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OPTICAL FIBER COMMUNICATION

LEARNING OBJECTIVES:

Concept of losses.

Study of dispersion.

About testing of losses.

To know about OTDR.

CHAPTER-3(LOSSES IN OPTICAL FIBER CABLE)

3.1. LOSSES

Fiber optic transmission has various advantages over other transmission methods like

copper or radio transmission. Fiber optic cable which is lighter, smaller and more flexible

than copper can transmit signals with faster speed over longer distance. However, many

factors can influence the performance of fiber optic. To ensure the nice and stable

performance of the fiber optic, many issues are to be considered. Fiber optic loss is a

negligible issue among them, and it has been a top priority for many engineers to consider

during selecting and handling fiber optic. This article will offer detailed information of

losses in optical fiber.

When a beam of light carrying signals travels through the core of fiber optic, the strength of

the light will become lower. Thus, the signal strength becomes weaker. This loss of light

power is generally called fiber optic loss or attenuation. This decrease in power level is

described in dB. During the transmission, something happened and causes the fiber optic

loss. To transmit optical signals smoothly and safely, fiber optic loss must be decreased.

The cause of fiber optic loss located on two aspects: internal reasons and external causes of

fiber optic, which are also known as intrinsic fiber core attenuation and extrinsic fiber

attenuation.

Intrinsic Fiber Core Attenuation

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Internal reasons of fiber optic loss caused by the fiber optic itself, which is also usually

called intrinsic attenuation. There are two main causes of intrinsic attenuation. One is light

absorption and the other one is scattering.

Light absorption is a major cause of losses in optical fiber during optical transmission.

The light is absorbed in the fiber by the materials of fiber optic. Thus light absorption in

optical fiber is also known as material absorption. Actually the light power is absorbed and

transferred into other forms of energy like heat, due to molecular resonance and wavelength

impurities. Atomic structure is in any pure material and they absorb selective wavelengths

of radiation. It is impossible to manufacture materials that are total pure. Thus, fiber optic

manufacturers choose to dope germanium and other materials with pure silica to optimize

the fiber optic core performance.

Scattering is another major cause for losses in optical fiber. It refers to the scattering of

light caused by molecular level irregularities in the glass structure. When the scattering

happens, the light energy is scattered in all direction. Some of them is keeping traveling in

the forward direction. And the light not scattered in the forward direction will be lost in the

fiber optic link as shown in the following picture. Thus, to reduce fiber optic loss caused by

scattering, the imperfections of the fiber optic core should be removed, and the fiber optic

coating and extrusion should be carefully controlled.

Extrinsic Fiber Attenuation

Intrinsic fiber core attenuation including light absorption and scattering is just one aspect of

the cause in fiber optic loss. Extrinsic fiber attenuation is also very important, which are

usually caused by improper handling of fiber optic. There are two main types of extrinsic

fiber attenuation: bend loss and splicing loss.

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Bend loss is the common problems that can cause fiber optic loss generated by improper

fiber optic handling. Literally, it is caused by fiber optic bend. There are two basic types.

One is micro bending, and the other one is macro bending (shown in the above picture).

Macro bending refers to a large bend in the fiber (with more than a 2 mm radius). To reduce

fiber optic loss, the following causes of bend loss should be noted:

Fiber core deviate from the axis;

Defects of manufacturing;

Mechanical constraints during the fiber laying process;

Environmental variations like the change of temperature, humidity or pressure.

fiber optic splicing is another main causes of extrinsic fiber attenuation. It is inevitable to

connect one fiber optic to another in fiber optic network. The fiber optic loss caused by

splicing cannot be avoided, but it can be reduced to minimum with proper handling.

Using fiber optic connectors of high quality and fusion splicing can help to reduce the fiber

optic loss effectively.

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The above picture shows the main causes of losses in optical fiber, which come in different

types. To reduce the intrinsic fiber core attenuation, selecting the proper fiber optic and

optical components is necessary. To decrease extrinsic fiber attenuation to minimum, the

proper handling and skills should be applied.

3.2 DISPERSION is a phenomenon that is an important factor in fiber optic

communications. It is the result of the different colors, or wavelengths, in a light beam

arriving at their destination at slightly different times. The result is a spreading, or

dispersion, of the on-off light pulses that convey digital information. Special care must be

taken to compensate for this dispersion so that the optical fiber delivers its maximum

capacity.

Chromatic dispersion is commonplace, as it is actually what causes rainbows - sunlight is

dispersed by droplets of water in the air. Sir Isaac Newton observed this phenomenon

when he passed sunlight through a prism and saw it diverge into a spectrum of different

colors. This dispersion occurs because different colors, or light frequencies, act slightly

differently as they pass through a medium such as glass. In fiber-based systems, an optical

fiber, comprised of a core and cladding with differing refractive index materials, inevitably

causes some wavelengths of light to travel slower or faster than others.

Chromatic dispersion is a serious consideration in long-haul optical fibers. Its effect is

essentially to stretch or flatten the initially sharply-defined binary pulses of

information. This degradation makes the signals (1s and 0s) more difficult to distinguish

from each other at the far end of the fiber. The result is that at any given length, the

effective information capacity, or bandwidth, of the fiber optic cable can be significantly

reduced. Dispersion is added as the modulated beam of light, consisting of a number of

closely spaced wavelengths, travels down this nearly transparent waveguide.

The bottom line is that chromatic dispersion becomes a major consideration and must be

accounted for when developing or deploying fiber optic equipment for use in

telecommunications, cable TV, or other high-speed optical networks.

Fortunately, techniques have been developed that help compensate for the negative effects

of chromatic dispersion. One method involves pre-compensating the signal for the

anticipated dispersion before it's sent down the optical fiber. Another method calls for

using dispersion compensating fiber at the end of a length to correct or reverse the

dispersion that was realized as the signal traversed the optical fiber. As a result, these

techniques are widely used to help solve the problem of chromatic dispersion.

3.3 FIBER OPTIC TESTING

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After the cables are

installed and terminated, it's time for testing. For every fiber optic cable plant, you will need to

test for continuity, end-to-end loss and then troubleshoot the problems. If it's a long outside

plant cable with intermediate splices, you will probably want to verify the individual splices

with an OTDR also, since that's the only way to make sure that each one is good. If you are the

network user, you will also be interested in testing power, as power is the measurement that tells

you whether the system is operating properly.

You'll need a few special tools and instruments to test fiber optics. See Jargon in the beginning

of Lennie's Guide to see a description of each instrument.

Getting Started

Even if you're an experienced installer, make sure you remember these things.

1. Have the right tools and test equipment for the job. You will need:

1. Source and power meter, optical loss test set or test kit with proper equipment adapters

for the cable plant you are testing.

2. Reference test cables that match the cables to be tested and mating adapters, including

hybrids if needed.

3. Fiber Tracer or Visual Fault Locator.

4. Cleaning materials - lint free cleaning wipes and pure alcohol.

5. OTDR and launch cable for outside plant jobs.

2. Know how to use your test equipment

Before you start, get together all your tools and make sure they are all working properly and

you and your installers know how to use them. It's hard to get the job done when you have

to call the manufacturer from the job site on your cell phone to ask for help. Try all your

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equipment in the office before you take it into the field. Use it to test every one of your

reference test jumper cables in both directions using the single-ended loss test to make sure

they are all good. If your power meter has internal memory to record data be sure you know

how to use this also. You can often customize these reports to your specific needs - figure

all this out before you go it the field - it could save you time and on installations, time is

money!

3. Know the network you're testing...

This is an important part of the documentation process we discussed earlier. Make sure you

have cable layouts for every fiber you have to test. Prepare a spreadsheet of all the cables

and fibers before you go in the field and print a copy for recording your test data. You may

record all your test data either by hand or if your meter has a memory feature, it will keep

test results in on-board memory that can be printed or transferred to a computer when you

return to the office.

A note on using a fiber optic source eye safety...

Fiber optic sources, including test equipment, are generally too low in power to cause any

eye damage, but it's still a good idea to check connectors with a power meter before looking

into it. Some telco DWDM and CATV systems have very high power and they could be

harmful, so better safe than sorry.

Fiber optic testing includes three basic tests that we will cover separately: Visual inspection

for continuity or connector checking, Loss testing, and Network Testing.

Visual Inspection

Visual Tracing

Continuity checking makes certain the fibers are not broken and to trace a path of a fiber

from one end to another through many connections. Use a visible light "fiber optic tracer"

or "pocket visual fault locator". It looks like a flashlight or a pen-like instrument with a

lightbulb or LED soure that mates to a fiber optic connector. Attach a cable to test to the

visual tracer and look at the other end to see the light transmitted through the core of the

fiber. If there is no light at the end, go back to intermediate connections to find the bad

section of the cable.

A good example of how it can save time and money is testing fiber on a reel before you pull

it to make sure it hasn't been damaged during shipment. Look for visible signs of damage

(like cracked or broken reels, kinks in the cable, etc.) . For testing, visual tracers help also

identify the next fiber to be tested for loss with the test kit. When connecting cables at patch

panels, use the visual tracer to make sure each connection is the right two fibers! And to

make certain the proper fibers are connected to the transmitter and receiver, use the visual

tracer in place of the transmitter and your eye instead of the receiver (remember that fiber

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optic links work in the infrared so you can't see anything anyway.)

Visual Fault Location

A higher power version of the tracer uses a laser that can also find faults. The red laser light

is powerful enough to show breaks in fibers or high loss connectors. You can actually see

the loss of the bright red light even through many yellow or orange simplex cable jackets

except black or gray jackets. You can also use this gadget to optimize mechanical splices or

prepolished-splice type fiber optic connectors. In fact- don't even think of doing one of

those connectors without one no other method will assure you of high yield with them.

Visual Connector Inspection

Fiber optic microscopes are used to inspect connectors to check the quality of the

termination procedure and diagnose problems. A well made connector will have a smooth ,

polished, scratch free finish and the fiber will not show any signs of cracks, chips or areas

where the fiber is either protruding from the end of the ferrule or pulling back into it.

The magnification for viewing connectors can be 30 to 400 power but it is best to use a

medium magnification. The best microscopes allow you to inspect the connector from

several angles, either by tilting the connector or having angle illumination to get the best

picture of what's going on. Check to make sure the microscope has an easy-to-use adapter

to attach the connectors of interest to the microscope.

And remember to check that no power is present in the cable before you look at it in a

microscope protect your eyes!

Optical Power - Power or Loss? ("Absolute" vs. "Relative")

Practically every measurement in fiber optics refers to optical power. The power output of a

transmitter or the input to receiver are "absolute" optical power measurements, that is, you

measure the actual value of the power. Loss is a "relative" power measurement, the

difference between the power coupled into a component like a cable or a connector and the

power that is transmitted through it. This difference is what we call optical loss and defines

the performance of a cable, connector, splice, etc.

Measuring power

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Power in a fiber optic system is like voltage in an electrical circuit - it's what makes things

happen! It's important to have enough power, but not too much. Too little power and the

receiver may not be able to distinguish the signal from noise; too much power overloads the

receiver and causes errors too.

Measuring power requires only a power meter (most come

with a screw-on adapter that matches the connector being

tested) and a little help from the network electronics to turn

on the transmitter. Remember when you measure power, the

meter must be set to the proper range (usually dBm,

sometimes microwatts, but never "dB" that's a relative

power range used only for testing loss!) and the proper

wavelengths matching the source being used. Refer to the

instructions that come with the test equipment for setup and

measurement instructions (and don't wait until you get to

the job site to try the equipment)!

To measure power, attach the meter to the cable that has the output you want to measure.

That can be at the receiver to measure receiver power, or to a reference test cable (tested

and known to be good) that is attached to the transmitter, acting as the "source", to measure

transmitter power. Turn on the transmitter/source and note the power the meter measures.

Compare it to the specified power for the system and make sure it's enough power but not

too much.

Testing loss

Loss testing is the difference between the power coupled into the cable at the transmitter

end and what comes out at the receiver end. Testing for loss requires measuring the optical

power lost in a cable (including connectors ,splices, etc.) with a fiber optic source and

power meter by mating the cable being tested to known good reference cable.

In addition to our power meter, we will need a test source. The test source should match the

type of source (LED or laser) and wavelength (850, 1300, 1550 nm). Again, read the

instructions that come with the unit carefully.

We also need one or two reference cables, depending on the test we wish to perform. The

accuracy of the measurement we make will depend on the quality of your reference cables.

Always test your reference cables by the single ended method shown below to make sure

they're good before you start testing other cables!

Next we need to set our reference power for loss our "0 dB" value. Correct setting of the

launch power is critical to making good loss measurements!

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Clean your connectors and set up your equipment like

this:

Turn on the source and select the wavelength you want for

the loss test. Turn on the meter, select the "dBm" or "dB"

range and select the wavelength you want for the loss test.

Measure the power at the meter. This is your reference

power level for all loss measurements. If your meter has a

"zero" function, set this as your "0" reference.

Some reference books and manuals show setting the

reference power for loss using both a launch and receive

cable mated with a mating adapter. This method is acceptable for some tests, but will

reduce the loss you measure by the amount of loss between your reference cables when you

set your "0dB loss" reference. Also, if either the launch or receive cable is bad, setting the

reference with both cables hides the fact. Then you could begin testing with bad launch

cables making all your loss measurements wrong. EIA/TIA 568 calls for a single cable

reference, while OFSTP-14 allows either method.

Testing Loss

There are two methods that are used to measure loss, which

we call "single-ended loss" and "double-ended loss". Single-

ended loss uses only the launch cable, while double-ended

loss uses a receive cable attached to the meter also.

Single-ended loss is measured by mating the cable you want

to test to the reference launch cable and measuring the

power out the far end with the meter. When you do this you

measure 1. the loss of the connector mated to the launch

cable and 2. the loss of any fiber, splices or other connectors in the cable you are testing.

This method is described in FOTP-171 and is shown in the drawing. Reverse the cable to

test the connector on the other end.

In a double-ended loss test, you attach the cable to test between two reference cables, one

attached to the source and one to the meter. This way, you measure two connectors' loses,

one on each end, plus the loss of all the cable or cables in

between. This is the method specified in OFSTP-14, the test

for loss in an installed cable plant.

What Loss Should You Get When Testing Cables?

While it is difficult to generalize, here are some guidelines:

- For each connector, figure 0.5 dB loss (0.7 max)

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- For each splice, figure 0.2 dB

- For multimode fiber, the loss is about 3 dB per km for 850 nm sources, 1 dB per km for

1300 nm. This roughly translates into a loss of 0.1 dB per 100 feet for 850 nm, 0.1 dB per

300 feet for 1300 nm.

- For singlemode fiber, the loss is about 0.5 dB per km for 1300 nm sources, 0.4 dB per km

for 1550 nm.

This roughly translates into a loss of 0.1 dB per 600 feet for 1300 nm, 0.1 dB per 750 feet

for 1300 nm. So for the loss of a cable plant, calculate the approximate loss as:

(0.5 dB X # connectors) + (0.2 dB x # splices) + fiber loss on the total length of cable

Troubleshooting Hints:

If you have high loss in a cable, make sure to reverse it and test in the opposite direction

using the single-ended method. Since the single ended test only tests the connector on one

end, you can isolate a bad connector - it's the one at the launch cable end (mated to the

launch cable) on the test when you measure high loss.

High loss in the double ended test should be isolated by retesting single-ended and

reversing the direction of test to see if the end connector is bad. If the loss is the same, you

need to either test each segment separately to isolate the bad segment or, if it is long

enough, use an OTDR.

If you see no light through the cable (very high loss - only darkness when tested with your

visual tracer), it's probably one of the connectors, and you have few options. The best one is

to isolate the problem cable, cut the connector of one end (flip a coin to choose) and hope it

was the bad one (well, you have a 50-50 chance!)

3.4 OTDR TESTING

As we mentioned earlier, OTDRs are always used on OSP cables to verify the loss of each

splice. But they are also used as troubleshooting tools. Let's look at how an OTDR works

and see how it can help testing and troubleshooting.

How OTDRs Work

Unlike sources and power meters which measure the loss of the fiber optic cable plant

directly, the OTDR works indirectly. The source and meter duplicate the transmitter and

receiver of the fiber optic transmission link, so the measurement correlates well with actual

system loss.

The OTDR, however, uses backscattered light of the fiber to imply loss. The OTDR works

like RADAR, sending a high power laser light pulse down the fiber and looking for return

signals from backscattered light in the fiber itself or reflected light from connector or splice

interfaces.

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At any point in time, the light the OTDR sees is the light scattered from the pulse passing

through a region of the fiber. Only a small amount of light is scattered back toward the

OTDR, but with sensitive receivers and signal averaging, it is possible to make

measurements over relatively long distances. Since it is possible to calibrate the speed of

the pulse as it passes down the fiber, the OTDR can measure time, calculate the pulse

position in the fiber and correlate what it sees in backscattered light with an actual location

in the fiber. Thus it can create a display of the amount of backscattered light at any point in

the fiber.

Since the pulse is attenuated in the fiber as it passes along the fiber and suffers loss in

connectors and splices, the amount of power in the test pulse decreases as it passes along

the fiber in the cable plant under test. Thus the portion of the light being backscattered will

be reduced accordingly, producing a picture of the actual loss occurring in the fiber. Some

calculations are necessary to convert this information into a display, since the process

occurs twice, once going out from the OTDR and once on the return path from the

scattering at the test pulse.

There is a lot of information in an OTDR display. The slope of the fiber trace shows the

attenuation coefficient of the fiber and is calibrated in dB/km by the OTDR. In order to

measure fiber attenuation, you need a fairly long length of fiber with no distortions on

either end from the OTDR resolution or overloading due to large reflections. If the fiber

looks nonlinear at either end, especially near a reflective event like a connector, avoid that

section when measuring loss.

Connectors and splices are called "events" in OTDR jargon. Both should show a loss, but

connectors and mechanical splices will also show a reflective peak so you can distinguish

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them from fusion splices. Also, the height of that peak will indicate the amount of reflection

at the event, unless it is so large that it saturates the OTDR receiver. Then peak will have a

flat top and tail on the far end, indicating the receiver was overloaded. The width of the

peak shows the distance resolution of the OTDR, or how close it can detect events.

OTDRs can also detect problems in the cable caused during installation. If a fiber is broken,

it will show up as the end of the fiber much shorter than the cable or a high loss splice at the

wrong place. If excessive stress is placed on the cable due to kinking or too tight a bend

radius, it will look like a splice at the wrong location.

OTDR Limitations

The limited distance resolution of the OTDR makes it very hard to use in a LAN or

building environment where cables are usually only a few hundred meters long. The OTDR

has a great deal of difficulty resolving features in the short cables of a LAN and is likely to

show "ghosts" from reflections at connectors, more often than not simply confusing the

user.

Using The OTDR

When using an OTDR, there are a few cautions that will make testing easier and more

understandable. First always use a long launch cable, which allows the OTDR to settle

down after the initial pulse and provides a reference cable for testing the first connector on

the cable. Always start with the OTDR set for the shortest pulse width for best resolution

and a range at least 2 times the length of the cable you are testing. Make an initial trace and

see how you need to change the parameters to get better results.

Restoration

The time may come when you have to troubleshoot and fix the cable plant. If you have a

critical application or lots of network cable, you should be ready to do it yourself. Smaller

networks can rely on a contractor. If you plan to do it yourself, you need to have equipment

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ready (extra cables, mechanical splices, quick termination connectors, etc., plus test

equipment.) and someone who knows how to use it.

We cannot emphasize more strongly the need to have good documentation on the cable

plant. If you don't know where the cables go, how long they are or what they tested for loss,

you will be spinning you wheels from the get-go. And you need tools to diagnose problems

and fix them, and spares including a fusion splicer or some mechanical splices and spare

cables. In fact, when you install cable, save the leftovers for restoration! And the first thing

you must decide is if the problem is with the cables or the equipment using it. A simple

power meter can test sources for output and receivers for input and a visual tracer will

check for fiber continuity. If the problem is in the cable plant, the OTDR is the next tool

needed to locate the fault.

QUESTIONS:

MULTIPLE CHOICE QUESTIONS:

1)which of following is loss

(a) joint (b) attenuation

(c) scattering (d)dispersion

2) which of following is a alignment:

(a)core (b) lateral

(c) gap (d) all of the above

SHORT ANSWER TYPES QUESTIONS:

1) What is joint loss?

2) LED stands for ……………..

3) What do you mean by splicing?

4) What is fiber connector?

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5) Write three types of splicing.

LONG ANSWER TYPES QUESTIONS:

1) Explain in detail components of connector..

2) Explain in detail losses.

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OPTICAL FIBER COMMUNICATION

LEARNING OBJECTIVES:

Concept of optical sources.

Study of LED.

About LASER.

To know about laser diodes.

CHAPTER-4(OPTICAL SOURCES)

4.1. CHARATERISTICS OF LIGHT SOURCES

4.1.1. LED Basics

Eye Protection - LEDs are very bright. DO NOT look directly into the LED light.

The light can be intense enough to injure human eyes.

Technical Index

4.1.2. How does a LED work?

This is a very simple explanation of the construction and function of LEDs. White LEDs need

3.6VDC and use approximately 30 milliamps of current, a power dissipation of 100 milliwatts. The

positive power is applied to one side of the LED semiconductor through a lead (1 anode) and a

whisker (4). The other side of the semiconductor is attached to the top of the anvil (7) that is the

negative power lead (2 cathode). It is the chemical makeup of the LED semiconductor (6) that

determines the color of the light the LED produces. The epoxy resin enclosure (3 and 5) has three

functions. It is designed to allow the most light to escape from the semiconductor, it focuses the

light (view angle), and it protects the LED semiconductor from the elements. As you can see, the

entire unit is totally embedded in epoxy. This is what make LEDs virtually indestructible. There are

no loose or moving parts within the solid epoxy enclosure.

Therefore, a light-emitting diode (LED) is essentially a PN junction semiconductor diode that

emits light when current is applied. By definition,

it is a solid-state device that controls current without heated filaments and is therefore very

reliable.

LED performance is based on a few primary characteristics:

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4.1.3. LED Colors

LEDs are highly monochromatic, emitting a pure color in a narrow frequency range. The color

emitted from an LED is identified by peak wavelength (lpk) and measured in nanometers (nm ).

Peak wavelength is a function of the LED chip material. Although process variations are ±10 NM,

the 565 to 600 NM wavelength spectral region is where the sensitivity level of the human eye is

highest. Therefore, it is easier to perceive color variations in yellow and amber LEDs than other

colors.

LEDs are made from gallium-based crystals that contain one or more additional materials such as

phosphorous to produce a distinct color. Different LED chip technologies emit light in specific

regions of the visible light spectrum and produce different intensity levels

White LED Light

When light from all parts of the visible spectrum overlap one another, the additive mixture of

colors appears white. However, the eye does not require a mixture of all the colors of the spectrum

to perceive white light. Primary colors from the upper, middle, and lower parts of the spectrum

(red, green, and blue), when combined, appear white. To achieve this combination with LEDs

requires a sophisticated electro-optical design to control the blend and diffusion of colors.

Variations in LED color and intensity further complicate this process.

Presently it is possible to produce white light with a single LED using a phosphor layer (Yttrium

Aluminum Garnet) on the surface of a blue (Gallium Nitride) chip. Although this technology

produces various hues, white LEDs may be appropriate to illuminate opaque lenses or backlight

legends. However, using colored LEDs to illuminate similarly colored lenses produces better

visibility and overall appearance.

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Intensity

LED light output varies with the type of chip, encapsulation, efficiency of individual wafer lots and

other variables. Several LED manufacturers use terms such as "super-bright," and "ultra-bright" to

describe LED intensity. Such terminology is entirely subjective, as there is no industry standard for

LED brightness. The amount of light emitted from an LED is quantified by a single point, on-axis

luminous intensity value (Iv). LED intensity is specified in terms of millicandela (mcd). This on-

axis measurement is not comparable to mean spherical candlepower (MSCP) values used to

quantify the light produced by incandescent lamps.

Luminous intensity

is roughly proportional to the amount of current (If) supplied to the LED. The greater the current,

the higher the intensity. Of course, there are design limits. Generally, LEDs are designed to operate

at 20 milliamps (mA). However, operating current must be reduced relative to the amount of heat

in the application. For example, 6-chip LEDs produce more heat than single-chip LEDs. 6-chip

LEDs incorporate multiple wire bonds and junction points that are affected more by thermal stress

than single-chip LEDs. Similarly, LEDs designed to operate at higher design voltages are subject to

greater heat. LEDs are designed to provide long-life operation because of optimal design currents

considering heat dissipation and other degradation factors.

The circuit symbol for the LED is relatively straightforward. The LED symbol comprises a diode

symbol with two arrows indicating outwards to signify that light emanated from the diode.

Light emitting diode, LED circuit symbol

Sometimes the light emitting diode symbol is shown only as an outline and without the filled in

shapes. The outline shape is equally acceptable,

Alternative light emitting diode, LED circuit symbol

Other versions of LED symbols may also be seen. Sometimes the light emitting diode symbol may

be enclosed in a circles. This symbol is not as widely used these days but may still be seen on many

circuits.

4.2 LED types

Since the introduction of the first LEDs, the technology has spawned a huge variety of different

types of LED, each with their own properties and applications.

Traditional inorganic LEDs: This type of LED is the traditional form of diode that has been

available since the 1960s. It is manufactured from inorganic materials. Some of the more widely

used are compound semiconductors such as Aluminium gallium arsenide, Gallium arsenide

phosphide, and many more – the colour of the light is often dependent upon the materials used.

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These LEDs are typified by the small LED lamps that are used as panel indicators, although

there are very many formats for LEDs of this type. However even within the inorganic LED

category, there are many different styles of LED that can be seen and used:

Single colour 5 mm, etc - the very traditional LED package

Surface mount LEDs

Bi-colour and multicolour LEDs - the types of LEDs contain several individual LEDs that

are turned on by different voltages, etc.

Flashing LEDs - with a small time integrated into the package

Alphanumeric LED displays

. . . . . more . . . .

All these different types of inorganic LED are used in very large quantities

High brightness LEDs: High brightness LEDs, HBLEDs, are a type of inorganic LED that are

starting to be used for lighting applications. This type of LED is essentially the same as the

basic inorganic LED, but has a much greater light output. To generate the higher light output,

this LED type requires to be able to handle much higher current levels and power dissipation.

Often these LEDs are mounted such that they can be mounted onto a heatsink to remove the

unwanted heat.

In view of their greater efficiency, this type of LED is being used as a replacement for many

more traditional forms of lighting. Domestic lighting along with automotive lamps are now in

widespread use. They have advantages in terms of efficiency and environmental factors over

incandescent and Compact Fluorescent Lightbulbs, CFLs. The HBLEDs have a greater

efficiency level and they also have a longer life, especially when being switched on and off

many times. However they do have a finite life, a factor that is sometimes overlooked.

Organic LEDs: Organic LEDs are a development of the basic idea for the light emitting

diode. This type of LED uses organic materials as the name indicates.

The traditional types of light emitting diode utilise traditional inorganic semiconductors with

varying dopant levels and they produce light from the defined PN junction - often this is a point

of light. The organic type of LED display is based on organic materials which are manufactured

in sheets and provide a diffuse area of light. Typically a very thin film of organic material is

printed onto a substrate made of glass. A semiconductor circuit is then used to carry the

electrical charges to the imprinted pixels, causing them to glow.

With LED technology improving all the time, the efficiency levels of all the different types of

LEDs is bound to improve, and their use will increase.

4.3 LED colours

Traditional inorganic LEDs are available in a variety of colours. The first LEDs to be produced

were red, but since then many other colours have been introduced. Now they are available in the

following colours:

Of the colours available the blue and white LED types are more expensive than LEDs in other

colours as a result of the higher manufacture costs.

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In addition to the LEDs that emit visible light, others are manufactured to emit infra-red. These

ones are often used for applications such as television remote controls where no visible light is

seen.

The colour of a light emitting diode is determined by the semiconductor material used in the diode.

Although the plastic body of the diode may appear to be coloured, this is not what gives the diode

its colour.

Multicolour LEDs

Sometimes it can be very useful to have a lamp that has more than one colour, indicating a different

colour to indicate a different state. This can be done using LEDs. There are two sorts:

Bi-colour LEDs A bi-colour LED is constructed by having two LEDs in parallel with each

other in the same package, but they are wired with one external connection of the package going

to the cathode of one diode, and the anode of the other. The other lead is again connected to the

anode of the first diode and the cathode of the second. In this way when a voltage is applied one

way round, one LED will light, and when it is applied the other way round, the other one will

light.

Tri-colour LEDs This type of LED has three leads enabling any combination of LEDs to be

light, i.e. the first LED, the second, or both. The most popular form of tri-colour LED uses a red

and green diode. This means that when one diode is on, then either red or green is produced. If

both are light, then the colours combine to form yellow.

Summary

Although LEDs will continue to be very widely used as small indicator lamps, the number of

applications they can find is increasing as the technology improves. New very high luminance

diodes are now available. These are even being used as a form of illumination, an application

which they were previously not able to fulfil because of their low light output. New colours are

being introduced. White and blue LEDs, which were previously very difficult to manufacture are

now available. In view of the ongoing technology development, and their convenience of use, these

devices will remain in the electronics catalogues for many years to come.

4.4 LASER DIODES: Injection Laser Diode (ILD) The laser is a device which amplifies the light;

hence the LASER is an acronym for light amplification by stimulated emission of radiation. The

operation of the device may be described by the formation of an electromagnetic standing wave

within a cavity (optical resonator) which provides an output of monochromatic highly coherent

radiation. Principle: Materials absorb light than emitting. Three different fundamental process

occurs between the two energy states of an atom. 1) Absorption 2) Spontaneous emission 3)

Stimulated emission. Laser action is the result of three process absorption of energy packets

(photons) spontaneous emission, and stimulated emission. (These processes are represented by the

simple two-energy-level diagrams). Where E1 is the lower state energy level. E2 is the higher state

energy level. Quantum theory states that any atom exists only in certain discrete energy state,

absorption or emission of light causes them to make a transition from one state to another. The

frequency of the absorbed or emitted radiation f is related to the difference in energy E between the

two states. If E1 is lower state energy level. and E2 is higher state energy level. E = (E2 – E1) =

h.f. Where, h = 6.626 x 10-34 J/s (Plank’s constant). An atom is initially in the lower energy state,

when the photon with energy (E2 – E1) is incident on the atom it will be excited into the higher

energy state E2 through the absorption of the photon. Dept of ECE, NIT Page 1 Optical Fiber

Communication 10EC72 When the atom is initially in the higher energy state E2, it can make a

transition to the lower energy state E1 providing the emission of a photon at a frequency

corresponding to E = h.f. The emission process can occur in two ways. A) By spontaneous

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emission in which the atom returns to the lower energy state in random manner. B) By stimulated

emission when a photon having equal energy to the difference between the two states (E2 – E1)

interacts with the atom causing it to the lower state with the creation of the second photon.

Spontaneous emission gives incoherent radiation while stimulated emission gives coherent

radiation. Hence the light associated with emitted photon is of same frequency of incident photon,

and in same phase with same polarization. It means that when an atom is stimulated to emit light

energy by an incident wave, the liberated energy can add to the wave in constructive manner. The

emitted light is bounced back and forth internally between two reflecting surface. The bouncing

back and forth of light wave cause their intensity to reinforce and build-up. The result in a high

brilliance, single frequency light beam providing amplification. Emission and Absorption Rates It

N1 and N2 are the atomic densities in the ground and excited states. Rate of spontaneous emission

Rspon = AN2 … Optical Fiber Communication 10EC72 Rstim = BN2 ρem … 3.1.14 Rate of

absorption Rabs = B’ N1 ρem … 3.1.15 where, A, B and B’ are constants. ρem is spectral density.

Under equilibrium condition the atomic densities N1 and N2 are given by Boltzmann statistics. …

3.1.16 … 3.1.17 where, KB is Boltzmann constant. T is absolute temperature. Under equilibrium

the upward and downward transition rates are equal. AN2 + BN2 ρem = B’ N1 ρem Spectral

density ρem Comparing spectral density of black body radiation given by Plank’s formula, …

3.1.18 … 3.1.19 … 3.1.20 Therefore, … 3.1.21 … 3.1.22 A and B are called Einstein’s coefficient.

4.5 COMPARISION BTEWEEN LED AND ILD

Following table compares LED and Laser with respect to various comparison

factors/specifications.

specification Light Emitting Diode Laser Diode

Working operation

It emits light by spontaneous

emission.

It emits light by stimulated

emission.

Coherent/Incoherent

The emitted light is incoherent i.e.

photons are in random phase among

themselves.

It possesses a coherent beam with identical

phase relation of emitted photons.

Output power

Emitted light power is relatively

low, Linearly proportional to drive

current

Output power is high (Few mW to GW) ,

Proportional to current above the threshold

Bias/Current

It requires small applied bias and

operates under relatively low

current densities.

It requires high driving power and high

injected current density is needed.

Coupled power Moderate High

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Speed Slower Faster

Output pattern Higher Lower

Fiber Type Multimode only Singlemode and multimode

Ease of use Easier Harder

Lifetime Longer Long

Spectral width

Wider, 25 to 100 nm

(10 to 50 THz)

Narrower, <10-5 to 5 nm

(<1 MHz to 2 MHz)

Modulation

Bandwidth

Moderate, Tens of KHz to tens of

MHz High, Tens of MHz to tens of GHz

Available

Wavelength 0.66 to 1.65 mm 0.78 to 1.65 mm

E/O Conversion

Efficiency 10 to 20 % 30 to 70 %

Eye Safety Generally considered eye-safe

Must be rendered eye-safe, especially for λ <

1400 nm

Cost Low Moderate to High

QUESTIONS:

MULTIPLE CHOICE QUESTIONS:

1)which of following is optical source:

(a) LED (b) LASER

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(c) ILD (d)ALL

2) which of following is a part of LED:

(a)lens (b) whisker

(c) envil (d) ALL OF THE ABOVE

SHORT ANSWER TYPES QUESTIONS:

1) What is LED?

2) ILD stands for ……………..

3) What do you mean by LASER?

4) What is function of LED?

5) Write three types of optical sources.

LONG ANSWER TYPES QUESTIONS:

1) Explain in detail optical sources..

2) Explain in detail LED.

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