some of the copper layers, rather than passing through the entire board. These holes are

called blind vias when they connect an internal copper layer to an outer layer, or buried vias

when they connect two or more internal copper layers.

The walls of the holes, for boards with 2 or more layers, are plated with copper to form

plated-through holes that electrically connect the conducting layers of the PCB. For

multilayer boards, those with 4 layers or more, drilling typically produces a smear comprised

of the bonding agent in the laminate system. Before the holes can be plated through, this

smear must be removed by a chemical de-smear process, or by plasma-etch.

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2.2.4 Exposed Conductor Plating and Coating

The pads and lands to which components will be mounted are typically plated, because bare

copper oxidizes quickly, and therefore is not readily solderable. Traditionally, any exposed

copper was plated with solder. This solder was a tin-lead alloy, however new solder

compounds are now used to achieve compliance with the RoHS directive in the EU, which

restricts the use of lead. Other platings used are OSP (organic surface protectant), immersion

silver, electroless nickel with immersion gold coating (ENIG), and direct gold. Edge

connectors, placed along one edge of some boards, are often gold plated.

2.2.5 Solder resist

Areas that should not be soldered to may be covered with a polymer solder resist (solder

mask) coating. The solder resist prevents solder from bridging between conductors and

thereby creating short circuits. Solder resist also provides some protection from the

environment.

2.2.6 Screen printing

Line art and text may be printed onto the outer surfaces of a PCB by screen printing. When

space permits, the screen print text can indicate component designators, switch setting

requirements, test points, and other features helpful in assembling, testing, and servicing the

circuit board.Screen print is also known as the silk screen, or, in one sided PCBs, the red

print.

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2.2.7 Test

Unpopulated boards may be subjected to a bare-board test where each circuit connection (as

defined in a netlist) is verified as correct on the finished board. For high-volume production,

a Bed of nails testeror fixture is used to make contact with copper lands or holes on one or

both sides of the board to facilitate testing. A computer will instruct the electrical test unit to

send a small amount of current through each contact point on the bed-of-nails as required,

and verify that such current can be seen on the other appropriate contact points. For small- or

medium-volume boards, flying-probe testers use moving test heads to make contact with the

copper lands or holes to verify the electrical connectivity of the board under test.

2.2.8 Populating

After the PCB is completed, electronic components must be attached to form a functional

printed circuit assembly, or PCA. In through-hole construction, component leads may be

inserted in holes and electrically and mechanically fixed to the board with a molten metal

solder, while in surface-mount construction, the components are simply soldered to pads or

lands on the outer surfaces of the PCB.Often, through-hole and surface-mount construction

must be combined in a single PCA because some required components are available only in

surface-mount packages, while others are available only in through-hole packages.

Again, JEDEC guidelines for PCB component placement, soldering, and inspection are

commonly used to maintain quality control in this stage of PCB manufacturing. After the

board is populated, the populated board may be tested with an in-circuit test system. To

facilitate this test, PCBs may be designed with extra pads to make temporary connections.

Sometimes these pads must be isolated with resistors. The in-circuit test may also exercise

boundary scan test features of some components. In-circuit test systems may also be used to

program nonvolatile memory components on the board. In boundary scan testing, test circuits

integrated into various ICs on the board form temporary connections between the pcb traces

to test that the ICs are mounted correctly. Boundary scan testing requires that all the ICs to be

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tested use a standard test configuration procedure, the most common one being the Joint Test

Action Group (JTAG) standard.

2.2.9 Protection and packaging

PCBs intended for extreme environments often have a conformal coat, which is applied by

dipping or spraying after the components have been soldered. The coat prevents corrosionand leakage currents or shorting due to condensation. The earliest conformal coats were wax.

Modern conformal coats are usually dips of dilute solutions of silicone rubber, polyurethane,

acrylic, or epoxy. Some are engineering plastics sputtered onto the PCB in a vacuum

chamber. Many assembled PCBs are static sensitive, and therefore must be placed in

antistatic bags during transport. When handling these boards, the user must be earthed; failure

to do this might transmit an accumulated static charge through the board, damaging or

destroying it. Even bare boards are sometimes static sensitive. Traces have gotten so fine that

it's quite possible to blow an etch off the board (or change its characteristics) with a static

charge. This is especially true on non-traditional PCBs such as MCMs and microwave PCBs.

2.3 STEPS FOR MAKING PCB

Prepare the layout of the circuit (positive). Cut the photofilm (slightly bigger) of the size of the layout. Place the layout in the photoprinter machine with the photofilm above it. Make sure

that the bromide (dark) side of the film is in contact with the layout.

Switch on the machine by pressing the push button for 5 sec.

http://en.wikipedia.org/wiki/JTAGhttp://en.wikipedia.org/wiki/Waxhttp://en.wikipedia.org/wiki/Electrostatic_dischargehttp://en.wikipedia.org/wiki/Antistatic_baghttp://en.wikipedia.org/wiki/Ground_%28electricity%29http://en.wikipedia.org/wiki/Multi-Chip_Modulehttp://en.wikipedia.org/wiki/Microwavehttp://en.wikipedia.org/wiki/Microwavehttp://en.wikipedia.org/wiki/Multi-Chip_Modulehttp://en.wikipedia.org/wiki/Ground_%28electricity%29http://en.wikipedia.org/wiki/Antistatic_baghttp://en.wikipedia.org/wiki/Electrostatic_dischargehttp://en.wikipedia.org/wiki/Waxhttp://en.wikipedia.org/wiki/JTAG

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Dip the film in the solution prepared (developer) by mixing the chemicals A & B inequal quantities in water.

Now clean the film by placing it in the tray containing water for 1 min. After this, dip the film in the fixer solution for 1 min. now the negative of the Circuit is ready.

Now wash it under the flowing water. Dry the negative in the photocure machine. Take the PCB board of the size of the layout and clean it with steel wool to make the

surface smooth.

Now dip the PCB in the liquid photoresist, with the help of dip coat machine. Now clip the PCB next to the negative in the photo cure machine, drying for

approximate 10-12 minute.

Now place the negative on the top of the PCB in the UV machine, set the timer forabout 2.5 minute and switch on the UV light at the top.

Take the LPR developer in a container and rigorously move the PCB in it. After this, wash it with water very gently. Then apply LPR dye on it with the help of a dropper so that it is completely covered

by it.

Now clamp the PCB in the etching machine that contains ferric chloride solution forabout 10 minutes.

After etching, wash the PCB with water, wipe it a dry cloth softly.

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Finally rub the PCB with a steel wool, and the PCB is ready.

2.4 PCB Layout

Figure 2.1 a original view of PCB

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Figure 2.1 b with componentslocation on PCB

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CHAPTER 3

FUNDAMENTAL PARTS AND OPERATION

To establish communication through fiber optics the three main sections are transmitter

section,receiver section and optical fiber.Besides these, we also require power supply and

repeaters.In this chapter we will see each of these sections in detail along with their working.

3.1 Transmitter

Fiber optic transmitters are typically composed of a buffer, driver, and optical source.

Often, optical connectors are also integrated into the final package. The buffer

electronics provide both an electrical connection and isolation between the driver

electronics and the electrical system supplying the data. The driver electronics provide

electrical power to the optical source in a fashion that duplicates the pattern of data

being fed to the transmitter. Finally, the optical source (LED in this kit) converts the

electrical power to light energy with the same pattern.

3.2 Receiver

Once light energy from the fiber optic transmitter reaches the destination (receiver) it must be

converted back to a form of electrical energy with the same information pattern that was fed

to the transmitter by the person sending the message. Analog fiber optic receivers typically

perform these functions using three elements: a photo detector, an amplifier and sometimes a

buffer. As with fiber optic transmitter, an amplifier and sometimes a buffer. As with fiber

optic transmitters, the optical connector is often integrated into the receiver package. The

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photo detector converts light energy (optical power) to an electrical current. Any pattern or

modulation imparted in the optical power (from, for instance, a fiber optic transmitter) will be

reproduced as an electric current with the same pattern. Long lengths of fibers and other

distribution losses can reduce the optical power, resulting in a comparatively weak electrical

signal from the photo detector. To compensate of the electrical signal. Finally, bufferelectronics isolate the photo detector and amplifier from any load the receiver is required to

drive.

3.3 Circuit Operation

Most fiber optic transmitters typically have and amplifier or buffer, driver, optical source, and

sometimes an optical connector or interface. The transmitter in this circuit has an acoustic

microphone for converting sound waves to an electrical signal, and requires a nine-volt

battery with holder to provide electrical power.

Pushing the momentary-close switch, SW1, activates the optical voice link by applying 9 volt

battery power to the indicator light, microphone, audio circuits and fiber optic LED. The

switch must be closed for the transmitter to operate by generating light to carry audio signals.

In the diagram are the receiver circuit functions: photo detector, amplifier, adjustable volumecontrol and miscellaneous electronics. Following a signal as it exits from the optical fiber, in

the form of light. The circuit is basically an amplifier. The IR signals are picked up by the

photodiode and converted into electrical variations which are amplified by the op-amp

(operational Amplifier) IC-741 used in the inverting mode with a single supply using divider

network of resistors. The gain of IC1 can be set by varying the feedback through VR1

resistance (can place a 2.2. M variable). Here the output of IC is further amplified by the push

pull amplifier using transistors BEL368/69 pair. The output of the amplifier is taken from

emitter of two transistors, with a filter C2 from speaker.

3.4 Advantages of Fiber Optics System

Fiber optic transmission systems a fiber optic transmitter and receiver, connected by fiberoptic cableoffer a wide range of benefits not offered by traditional copper wire or coaxial

cable. These include:

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1. The ability to carry much more information and deliver it with greater fidelity than

either copper wire or coaxial cable.

2. Fiber optic cable can support much higher data rates, and at greater distances, thancoaxial cable, making it ideal for transmission of serial digital data.

3. The fiber is totally immune to virtually all kinds of interference, including lightning,

and will not conduct electricity. It can therefore come in direct contact with high

voltage electrical equipment and power lines. It will also not create ground loops of

any kind.

4. As the basic fiber is made of glass, it will not corrode and is unaffected by most

chemicals. It can be buried directly in most kinds of soil or exposed to most corrosive

atmospheres in chemical plants without significant concern.

5. Since the only carrier in the fiber is light, there is no possibility of a spark from a

broken fiber. Even in the most explosive of atmospheres, there is no fire hazard, and

no danger of electrical shock to personnel repairing broken fibers.

6. Fiber optic cables are virtually unaffected by outdoor atmospheric conditions,

allowing them to be lashed directly to telephone poles or existing electrical cables

without concern for extraneous signal pickup.

7. A fiber optic cable, even one that contains many fibers, is usually much smaller andlighter in weight than a wire or coaxial cable with similar information carrying

capacity. It is easier to handle and install, and uses less duct space. (It can frequently

be installed without ducts.)

8. Fiber optic cable is ideal for secure communications systems because it is very

difficult to tap but very easy to monitor. In addition, there is absolutely no electrical

radiation from a fiber.

3.5 Optical Fiber Communication

The birth of optical communications occurred in the 1970s with two key technologybreakthroughs. The first was the invention of the semiconductor laser in 1962. The laser

generates a tightly focused beam of light at a single pure wavelength, a spot small enough to

be connected to fiber optics. The second breakthrough happened in September 1970, when aglass fiber with an attenuation of less than 20 dB/km was developed. In the 1960s, glass-clad

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fibers had an attenuation of about 1 dB/m, which was sufficient for medical imaging

applications, but was too high for telecommunications. With the development of optical

fibers with an attenuation of 20 dB/km, the threshold to make fiber optics a viable technology

for telecommunications was crossed. In 1977, AT&T installed the first optical fiber cables in

Chicago. The first field deployments of fiber communication systems used Multimode Fibers(MMFs) with lasers operating in the 850 nm wavelength band. These systems could transmit

several kilometers with optical losses in the range of 2 to 3 dB/km. A second generation of

lasers operating at 1310 nm enabled transmission in the second window of the optical fiberwhere the optical loss is about 0.5 dB/km in a Single-Mode-Fiber (SMF). In the 1980s, thetelecom carriers started replacing all their MMFs operating at 850 nm. Another wavelength

window around 1550 nm was developed where a standard SMF has its minimum optical loss

of about 0.22 dB/km. The development of fiber-based telecommunication systems in the

1990s focused on increasing their transmission capacity. This was done first by increasing

the signal modulation speed from 155 Mb/s to 622 Mb/s, to 2.5 Gps, and finally to 10 Gb/s,todays modulation speed. The total available bandwidth of standard optical fibers isenormous; it is about 20 THz. Since it is impossible for a single-wavelength laser to utilize

this enormous bandwidth, multiple single-wavelength laser transmitters are typically

multiplexed and transmitted on a single fiber. This scheme, which was developed in the mid

1990s, is called Wavelength-Division-Multiplexing (WDM). Dense WDM (DWDM) opticalcommunication systems with more than 60 wavelengths, where each wavelength carries 40

Gb/s data, have been demonstrated. Thus, the demonstrated total transmission capacity of an

SMF is more than 2.5 Tb/s.

Today, MMFs operating at 850 nm are primarily used for short distances in the enterprise as

the least expensive method. An SMF at the 1310 nm wavelength band is primarily used for

medium distances ranging from 2 km to 40 km. For long-haul telecommunications, WDM

systems operating in the 1550 nm wavelength band windows are deployed. From 850 nm to

long wavelength and WDM, higher performance is being offered, but each one comes with a

higher price tag. Nowadays, the entire telecom infrastructure is fiber-based with the exception

of the famous last miles to homes, which is still based on coaxial cables and copper-twisted

pairs. Inside enterprise networks, fiber has been deployed since the early 1980s initially withsupercomputers, and later in Local Area Networks (LANs) as well as more recently in

Storage Area Networks (SANs). With continuously increasing demands for high-speed data,

optical fibers and interconnects will continue to play an increasing role within the enterprise

network.

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3.6 The Fundamentals of Optical Components

Diode (LD) and the Light Emitting Diode (LEDs). All light emitters that convert electrical

current into light are semiconductor based. They operate with the principle of the p-n

semiconductor junction found in transistors. Historically, the first achievement of laser action

in A basic optical communication link consists of three key building blocks: optical fiber,

light sources, and light detectors. We discuss each one in turn.

3.6.1 Optical Fibers

In 1966, Charles Kao and George Hockmam predicted that purified glass loss could be

reduced to below 20 dB per kilometer, and they set up a world-wide race to beat this

prediction. In September 1970, Robert Maurer, Donald Keck, and Peter Schultz of Corning

succeeded in developing a glass fiber with attenuation less than 20 dB/km: this was the

necessary threshold to make fiber optics a viable transmission technology. The silica-based

optical fiber structure consists of a cladding layer with a lower refractive index than the fiber

core it surrounds. This refractive index difference causes a total internal reflection, whichguides the propagating light through the fiber core. There are many types of optical fibers

with different size cores and cladding. Some optical fibers are not even glass-based such as

Plastic Optical Fibers (POFs), which are made for short-distance communication. For

telecommunications, the fiber is glass based with two main categories: SMF and MMF.

SMFs typically have a core diameter of about 9 m while MMFs typically have a core

diameter ranging from 50 to 62.5 m. Optical fibers have two primary types of impairment,

optical attenuation and dispersion. The fiber optical attenuation, which is mainly caused by

absorption and the intrinsic Rayleigh scattering, is a wavelength-dependent loss with optical

losses as low as 0.2 dB/km around 1550 nm for conventional SMF (i.e., SMF-28*).

The optical fiber is a dispersive waveguide. The dispersion results in Inter Symbol

Interference (ISI) at the receiver. There are three primary types of fiber dispersions: modal

dispersion, chromatic dispersion, and polarization-mode dispersion. The fiber modal

dispersion depends on both the fiber core diameter and transmitted wavelengths. For a single-

mode transmission, the step-index fiber core diameter (D) must satisfy the following

condition:

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where is the transmitted wavelength and n1 and n2 are the refractive indices of fiber coreand cladding layer, respectively. Consequently, for a single-mode operation at 850 nm

wavelength, the fiber must have a core diameter of 5 m. Since a conventional SMF has

typically a core diameter of 9 m, single-mode operation can be only supported for

wavelengths in the 1310 nm wavelength band or longer.

The fiber chromatic dispersion is due to the wavelength-dependent refractive index with a

zero-dispersion wavelength occurring at 1310 nm in conventional SMF. At 1550 nm, the

fiber dispersion is about 17 ps/nm/km for SMF-28. When short duration optical pulses are

launched into the fiber, they tend to broaden since different wavelengths propagate atdifferent group velocities, due to the spectral width of the emitter. Optical transmission

systems operating at rates of 10 Gb/s or higher and distances above 40 km are sensitive to

this phenomenon. There are other types of SMFs such as Dispersion Shifted Fibers (DSFs)

where the zero dispersion occurs at 1550 nm.

Polarization-Mode Dispersion (PMD) is caused by small amounts of asymmetry and stress in

the fiber core due to the manufacturing process and environmental changes such as

temperature and strains. This fiber core asymmetry and stress leads to a polarization-

dependent index of refraction and propagation constant, thus limiting the transmissiondistance of high speed ( 10 Gb/s) over SMF in optical communication systems. StandardSMF has a PMD value of less than 0.1 ps/km. Special SMFs were developed to address thisissue.

Optical fiber is never bare. The fiber manufacturer coats the fiber with a thin primary coating;

then a cable manufacturer, not necessarily the fiber manufacturer, cables the fiber. There is a

wide variety of cable construction. Simplex cable has a single fiber in the center while duplex

cables contain two fibers. Composite cable incorporates both single-mode and multimode

fiber. Hybrid cables incorporate mixed optical fiber and copper cable. In the enterprise, the

MMF is housed in a cable with an orange colored jacket, and the SMF is housed in a yellow

jacket cable.

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3.6.2 Light Sources

The light source is often the most costly element of an optical communication system. It has

the following key characteristics: (a) peak wavelength, at which the source emits most of itsoptical power, (b) spectral width, (c) output power, (d) threshold current, (e) light vs. current

linearity, (f) and a spectral emission pattern. These characteristics are key to system

performance.

There are two types of light sources in widespread use: the Laser GaAs p-n junction was

reported in 1962 by three groups. Both LEDs and LDs use the same key materials: Gallium

Aluminum Arsenide (GaAIAs) for short-wavelength devices and Indium Gallium Arsenide

Phosphide (InGaAsP) for long-wavelength devices.

Semiconductor laser diode structures can be divided into the so-called edge-emitters, such as

Fabry Perot (FP) and Distributed Feedback (DFB) lasers and vertical-emitters, such as

Vertical Surface Emitting Lasers (VCSELs). When edge-emitters are used in optical fiber

communication systems, they incorporate a rear facet photodiode to provide a means to

monitor the laser output, as this output varies with temperature.

In todays optical networks, binary digital modulation is typically used, namely on (i.e., lighton) and off (no light) to transmit data. These semiconductor laser devices generate output

light intensity which is proportional to the current applied to them, therefore making them

suitable for modulation to transmit data. Speed and linearity are therefore two importantcharacteristics.

Modulation schemes can be divided into two main categories, namely, a direct and an

external modulation. In a direct modulation scheme, modulation of the input current to the

semiconductor laser directly modulates its output optical signal since the output optical power

is proportional to the drive current. In an external modulation scheme, the semiconductor

laser is operating in a Continuous-Wave (CW) mode at a fixed operating point. An electrical

drive signal is applied to an optical modulator, which is external to the laser. Consequently,

the applied drive signal modulates the laser output light on and off without affecting the laser

operation.

One important feature of the laser diode is its frequency chirp. The frequency of the output

laser light changes dynamically in response to the changes in the modulation current. A

typical DFB has a frequency chirp of about 100-MHz/mA. This spread of the wavelength

interacts with the fiber dispersion. As previously mentioned, as the data rate is increased, this

interaction limits the transmission distance of optical transmission systems due to the

additional ISI generated at the receiver.

Optical back-reflection is one of key issues when coupling the output light from a lasersource to a fiber. The optical back-reflection disturbs the standing wave in the laser cavity,

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increasing its noise floor, and thus making the laser unstable. One practical way to reduce the

phenomenon of back-reflection is to place an isolator between the laser cavity and the fiber,

which adds a significant additional cost to the laser [1, 4]. Temperature also affects the peak

wavelength of the laser; threshold current also increases with temperature as slope efficiency

decreases. For DWDM applications, which require very precise operating wavelengths, mostof the current laser diode designs need to be cooled to within 0.3 C.

As previously explained, the direct modulation of a laser diode has several limitations,

including limited propagation distance due to the interaction between the laser frequency

chirp and fiber dispersion. This is not an issue for enterprise networks which are short

distance, but could be a serious limiting factor for telecommunications applications. To

overcome this limitation, the laser diode is operated in a CW mode, and output light is

externally modulated by an optical modulator. Intensity modulators can be divided into two

main groups: Mach-Zehnder Interferometer (MZI) and Electro-Absorption (EA) modulators.

In an MZI modulator, a single input waveguide is split into two optical waveguides by a 3 dB

Y junction and then recombined by a second 3 dB Y junction into a single output. A Radio

Frequency (RF) signal, which is applied to a pair of electrodes constructed along the

waveguides, modulates the propagating optical beam. The modulator key parameters are its

modulation bandwidth, linearity, and the required drive signal voltage for phase shift. MZImodulators based on LiNbO3 are high-performance modulators with a large form-factor

(about 2.5 inches) that are not suitable for optical integration. EA modulators are based on a

voltage-induced shift of the semiconductor bandgap so that the modulator becomes absorbing

for the lasing wavelength. The advantages of an EA modulator are its low driving voltage,

high-speed operation, and suitability for optical integration with InP-based laser diodes.

A tunable laser is a new type of laser where its main lasing longitudinal mode can be tuned

over a wide range of wavelengths such as the C band (1510 -1540 nm) of an Erbium-Doped

Fiber Amplifier (EDFA), which is commonly used for DWDM systems.

The use of tunable lasers is driven by the potential cost savings in DWDM transport networks

since a significantly reduced inventory of fixed-wavelength lasers could be maintained for a

robust network operation. The technical challenges are to provide both broad wavelength

tunability and excellent wavelength accuracy over the laser life. A broadly tunable External

Cavity Laser (ECL) employing micromachined, thermally tuned silicon etalons has beendesigned to achieve these goals.

3.6.3Light Detectors

Light detectors convert an optical signal to an electrical signal. The most common lightdetector is a photodiode. It operates on the principle of the p-n junction. There are two main

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categories of photodetectors: a p-i-n (positive, intrinsic, negative) photodiode and an

Avalanche Photodiode (APD), which are typically made of InGaAs or germanium. The key

parameters for photodiodes are (a) capacitance, (b) response time, (c) linearity, (d) noise, and

(e) responsivity. The theoretical responsivity is 1.05 A/W at a wavelength of 1310 nm.

Commercial photodiodes have responsivity around 0.8 to 0.9 A/W at the same wavelength[1-4]. The dark photo-current is a small current that flows through the photo-detector even

though no light is present because of the intrinsic resistance of the photo-detector and the

applied reverse voltage. It is temperature sensitive and contributes to noise. Since the output

electrical current of a photodiode is typically in the range of A, a Transimpedance Amplifier

(TIA) is needed to amplify the electric current to a few mA.

APDs provide much more gain than the pin photodiodes, but they are much more expensive

and require a high voltage power to supply their operation. APDs are also more temperature

sensitive than pin photodiodes.

3.7 Packaging: Optical Sub Assembly (OSA) and Optical Transceivers

As previously described, laser diodes and photodiodes are semiconductor devices. To enable

the reliable operation of these devices, an optical package is required. In general, there aremany discrete optical and electronic components, which are based on different technologies

that must be optically aligned and integrated within the optical package. Optical packaging of

laser diodes and photodiodes is the primary cost driver. These packages are sometimes called

Optical Sub-Assemblies (OSAs). The Transmitter OSA package is called a TOSA and the

Receiver OSA package is called a ROSA.

Figure 3.1 shows, for example, a three-dimensional schematic view of a DFB laser diode

mounted on a Thermo-Electric Cooler (TEC) inside a hermetically sealed 14-pin butterfly

package with an SMF pigtail. Most of the telecom-grade laser diodes are available in the so-

called TO can or butterfly packages. The standard butterfly package is a stable and high-

performance package, but it has a relatively large form-factor and it is costly to manufacture.

These packages are typically used for applications where cooling is required using a TEC.

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Figure 3.1: Three-dimensional view of a DFB laser diode configuration with single-mode fiber pigtail

The TEC requires a large amount of power to regulate the temperature of a laser inside the

package. This type of optical packaging was used for the early 10 Gb/s modules. More

recently, tunable 10 Gb/s lasers are using a similar butterfly optical package. The butterfly

package design uses a coaxial interface for passing broadband data into the package, which

requires the use of a coaxial interface to the host Printed Circuit Board (PCB). Although

coaxial cables and connectors have been reduced in size, they still consume valuable real

estate in the optical transceiver.

The evolution of optical module packages is toward smaller footprint packages. If relatively

easy for receivers, the trend toward smaller packages is particularly challenging for laser

transmitter modules due to the power and thermal dissipation constraints. Figure 3.2 shows

the evolution of 10 Gb/s optical module packaging technology. To operate with high-

performance, uncooled designs must be implemented with more advanced control systems

that can adjust the laser and driver parameters over temperature. The smaller packages utilize

a coplanar approach to the broadband interface, which more closely resembles a surface-

mount component and enables much smaller RF interfaces.

TO-can-based designs, which have been used extensively in lower data rate telecom anddatacom systems up to 2 Gb/s as well as CD players and other high-volume consumer

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applications, are now maturing to support high-performance 10 Gb/s optical links.

Leveraging the fact that these packages are already produced in high volume will further

reduce the cost of the 10 Gb/s optical modules in optical transceiver designs.

Figure 3.2: Trends in 10 Gb/s optical transmitter packaging technology. To decrease size and power dissipation, the trends are from cooled

to uncooled packages, from coaxial to planar RF interfaces, and from pigtailed to pluggable optical interfaces.

The current TOSA/ROSA package form factors are trending toward smaller packages, and it

will not end at the 10 Gb/s TO-can implementation. These TOSA/ROSA form factors are still

too large and too expensive to compete in the market segment where today copper

interconnects dominate. Leadframe-type packages could be an attractive choice for high-

speed optical modules since similar packages are already in use in the semiconductor industry

[10]. Using insert molded or pre-molded thermoplastic housing, different optical components

can be passively aligned in a fully automated manufacturing process. For example, integrated

modules with VCSEL and photodiodes in leadframe packages have been developed for the

automobile industry, but are limited today in the 20 Mb/s bit rate. Additional research and

development is needed to define the packaging specifications for optoelectronic modules

based on size and cost.

3.7.1Optical Transceivers

For telecommunication applications, the optical transmitter and receiver modules are usually

packaged into a single package called an optical transceiver. Figure 3 shows an example of

different transceivers and Figure 4 shows an example of the printed circuit board of a

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transceiver. There are several form factors for this optical transceiver depending on their

operating speed and applications. The industry worked on a Multi-Source Agreement (MSA)

document to define the properties of the optical transceivers in terms of their mechanical,

optical, and electrical specifications. Optical transponders operating at 10 Gb/s, based on

MSA, have been in the market since circa 2000, beginning with the 300-pin MSA, followedby XENPAK, XPAK, X2, and XFP. Table 1 summarizes the key MSA specifications for the

different form-factor 10 Gb/s optical transceivers and their release dates.

The Most Popular Form-Factor Transceivers in the Enterprise_ For the 1/2/4 Gb/s

transceivers, the Small Form-Factors (SFFs) and the small Form-Factor Pluggables (SFPs)

are the most recently developed and the ones that are finding new sockets into systems. It

should be noted, however, that the older GBIC form factors for 1 Gb/s Ethernet (GbE),

despite no new development, is still shipping in large volumes due to the large installed base

of this design. The SFF transceiver is used in a Network Interface Card (NIC) for the LAN or

in the Host Bus Adaptor (HBA) in SANs. The SFP transceiver is typically used for enterprise

switches such as Ethernet or Fiber-Channel (FC) switches. In these high-capacity switches,

switching is done by electrical ICs while the optical transceivers provide optical-to-electrical

electrical (O-E) or electrical-to-optical (E-O) conversion.

Figure 3.3 Next-generation 10 Gb/s enterprise optical transceivers: (from left) XFP, XPAK/X2, XENPAK. These modules are electrically

hot-pluggable and optically pluggable

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In general, not all the switches ports are populated with transceivers when they are shippedto customers. The customer has the option to buy these transceiver modules as the demand

for ports increases. It also gives the customer the choice of optics: MMF or SMF. Therefore,

these modules have been designed to be pluggable.

The choice between the different 10 Gb/s form-factor optical transceiver packages is guided

by reach, cost, and thermal and size constraints and requirements.

Table 3.1: Summary of dif ferent form-factor 10 Gb/s optical tr ansceiver packages

MSA XENPAK XPAK/X2 XFP

MSA Date March 2002 March 2003 April 2003

Application Enterprise switch Enterprise switch NIC

Storage

Telecom

Datacom

Electrical

Interface

4 bit XAUI 4 bit XAUI 1 bit XFI

Optical interface SC pluggable SC or LC

pluggable

LC pluggable

Dimension 4.8x1.4x0.7 2.7x1.4x0.4

Max Power 11W 5W 3W

3.8 Photonics:A Key Future Prospect

Photonics is the technology associated with generating and harnessing light. One of the most

important applications of photonics is the transmission of information as light through optical

fibers. Photonics and optical communications should be seen as the principle enabling

technology for developing broad-band communication systems, because optical fibers can

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achieve data transmission at speeds far in excess of what can be obtained with conventional

electronic communication systems.

Some of the main projects currently being undertaken within the group are as follows:

3.8.1 Hybrid Radio/Fibre Systems for Broadband Access

As the demand for broadband mobile services such as mobile computing increases, so does

the need to develop high capacity mobile communication networks. High capacity mobile

networks of the future are likely to use microwave radio, operating at frequencies from

10GHz up to 100GHz as the access medium. To develop wireless networks of this type, it is

anticipated that optical fibre will be used for distributing the microwave data from a central

station to remote base stations. The goal of this project is to develop and test a complete radio

over fibre communication system.

3.8.2 Optical Time Division Multiplexing Technologies

Limitations on the overall capacity of optical communication systems will be encountered in

the near future due to rapid growth in demand for broadband services. The main approaches

to increase the capacity of optical fibre are Wavelength Division Multiplexing (WDM) and

Optical Time Division Multiplexing (OTDM). The main aim of this project has been to

develop high-speed optical transmitter and receiver circuits that may be employed for data

transmission in the high capacity optical communication systems of the future, and to

examine the development of optical systems using hybrid WDM/OTDM technologies.

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CHAPTER 4

COMPONENTSDESCRIPTION

Various components are used to establish communication through optical fiber.The

components such as resistors,capacitors,transistors,LEDs and speaker are explained in detail

in this chapter.

4.1 Capacitors

A capacitor or condenser is a passive electronic component consisting of a pair ofconductors separated by a dielectric (insulator). When apotential difference (voltage) exists

across the conductors, an electric field is present in the dielectric. This field stores energy andproduces a mechanical force between the conductors. The effect is greatest when there is anarrow separation between large areas of conductor, hence capacitor conductors are oftencalled plates.

An ideal capacitor is characterized by a single constant value, capacitance, which is measuredin farads. This is the ratio of the electric charge on each conductor to the potential difference

between them. In practice, the dielectric between the plates passes a small amount ofleakagecurrent. The conductors and leads introduce an equivalent series resistance and the dielectrichas an electric field strength limit resulting in a breakdown voltage.

Capacitors are widely used in electronic circuits to block the flow of direct current whileallowing alternating current to pass, to filter out interference, to smooth the output ofpowersupplies, and for many other purposes. They are used in resonant circuits in radio frequencyequipment to select particularfrequencies from a signal with many frequencies.

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Figure 4.1 Types of Capacitors

4.1.1Theory of operation

Figure 4.2 Working of Cpacitor

Charge separation in a parallel-plate capacitor causes an internal electric field. A dielectric

(orange) reduces the field and increases the capacitance.

Figure 4.3 A Practical Approach

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A capacitor consists of two conductors separated by a non-conductive region.The non-conductive substance is called the dielectric medium, although this may also mean a vacuumor a semiconductor depletion region chemically identical to the conductors. A capacitor isassumed to be self-contained and isolated, with no net electric charge and no influence froman external electric field. The conductors thus contain equal and opposite charges on their

facing surfaces, and the dielectric contains an electric field. The capacitor is a reasonablygeneral model for electric fields within electric circuits.

An ideal capacitor is wholly characterized by a constant capacitance C, defined as the ratio ofcharge Q on each conductor to the voltage Vbetween them

Sometimes charge buildup affects the mechanics of the capacitor, causing the capacitance to

vary. In this case, capacitance is defined in terms of incremental changes:

In SI units, a capacitance of one farad means that one coulomb of charge on each conductorcauses a voltage of one volt across the device.

4.1.2 Energy storage

Work must be done by an external influence to move charge between the conductors in acapacitor. When the external influence is removed, the charge separation persists and energyis stored in the electric field. If charge is later allowed to return to its equilibriumposition, theenergy is released. The work done in establishing the electric field, and hence the amount ofenergy stored, is given by:

4.1.3Current-voltage relation

The current i(t) through a component in an electric circuit is defined as the rate of change ofthe charge q(t) that has passed through it. Physical charges cannot pass through the dielectriclayer of a capacitor, but rather build up in equal and opposite quantities on the electrodes: aseach electron accumulates on the negative plate, one leaves the positive plate. Thus theaccumulated charge on the electrodes is equal to the integral of the current, as well as being

proportional to the voltage (as discussed above). As with any antiderivative, a constant of

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integration is added to represent the initial voltage v (t0). This is the integral form of thecapacitor equation,

.

Taking the derivative of this, and multiplying by C, yields the derivative form,

.

The dual of the capacitor is the inductor, which stores energy in the magnetic field rather thanthe electric field. Its current-voltage relation is obtained by exchanging current and voltage inthe capacitor equations and replacing Cwith the inductanceL.

4.1.4 DC circuits

Figure 4.5 A dc Circuit

A simple resistor-capacitor circuit demonstrates charging of a capacitor.

A series circuit containing only a resistor, a capacitor, a switch and a constant DC source ofvoltage V0 is known as a charging circuit.

If the capacitor is initially uncharged while the

switch is open, and the switch is closed at t= 0, it follows from Kirchhoff's voltage law that

Taking the derivative and multiplying by C, gives a first-order differential equation,

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At t= 0, the voltage across the capacitor is zero and the voltage across the resistor isV0. The initial current is then i (0) =V0 /R. With this assumption, the differential equationyields

where 0 =RCis thetime constantof the system.

As the capacitor reaches equilibrium with the source voltage, the voltage across the resistorand the current through the entire circuit decay exponentially. The case of discharging acharged capacitor likewise demonstrates exponential decay, but with the initial capacitorvoltage replacing V0 and the final voltage being zero.

4.2 Resistor

Resistors are used to limit the value of current in a circuit. Resistors offer opposition to theflow of current. They are expressed in ohms for which the symbol is . Resistors are

broadly classified as

(1) Fixed Resistors(2) Variable Resistors4.2.1 Fixed Resistors

The most common of low wattage, fixed type resistors is the molded-carbon composition

resistor. The resistive material is of carbon clay composition. The leads are made of tinned

copper. Resistors of this type are readily available in value ranging from few ohms to about

20M, having a tolerance range of 5 to 20%. They are quite inexpensive. The relative size of

all fixed resistors changes with the wattage rating.

Another variety of carbon composition resistors is the metalized type. It is made by

deposition a homogeneous film of pure carbon over a glass, ceramic or other insulating core.

This type of film-resistor is sometimes called the precision type, since it can be obtained with

an accuracy of 1%.

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Lead Tinned Copper Material

Colour Coding Molded Carbon Clay Composition

Figure 4.6 Fixed Resistor

A Wire Wound Resistor _It uses a length of resistance wire, such as nichrome. This wire is

wounded on to a round hollow porcelain core. The ends of the winding are attached to these

metal pieces inserted in the core. Tinned copper wire leads are attached to these metal pieces.

This assembly is coated with an enamel coating powdered glass. This coating is very smooth

and gives mechanical protection to winding. Commonly available wire wound resistors have

resistance values ranging from 1 to 100K, and wattage rating up to about 200W.

Coding Of Resistor _Some resistors are large enough in size to have their resistance printed

on the body. However there are some resistors that are too small in size to have numbers

printed on them. Therefore, a system of colour coding is used to indicate their values. For

fixed, moulded composition resistor four colour bands are printed on one end of the outer

casing. The colour bands are always read left to right from the end that has the bands closest

to it. The first and second band represents the first and second significant digits, of the

resistance value. The third band is for the number of zeros that follow the second digit. In

case the third band is gold or silver, it represents a multiplying factor of 0.1to 0.01. The

fourth band represents the manufactures tolerance.

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RESISTOR COLOUR CHART

Figure 4.7 Colour Coding of Resistors

For example, if a resistor has a colour band sequence: yellow, violet, orange and gold

Then its range will be

Yellow=4, violet=7, orange=10, gold=5% =47K 5% =2.35K

Most resistors have 4 bands_

The first band gives the first digit. The second band gives the second digit. The third band indicates the number of zeros. The fourth band is used to show the tolerance (precision) of the resistor.

This resistor has red (2), violet (7), yellow (4 zeros) and gold bands.So its value is 270000 = 270 k .

5 green

0 black

1 brown

2 red

3 orange

4 yellow

6 blue

7 purple

8 silver

9 white

0 black

1 brown

2 red

3 orange

4 yellow

6 blue

7 purple

8 silver

9 white

5 reen 5 green

0 black

1 brown

2 red

3 orange

4 yellow

6 blue

7 purple

8 silver

9 white

5 green

0 black

1 brown

2 red

3 orange

4 yellow

6 blue

7 purple

8 silver

9 white

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The standard colour code cannot show values of less than 10 . To show these small values

two special colours are used for the third band: gold, which means 0.1 and silver which

means 0.01. The first and second bands represent the digits as normal.

For example_

red, violet, gold bands represent 27 0.1 = 2.7blue, green, silver bands represent 56 0.01 = 0.56

The fourth band of the colour code shows the tolerance of a resistor. Tolerance is the

precision of the resistor and it is given as a percentage. For example a 390 resistor with a

tolerance of 10% will have a value within 10% of 390 , between 390 - 39 = 351 and 390

+ 39 = 429 (39 is 10% of 390).

A special colour code is used for the fourth band tolerance:silver 10%, gold 5%, red 2%, brown 1%.If no fourth band is shown the tolerance is 20%.

4.2.2VARIABLE RESISTOR

In electronic circuits, sometimes it becomes necessary to adjust the values of currents and

voltages. For n example it is often desired to change the volume of sound, the brightness of a

television picture etc. Such adjustments can be done by using variable resistors.

Although the variable resistors are usually called rheostats in other applications, the smaller variable

resistors commonly used in electronic circuits are called potentiometers.

Resistor shorthand_

Resistor values are often written on circuit diagrams using a code system which avoids usinga decimal point because it is easy to miss the small dot. Instead the letters R, K and M are

used in place of the decimal point. To read the code: replace the letter with a decimal point,

then multiply the value by 1000 if the letter was K, or 1000000 if the letter was M. The letter

R means multiply by 1.

For example:560R means 560

2K7 means 2.7 k = 2700

39K means 39 k

1M0 means 1.0 M = 1000 k

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2. p-n-p transistor

Figure 4.9 Transistors

An n-p-n transistor is composed of two n-type semiconductors separated by a thin section of

p-type. However a p-n-p type semiconductor is formed by two p-sections separated by a thin

section of n-type.

Transistor has two pn junctions one junction is forward biased and other is reversed biased.The forward junction has a low resistance path whereas a reverse biased junction has a high

resistance path.

The weak signal is introduced in the low resistance circuit and output is taken from the high

resistance circuit. Therefore a transistor transfers a signal from a low resistance to high

resistance.

Transistor has three sections of doped semiconductors. The section on one side is emitter and

section on the opposite side is collector. The middle section is base.

4.3.1 Emitter

The section on one side that supplies charge carriers is called emitter. The emitter is always

forward biased w.r.t. base.

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4.3.2 Collector

The section on the other side that collects the charge is called collector. The collector is

always reversed biased.

4.3.3 Base

The middle section which forms two pn-junctions between the emitter and collector is called

base.

A transistor raises the strength of a weak signal and thus acts as an amplifier. The weak signal

is applied between emitter-base junction and output is taken across the load Rc connected in

the collector circuit. The collector current flowing through a high load resistance Rc produces

a large voltage across it. Thus a weak signal applied in the input appears in the amplified

form in the collector circuit.

4.4 Light Emitting Diode

A junction diode, such as LED, can emit light or exhibit electro luminescence. Electro

luminescence is obtained by injecting minority carriers into the region of a pn junction where

radiative transition takes place. In radiative transition, there is a transition of electron from

the conduction band to the valence band, which is made possibly by emission of a photon.

Thus, emitted light comes from the hole electron recombination. What is required is that

electrons should make a transition from higher energy level to lower energy level releasing

photon of wavelength corresponding to the energy difference associated with this transition.

In LED the supply of high-energy electron is provided by forward biasing the diode, thus

injecting electrons into the n-region and holes into p-region.

The pn junction of LED is made from heavily doped material. On forward bias condition,

majority carriers from both sides of the junction cross the potential barrier and enter the

opposite side where they are then minority carrier and cause local minority carrier population

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to be larger than normal. This is termed as minority injection. These excess minority carrier

diffuse away from the junction and recombine with majority carriers.

In LED, every injected electron takes part in a radiative recombination and hence gives rise toan emitted photon. Under reverse bias no carrier injection takes place and consequently no

photon is emitted. For direct transition from conduction band to valence band the emission

wavelength.

In practice, every electron does not take part in radiative recombination and hence, the

efficiency of the device may be described in terms of the quantum efficiency which is defined

as the rate of emission of photons divided by the rate of supply of electrons. The number of

radiative recombination, that take place, is usually proportional to the carrier injection rate

and hence to the total current flowing.

4.4.1 LED Materials

One of the first materials used for LED is GaAs. This is a direct band gap material, i.e., itexhibits very high probability of direct transition of electron from conduction band to valence

band. GaAs has E= 1.44 eV. This works in the infrared region.GaP and GaAsP are higher band gap mat