Reyson Proposal

7/31/2019 Reyson Proposal

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1 INTRODUCTION

Communication is one of the most important parts of human life. Communicating with each

other and passing information and understanding each other are of vital importance if you

are to survive in society. Methods of communication, verbal and non-verbal have come a

long way since the time we communicated for simple needs like hunger and illness.

Since Alexander Graham Bell the methods of communication technology has developed at

an alarming rate. This actually shows us that mankind was actually waiting for a

technological breakthrough that would push us to the next step in communication. Imagine if

Bell did not invent the telephone, what kind of world we would be living in today. Almost

every thing that concerns communication uses the technology. Graham Bell discovered not

only the telephone, but the base for all modern communication methods.

Today we can pick up a telephone and call any one around the world or in some exceptional

cases astronauts who are based in space stations. We have the internet, satellite

communication and so many other ways that we can communicate.

A couple of centuries ago mankind did not even dream of communicating with the use of the

sky, much less dream of something like satellite communication. A couple of decades ago

we did not dream of what we could do with satellite communication. Today satellite

communication has become one of most powerful methods of communication. Everything

from phones to the internet to the TV and radio can be used through satellite

communication. Through satellite communication it is possible for an individual to

communicate with anyone who has a similar communication device. This can be done from

anywhere in the world. The latest GPS (General Positioning System) can show an

individuals position with pinpoint accuracy. An individual can ask for directions from the

system and it will show you the closest route that you can get there If the road to the


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destination is under construction or if you cannot go in that path the GPS will choose a

different route for you.

The best and worst part about communication is that it evolves, develops and expands. The

way we see the world today will be totally different to how we might see the world in

another 10 years time. Communication methods and ways might change and expand in ways

that we never thought possible. Now it is even possible to move and control other objects by

the movement of your hands, movement of your eyes and even by just thinking it. In the

future we might start to communicate by just thinking about it. All this might be in the

future, but for the moment talking and body expressions are some of the simplest and best

ways to express yourself.

1.1 BACKROUND RESEARCH

The telephone has changed the world we live in. Just like in recent years the Internet has

completely changed the way we live and learn, the telephone allowed people to

instantaneously get in touch with other people the other side of the world, well before the

Internet. This is the main advantage, in that these days anybody can get in touch with their

friends or loved ones within a matter of seconds or minutes.

However, there is actually growing concern that mobile telephones are emitting waves and

signals that could potentially damage the brain. Given that mobile telephones, in historical

terms, are relatively new we do not have any extended research on what these waves and

signals are able to do to the brain. Hence, all we can do is carry-on using the mobile phones

and see what happens in the future. For all we know, these mobile phones could be causing

cancers in a large proportion of their users.

The internet, cell phones and telephones are a way of life in modern society. Using Laser as

a communication medium can be a good substitute for the present day communication


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router requirements. Besides there are no recurring line costs, portability, transparency to

networks or protocols, although range is limited to a few hundred meters. Also the

laser transmission is very secure because it has a narrow beam. Also it cannot be detected

with use of spectrum analyzers and RF meters and hence can be used for diverse

applications including financial, medical and military. Lasers can also transmit through

glass, however the physical properties of the glass have to be considered. Laser transmitter

and receiver units ensure easy, straightforward systems alignment and long-term stable,

service free operation, especially in inaccessible environments, optical wireless systems

offer ideal, economical alternative to expensive leased lines for buildings. The lasers can

also be commissioned in satellites for communication, as laser radar requires small aperture

as compared to microwave radar. Also there is high secrecy and no interference like in EM

waves. Further, potential bandwidth of radar using lasers can translate to very precision

range measurement. For these reasons, they can be used as an alternative to present modes of

communication, which is both wide-band and high-speed.

1.2 PROJECT AIMS AND OBJECTIVES

The aim of this project is to design and develop a laser voice transmitter and receiver as an alternative way of

communication.

The project will specifically provide emphasis on the following items:

1. To identify the design features of the transmitter and receiver modules;2. To design and develop the hardware components of the prototype device;3. To evaluate the effectiveness of transmitting and receiving voice signals;4. To compare the results of the prototype model with that of existing communication device.

1.3 PROJECT SPECIFICATIONSThe laser beam can transmit light up to a distance of about 500 meters. (depending on the


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1.4 SCOPE AND DELIMITAITONSThe study consists of transmitter and receiver circuits. The circuit is based upon the principle

of light modulation where instead of radio frequency signals; light from a laser torch is used

as the carrier in the circuit. Using this circuit we can communicate with your neighbors

wirelessly. The laser torch can transmit light up to a distance of about 500 meters. The

phototransistor of the receiver must be accurately oriented towards the laser beam from the

torch. If there is any obstruction in the path of the laser beam, no sound will be heard from

the receiver.

This study is limited to

1.5DEFENITIONS OF TERMS1.5.1 Laser

A laser is a device that emits light (electromagnetic radiation) through a process ofoptical

amplification based on the stimulated emission ofphotons. The term "laser" originated as

an acronym for Light Amplification by Stimulated Emission of Radiation.

1.5.2 Communication

Communication is the exchange of thoughts, messages, or information, as by speech,

visuals, signals, writing, or behavior. Derived from the Latin word "communis", meaning to

share. Communication requires a sender, a message, and a recipient.

1.5.3 Transmitter

In electronics and telecommunications a transmitter or radio transmitter is an electronic
http://en.wikipedia.org/wiki/Electromagnetic_radiationhttp://en.wikipedia.org/wiki/Optical_amplificationhttp://en.wikipedia.org/wiki/Optical_amplificationhttp://en.wikipedia.org/wiki/Stimulated_emissionhttp://en.wikipedia.org/wiki/Photonhttp://en.wikipedia.org/wiki/Acronymhttp://en.wikipedia.org/wiki/Messagehttp://en.wikipedia.org/wiki/Electronicshttp://en.wikipedia.org/wiki/Telecommunicationshttp://en.wikipedia.org/wiki/Electronic_devicehttp://en.wikipedia.org/wiki/Electronic_devicehttp://en.wikipedia.org/wiki/Telecommunicationshttp://en.wikipedia.org/wiki/Electronicshttp://en.wikipedia.org/wiki/Messagehttp://en.wikipedia.org/wiki/Acronymhttp://en.wikipedia.org/wiki/Photonhttp://en.wikipedia.org/wiki/Stimulated_emissionhttp://en.wikipedia.org/wiki/Optical_amplificationhttp://en.wikipedia.org/wiki/Optical_amplificationhttp://en.wikipedia.org/wiki/Electromagnetic_radiation


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1.5.4 Receiver

Receiver is the receiving end of a communications channel.


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2 LITERATURE REVIEW

2.1 CONCEPTUAL LITERATURE

2.2 RELATED LITERATURE

2.2.1 Optical-fiber Telecommunications

Optical communication has a long history, dating back to signal fires in ancient times.

Optical communication reached a peak in the 1800s with the use of the heliograph for

signaling during military operations in the American southwest. In the late 1800s, electronic

communication developed very rapidly, and virtually eliminated interest in optical

communication for many years.

By the middle of the 20th century, the electronic communication system had become very

crowded. The radio spectrum was virtually filled. Telephone lines were heavily loaded,

especially in large cities. The cost of adding new telephone lines in urban areas was very

high. When the laser was invented in 1960, interest in optical communication was revived.

The invention of the laser made it possible to build optical communication systems with

significant advantages:

1. Very high concentration of optical power and very little spread of that power with

distance (low beam divergence).

2. Ability to carry huge amounts of information (high information bandwidth).

3. Small antennas required (compared to radio-frequency communication systems).

4. Narrow spectral linewidth, allowing the rejection of light except at the laser wavelength.


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Because of these characteristics, useful information can be impressed on a beam of light, and

transmitted to a remote location, where the information can be recovered. There was great

initial interest in optical communication based on laser beams transmitted through the

atmosphere. However, this type of optical communication system has not attained wide use

because of the nature of the atmosphere. The atmosphere often can be turbulent, causing

beam wander and scintillation. Molecular absorption bands cut off some wavelengths

completely. Scattering by haze and dust is also a problem. Poor weather conditions, like fog,

clouds, and rain, might shut down a communication link completely. So, only a few laser-

based communication systems have been developed for free atmospheric propagation.

Optical fibers offer an attractive alternate choice for a transmission medium to eliminate

problems with atmospheric transmission. Thin optical fibers can be fabricated with lengths

of many kilometers. A light beam coupled into one end of the fiber can propagate through

the fiber without atmospheric interference, and can be detected at the other end of the fiber.

Figure 1 shows the structure of a fiber. It has a core surrounded by a cladding with lower

index of refraction. The numerical aperture (NA) is defined as NA sin a max where a max is

the angle between the incident light and the fiber axis. The NA is a measure of how much

light can be coupled into the fiber.

Fig. 2.1 Fiber design

In practice, laser based communications are dominated by fiber-optic transmission. A fiber-

optic telecommunication system is used to transfer information (such as conversations, TV

pictures, telemetry data) over some distance. The optical communication system is generally


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several respects. This chapter will describe briefly the areas of concern in communication

systems in general. It will emphasize those characteristics of the communication system that

are peculiar to optical (or laser) communication. In addition, it will emphasize the systems

aspects of fiber-optic communication systems. It assumes that the principal components

(lasers, fibers, and detectors) are familiar to the student.

The laser used in fiber-optic telecommunication systems is the semi-conductor laser.

Semiconductor lasers are especially well-suited for use in this type of communication

system. Semiconductor lasers have suitably small size and configuration for coupling into

the small-diameter core of an optical fiber. Modern AlxGa1xAs lasers operate continuously

at mill watt power levels sufficient for fiber-optic communications. They can be modulated

easily, through modulation of an electric power supply, at frequencies up to the gigahertz

range. This makes it possible to transmit information, by modulating a beam of light from a

laser, through optical fibers. The semiconductor laser, with its small emitting area, is a

natural choice as a source for fiber communications. But for a number of years, laser lifetime

was too short and fiber losses too high to make laser-based fiber communications a success.

The status of both lasers and fibers has advanced considerably, with both fiber loss and laser

lifetime undergoing improvements by factors of ten. Figure 2.2 shows the improvement of

these parameters over the years. Before laser-based fiber-optic telecommunications could be

regarded seriously, laser lifetime had to reach 105

hours or more, and fiber loss had to be

reduced to a few dB/km or less. Both these levels were reached by the late 1970s. In recent

years, fiber-optical telecommunication systems have become practical realities, carrying

information for intracity telephone links, telephone trunk lines, video data links, and

information links between computers.


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Fig. 2.2 Fiber loss and laser lifetime

Figure 2.2 schematically represents that loss in fiber will decrease and laser lifetime will

increase as the telecommunications industry matures.

The loss in optical fibers is expressed commonly in terms of the number of decibels (dB)

loss per kilometer of length of the fiber.

Loss in decibels is described on a logarithmic scale. If a signal P0 is input to a fiber and a

signal P is transmitted, the loss in decibels is expressed as

dBloss = 10 log10 ( P0/P)

So, ten decibels corresponds to a decrease in signal level by a factor of 10, twenty decibels

by a factor of 100, etc.

Example A: Fiber Loss over Distance

Given:

A signal of 10 mW coupled into an 10-km-long fiber with a

signal of 1 microwatt detected at the end of the fiber.

Find: The dB loss of the fiber.

Solution: The loss, in dB, is given by:


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10 log10 (P0/P) = 10 log10 (10 mW/103

mW)

P0/P = 10 log10 104

= 40 dB

So the dB loss of the fiber per kilometer of fiber length is:

40 dB/10 km = 4 dB/km.

A common nomenclature associated with optical-fiber telecommunication is that of light

wave and waveguide.Light wave refers to the components used to generate and receive the

light (lasers, detectors, etc.). Waveguide refers to the media through which the light is

transmitted (fibers, connectors, and so on).

A basic fiber-optic telecommunication link is shown in Figure 3. The laser output is

modulated to yield a digital pulse-code-modulated (PCM) signal, that is, a series of ones and

zeros. The input signal drives the laser power supply (the driver), which in turn pulses the

laser on and off. The light from the laser is coupled into the fiber. The end of the fiber is

positioned by a connector to maximize the input. This part of the system constitutes an

optical transmitter.

The fiber carries the light toward the receiver, where the light is detected and the digital

signal is recovered. But the link may be long, perhaps many kilometers. Absorption,

scattering and dispersion in the fiber may degrade the signal. Optical amplifiers are needed

to regenerate the signal every 50 to 100 km. Early fiber-optic telecommunications systems

included signal repeaters that consisted of a detector, amplifier, and a signal regenerator that

restored the shape and intensity of the pulses. In the more modern networks, the repeater

system is replaced by an optical amplifier which consists of laser gain material and

replicates and reinforces the signal optically.


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Fig. 2.3

Schematic diagram for fiber-optic communication system

Several repeaters may be needed between the original source and the final receiver. Current

technology usually requires repeaters every few kilometers. The distance between repeaters

is an important parameter that affects the cost and practicality of a fiber-optic

communication system.

At the end of the fiber is the receiver, which consists of an optical detector that detects the

light and turns it back into an electrical signal, plus an amplifier and regenerator that restore

the pulse shape. The output is a PCM train of digital information, the same as the input at the

transmitter.

Figure 2.3 also shows a splice. Its important for installation and for repair to be able to cut

fiber cables and to splice them. Low-loss splices for optical cables have been developed.

These splices can be used successfully to repair cables in the field. A splice may add a loss

of around 0.2 dB to the system.

Fibers are available as multifiber cables with protective coatings, metal strands for strength

and outside covers. Multifiber stranded cables, for example, include interwoven strands of 1

to 16 fibers. Multifiber ribbon cables are based on ribbons manufactured by packing fibers

between adhesive-backed plastic tapes Such ribbons can contain up to 140 fibers


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Cables can be obtained complete with connectors that allow simple plug-in connection to

optical components or to other lengths of cable. Such connectors are available with a loss of

only a few tenths of one decibel. High-quality multifiber cables and connectors are

manufactured in large volume.

To understand the factors that affect the design of fiber-optic communication systems, we

first describe the causes of signal degradation in these systems.

Fiber-optic telecommunication systems use pulse code modulation. This means that the

information is transmitted as a series of pulses that represent binary bits of informationthat

is, ones and zeros. The presence of a pulse in a given time interval will represent a one. The

absence of a pulse represents a zero. Information can be lost if the amplitude of the pulse

becomes so small that it cannot be detected, or it can be lost if the pulse shape becomes

spread out so that it does not fall within the proper time interval. In either case, the

information represented by the pulse cannot be received by a receiver looking within the

specified time interval.

Causes of signal degradation are shown in Figure 2.4. The top part of the figure represents

what is commonly called attenuation or fiber loss. The intensity of the light pulse decreases

as the pulses travel along the length of the fiber. This is the number that usually is expressed

in terms of decibels per kilometer. Fiber loss will decrease the amount of light that is

available for the receiver, but it does not cause the signal to move out of its proper time

interval.


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Fig. 2.4

Causes of signal degradation

The lower part of the figure shows two mechanisms of signal degradation that do not involve

loss of light intensity, but that do cause the pulse to broaden and to move out of its time slot.

The first, called modal dispersion, results from the fact that light can travel along different

paths down the length of the fiber. This means that the initial short pulse will be broadened,

and will spread out of its time slot.

The second, called chromatic dispersion, results from the variation of index of refraction

with wavelength, so that light of different wavelengths travels through the fiber at different

velocities.

We now discuss these three causes of signal degradation and their effect on system

performance.

Choice of Components for Fiber-optic Systems

Table 2.1 describes the components used in fiber-optical systems. The losses for fibers are

those of production fibers. Experimental fibers with lower losses have been demonstrated.

Laser sources offer high performance, but LED sources may be suitable as lower-cost

sources in systems that require lower performance.

Most existing systems use AlxGA1xAs source (either lasers or LEDs) operating near

0.85 m m. Such systems are termed "short-wavelength" systems. There is much interest in

InGaAsP sources operating near 1.3 m m and 1.55 m m. These are called "long-wavelength"

systems. The long wavelengths have two advantages: The loss in optical fiber is lower than

at 0.85 m m, and the dispersion (the variation of refractive index with wavelength) is near


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experimental work is emphasizing long-wavelength systems, and we may expect that future

installed fiber-optic systems will be dominated increasingly by long-wavelength devices.

Table 2.1 Typical Components for Fiber-optic Systems

Fibers

2 dB/km at 850 nm, 0.5 dB/km at 1300 nm, 0.2 dB/km at

1550 nm

Bandwidthdistance products to 3000 MHz-km

Sources

Lasers give 2-10 mW into cable, up to 6000-MHz

modulation rates, 106 hour life.

LEDs give 0.1 mW into cable, 200-MHz modulation,

greater than 106

hour life.

Detectors

PIN photodiodes give responsivity 0.5 A/W, noise

equivalent power (NEP) 1012

W/(Hz)1/2

.

APDs give responsivity 75 A/W, NEP 1014

W/(Hz)1/2

.

Connectors

and Splices

0.1-0.5 dB insertion loss.

Table 2.2 lists typical properties of sources that could be chosen for use in fiber-optic

systems.

Table 2.2 Typical Selected Semiconductor Sources

Type

Wavelength

(nm)

Power

(mW)

Current

(mA)

Spectral

Width

(nm)

Beam

Divergence

(degrees)


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AlGaAs LED 830 1 200 40 30 42

AlGaAs

Double

heterojunction

laser

850 10 300 2.5 5 20

AlGaAs TJS

laser

830 15 65 0.1 13 40

InGaAsP laser 1300, 1550 7 250 4 10 30

Multiple-stripe

AlGaAs laser

850 to 500 1600 2 10 35

Some properties of commercially available fibers are illustrated in Table 2.3 An important

parameter is the distance-bandwidth product, expressed in MHz-km. This gives the product

of the maximum data-transmission rate and the maximum distance between repeaters. It is a

common figure of merit used to characterize system performance.

Table 2.3 Types of Fibers

Type

Diameter

Core/

Cladding

(Micrometers)

NA

Attenuation

(dB/km)

Distance

-Bandwidth

Product

(MHz-km)

Short

distance

lti d )

100/200 0.3 5 - 10 20 - 200


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Single

mode

6/125 0.03 < 1 > 1000

Long

distance

graded

index

50/125 0.2 1 - 5 500 - 1500

The detectors are another important part of the system. Detector technology is well

developed. For AlxGA1xAs sources, silicon photodiodes are suitable detectors. Silicon

photodiodes offer excellent high-frequency response at wavelengths to 1.1 m m. They have

peak spectral response near 0.9 m m, close to the wavelength of AlxGA1xAs lasers. At

longer wavelengths, in particular at 1.3 m m, silicon photodiodes are no longer useful. For

long-wavelength (i.e., 1.3-m m) fiber-optic systems, germanium or InGaAsSb photodiodes

must be used. However, they are less well developed than silicon photodiodes.

Detectors for fiber-optic communications systems are of two types, either PIN or APD. The

PIN construction features a layer of P-type material, a layer of Intrinsic silicon, and a layer

of N-type material. So the initials PIN are used. The APD detector is

an Avalanche PhotoDiode, which features a high applied voltage. When light is incident on

the APD, an avalanche of impact ionization is produced by high-energy carriers, so the APD

delivers a large signal.

The detectors are characterized by a parameter called responsivity. Responsivity is the

amperes of electrical current output per unit of optical power input. The APD offers better

performance (that is, higher responsivity) but at higher cost, than the PIN photodiode.


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The receiver used for detection of the light and recovery of the information contains a

detector, which converts the received optical energy to an electrical signal, and the

electronics to convert the detector output to a usable electronic signal.

Even though the detector has provided an adequate signal voltage corresponding to the

received optical signal, care must be exercised in the electronic circuitry that follows the

detector if optimum system performance is to be realized.

The first stage of amplification following the detector, called thepreamplifieror, more

commonly "preamp," is especially important. Any external noise introduced at the preamp

will be amplified in succeeding stages, and will degrade ultimate system performance. In

most cases, the detector and preamp are located as close to each other as possible to reduce

the amount of interference or noise picked up in this stage.

Random fluctuations in the output of the receiver are called noise. Noise can be categorized

on the basis of its source. Noise may be associated with the optical signal itself. For

example, if dispersion causes part of the pulse energy to spill over into the time slot for

another pulse, this will represent a source of noise. Other types of noise are generated by the

detector and the detector load resistor. A third type of electronic noise is that induced by

amplifiers in the receiver.

One measure of the performance of an optical communication system is the magnitude of

the signal in comparison to the noise. This is expressed as the signal-to-noise ratio (SNR).

SNR is calculated from the formula:

SNR =

In digital communication systems, a different figure-of-merit is also used. It is called the bit

error rate (BER). The BER is the ratio of the number of wrong decisions made by the


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BER =

In practical modern optical communication systems, bit error rates less than 109

are desired.

Figure 2.5 shows sensitivities for state-of-the-art optical receivers, plotted as a function of

bit rate. The sensitivity represents the number of photons required to achieve a bit error rate

of 109

. Sensitivities are given in terms of the average number of signal photons per bit to

achieve a bit error rate of 109

. The bands show expected performance for APD and PIN

receivers. The dots and squares represent experimental results for PIN devices and APD

devices, respectively.

Fig. 2.5

Receiver sensitivities for optical receivers versus bit rate.

Trade-offs in Optical Systems

Now let us consider trade-offs that may be made in the choice of components for a system.


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telecommunication system. Systems that require relatively low bandwidth and/or short

distances between repeaters may use low-cost components: LEDs, PIN photodiodes and

multimode fibers. These components are adequate for systems that operate with a

bandwidth-distance product less than 100 MHz-km, higher-cost components (laser diodes,

APDs as detectors and single-mode fibers) should be used.

Table 2.4 Performance/Cost Trade-offs

High cost/high performance

(large bandwidth, long distance between repeaters)

Sources: Laser diodes

Detectors: Avalanche photodiodes

Fibers: Single mode

Low cost/low performance

(small bandwidth, short distance between repeaters)

Sources: LEDs

Detectors: PIN photodiodes

Fibers Multimode

Table 2.6 illustrates the trade-offs for the choice of the wavelength of a system. Most

operating systems now operate at 0.85 m m. But for high performance future systems, the

dominant factor may be fiber loss, which is lowest at 1.55 m m. We can expect most high-


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Table 2.6 Wavelength/Performance Trade-offs

Short wavelength (0.85 m m)

well-developed sources and detectors

Long wavelength (1.3 m m and 1.55m m)

lower fiber loss and higher bandwidth fibers

2.2.2 Free space optical communication:

Free space optical communication is used to transmit data between two stations. Free

space optical communication is the part of technologies used in telecommunication and

referred as the line of sight communication which transmits a modulated beam of light in

free space.

Light emitting diode (LED) or Laser diode is used in free space optics. Beam energy in

free space optics is collimated and transmitted via free space compared to the optical

cables in which beam is guided.

There are less chances of light being distorted outside the atmosphere that is why

it is used for communication between spacecrafts.

For short distance optical communication, LEDs are used and infrared laser lights

are used for long distance communications.


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1.1.1.1 Applications:

Following are basic applications of free space optics.

It facilitates LAN connections between campuses over fast Ethernet. Same

function can be performed through optical communication using optical fibre but

it demands budget to be expanded.

It can provide connections between different LANs in a city.

It can be used to upgrade existing wireless technologies.

It may provide help in re-establishment of high speed connections.

Interconnection between two spacecrafts can be achieved by this technology.

1.1.1.2 Advantages:

Advantages of free space optical communication are described below.

Very quick link establishment.

Bit error rate is very low.

Transmission facilitates full duplex communication.

Transparent protocol.

Dispersion rate is low.

Light beam may be visible or invisible to provide help for aiming and detection of

failures. [1]

1.2 Laser:

1.2.1 Why use a Laser:


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other types of media, laser has unique characteristics as a communication media. Line of

sight laser communication can be used when it is hard for wires to be physically

connected with a remote location. There is also no need of laser light shielding for long

distances which is main requirement for cables. Longer distance communication is

possible with the use of laser. It is also possible to make communication possible by using

RF but it may face interference due to other RF transmitters. As diameter of laser is few

millimetres and also it is line of sight technology, it is harder to tap the data. Due to it s

this characteristic it makes communication safe and secure.

1.3 Proposed project:

Our aim was to design and implement free space laser communication system by dividing

it into two sections of transmitter and receiver. Main aim of the project was to transmit

analog and digital data simultaneously using the same laser beam that is also bandwidth

efficient. As bandwidth of the laser can be described in frequency so if a laser for

example has a wavelength of 532 nm it corresponds to frequency of 5.61014 Hz by

using following formula.

= C/F

Frequency of 5.61014 Hz is the bandwidth of a laser whose wavelength is 532 nm. So it

follows that use of laser for simultaneous transmission of voice and text results in

efficient bandwidth utilization. Diagram shown in the next page is of the proposed

communication system.


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Figure#1.1: System Overview

It shows that we wanted to transmit voice and pc to pc text through same laser beam.

Hyper terminal was proposed to be used for text communication between two stations.

1.4 Basic working principle:

Working on this project required a comprehensive knowledge about the mixing of voice

and text signals and then separating them at the receiver side. As project working was

divided into two tasks, there was need to first design block diagrams of these two

sections.

1.4.1 Transmitter:

We studied about different methods such as optical wavelength division multiplexing and

frequency division multiplexing to multiplex analog and digital signals but these methods

required components which were not suited to budget constraints. So we presented a way

to mix both signals which was according to the budget constraints. Block diagram

designed for project represented the way of mixing both signals which is given below.


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Figure#1.2: TransmitterBlockDiagram

Main components of transmitter are transformer, bridge rectifier, voltage regulator,

microphone, transistors, audio amplifier, max-232 and laser.

1.4.2 Receiver:

At the receiver side, aim was to detect the mixed signal and separate both of voice and

text signals. It was proposed to use a separator to separate voice and text signals.

Phototransistor was proposed to be used to detect laser beam. Block diagram better

defines the use of separator which is given below.


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Figure#1.3: ReceiverBlockDiagram

Main components used in the receiver are Transformer, Bridge rectifier, Voltage

regulator, Audio amplifier, Max-232, DB-9 connector, Phototransistor and Earphone.

1.5 Hyper terminal:

Hyper terminal is the windows application which enables to transfer text and files from

one PC to a remote PC. Hyper terminal supports two type of connections for text and file

transfer, these are by using a modem and Ethernet connections. It is an easy to use tool of

windows.


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PCB wizard was used in the transmitter and receiver designing of the system. PCB wizard

has the complete component library. Values of the components can be changed

accordingly. We simply selected the components needed, dragged them into the working

sheet and connected them.

2.3 Phase 2: Component study

In this phase, we studied about the different components used in the project which were

mentioned in the block diagrams.

2.3.1 Laser diode:

Laser diode used in the system had to transmit voice and text data simultaneously.

2.3.1.1 Production of laser light:

Light amplification by stimulated emission of radiation makes use of processes that

increase or amplify the light signals after those signals have been generated by other

processes. These processes include:

Stimulated emission: It is the process by which an atom interacts with the

electromagnetic wave of a specified frequency and may drop to a level of lower

energy.

Optical feedback: It is the process in which feedback is provided by the use of

mirrors.

Thus it is clear that a laser consists of an amplifying medium and a set of mirrors to feed

the light back into the amplifier for continued growth of the developing beam.[3]

Production of laser light can be better described by the following diagram.


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Figure#2.1: production of laser

light [4]

2.3.1.2 Electrical specifications:

Laser diode used in the system was a class 2 laser which was safe one to use. Pin diagram

of the laser diode is given below.

Figure#2.2: laser

diode[5]

Its electrical specifications are as follows.

Optical output: 5 mW

Wavelength: 650 nm

Th h ld 70 A


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Operating voltage: 4.5v min

Divergence: >0.5 m rad

2.3.2 Microphone:

Microphone was used to take audio input from the user. It converts voice into an

electrical signal. For this purpose we used Electric Condenser microphone. Power

requirement of condenser microphones is fulfilled by providing power from a battery or

external source. These microphones are also more sensitive and responsive. [6]

2.3.2.1 Principle of operation:

Condenser microphone works like a capacitor. As capacitor has two plates, condenser

microphone also has two plates. One of these plates is very thin and its action is as a

diaphragm. When sound signal is received, diaphragm vibrates resulting in changing the

distance between the two plates and also resulting in change of capacitance. Capacitance

increases and current occurs when plates are closer. When plates are farther apart, there

happens a decrease in capacitance and a discharge current occurs.

Figure#2.3: Structure of Electric Condenser

microphone[6]


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Transistor is constructed with three semiconductor doped regions, which are separated by

two PN junctions. Transistor that we used in our project is C1815, which is an NPN

transistor. There are three regions in a transistor, which are Base, Emitter and Collector.

Pin diagram of transistor used in our project, is as below:

Figure#2.4: Pin Diagram of

transistor [7]

2.3.3.1 Transistor as an amplifier:

A useful mode of operation of transistor in our project is the common emitter

configuration, which is shown in the given figure.


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Figure#2.5: Common emitter

configuration[7]

The DC current gain is defined by dc which is equal to the ratio of collector current

to the base current. The collector current is much greater than base current that is why it

exhibits DC current gain. [7]

2.3.4 Transformer:

Transformers are characterised into two types i.e. step down transformer and step up

transformer. Transformer used in this project is the step down transformer. This

transformer was used to convert 220v AC supply into 9V AC. It provided main power

supply for transmitter unit. We attached a bridge rectifier with the transformer to convert

AC into DC and that reverse polarity may not affect the remaining circuitry. Diagram

given is of step down transformer, which is used in transmitter unit.

Figure#2.6: 9v Transformer [8]


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other is called secondary coil. Transformer transfers electrical energy from primary coil

to secondary coil due to the mutual induction. If we connect load with the secondary coil

then varying current in primary coil will produce electrical energy in secondary coil i.e.

load.

2.3.5 Bridge rectifier:

Leo Graetz was the inventor of bridge rectifier. Bridge rectifier is basically a diode bridge

circuit. Main feature of bridge rectifier is that the output polarity remains the same

ignoring the polarity at the input.

Figure#2.7: Bridge rectifier[9]

2.3.5.1 Principle of operation:

Usually bridge rectifier is connected with the secondary coil of the transformer. During

positive half cycle, diodes D2 and D4 act as forward bias and conduct current from high

potential to low potential. D1 and D3 are reversed bias. In negative half cycle diodes D1

and D3 act as forward bias and conduct current, while D2 and D3 are reversed bias. So it

is clear that a bridge rectifier follows two voltage cycles that are positive half cycle and

i h lf l i i h lf l l d l i i i hil i i h lf
http://en.wikipedia.org/wiki/Leo_Graetzhttp://en.wikipedia.org/wiki/Leo_Graetz


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2.3.5.2 Smoothing:

Usually a capacitor is attached with the output of the bridge rectifier, especially in

situations where its task is to convert AC into DC. The capacitor used is called the

smoothing capacitor whose purpose is make variations less in the rectified AC output

voltage waveform from the bridge rectifier. [9]


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2.3.6 Voltage regulator:

The L-7805 is a simple voltage regulator which has 3 pins. Its function is to take 9v and

produce a constant 5v at the output. Its input is 9v of bridge rectifier. Its purpose of use is

to provide constant power supply of 5v to those components whose operating voltage is

5v. Below is the diagram of voltage regulator.

Figure#2.8: Pin configuration ofVoltageRegulator[10]

2.3.7 Audio amplifier:

Audio amplifier of this type is usually used in low voltage applications. Minimum voltage

required for its operation is 4 volt. Pin diagram of the audio amplifier is given at the next

page.


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Figure#2.9: Pin configuration ofAudioamplifier [10]

Gain of 20 is set internally but it can be increased by putting a capacitor between the

pin#1 and pin#8. LM-386N only amplifies voice signal because it only allows frequency

band of less than 4 kHz to pass at its input.

2.3.8 Max-232:

This IC is very much of interest for people who want to build their own electronic devices

and want to interface them with RS-232. Its purpose of use is to convert CMOS or TTL

logic levels into RS-232 and vice versa.

2.3.8.1 Voltage logic:

Digital devices in our daily use require either CMOS or TTL logic levels. Therefore the

first step of connecting these devices to the RS-232 port is to convert the RS-232 logic

levels back into 0 and 5 volts logic. This IC is low power driver and receiver, which

requires only +5v for its operation.


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inputs are added to give one output. And at the receiver side, CMOS/TTL logic is

converted into RS-232 logic. Logic conversion function of Max-232 can defined with the

help of following table.

Max-232 conversion level

TTL +5v -9v RS-232

TTL 0v +9v RS-232

-9v RS-232 TTL +5v

+9v RS-232 TTL 0v

Table#2.2: Max-232Level Conversion

2.3.8.2 Pin configuration:

Max-232 is dual transmitter/receiver IC. Pin diagram of Max-232 is given at the next

page.


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Figure#2.10: Pin configuration ofMax-232[11]

2.3.9 Serial port communication:

In parallel port communication, 8 bit data is sent and received at a time over 8 separate

cables. This makes data transfer very quickly. While in serial port data transmission and

reception happens as one bit at a time over one wire. So by this way it takes 8 times more

than parallel cable for one byte. Serial port communication is used for long distance

communication.

Serial port standard is called the EIA/TIA-232-E standard. It is the Interface between

Data Terminal Equipment (DTE) and Data Circuit-Termination Equipment (DCE)

facilitating Serial Binary Data Interchange. RS-232 standard is apprehensive with serial

communication between a host (DTE) and a peripheral system (DCE). [12]

2.3.9.1 RS-232 Specifications:

EIA/TIA-232-E (RS-232) standard is a complete standard. This means that standard

establishes a complete relation between host (DTE) and Peripheral system (DCE) by

specifying:

Voltage and signal levels.

Pin wiring configuration.

Quantity of control information between DTE and DCE.

Valid signals are plus or minus 3 to 15. Positive is logic zero and minus is logic 1.

2.3.9.2 DB-9 connector:

DB-9 connector is characterised into two categories, which are: 1) Male and 2) Female

connector. DB-9 male connector has pins in it which are inserted in female connector.

DB-9 male connector Diagram is given below.


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Figure#2.11: DB-9 male

connector [12]

DB-9 female connector has holes in it in which pins of male connector are inserted.

Diagram of DB-9 female connector is given below.

Figure#2.12: DB-9 female

connector [12]

There are 9-pins in a DB-9 connector. Their functions are defined in the following table.

Pin# Function

1 Data carrier detect from peripheral system


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3 Transmit data to peripheral system

4 Data terminal ready outgoing handshaking signal

5 Ground for signal

6 Data set ready incoming handshaking signal

7 Request to send outgoing flow control

8 Clear to send incoming flow control

9 Ring indicator incoming signal from peripheral system

Table#2.3: DB-9 pin

specifications

Below is the diagram, which shows the pin configuration of DB-9.

Figure#2.13: DB-9 pin

configuration[13]

2.3.10 Phototransistor:

Phototransistors are photodiode amplifier combinations integrated within a single silicon

chip It is required in many applications that output of photo detector should be greater as


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compared to the photo diode. It is also possible to use a photo diode and then amplify its output

by using an external amplifier. But this technique is even not practical and cost effective than

phototransistors. The phototransistor can be viewed as a photo diode whose output photo

current is fed into the base of a conventional small signal transistor. [14]

Phototransistor used in our project is NPN phototransistor, whose part number is 55c

qt804 and its pin diagram is as follows.

Figure#2.14: phototransistor

[15]

Phototransistor used in our project receives laser light of 650 nm through the base region and

produces corresponding electrical signal, which is taken out from the collector region.

2.3.10.1 Performance parameters:

There are following three main reasons which inspired us to use phototransistors instead of


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the photo diodes. [14]

Responsivity: Phototransistors are highly sensitive due to the lower response time than

photo diode. Response time of 55c qt804 is 50ns.

Dark current: There is always some amount of current flows through the

phototransistor even when no current is detected; this is called the dark current.

Amount of dark current for 55c qt804 is less than or equal to the 10nA.

Noise equivalent power: Noise equivalent power is the signal power of a

phototransistor which gives a signal to noise ratio of 1 for half a second. Noise

equivalent power for 55c qt804 is 30pw.

2.4 Phase 3: Hardware design:

After the selection of project and related component study, we worked on the hardware of the

project. We made analysis of the project from different aspects. First we worked on the

designing of the circuit diagram. Then after it we assembled all the components according to

the circuit diagram.

2.4.1 Transmitter design:

Transmitter description can be justified by dividing the circuit diagram into its major

parts. These major parts and their relationships are described below.


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Step down AC to DC supply: it consisted on a step down transformer and a bridge

rectifier. Step down transformer was used to transform 220v AC into 9v AC and 9v

output of transformer was rectified by bridge rectifier. It converted 9v AC into 9v DC.

Circuit diagram of this section is given at the next page.

Figure#2.15: Step down AC to DC supply.

Microphone pre-amplifier: Human voice input was taken through microphone, which

converted voice signal into electrical signal. This electrical signal was fed into the

transistor, which was used as an amplifier. Amplification processes included a Zobel

network which was used for stability of voice signals. Zobel network is the

combination of capacitor and resistor connected with the ground. Below is the diagram

of microphone pre-amplifier.


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Figure#2.16: Microphonepre-

amplifier

Voltage regulator: It was used to take 9v Dc input from the bridge rectifier and

convert it into 5v DC. Reason to convert 9v into 5v was that some components

required 5v for their operation.

Audio power amplifier: Audio power amplifier was used to amplify voice

signals received from the transistor output. Amplified voice signal was then fed to

max-232 to convert voice signal into serial supported voice signal. Audio

amplifier section diagram is given at the next page.


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Figure#2.17: Audiopower amplifier

Serial supported voice and data transmission: Data input and amplified voice signal

was taken by max-232. By this way voice became serially supported and serial

supported voice and data signal was then transmitted through laser diode.


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Figure#2.18: Serial supported voiceand data transmission

2.4.1.1 Transmitter circuit diagram:

Complete circuit diagram designed in PCB wizard is given at the next page.

Figure#2.19: Transmittercircuit diagram


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2.4.2 Receiver design:

Light signal transmitted by the transmitter carried the voice and data information. It was

detected by the phototransistor, which produced electrical signal. This electrical signal was

amplified using the same pre-amplifier configuration. Configuration and function of

transformer, bridge rectifier, voltage regulator and audio power amplifier was the same.

Output of audio amplifier was fed into audio jack which produced voice at the output

while data was received by max-232, whose logic level was of CMOS. So its logic level was

converted into RS-232 logic level. Data output was then sensed by serial port through the use

of hyper terminal.

2.4.2.1 Receiver circuit diagram:

The receiver diagram is given below, which shows all the components and their

connection.


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Figure#2.20: Receivercircuitdiagram

2.5 Phase 4: Hyper terminal configuration:

When hardware was assembled then there was a need to configure hyper terminal for the first

time use. Configuration of hyper terminal is easy and can be done in limited number of steps.

Below is the descriptive information about how we configured hyper terminal.


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1 Opened hyper terminals exe from the start menu.

Figure#2.21: openinghyper terminal


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1. When we clicked on the hyper terminal there opened a window of location

information, we simply cancelled that window.

Figure#2.22: Location

information

2. After cancelling the location information window, there appeared phone and

modem options window. Then we clicked on OK.


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Figure#2.23: phone and modem options

3. Next window opened was of the connection description. We chose a connection

name and then selected the icon.


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Figure#2.24: Locationdescription

4. When we were finished with the selection of connection name and icon, there

opened another window from which we selected the port number of serial port e.g.

com1 or com2.

Figure#2.25: Serial port selection

5. There opened window of serial ports properties. We selected the data rate from this

window.


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Figure#2.26: Serial port propertiesselection

6. After we have completed the procedures discussed above, there opened a blank

window of hyper terminal. We clicked in the File tab and selected the properties icon.


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Figure#2.27: Hyper terminal connectionproperties

7. There opened connection properties window, we clicked on the setting tab of the

window and selected ASCII setup option.

Figure#2.28: ASCII setup in hyper

terminal

8. In ASCII setup, we simply checked on all the boxes and clicked on OK.


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Figure#2.29: ASCII setup

procedure

9. These were all the steps that we performed while configuring the hyper terminal.

At the last we were able to write on the hyper terminals window that was

transmitted to the other computer.

4.1 Scope of the project:

Free space laser communication system has much more applications in connecting two

building, in airports, in defense and sensitive areas etc.

In the future free space laser communication system can be enhanced by including the

functions of self alignment of laser beam and range enhancement. Communication range can

be increased by the use of high power infrared laser, so that it may provide more enhanced


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security for communication. Self alignment of laser beam may be achieved by using a separate

laser beam with the combination of a servo motor.

4.2 System features:

Features of free space communication system are summarized in the following steps.

Free space laser communication system supports human voice communication.

Text data communication is also possible from a PC to another PC.

Although the system supports short range communication but it is possible to

improve its range by using high power laser diodes.

4.3 System limitations:

Free space laser communication system is a trustworthy system but it only has one

limitation due to low power laser diode.

Free space laser communication system has limited range of communication.

4.4 Conclusion:

Free space laser communication system paved the way for us to learn about free space optics,

Laser physics, its use for communication and optical multiplexing techniques. Project was

helpful to understand the use of hyper terminal for communication purposes. System

designing helped to learn PCB wizard.


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BLOCK DIAGRAM

The basic circuit includes a transmitter and a receiver section. The above shown is a schematic representation of a

Laser torch based voice transceiver. The input is fed to a condenser microphone which is modulated at the

transmitter end and then detected by a photo transistor at the receiver end and then its fed to a loud speaker where

the voice is regenerated..

BLOCK DIAGRAM EXPLANATION

1. Condenser MicrophoneIt is also called a capacitor or electrostatic microphone. Condenser means capacitor,

which stores energy in the form of an electric field. Condenser microphones require

power from a battery or external source. Condenser also tends to be more sensitive and

responsive than dynamic, making them well suited to capturing subtle nuances in

a sound. The diaphragm vibrates when struck by sound waves, changing the

distance between the two plates and therefore changing the capacitance. Specifically

when the plates are closer together capacitance increases and a charge current occurs and

this current will be used to trigger the transmitting section.

CONDENSER

MICHROPHONE

TRANSMITTING

SECTION LASER TORCH

RECEIVING

SECTION LOUD SPEAKER


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doesn't need any complicated alignment. Just keep the phototransistor oriented towards

the remote transmitter's laser point and adjust the volume control for a clear sound.

5. Loud SpeakerSpeaker is an electro acoustic transducer that converts an electrical signal into sound. The

speaker moves in accordance with the variations of an electrical signal and causes sound

waves to propagate through a medium such as air or water.


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CIRCUIT DIAGRAM

TRANSMITTER

The transmitter circuit (Fig.1) comprises condenser microphone which receives audio signal (or

voice) of the non-sinusoidal waveform as input. It is of the order of few mV of amplitude. It is

followed by transistor amplifier BC548 along with op-amp stage built around UA741. Transistor

BC548 is connected in common emitter configuration. The gain of the op-amp can be controlled

with the help of 1-mega-ohm pot-meter VR1. The AF output from IC1 is coupled to the base of

transistor BD139 (T2), which, in turn, modulates the laser beam. The transmitter uses 9V power

supply. However, the 3-volt laser torch (after removal of its battery) can be directly connected to

the circuitwith the body of the torch connected to the emitter of BD139 and the spring-loaded

lead extended from inside the torch to circuit ground.

Resistor R1 is the source resistor, which is directly connected to the power-supply. Capacitor C1 is the coupling capacitor. Since audio input signal is having a non-sinusoidal

waveform of different amplitude and frequency, coupling capacitor is used to reject some of the

dc noise as well as level from audio input signal.

R2, R3 are acting as self-biasing circuits, which is used for the biasing transistor. These circuitarrangements provide or establish a stable operating point. The biasing voltage is obtaining by

R2 and R3 resistors network. Self-bias is used for obtaining entire audio signal as input.

The self-biased circuit is connected with the BC548 in CE configuration. It is transistor amplifierstage, where the low amplitude audio signal is amplified to the desired voltage.

The output is taken from the collector terminal; so inverted audio input signal is obtained.


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Transistor pre-amplifier stage is coupled with op-amp stage built by UA741. C2 is the blockingcapacitor while R4 is the op-amp stage resistor. Op-amp ua741 is easily available general-

purpose operational amplifier.

Pin no. 1 and 5 are not connected in order to nullify input-offset voltage. Pin no. 7 and 4 areVCC as well as VEE supply voltage. Pin no. 3 is non-inverting input while pin no. 2 is

inverting input. Between pin no. 2 and 6, 1 mega-ohm potmeter is connected as voltage series

negative feedback, which controls the infinite gain of the op-amp.

Resistors R5 and R6 acts as a voltage-divider network, thus it gives a fixed voltage at the non-

inverting pin.

Input inverted audio signal is applied to the inverting pin. Op-amp works on the differences intothe applied two input voltage and provide a output at pin no. 6. Since, input is applied to the

inverting pin the output is also an inverting one. Thus, again we get in phase high power and

high amplitude level audio signal.

Capacitors C3, C4 and resistor R7 are acting as diffusion capacitors and feedback resistorrespectively. These diffusion capacitors stored the carriers like holes and electrons in the base

and thus provide self-biasing of the transistor.

Power dissipation rate of UA741 is very high, which is not practical for driving other electronicsdevices, so heat sink power transistor BD139 is used. Power transistor BD139 absorbs most of

the power and supplies the suitable power to drive the laser torch.

In the end the laser beam is modulated. Here laser torch acts like a modulator, where two signalsone is message signal (audio signal) and carrier laser signal, are superimposed. So, laser beam

modulates and transmits the signals to large distances.


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Fig. 1

RECEIVER

The receiver circuit (Fig. 2) uses an NPN phototransistor as the light sensor. It receives the audio

signal of low power and amplitude and hence followed by a two-stage transistor preamplifier and

LM386-based audio power amplifier. The receiver does not need any complicated alignment.

Just keep the phototransistor oriented towards the remote transmitters laser point for a clear

sound.

In the pre-amplifier stage R8 is a source resistor, which is directly connected to the powersupply.

The pre amplifier stage is RC coupled amplifier in CE configuration.

C5, C6 are the junction capacitances, which are taken in to the account when we consider highfrequency response, which is limited by their presence.

Resistors R9 and R12 are used to establish the biasing of the transistor BC549.


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R11 is self-bias resistor, which is used to avoid degeneration. C7 is a bypass capacitor, which acts as to prevent loss of amplification due to negative feedback

arrangement.

C8 is the blocking capacitor, which is connected to the variable resistor VR2. Pin no. 1 and 10 is followed by C10, which is an external capacitor, used to compensate internal

error amplifier and thus avoid instability.

Volume control can be adjusted from variable resistor VR2 of 10 kilo- ohms. LM386 provides suitable power output useful for drive the loudspeaker of 0.5W.

R14 and C13 are bypass arrangement used to prevent loss of amplification.

C12 capacitor is used for preventing the noise as well as the hum produced by the ac sources. From the loudspeaker, the audio output is heard.

Fig. 2


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ADVANTAGES

1. Wireless2. Portability and quick deployment3. Less costly4. Circuit can be easily constructed5. High data rate6. No communication licenses required.7. There are no recurring line costs8. Laser can also transmit trough glass, however the physical properties of the glass have to be considered.9. Service free operation especially in inaccessible environments10. No chance of hacking & theft11. Data transfer at the speed of light

DISADVANTAGE

To avoid 50Hz hum noise in the speaker, keep the phototransistor away from AC light sources such as bulbs. The

reflected sunlight, however, does not cause any problem. But the sensor should not directly face the sun.

1. Point to point transmission alignment2. Atmospheric particles such as smoke, fog, dust etc.


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