Free Space Laser Communication system

Free space laser communication system

Free Space Laser Communication System

Submitted By Tallat Masood (9574) Supervised By Mr. Farhan Sohail

Submitted for the partial fulfillment of B.E in Telecomm Engineering degree to the Faculty of Engineering & IT NATIONAL UNIVERSITY OF MODERN LANGUAGES ISLAMABAD January, 2011

National university of modern languages, Islamabad

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Chapter# 1 Introduction

1.1 Used Concept: This section determines the concept used in the project. National university of modern languages, Islamabad

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1.1.1 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.

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There are less chances of light being distorted outside the atmosphere that is why it is used for communication between spacecrafts.

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For short distance optical communication, LEDs are used and infrared laser lights are used for long distance communications.

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Distance and data rate of free space optical communication are highly dependent on atmospheric conditions.

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.

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It can be used to upgrade existing wireless technologies.

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

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Interconnection between two spacecrafts can be achieved by this technology.

1.1.1.2 Advantages:


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Advantages of free sapce optical communication are described below. •

Very quick link establishment.

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Bit error rate is very low.

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Transmission facilitates full duplex communication.

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Transparent protocol.

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Dispersion rate is low.

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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: Many different types of media are used for communication purposes. But compared to 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.

1.2.2 Characteristics of Laser Light: There are three main characteristics of laser light, which are described in the coming sections. 1.2.2.1 Coherent: Two waves travelling can add up to give larger wave or can be subtracted from one another to give shorter waves. It all depends on the relative phase of the waves. If two


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waves have constant relative phase, they are said to be coherent. Similarly laser exhibits this property.

1.2.2.2 Monochromatic: Laser light has one wavelength which has its origin in stimulated emission from one set of atomic energy levels. So the laser light has single colour and is monochromatic. 1.2.2.3 Collimated: Laser light bounces back many times between the reflecting mirrors of laser cavity. It results in narrowing the beams and light do not spread very much.

1.2.3 Laser Classes: Due to wavelength and maximum output power, lasers have been classified into four classes. This classification categorises the lasers according to their ability to damage human eye. Classification of laser is dependent on the concept of accessible emission limit (AEL) that defines each laser class. This is normally maximum power and energy that can be emitted about a specific range of wavelengths and time of exposure. 1.2.3.1 Class 1: Laser of this class is safe one to use under estimated conditions of operation. Safety under every condition is not guaranteed.

1.2.3.2 Class 2: Class 2 lasers have output light in the visible spectrum, which starts from 400 nm and ends at 700 nm. It is assumed that the blink reflex will close the eyes within fraction of a second and it is necessary to provide protection. Long-standing exposure of this class results in damage.

1.2.3.3 Class 3a: National university of modern languages, Islamabad

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Class 3 lasers are safe when viewed by the unaided eye but when viewed by the instruments, result may be unsafe.

1.2.3.4 Class 3b: This class of laser is safe for reflected light but direct viewing of light is hazardous. It is directed to users that not use instruments for viewing. 1.2.3.5 Class 4: Laser of this class are horribly dangerous. Its direct beam causes fire and skin injury. It is avoided for communication uses. [2]

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 5.6×1014 Hz by using following formula.

λ= C/F Frequency of 5.6×1014 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: National university of modern languages, Islamabad

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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.

Figure# 1.2: Transmitter Block Diagram

Main components of transmitter are transformer, bridge rectifier, voltage regulator, microphone, transistors, audio amplifier, max-232 and laser.

1.4.2 Receiver:


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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.

Figure# 1.3: Receiver Block Diagram

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 window’s 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 National university of modern languages, Islamabad

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

1.6 PCB wizard: 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.


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Chapter# 2 Project work

2.1 Phases: For our simplicity and manageability, we divided the project into different phases. The Gantt chart is made to schedule project. Chart represents milestones to be achieved by following the schedule. National university of modern languages, Islamabad

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There are four phases of our project, which are:

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Project planning and Knowledge gathering

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Component study

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Hardware design

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Hyper terminal configuration

2.1.1 Project planning chart:

Table# 2.1: Gantt chart

2.2 Phase 1: Project planning and knowledge gathering: This was the phase in which we discussed areas of interest and planned project. This was the major part as whole degree was dependent on the selection of the area of project. It was kept in mind that all the group members are agree and willing for this project.


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Keeping in mind the planned project, we gathered knowledge about different technologies and very much related to the project

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:

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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.

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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.

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Optical output: 5 mW

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Wavelength: 650 nm

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Threshold current: 70 mA

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Operating current: 85 mA


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Operating voltage: 4.5v min

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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]

2.3.3 Transistor:

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 National university of modern languages, Islamabad

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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.

Figure# 2.5: Common emitter configuration [7]

The DC current gain is defined by β 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: National university of modern languages, Islamabad

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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]


Transformer uses two coils which are coupled together, one coil is called primary coil and 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:


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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]


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 negative half cycle. In positive half cycle load voltage is positive while in negative half cycle load voltage is negative. 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 of Voltage Regulator [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 of Audio amplifier [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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At the transmitter side this IC is used to drive CMOS/TTL and RS-232 input. Both driver 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-232 Level 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 of Max-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.


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Pin wiring configuration.

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Quantity of control information between DTE and DCE.

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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.

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.


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Pin#

Function

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Data carrier detect from peripheral system

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Receive data from peripheral system

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Transmit data to peripheral system

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Data terminal ready outgoing handshaking signal

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Ground for signal

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Data set ready incoming handshaking signal

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Request to send outgoing flow control

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Clear to send incoming flow control

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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 National university of modern languages, Islamabad

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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 the photo diodes. [14]

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Responsivity: Phototransistors are highly sensitive due to the lower response time than photo diode. Response time of 55c qt804 is 50ns.

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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.

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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.


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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.

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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: Microphone pre-amplifier

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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.

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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.

Figure# 2.17: Audio power amplifier


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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.

Figure# 2.18: Serial supported voice and data transmission

2.4.1.1 Transmitter circuit diagram:

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


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Figure# 2.19: Transmitter circuit diagram

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: National university of modern languages, Islamabad

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

Figure# 2.20: Receiver circuit diagram

2.5 Phase 4: Hyper terminal configuration:


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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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Opened hyper terminal’s exe from the start menu.

Figure# 2.21: opening hyper terminal

1. When we clicked on the hyper terminal there opened a window of location information, we simply cancelled that window.


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Figure# 2.22: Location information

2. After cancelling the location information window, there appeared phone and modem options window. Then we clicked on OK.

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: Location description

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 port’s properties. We selected the data rate from this window.


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Figure# 2.26: Serial port properties selection

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.

Figure# 2.27: Hyper terminal connection properties

7. There opened connection properties window, we clicked on the setting tab of the window and selected ASCII setup option.


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Figure# 2.28: ASCII setup in hyper terminal

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

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 terminal’s window that was transmitted to the other computer. National university of modern languages, Islamabad

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Chapter# 3 Results

3.1 Result: Proper planning at the beginning of project resulted in the completion of project before the deadline. Voice and text data communication through this system was very reliable and efficient. Resulted security system is better capable of to be implemented for communication where there is need to jam RF frequencies. National university of modern languages, Islamabad

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3.2 Snapshots of the project: •

Transmitter: Below given diagram is of the transmitter used for free space laser communication system. The figure contains complete and practical view of the transmitter. Every component used in the circuit e.g. condenser microphone, transistors, voltage regulator, transformer, audio amplifier, Max-232, DB-9 connector and laser etc. is shown in the below given figure.

Figure# 3.1: Transmitter’s snapshot

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Receiver: Next snapshot is of the receiver used for free space laser communication system. Figure is complete and practical view of the receiver. It shows each and every component used in the project e.g. phototransistor, variable resistor, transformer, voltage regulator, audio amplifier, Max-232 and ear-phone pin etc.


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Figure# 3.2: Receiver’s snapshot


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Chapter# 4 Conclusion and scope of the project

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.


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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 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.

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Free space laser communication system supports human voice communication.

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

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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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Bibliography and References [1]. Free space optical communication by Olivier Bouchet, Christian Boisrobert [2]. Introduction to fiber optics by john crisp, 2nd edition. ISBN: 07506 50303 [3]. Laser fundamentals by William T. Silfvast, 2nd edition. ISBN: 0-521-83345-0 [4]. www.scribd.com [5]. www.mi-lasers.com National university of modern languages, Islamabad

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[6]. www.mediacollege.com [7]. www.forum.arcadecontrols.com [8]. www.qy6.net/upimg/prodimg/base/12053.jpg [9]. www.images.blog-u.net/img/diodes-bridge-rectifier/ [10]. www.freedatasheets.com/blog/labels/datasheet.html [11]. www.focus.ti.com [12]. www.navtechgps.com [13]. www.lammertbies.nl [14]. www.sunrom.com/sensors/light-infrared/phototransistor [15]. www.electricalwhat.com

Appendix A Datasheet


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Datasheet National university of modern languages, Islamabad

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Free Space Laser Communication system

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