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ACKNOWLEDGEMENT

At first, well it is appeared a copmlicated task but if some one bestows us direction then its not

difficult task for us. So after started our project we have found lots of assitance from our intrenal

project guide and also our H.O.D.Well actually on this acknowledgement we are only two in this

project. But the reallty of this matter is, there are lots of people, working “behind this scenes”

helping in every possible way in making our project.

We are thankful to our internal project guide Mr. Dharmendra chauhan of our colloge, for

providing proper guidance in successful completion of this project. As well as also thankful to

our H.O.D Mr. Jaymin bhalani ,who trust us as asuccesful project learner.

We can’t forgot to thank our parents who are always with us to help us and give full support in

every good work we do.So we give exceptional thanks to our loving parents.

Maulik B. Patel(17/EC/63)

Parth P. Patel(07/EC/70)

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ABSTRACT

This kit can be used to send signal in well over 200 yards of cable.Note that the plastic FO cable

used here is not the glass 1 micron FO cable which is used in long distance (say 20 miles)

communication.

However, Plastic cable introduces most FO concept, is far easier for the experimenter to use and

certainly has definite uses for short distance communication in electrically noisy environment.

This Kit allows you to send sound through 1mm plastic fiber optic (FO) cable. On the

transmitter (Tx) circuit board there is a microphone and a circuit to modulate the light emitted

from an LED. The LED is contained in a plastic case which allows easy connection of the FO

cable. This Kit can be used to send a signal in well over 200 yards of cable. Note that the plastic

FO

On the receiver (Rx) boards there is the photo-Darlington receiver unit, a speaker and a circuit

and amplify the detected signal back into a sound wave. Because the signal travels in the FO

cable as a light wave it is unaffected by any electric or magnetic fields that it travels through.

Each board requires a 9V battery. However, for continuous use pluf packs packs would be

advised.

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CHAPTER 1

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1. Introduction of Project

1.1General description of project:

This kit allows you to send sound through 1mm plastic fiber optic cable on the transmitter TX

circuit board there is microphone & a circuit to modulate the light emitted from an LED is

contained in a plastic case which allows easy connection of the fiber optic cable. On the receiver

RX board there photo darlington receiver unit, a speaker and a circuit to convert and amplify the

detected signal back in to a sound wave.

Because the signal travels in the fiber cable as light wave it is unaffected by any electric or

magnetic fields that it travels through.

Each board requires a 9V battery. However for continuous use plug packs would be advised.

1.2 Basic description of fiber optic cable:

To understand how a fiber optic cable works, imagine an immensely long drinking straw or

flexible plastic pipe. For example imagine a pipe is several miles long. Now imagine that the

inside surface of the pipe has been coated with a perfect mirror.

Now imagine that you are looking in to one end of the pipe. Several miles away at the other end

a friend turns on a flash light and shines it in to pipe.

Because the interior of the pipe is the perfect mirror the flash light’s lights will reflect off the

sides of pipe (even though the pipe may curve and twist) and you will see it at the other end.

If your friend were to turn the flash light on and off in a morse code fashion your friend could

communicate with you through pipe. That is the essence of a fiber optic cable.

Making a cable out of a mirrored tube would work, but it would be bulky and it would also be

hard to coat the interior of the tube with a perfect mirror. A real fiber optic cable is therefore

made out of glass. The glass is incredibly pure so that even though it is several miles long, light

can still make it through imagine glass so transparent that window severa l miles thick still looks

clear.

The glass is drawn in to very strand with a thickness comparable to that of a human hair. The

glass strand is then coated in two layers of plastic.

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By coating the glass in the plastic, you get the equivalent of the mirror around the glass strand.

This mirror creates total internal reflection, just like a perfect mirror coating on the inside of the

tube does.

You can experience this sort of reflection with the flash lighth and a window in the dark room. If

you direct the flash light through the window at 90 degree angle,it passes straight through the

glass.

However,if you shine the flashlight at a very shallow angle(nearly parallel to glass),the glass will

act as a mirror and you will see the beam reflect off the window and hitthe wall inside the room.

Light traveling through fiber bounces at shallow angels like this and stays completely within the

fiber.

To send telephone conversations through a fiber optic cable analog voice signal are translated in

to digital signals. A laser at one end of the pipe switches on and off to send each bit. Modern

fiber systems with a single laser can transmit billions of bits per second. The laser can turn on

and off several billions of time per second.

The newest systems use multiple laser with different colors to fit multiple signals in to the same

fiber.

Modern fiber optic cables can carry a signal quite a distance (perhaps 60miles) on a long distance

line, there is an equipment hut every 40 to 60 miles. The hut contains equipment that picks up

and retransmits the signal down the next segment a full strength.

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Chapter 2

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2.Hardware

2.1 Block Diagram:

When designing a fiber optic system there arre many factors that must be considered- all of

which contribute to the final goal of ensuring that enough light reaches the receiver. Without the

right amount of light, the entire system will not operate properly. Figure 2.1 identifies many of

these factors and consideration.

Figure 2.1 Importants Parameters to Consider When Specifying F/O System

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2.2 Elements of fiber optic link:

Fiber optic transmitter uses the same basic element as copper-based transmission system: A

transmitter, a receiver and a medium by which the signal is passes from one to the other, in this

case optical fiber, Figure 2.2 illustrates these elements.

The transmitter uses an electrical interface to encode the use information through AM,FM or

DIGITAL modulation. A laser diode or an LED do the encoding to allow an optical output of

850 nm, 1310 nm, or 1550 nm(typically).

Figure 2.2 –elements of a fiber optic link

The optical fiber connects the transmitter and the receiver. This fiber may be either singal-mode

or multimode.

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The fiber consists of three main regions, as illustrated in figure 2.3 the core, the center of the

fiber, actually carries the light. The cladding surrounds the core in a glass with a different

refractive index than the core, allowing the light to be confined in the fiber core. A coating or

buffer typical plastic provides strength and protection to the optical fiber.

Figure 2.3- Cross section of an optical fiber

The receiver uses either a PIN photodiode or an APD to receive the optical signal and convert it

back signal qun to an electrical signal. A data demodulator convert the data into its original

electrical from. These element comprise the simplest link, but other elements may also appear in

a fiber optic transmission system.

For example, the addition of WDM components allows two separate signals to be joined into a

composite signal for transmission, and then can be separated into their original signals at the

receiver end. Other wavelength-division multiplexing techniques allow up to eight

signal(CWDM) or more (DWDM) to be combined onto a signal fiber. These are discussed in

separate articles as linked in the paragraph.

Long distance fiber optic transmission leads to further system complexity. Many long-haul

transmission systems require signal regenerators, signal repeaters, or optical amplifiers such as

EDFAs in order to maintain signal quality. System drop/pepeat/add requirement, such as those

in multichannel broadcast networks, further add to the fiber optic system, incorporating add-drop

multiplexers, couplers/splitters, signal fanouts,dispersion management equipment, remote

monitoring and error-correction components. See the linked articles for additional on these

components.

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2.3 Circuit Diagram:

Figure 2.4 Power supply of Transmitter

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Figure 2.5 Power supply of receiver

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Figure 2.7 Optical Transmitter

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Figure 2.8 Opical receiver

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2.4 Circuit Operation

First we have to give 230 v supply to trans former which will convert the 230v supply to 12v AC

supply.

Now, we are using full-wave rectifier circuit to convert 12v AC to DC.

And, this ckt we use for both transmitter and receiver power supply circuit.

The voice signal begins as a sounf wave. It is converted to an electrical signal by the electrets

microphone in the TX circuit.

This signal is amplified by the LM741 audio amplifier and converted to an optical signal by

switching the voltage to the fiber optic transmitter via a signal transistor.

This optical signal is fed in to the plastic fiber optic cable. At the other end of the cable the

optical signal is directed at a photo darlington in the receiver, which converts it to an electrical

signal again.

The signal is amplified and fed into a speaker where it becomes to a sound wave. A voltage

regulator has been used in the circuit to overcome feed back in the circuit.

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Chapter 3

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3. Optical transmitter

Defination:

A device that accepts an electrical signal as its input, processes this signal, and uses it to

modulate an opto-electronic device, such as an LED or an injection laser diode, to

produce an optical signal capable of being transmitted via an optical transmission

medium.

3.1 Introducation to transmitter:

The basic optical transmitter converts electrical input signals into modulated light for

transmission light over an optical fiber. Depending on the nature of this signal, the

resulting modulated light may be turned on and off or may be linearly varied in intensity

between two predetermined levels. Figure 3.1 shows a graphic representation of these

two basic schemes.

The most common devices used as the light source in optical transmitter are the light

emitting diode(LED) and the laser diode(LD). In a fiber optic system, these devices are

Mounted in a package that enables an optical fiber to be placed in very close proximity to

the light emitting region in order to couple as much light as possible into the fiber.

In some cases, the emitter is even fitted with a tiny spherical lens to collect and focus

“every last drop” of light onto the fiber and in other cases, a fiber is “pigtailed” directly

onto the actual surface of the emitter.

Figure 3.1 Basic optical modulation methods

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LEDs have relatively large emitting areas and as a result are not as good light sources as

LDs. However, they are widely used for short to moderate transmission distance because

they are much more economical, quite liner in terms of light output versus electrical

current input and stable in terms of light output versus ambient temperature.

LDs on the other hand have very small light emitting surface and can couple many times

more power to the fiber than LEDs. LDs are also linear in terms of light output versus

electrical current input,but unlike LEDs they are not stable over wide operating temperate

ranges and requires more elaborate circuity to achieve acceptable stability. In addition,

their added cost makes them primarily useful for applications that requires the

transmission of signal over long distance.

LEDs and LDs operate in the infrared portion of the electromagnetic spectrum so that

their light output is usually invisible to the human eye. Their operating wavelength are

chosen to be compatible with the lowest transmission loss wavelength of glass fibers and

highest sensitivity ranges of photodiodes.

The most common wavelength in use today are 850nm,1300nm and 1550 nm. Both LEDs

and LDs are available in all three wavelength.

LEDs and LDs, as previously stated, are modulated in one of two ways, on and off, or

lineary. Figure 3.2 shows simplified circuitry to achieve either method with an LED or

LD.

A transistor is used to switch the LED or LD on and off in step with an input digital

signal. This signal can be converted from almost any digital format by the appropriate

circuitry, into the correct base drive for the transistor. Overall speed id then determined

by the circuitry and the inherent speed of LED or LD.

Used in this manner, speeds of several hundred megahertz are read ily achieved for LEDs

and thousands of megahertz for LEDs. Temperature stabilization circuitry for the LD has

been omitted from this example for simplicity. LEDs do not normally require any

temperature stabilization.

Liner modulation of an LED or LD is accomplished by the operational amplifier circuit

of figure 3B.

The inverting input is used to supply the modulating drive to the LED or LD while the

non- inverting input supplies a DC bias reference. Once again temperature stabilization

circuitry for the LED has been omitted from this example for simplicity

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Figure 3.2 Methode of modulating LED or Laser Diodes

Digital on/off modulation of an LED or LD can take a number of forms. The simplest as

we have already seen is light-on for a logic “1” and light off for a logic “0”. Two other

common forms are pulse width modulation and pulse rate modulation.

In the former a constant stream of pulses is produced with signifying a logic “1” and

another width a logic “0”. In the latter the pulse are all of the same width but the pulse

rate changes to differentiate between logic “1” and logic “0”.

Analog modulation can also take a number of forms. The simplest is intensity modulation

where the brightness if the LED is varied in direct step with the variation of the

transmitted signal.

In the other methods, an RF carrier is first frequency modulated with another signal or, in

some cases, several RF carriers are separately modulated with separate signals, then all

are combined and transmitted as one complex wave form. Figure 3.3 shows all of the

above modulation methods as a function of light output.

The equivalent operating frequency of light, which is, after all, electromagnetic eadiation,

is extremely high – on the order of 1,000,000 GHz. The output bandwidth of the light

produced by LEDs and laser diodes is quiet wide.

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Figure 3.3 Various Methodes to |Optically Trasmit Analog Information

Unfortunately today’s technology does not allow this bandwidth to be selectively used in

the way that conventional radio frequency transmissions are utilized.

Rather, the entire optical bandwidth is turned on and turn off in the same way that early

“spark transmissions” (in the infancy of radio), turned wide portions of the RF spectrum

on and off. However, with time, researchers will overcome this obstacle and “coherent

transmissions”, as they are called, will become the direction in which the fiber optic field

progresses.

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Chapter 4

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4. Optical receiver

Defination:

A device that detects an optical signal, converts it to an electrical signal, and processes

the electrical signal as required for further use.

4.1 introduction of receiver

The basic optical receiver converts the modulated light coming from the optical fiber

back into a replica of the original signal applied to the transmitter.

The detector of this modulated light is usually a photodiode of either the PIN or the

Avalanche type. This detector is mounted in a connector similar to the one used for the

LED or LD. Photodiodes usually have a large sensitive detecting area that can be several

hundred micron in diameter. This relaxes the need for special precaution in centering the

fiber in the receiving connecter and makes the “alignment” concern much less critical

than it is in optical transmitter.

Since the amount of light that exits a fiber is quite small, optical receiver usually employ

high gain internal amplifiers. Because of this, optical receiver can be easily overloaded.

For this reason, it is important only to the size fiber specified for use with a given system.

If, for example, a transmitter/receiver pair designed for use with signal-mode fiber were

used with multimode fiber, the large amount of light present at the output of the fiber

(due to over-coupling at the light source) would overload the receiver and cause a

severely distorted output signal. Similarly, if transmitter/receiver pair designed for use

with multimode fiber were used with signal-mode fiber, not enough light would reach the

receiver, resulting in either an excessively noisy output signal or no signal at all.

The only time any sort of receiver “mismatching” might be considered is when there is so

much excessive loss in the fiber that the extra 5 to 15 dB of light coupled into a

multimode fiber by a signal-mode light source is the only chance to achieve proper

operation. However, this is an extreme case and is not normally recommended.

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As in the case of transmitter, optical receivers are available in both analog and digital

versions. Both types usually employ an analog preamplifier stage, followed by either an

analog or digital output stage (depending on the type of receiver). Figure 4.1 is a

functional diagram of a simple analog optical receiver.

The first stage in an operational amplifier connected as a current-to-voltage convert. This

stage takes the tiny current from the photodiode and converts it into a voltage,

Usually in the millivolt range. The next stage is a simple operational voltage amplifier.

Here the signal is raised to the desired output level.

Figure 4.1 Basic Analog Fiber Optic Receiver

Figure 4.2 is a functional diagram of a simple digital optical receiver. As in the case of

the analog receiver, the first stage is a current-to-voltage converter. The output of this

stage, however, is fed to a voltage comparator, which produces a clean, fast rise-time

digital output signal.

The trigger level adjustment, when it is present, it used to “touch up” the point on the

analog signal where the comparator switches. This allows the symmetry of the recovered

digital signal to be trimmed as accurately as desired.

Additional stages are often added to both analog and digital receiver to provide drivers

for coaxial cable, protocol converters or a host of other function in efforts to reproduce

the original signal as accurately as possible.

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It is important to note that while fiber optic cable is immune to all forms of interference,

the electronic receiver is not, because of this, normal precautions, such as shielding and

grounding, should be taken when using fiber optic electronic components.

Figure 4.2 Basic Digital Fiber Optic Receiver

4.2 Basic section of receiver:

The typical through-the-air communication receiver can be broken down into five

separate section.

These are:

1.light collector (lens)

2.light detector (PIN)

3.current to voltage converter

4.signal amplifier

5.pulse discriminator.

There may also be additional circuits depending on the kind of the signal being received.

As an example, a receiver that is extracting voice information will need a frequency to

voltage converter and an audio amplifier to reproduce the original voice signal. Computer

data receiver will also need some decoding circuits that would configure the transmitted

serial data into 8 bit words. However, this section will concentrate on the circuits needed

for processing voice information.

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4.2.1 Light collector:

For long-range application it is essential to collect the weak modulated light from the

distance transmitter with a glass or plastic lens and focus it onto a silicon PIN

photodiode. Although mirrors could also be used to collect the light, glass or plastic

lenses are easier to use and cost less. Plastic lenses measuring from a fraction of an inch

to six inches are available. For a system that demands a large lens, the flat “Fresnel” lens

is much less expensive than a solid lens. Forming special concentric bumps in a clear

plastic sheet makes Fresnel lenses. The bumps bend the light just as a conventional thick

lens would. Fresnel lenses are available with diameter or several feet.

For contain short-range application it may also be possible to use a naked light detector

without any lens. Distances up to several hundred feet are possible with system that don’t

rely on lenses at either the transmitter or the receiver. Lens- less system are especially

useful when very wide acceptance angles are required. Many cordless IR stereo headset

use two or more naked detector to provide acceptance angles approaching 360 degrees.

The lens chosen should be as large as possible but not too large. A lens that is too large

can produce a half angle acceptance angles that is too small. Acceptance angles less than

about 0.3 degrees will result in alignment difficulties. Building sway and atmospheric

disturbances can cause signal disruption with narrow acceptance angles.

A rough rule-of-thumb might be that the lens diameter should not be more than 100 times

larger than diameter of the active area of the PIN detector. Also, the receiver should

never be positioned so sunlight will destroy the sensor. A north/south alignment for the

transmitter and the receiver will usually prevent an optical system from going blind from

focused sunlight.

4.2.2 Light detector:

As discussed in the section on light detectors, the silicon PIN photodiode is the

recommended detector for most all through-the-air communications. Such a detector

works best when reversed biased. In the reversed biased mode it becomes a diode that

leaks current in response to the light striking it. The current is directly proportional to the

incident light power level (light intensity).

When detecting light at its peak spectrum response wavelength of 900 nanometer, the

silicon PIN photodiode will leak about 0.5 micro amps of current for each microwatts of

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light striking it. This relationship is independent to the size of the detector. The PIN

photodiode size should be chosen based on the required frequency response and the

Desired acceptance angle with the lens being used. Large PIN photodiodes will have

slower response times than smaller devices.

For example, 1cm X 1cm diodes should not be used for modulation frequency beyond

200KHz, while 2.5cm X 2.5cm diodes will work beyond 50MHz. If a long range is

desired, the largest photodiode possible that will handle the modulation frequency should

be used.

4.2.3 Stray light filter:

Some systems can benefit from the placement of an optical fiber between the lens and the

photodiode. The filter can reduce the effects of sunlight and some stray light from distant

street lamps. Filters can be specially effective if the light detector is going to be

processing light from a diode laser. Since laser light has a very narrow bandwidth, an

optical band pass filter that perfectly matches the laser light can make a light receiver

nearly blind to stray sunlight.

If light emitting diode light sources are used, optical filters with a much broader

bandwidth are needed. Such a filter may be needed for some situation where man-made

light is severe. Many electronically controlled fluorescent and metal vapor lamps can

produce unwanted modulated light that could interfere with the light from the distance

transmitter.

But, in all but a few rare exceptions, band pass filter produce few overall improvements if

the correct detector circuit is used. Since be optical filter is perfectly transparent, the

noise reduction benefits of the filter usually do not outweigh the loss of light through the

filter. Also, if the detector is going to process mostly visible light, no optical filter should

be used.

4.2.4 Current to voltage converter:

The current from the PIN detector is usually converted to a voltage before the signal is

amplified. The current to voltage convert is perhaps the most important section of any

optical receiver circuit. An improperly designed circuit will often suffer from excessive

noise associated with ambient light focused onto the detector.

Many published magazine circuit and even many commercially made optical

communication system fall short of achievable goals from poorly designed front-end

circuits. Many of these circuit are greatly influenced by ambient light and therefore suffer

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from poor sensitivity and shorter operating range when used in bright light condition. To

get the most from your optical through-the-air system you need to use the right front-end

circuit.

4.2.5 Signal amplifier:

As discussed above, the transimpedance amplifier converts the PIN current to a voltage.

However, it may be too much to expert one amplifier stage to boost the signal o f interest

to a useful level. Typically, one or more voltage amplifier stage after the front end circuit

are needed.

Often the post amplifiers will include some additional signal filter son only the desired

signals are amplified, rejecting more of the undesired noise. The circuit uses a quality

operational amplifier in conjunction with some filter circuit designed to process light

pulses lasting about 1 micro second. The circuit boosts the signal by a factor of X20.

4.2.6 Pulse discriminator:

Once the signal has been sufficiency amplified and filtered, it often needs to be separated

completely from any background noise. Since most system use pulse frequency

modulation techniques to transmit the information, the most common method to sepa rate

the signal from noise is with the use of a voltage comparator.

The comparator can produce an output signal that is thousand of time higher in amplitude

than the input signal. As an example, a properly designed comparator circuit can produce

a 5 volt peak to peak TTL logic output signal from a input of only a few millivolts.

But, to insure that the comparator can faithfully extract the signal of interest, the signal

must be greater in amplitude than any noise by a sizeable margin. For most applications, I

recommend that the signal to noise ratio exceed a factor of at least 10:1 (20 db). Then,

with a properly designed comparator circuit, the comparator output would change state

(toggle) only when signal is present and will not be effected bt noise.

4.2.7 Audio power amplifier:

The final circuit needed to complete a voice grade light pulse receiver is an audio power

amplifier. It cause a signal inexpensive LM386 IC. The circuit is designed to drive a pair

of audio headphones. The variable resistor shown is used to adjust the audio volume.

Since the voice audio system described above dies not transmit stereo audio, the left and

right headphones are wired in parallel so both ears receive the same audio signal.

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4.2.8 Noise considerations:

One of the most difficult problems to overcome in an optical through the air

communication system in ambient light. Any stray sunlight or bright background light

that is collected by the receiver optics and focused onto the light detector will produce a

large stedy state DC level through the detector circuit.

Although much of the DC is ignored with the use of an inductive feedback amplifier

method in the front-end circuit, the large DC component in the light detector will produce

some unwanted broadband noise.

The noise is very much like the background static you may hear on an AM radio when

tunning the dial between stations. As discussed in the section on light detector, the

amount of noise produces by the detector is predictable.

The equation describe how the detector noise varies with ambient light. The relationship

follows a square root function. That means if the ambient light level increases by a factor

of four, the noise produced at the detector only doubles. This characteristic both helps

and hurts a light receiver circuit, depending on whether the system is being used during

the light of day or during the dark of night.

The equation predicts that for high ambient daytime condition, you will have to

dramatically reduce the amount of ambient light striking the detector in order to see an

significant reduction in the amount of noise produce at the detector circuit.

The equation also describes that under dark nighttime conditions, the stray light has to

dramatically increase in order to produce a sizable elevation in noiself the system must

work during both day and night, it will have to contend with worst daytime noise

condition.

Conversely, some light receivers could take advantage of the low stray light conditions

found at night and produce a communication system with a much longer range than

would be otherwise possible if it were used during daylight.

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LIGHT DETECTOR NOISE

Ld= (𝟑.𝟐× 𝟏𝟎−𝟏𝟗)(𝑩𝒘)(𝑬)(𝒍𝒂)

Ld = RMS NOISE CURRENT FOR DETECTOR IN AMPS

Bw = RECIVER BANDWIDTH IN HERTZ

E = DETECTOR CONVERSION EFFICINCY (TYP 0.5)

la = DETECTOR DC CURRENT FROM AMBIENT LIGHT IN AMPS

NOTE: TYPICAL PEAK NOISE IS APPROX. 5X THE RMS

As mentioned above, interesting an optical filter between the lens and the light detector

can reduce the effects of ambient light. But, as shown by the noise equation, the amount

of light hitting the detectors needs to be dramatically reduce to produce a sizeable

reduction in the induced noise

Since most sunlight contains a sizable amount of infrared light, such filter do not reduce

the noise level very much. However, very narrow band filters that can be selected to

match the wavelength of laser diode light source, are effective in reducing ambient light

and therefore noise.

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Chapter 5

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5.Fiber Optic Cable

5.1 Basic principle of fiber optic cable:

5.1.1 Total internal reflection:

An unusual observation- a discrepant event –was observed in a recent lab-the index of

Refraction lab.the refraction of light yhough a glass block in the shape of an isosceles Tringle

was investigated (at least, so we thought ). In the lab, a ray of light entered the Face of the

triangular without refraction since it was incident along the normal.

The ray of light then traveled in a straight line through the glass untill it reached the Secondary

boundary . Only instead of transmitting across this boundary , the entirety of the light seemed to

reflect off the boundary and transmitting out the opposite face of the isosceles triangle .This

discrepant event bothered many as htey spent several minutes looking for the light to reflract

through the second boundry.

Then finally ,to their amazement ,they looked through the third face of block and could clearly

seee the ray. What happened ? Why did light not refracted through the second face

Figure 5.1.1

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The phenomenon observed in this part of the lab is known as total internal reflection.Total

internal reflaction , or TIR as it is intimately called , is the reflaction of the total Amount of

incident light at the boundary between two mediam.To understand total Internal reflection, we

will begin the thought experiment.

Suppose that a laser beam is submerged in a tank of water (dont do this at home ) and Pointed

upward toward water-air buondary .Then suppose that the angle at which the beam is directed

upward is slowly alterd,beginning with samll angle of incidence and procedding toward larger

and larger angle of incidence.

What would be observed in such experiment? If we understand the principle of Boundry

behaviour, we would expect that we would observed both refletion and refraction. And indeed

that is what is observed(mostly).

But that”s not only obervation which ehich we could make. We would also observed that the

intensity of the reflection and reflacted rays do not remain constant , At angle of incidence close

to 0 degree ,most of the light energy is transmitted across the boundry and very littel of it is

reflacted.

As the angle is incresed to greater and greater angle , we would begin to observed less reflaction

and more reflaction . That is, as the angle of inciedence is increased, the brightness of the

reflacted ray decreases and the brightness of the reflacted ray increases.

Finally ,we would observed that the angle of the reflaction are not equal. Since the light waves

would refract away from the normal (a case of the SFA principle of refraction.) the angle of the

refraction would be greater than the angle of incidence.

And if this the case , the angle of refraction would also be greater than the angle of Reflaction

(since the angle of reflaction and incidence are the same). As the angle of Incidence is increased

,the angle of reflaction would eventually reched a 90 degree

Angle.These principle are depicted in the diagram below.

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Figure 5.1.2

The maxiaum possible angle of reflection is 90-degree . Ifyou think about it (a practical which

always helps),you recognize that if the angle of refraction were greater than 90 degrees,then

refracted ray would lie on the incident side of the medium – that’s just not possible .So in the

laser beam in the water there is some specific value for the angle of incident (we’ll call it the

“crtical angle”) which yeild an angle of refraction of 90-degree. This particular value for the

angle of incident could be calculated using Snell’s law (ni=1.33,nI=1.000,ӨI=90 degree, Өi=???)

and would be found to be 48.6 degree.

Any angle of incidence which is greater than 48.6 degree would not result in any refraction.

Instead when the angles of incidence is greater (the critical angle ) all of the energy the (the totol

energy ) carried by the incident waveto the boundary stays within the water( internal to the

origianl medium) and undergoes reflaction off the boundary.When this happens, totol internal

reflaction occurs.

Total internal reflection (TIR) is the phenomenon which involves the reflection of all the incident

light off the boundary. TIR only takes place both of the following two condition are met:

The light is themore dense medium and approching the less dense medium.

The angle if incidence is greater than the so called criacal angle.

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Total internal reflection will not take place unless the incident light is traveling within the more

optical dense material medium toward the less optically dense medium.TIR will happen for light

traveling from water toward air,but it will not happen for light traveling from air toward

water.TIR would happen for light traveling from water .TIR would happen for light traveling

from (n=1.333) toward crown glass(n=1.52) TIR occurs becuase the angle of reflection reaches a

90 degree before the angle of reflection reaches a 90-degree angle.

The only way for the angle of reflection to be greater than the angle of reflection , is for light to

be bend away to the normal. Since light only bend away from the normal when passing from a

more dense medium to the less medium . then this would be a necessary condition for total

internal reflection.

Total internal refletion occurs with large angle of incidence . Question: How large iv large?

Answer: larger than critical angle for water air boundary is 48.6 degree.So for angle of incidence

greater than 48.6-degreee.TIR occurs .but 48.6 degree is the critical angle only for the water-air

boundary .The actual value of critical angle is dependant upon the two material on either side of

the boundary .For the crown glass-air is 41.1 degree.

For the diamond-air boundary, the critical angle is 24.4 degree.For the diamond-water buondary,

the critical angle is 33.4 degree . The critical angle is diffrent for diffrent media, we will

investigate how to determine the critical angle for any two material. For now, let’s internalize

the idea that TIR can only occurs if the idea that TIR can only occurs if the anglr of incidence is

greater than the critical angle for the particular combination of materials.

Figure 5.1.3

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Total internal reflection was demonstrated in class through a variety of demonstrated.In one

such demonstation , a beam of laser light was directed into a coiled plastic thing-a-majig. The

plastic served as “light pipe,” directed the light through the coiled untill it final existed out the

opposite end .Once the light entered the plastic , it was in the more dense med ium. Every time

the light approached the plastic-air boundary, it was approching at angle greater than the critical

angle .The two condition necessary for TIR were met,and all of the incident light at the plastic-

air boundary stayed internal to the plastic and underwent reflection . And with the room light off.

Every becomes quickly awre that once more physics is better than druges.

Figure 5.1.4

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This demonstratin illustrated the principle by which optical fiber work the use of a long strand of

plastic (or other material sush as glass) to pipe light from one end of the medium to the other is

the for the modern day use of optical fiber.

Optical figers are used in the communication system and micro-surgeries.Since total internal

reflection takes place within the fiber ,no incident energy is ever lost due to the transmission of

the light across the boundary . The intensity of the signal remains constant.

5.2 optical fiber launching light:

Once the transmitter has converted the electrical input signal from whatever form of modulated

light is desired , the light must be “launched” into the optical fiber .

As previously mentioned,there are two mwthods whereby light is coupled into a fiber onse ics by

pigtailing. The other is by placing the fibre’s tip in very close proximity to an LED or LD.

When the proximity type of coupling is employed ,the amount of light that will enter the fiber is

a function of one of four factors: the intensity of the LED or LD,the area of the light emitting

surface,the acceptance angle of the fiber, the fiber ,and the losses due to reflection and scattering.

Following is a short discussion on each:

Intensity : The intensity of an LED or LD is a function of its design and is usuaiiy specified in

terms of total power output at a particular drive current .sometime,this figure is given a actual

power taht is delivered into a particular type of fiber of all other factor being equal,more power

provided by an LED or LD translates to more power “launched” into fiber.

Area: The amount of light “launched” into a fiber is a function of the a area of the light emitting

surface compared to the area of the light accepting core of the fiber . The smaller this ratio is ,the

more light that is launched into the fiber

Acceptance Angle: The acceptance angle of a fiber is expressed in terms of numeric aperture.The

numerical aperture (NA ) is defined as the sine of one half of the acceptnace of the fiber .Typical

NA values are 0.1 to0.4 which correspond the acceptance angle of 11 degree ti 46 degree.Optical

fiber will only transmitte light that enters at an angle that is an equal to or less than the

acceptance angle for the particular fiber.

Other losses: other than opaque obesruction on the surface of a liber, there is always a loss due to

reflaction from the entrance and exit surface of any fiber. This loss is called the Fresnall loss and

is equal to about 4% for each transition between a air and glass. There are special coupling gels

that can be applied between glass surface to reduce this loss when necessary.

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5.3 Losses in the optical fiber :

Other than losses ehibited when coupling LEDs or LDs into a fiber , there are losses that occur as

the light travells through the actual fiber. The cores of optical fiber is made of ultra-pure low-

loss glass. Considering that light has to pass through of feet or more fiber core, The purity of the

glass must be extremely high

To appreciate the purity of the glass ,consider the glass in common windowpanes.We think of

windowoanes as “clear,” allowing light to pass fereely through, but this is bicuase they are only

1/16 to ¼ inch thick ,in contrast to this clear apperance ,the edges of a broken windowpanes

look green and almost opaque .

In this case ,the light is passing edgewise into the glass, through several inches. Just imagine how little light would be able to pass through a thosand feet of window glass!

Most general pupose optical fiber exhibits losse of 4 to 6 dB per km ( a 60% to 75% loss per km) at a wavelength of 850 nm.When the wavelength is changed to 1300 nm the loss drops to about

3 to 4 dB (50% to 60%) per km. AT 1550nm, it is evev lower. Premium fiber are available with loss figures of 3 dB (50%) per km at 850nm and 1 db(20%) per km at 1300nm. Losses of

0.5dB(10%) peer km at1550nm are not uncommon.

Another source of loss within the fiber is due to exessive bending ,which causes some of the light

to leave the core area of the fiber.

The smaller the bend radius,the greater the loss.Becuase of this, bends along a fiber optic cable should have a turning radius of at least an inch.

5.4 Optic Fiber Bandwidth:

All of the above attenution factors result in simple attenution that is indepedent of bandwidth .

In other words, a 3 dB loss means that 50% of the light will be loss whether it is being modulated at 10 Hz or 100 MHz.

There is an actual bandwidth limitation of optic fiber however , and this is measured in MHz per km. The easiest way to understand why this loss occurs is to refers to figure 5.4.1

Figure 5.4.1 Diffrerent Light Path Length Determine The Bandwidth of Fiber

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As figure 5.4.1 illustrated, a ray of light that enters a fiber at a small angle (M1) has a shorter path through the fiber than light which enters at an angle close to the maximum acceptance angle

(M2), as a result ,diffrent .”rays” (or modes) of light reach the end of the fiber at different times, even through the original source is same LED or LD.

This produces a “smearing” effect or uncertainty as to where the start and end of a pulse occurs at the output end of the fiber- which in turn limits the maximum frequency that can be transmitted.

In short, the less modes, the higher the bandwidth of the fiber. The way that the number of

modes is reduced is making the core of the fiber as small as possible. Single-mode fiber, with a core measuring only 8 to 10 microns in diameter, has a much higher bandwidth becuase it allows only a few modes of light to propogate along its core.

Fiber with a wider core diameter such as 50 and 62.5 microns, allows many modes to propagate

and are therefore referred to as “multimode” fibers.

Typical bandwidth for common fiber range from a few MHz per km for verry large core fibers,

to hundreds of MHz per km for standard multimode fiber, to thousands of fiber increase, its bandwidth will decrease proportionally. For example, afiber cable that can support 500MHz bandwidth at a distance of one km only be able to support 250MHz at 2 km and 100MHz at 5

km.

Because single –mode fiber has such a high inherent bandwidth, the “bandwidth reduction as a function of length” factor is not real issue of concern when using this type of fiber. However, it is a consideration when using multimode fiber, as its maximum bandwidth often falls within the

range of the signals most often used in point-to-point transmission systems.

5.5 Fiber Optic Cable Construction:

Fiber optic cable comes in all sizes and shapes. Like coaxial cable, its actual construction is a function of its intended application. It also has a similar “feel” and appearance. Figure 5.5.1 is a

sketch of a typical fiber optic cable.

The basic optic fiber is provided with a buffer coating which is mainly used for proctection during the manufacture praocess. This fiber is then enclosed in a central pvc loose tube which allows the fiber to flex and bend, particularly when going around corners or when being pulled

through conduits.around the loose tube is a braided kevlar yarn strength member which absorb most of the strain put on the fiber during installtion.

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Figure 5.5.1 Constuction of a Typical Fiber Optic Cable

Finally , a PVC outer jacket seals the cable and prevents moisture from entering.

Basic optical fiber is ideal for most inter-building application where extreme ruggedness is not required . In addition to the “basic” variety. It is also available for just about any application ,

including direct biried ,armored,rodent resistant cable with steel outer jacket, and UL approved plenum grade cable. Colored-coded, multi- fiber cable is also available.

5.6 Other Types Of Fiber :

Two additional types of fiber – very large core diameter silica fiber and fiber made completely of

plastic- are normally not employed for data transmission.

Silica fiber it typically used in application involving high power lase and sensors, such as medical laser-surgery.

All- plastic fiber is useful for very short data links within eqquipment beacuse it may be used with relatively inexpensive LEDs . An isolation system for use as part of a high voltage power supply would be typical exmple of application for plasic fiber.

5.7 Optical connectors:

Optical connectors are the means by which fiber optic cable is usually connected to peripheral equipment and to other fiber. These connectors are similar to their electrical counterpart in

function and outward apperance but are actually high precision devices.

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In operation,the connector centers the small fiber so that its light gathering cores lies directly core lies directly over and in line with light source (or other fiber) to tolerances of a few ten

thousandths of an inch. Since the core size of common 50 micron fiber is only 0.002 inches, the need for such extreme tolerances is obvious.

There are many different types of optical connectors in use today. The SMA connectors, which was first develope before the invention of single-mode fiber, was the most popular types connectors until recently. Figures 5.7.1 shows an exploded view of the parts of this connector.

Figure 5.7.1 Construction of SMA Connector

The most popular type of multimode connector in use today is the ST connectors. Initially developed by AT&T for telecommunication purposes, this connectors will exhibits less than 1

dB (20%) of loss and does not require aligment sleeves or similar devices.

The inclusion of an “anti-rotaion tab” assures that every times connectors are mated, the fibers always return to the same rotational position assuring constant, uniform performance.

ST connectors are available for both multi-mode and single-mode fibers, the primary difference being the overall tolerances. Note that multi-mode ST connectors will only perform properly

with multimode fibers. More expensive single-mode ST connectors will perform with both singlemode anr multimode fibers.

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The installation procedure for the ST connector is not difficult and can be easily mastered by any system installer. Figure 5.7.2 shows same of the major feature of the typical ST connector.

Figure 5.7.2 Major Feature of The Standard ST Connector

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Chapter 6

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6. User guide

6.1 Operating Manual:

First of all check off the component against the component listing on the next page. Especially

make sue toget the resistors correctly identified.

Note that the FO emitter and detector modules are very similar. The components for both board

are mixed together. Make one board at a time.

Look at the circuit schematic diagram, identify the component them place it in the board. It is

generally best to add the lowest hight component first.

So place the resistor first. The elecret microphone must be inserted with a pin connected to the

metal case connected to the negative rall (that is, to the ground of zero voltage side of the

circuit) .

This is marked with a sign on the Mic on the transmitting circuit board. Make sure to get the

electrolytic capacitors around the correct way.

6.2 Technical Specification:

1. Working voltage: 230v AC

2. Operating current: 250MA

3. Range : 1.5m

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6.3 Part(Components) List [Tx]

R1 3K3

R2 4K7

R3,R4 10K

R5 100E

R6 1K

P1 1M/ 3006 TRIMPOT

C1 1000 UF / 16V

C2 22 UF/ 16V

C3 47KPF, DISC

C4 10UF/25V

D1-D4 4007 DIODE(4 nos.)

U1 LM 7809

U2,U3 LM 741

Q1 BC 547

TX1 FBO TRANSMITTER

2 nos. 8 PIN IC SOCKET

1 nos. CON .MICE

1 nos. 0-12 V /250MA TRASFORMER

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6.4 Part(Component) List [Rx]

R1 100K

R2 1M

R3 1K

R4 100K

R5 100K/3006 TRIMPOTE

R6 10K LOG VOLUME CONTROL

R7 220K

R8 100K

R9 10K

R10 4.7E

C1,C3,C4,C5,C8,C9 0.1UF DISC (104)

C2 100/16V

C6 10UF/25V

C7 10UF/25V

C10 220UF/16V

D1-D4 4007 DIODE( 4 nos.)

U1 LM 7805

U2 LM 358

U3 LM 386

RX1 FIBER OPTIC RECEIVER

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2nos. 8 PIN IC SOCKET

1nos. 0-12 / 250MA X’MER

1nos. 3MM FIBER OPTIC CABLE [1.5M]

1nos. 2” ½ SPEAKER

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Referances:

Books:-

(1) Optical Fiber Communication by Gerd Kaiser

(2) Optic Fiber Communication by Senior

WEBSITE:-

@www.alldatasheet.com

@www.google.com

@www.nationalsemiconductor.com

@www.atmel.com

@www.kitrus.com

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Appendix