Free Space Optical Communication

Published Date: 02 Nov 2017

This report gives the details about Free-Space Optical Communication. FSO communication is mostly used in the areas where it is impossible to set a physical connection. FSO is very useful these days in the aircrafts for making them to communicate at short distances. To make a connection between two buildings through wireless network, FSO is functional at that part. The type of services provide by FSO, cost and speed is described in this report. This report also tells about the technologies used in this FSO and their architecture in detail. At the end of report, I have described some advantages and disadvantages for using this service.

1. INTRODUCTION

Free space optical communication is known to be an optical communication based communication, which transmits data in free space using light propagation. Free space can be an air, vacuum or any outer space. Over the last two decades, free space communication (FSO) is becoming more popular over the Radio Frequency Communication. FSO provides secure and high bandwidth data communications.

This project describes the gains and losses along transmitter path passing across medium to the receiver. This system has developed for high-speed communication systems. Basically free space optics is used to connect bridges and towers together with gigabit capacity and intrusion free data transmission. This technology can be installed anywhere in licensed-free global. FSO uses invisible beam of light to provide optical bandwidth connections. Data, voice or video can be sent over with the speed of 1.25 Gbps. This communication also authorizes optical communications with the speed of light.

Mainly Free Space Optical communication is a line of sight communication that provides optical bandwidth connections to send and receive data, voice or video. This communications doesn’t require fiber optics instead uses light for the transmission of data. FSO is used where it is unfeasible to make any physical connections. The invention of LASER transform into free space optics. FSO can offer Tb/s of throughput wirelessly.

Definition

Free-space optical communication in a technology that is based on light to transmits data in the field of computer networking. Free Space can be anything either vacuum or air. It also shows some similarities with the optical fiber by using optical fiber cable or transmission line.

This technology as very clear from the name is based and works where it is impossible to make physical connections. It can be due to high range of costs or impractical due to large distance.

Why FSO.

Free-space optical communication involves optical links such as infrared or laser light. There were cases in which the source and the transmitter fails to make a physical connection so, this wireless communication was taken into consideration. FSO can also travels to long distances through the space as name says. Previously, Infrared laser light was used but it was only applicable for short distances with low cost. Aircraft uses FSO to communicate to each other or to the main station.

1.3 Factors affecting FSO.

The stability and quality of the links undoubtedly works perfect but it is mostly affected by the atmospheric conditions such as rain, fog or heat. It uses high sensitive detectors to travel through long distances and also uses low-pass filters with a cut-off frequency of 4 khz.

Outline

1.5 Summary

This chapter gives us the outline of Free-space communication and tells various techniques which are used and why it is in great demand these days.

CH 2 FSO- A PRESENT DAY COMMUNICATION

2.1 Relevance of FSO

With the increase in demand of High bandwidth and data services, new technologies are introduced to fulfill these demands. Network traffic is increasing at a rapid speed every year. Introduction of Internet and wireless based transmission has fulfill this demand to some extent. But the large companies are still in wait for high bandwidth applications as the last mile option. With the polishing technology for the current period, costs is also increasing. As a current option, FSO has find its place in the field of networking.

2.2 Origin of FSO

Greeks first introduced the idea behind FSO in 8th century where they used light source- fire, Transmission medium- atmosphere, and receiver as a human eye. It became more popular and in demand when Graham bell introduced telephone. He converted voice signals into telephone signals and transmitted over free space with a beam of light and named it as photophone. Now this turned into great technology by using it over thousands kilometers through wireless means at high bandwidth and data rate.

2.3 Architecture of FSO

The architecture of FSO is of three types.

2.3.1 Point-to-point Architecture.

Point to point is a dedicated connection between two sites. It uses a single line and offers high bandwidth. This type of Free-space communication is less scalable.

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Figure 2.1 Point-to-point Architecture.

2.3.2 Mesh Architecture

In case of Mesh architecture, a single site is connected to other sites. This type of link has high redundancy but is more reliable. Other nodes can also be added later on.

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figure 2.2 Mesh Architecture

2.2.3 Point-to-multipoint Architecture

In point-to-multipoint architecture, we have lower bandwidth as it has multiple links on single node.

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Figure 2.3 point to multipoint architecture.

2.4 Technologies Involved

FSO mainly works for the buildings, which are at long distances and mostly work in the metropolitan areas. The links mainly involved are satellite, ground stations. Mainly optical communication involves fiber. Lasers can be taken into consideration for this type of communication. Free-space optical communication offers a guaranteed speed of 2.5 Gb/s. The first practical use of FSO is a military communication. The LASER when invented in 1960 revolutioned free space optics. The use of infrared can be useful as such for short distances. Infrared is of low costs but is useful in short distance areas.

2.4.1 Infrared Data Association (IrDA).

Infrared Data Association is an initial state of FSO communication. It was first used in aircrafts. To make a link between aircrafts, infrared was used but it was only for limited distance from 2 to 3 km. This type of link is highly stable but is the same time is also affected by some climatic conditions such as fog, rainfall, or heat.

2.4.2 Optical Telescopes.

For a communication over a long distance, another device is used known as optical telescope. Optical telescope uses Beam expanders which were used by NASA to laser image of Mona Lisa to the Lunar reconnaissance Orbiter. The distance recorded was 240,000 miles. We can communicate over thousands of kilometers.

2.4.3 Mercury Laser Altimeter

To record a two-way distance, a device was used called Mercury laser Altimeter. This laser was introduced for mercury orbit mission and it can communicate over 24 million kilometers.

2.5 Visible Light Communication for LAN

The researchers for the LAN network also used LED based system. This system has been used as of its low cost and high bandwidth. Data can transmit over 100Mbit/s. In practical this system was used for controlling the traffic with LED lights.

It is a wireless communication and uses visible light for communication.

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figure 2.4 Visible Light communication.

It uses image sensors for receiving the incoming data in the right direction from the transmitter. Devices that use visible light communication are Indoor Navigation, control of Robots or Vehicles.

2.5.1 Example

On the transmitter side, the intensity of LED light can be modulated by controlling its current. On other hand, for receiving side, the PIN Photo diodes are used.

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Figure 2.5 Devices showing Visible Light Communication.

2.5.2 Image Sensor

Camera that acts as transmitter continuously takes image of a scene by using LED light and on other hand receiver detects optical intensity at pixel as LED light is focused here.

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figure 2.6 Image sensor.

2.6 Summary

This chapter describes the different types of FSO and their functions. The type of technologies used in FSO is described in this chapter. Basically, Visible light is stressed and explained clearly with detail features and its uses by giving some actual facts.

CH 3 WORKING OF FSO

3.1 How FSO works

To make a connection between receiver and the transmitter, FSO uses wireless means for making the connectivity. Signals are transmitted in the form of binary numbers and they then modulated into the light source. The light source can be anything either LED or Laser. These signals are transferred to the receiver side via light pulses and is received or sensed using photonic energy detector especially photo diode. In this case the technology used in the Line-Of-Sight communication and medium of transmission is free space or air.

Optical systems if transmitting signals at short distances use INFRARED and for long distances, it basically relies upon LED/LASER diodes. In the present era, a Laser system can provide upto 622 Mb/sec of speed to travel a signal with proper reliability and accuracy.

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figure 3.1 system showing working of FSO

In the transmitter side, the data is modulated and transmitted using a laser. In the receiver side, optical signals are detected and converted to electric signals. Amplifiers are used to amplify the signal and are demodulated for the getting the original signal.

3.2 Free Space Optical Design

In this section of FSO, I will discuss the various parts for creating a LASER BEAM, which carries information via Air.

The main parts involved in this are:

3.2.1 Transmitter Type

The main source by which the optical signal is modulated is Optical Light source. The light signal is modulated to generate a logical zero or one. So to transmit this signal over the short distance, Infrared is used and for long distances LED IS USED.

LASER used is of three types.

FP- Fabryperot or DFB (Distributed-feedback laser is used for long distances).

VCSEL – Vertical Cavity Surface Emitting Laser is used for wavelength of 850nm. It is low in cost and has high performance level.

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Figure 2.7 working of FSO using Light pulses.

3.2.2 Transmitter Power

Transmitter power is the power for transmitting device. The device having more power has higher cost and more stability.

3.2.3 Beam Divergence

Beam divergence shows the function of transmitting lens. If the beam is more focused then it has higher cost. Also if the diameter is larger the then it will give better image.

3.2.4 Receiver Diode Types

Performance and cost of system depends upon the type of diode used. Here, we used photodiode is used to convert the incoming light signal into electrical signal. Gallium or arsenide detectors are used for longer wavelengths. This case is also same as higher the cost it will have higher sensitivity.

3.3 Free Space Optical Link Equation

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Preceived = received power

Ptransmit = transmit power

Areceiver = receiver area

Div = beam divergence

Range = link length

3.4 Summary

This chapter gives us knowledge about the working of FSO. It also tells about the design and provides the function of various devices used in the communication. At the end we can see the Free space optical link equation.

Ch 4 Free-Space Optical Communication Issues and comparison

4.1 FSO Issues

FSO has made its place in the market of wireless communication and is a fast and reliable approach for using it on the practical basis. With emerging broadband, FSO seems to be a last mile. It offers high bandwidth, operational costs and are compatible with almost all the applications and markets.

A balanced approach was used by the engineers to a optical wireless system to understand the difference between other systems available. Various elements were considered by the engineers while designing a FSO system are discussed below.

4.1.1 Wavelength

The hardware used in the FSO can be divided into two categories as 800nm and 1550 nm.

There were valid reasons for using 1550 nm wavelength as laser eye safety, compatibility with existing technologies and reduction in solar background radiation.

4.1.2 Atmospheric Attenuation

FSO systems are designed for the heavy atmospheric attenuation especially by Fog. The systems having 1550 nm wavelength an transmit more than 50 times eye-safer.

4.1.3 Performance – Transmit power and receiver sensitivity.

The performance of Free-space optical systems can be defined by four main variables.

Total transmitted Power.

Transmitted Beamwidth.

Receiving Optics collecting area.

Receiving Sensitivity.

4.2 Comparison

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Figure 4.1 shows comparison between RF, FSO and Optical fiber.

4.3 FSO Applications

Free-Space Optical communication has various applications. Mostly FSO is in great demand as it can be used anywhere where no physical connections can be set up.

FSO include:-

Connecting various sites.

It can also be used to extend a fiber cable network between different buildings.

Local loop bypass

Backhaul

Disaster recovery

4.4 Summary

Ch 5 Advantages and Disadvantages

5.1 Advantages

5.2 Disadvantages

5.3 Development in FSO

5.4 Summary