free space optical communication

Free Space Optical Communication

Seminar report submitted for the award of the Degree of

Bachelor of Technology in

Electronics and Communication Engineeringof Siksha ‘o’ Anusandhan University

by

Sweta Mohanty

Department of Electronics & Communication Engineering

Institute of Technical Education & Research

Siksha ‘o’ Anusandhan University, Bhubaneswar - 751030

September,2013

DECLARATION

I hereby declare that the report entitled “Free Space

Optical Communication” submitted to Siksha ‘o’

Anusandhan University for the award of the degree of

Bachelor of Technology in Electronics and

Communication Engineering is absolutely based on my

own literature review and extensive survey. Wherever

contributions of others are involved, every effort has been

made to indicate this clearly, with due reference to the

literature. I also declare that this report in the present

form has not been submitted for award of any degree or

diploma or any other academic award anywhere else

before.

i

Sweta Mohanty

Reg. No.: 1011016060, Section: E

Department of

Electronics and Communication Engineering

ABSTRACT

Free Space Optics (FSO) or Optical Wireless, refers to the transmission of

modulated visible or infrared (IR) beams through the air to obtain optical

communications. Like fiber, Free Space Optics (FSO) uses lasers to transmit data, but

instead of enclosing the data stream in a glass fiber, it is transmitted through the air. It

is a secure, cost-effective alternative to other wireless connectivity options. This form

of delivering communication has a lot of compelling advantages.

Data rates comparable to fiber transmission can be carried with very low error

rates, while the extremely narrow laser beam widths ensure that it is possible to co-

locate multiple transceivers without risk of mutual interference in a given location.

FSO has roles to play as primary access medium and backup technology. It could

also be the solution for high speed residential access. Though this technology sprang

into being, its applications are wide and many. It indeed is the technology of the

future...

Keywords: Free space Optics(FSO), Infrared, Optical Communication, Data rates, Error

rates.

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CONTENTS

DECLARATION...............................................................................................................................i

ABSTRACT.......................................................................................................................................ii

CONTENTS.....................................................................................................................................iii

(Free Space Optical Communication).............................................................................................5

1. Introduction..........................................................................................................................5

2. Free Space Optics.................................................................................................................6

3. Relevance of FSO in Present Day Communication.............................................................7

4. Origin of FSO.......................................................................................................................8

5. The Technology of FSO.......................................................................................................8

6. The Working of FSO system................................................................................................9

7. Block Diagram of FSO.......................................................................................................10

7.1 The Transmitter ...........................................................................................................11

7.2 The Atmospheric Channel...........................................................................................12

7.3 The Receiver...............................................................................................................15

8. Architectures of FSO..........................................................................................................17

8.1 Point to Point Architecture..........................................................................................17

8.2 Mesh Architecture.......................................................................................................17

8.3 Point to Multi-Point Architecture.............................................................................189.

FSO Security......................................................................................................................18

10. Merits of FSO.....................................................................................................................20

11. Applications of FSO...........................................................................................................21

12. Challenges of FSO.............................................................................................................22

12.1 Fog.............................................................................................................................22

12.2 Physical Obstruction.................................................................................................22

iii

12.3 Scintillation...............................................................................................................22

12.4 Solar Interference......................................................................................................23

12.5 Scattering...................................................................................................................23

12.6 Absorption ................................................................................................................24

12.7 Building Sway/Seismic .......................................................................................2413.

FSO! as a Future Technology.............................................................................................25

14. Conclusion..........................................................................................................................26

15. References..........................................................................................................................27

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Free Space Optical Communication

1. INTRODUCTION

Communication, as it has always been relied and simply depended upon

speed. The faster the means! The more popular, the more effective the

communication is! Presently in the twenty-first century wireless networking is

gaining because of speed and ease of deployment and relatively high network

robustness. Modern era of optical communication originated with the invention of

LASER in 1958 and fabrication of low-loss optical fiber in 1970.

When we hear of optical communications we all think of optical fibers, what we

have today is AN OPTICAL COMMUNICATION SYSTEM WITHOUT FIBERS

or in other words WIRE FREE OPTICS. Free space optics or FSO –Although it

only recently and rather suddenly sprang in to public awareness, free space optics

is not a new idea. It has roots that 90 back over 30 years-to the era before fiber

optic cable became the preferred transport medium for high speed communication.

FSO technology has been revived to offer high band width last mile connectivity

for today’s converged network requirements.

(Free Space Optical Communication)

2. FSO! FREE SPACE OPTICS

Free space optics or FSO, free space photonics or optical wireless, refers to

the transmission of modulated visible or infrared beams through the atmosphere to

obtain optical communication. FSO systems can function over distances of several

kilometers.

FSO is a line-of-sight technology, which enables optical transmission up to

2.5 Gbps of data, voice and video communications, allowing optical connectivity

without deploying fiber optic cable or securing spectrum licenses. Free space optics

require light, which can be focused by using either light emitting diodes (LED) or

LASERS(light amplification by stimulated emission of radiation). The use of lasers

is a simple concept similar to optical transmissions using fiber-optic cables, the only

difference being the medium.

As long as there is a clear line of sight between the source and the destination

and enough transmitter power, communication is possible virtually at the speed of

light. Because light travels through air faster than it does through glass, so it is fair to

classify FSO as optical communications at the speed of light. FSO works on the

same basic principle as infrared television remote controls, wireless keyboards or

wireless palm devices.

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3. RELEVANCE OF FSO IN PRESENT DAY

COMMUNICATION

Presently we are facing with a burgeoning demand for high bandwidth and

differentiated data services. Network traffic doubles every 9-12 months forcing the

bandwidth or data storing capacity to grow and keep pace with this increase. The

right solution for the pressing demand is the untapped bandwidth potential of optical

communications.

Optical communications are in the process of evolving Giga bits/sec to

terabits/sec and eventually to pentabits/sec. The explosion of internet and internet

based applications has fuelled the bandwidth requirements. Business applications

have grown out of the physical boundaries of the enterprise and gone wide area

linking remote vendors, suppliers, and customers in a new web of business

applications. Hence companies are looking for high bandwidth last mile options. The

high initial cost and vast time required for installation in case of OFC speaks for a

wireless technology for high bandwidth last mile connectivity there FSO finds its

place.

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4. ORIGIN OF FSO

It is said that this mode of communication was first used in the 8 th century by

the Greeks. They used fire as the light source, the atmosphere as the transmission

medium and human eye as receiver.

FSO or optical wireless communication by Alexander Graham Bell in the late

19th century even before his telephone! Bells FSO experiment converted voice

sounds to telephone signals and transmitted them between receivers through free air

space along a beam of light for a distance of some 600 feet, this was later called

PHOTOPHONE. Although Bells photo phone never became a commercial reality, it

demonstrated the basic principle of optical communications.

Essentially all of the engineering of today’s FSO or free space optical

communication systems was done over the past 40 years or so mostly for defense

applications.

5. THE TECHNOLOGY OF FSO

The concept behind FSO is simple. FSO uses a directed beam of light

radiation between two end points to transfer information (data, voice or even video).

This is similar to OFC (optical fiber cable) networks, except that light pulses are sent

through free air instead of OFC cores.

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An FSO unit consists of an optical transceiver with a laser transmitter and a

receiver to provide full duplex (bi-directional) capability. Each FSO unit uses a high

power optical source (laser) plus a lens that transmits light through the atmosphere

to another lens receiving information. The receiving lens connects to a high

sensitivity receiver via optical fiber. Two FSO units can take the optical connectivity

to a maximum of 4kms.

6. WORKING OF FSO SYSTEM

Optical systems work in the infrared or near infrared region of light and the

easiest way to visualize how the work is imagine, two points interconnected with

fiber optic cable and then remove the cable. The infrared carrier used for

transmitting the signal is generated either by a high power LED or a laser diode.

Two parallel beams are used, one for transmission and one for reception, taking a

standard data, voice or video signal, converting it to a digital format and transmitting

it through free space.

Today’s modern laser system provide network connectivity at speed of 622

Mega bits/sec and beyond with total reliability. The beams are kept very narrow to

ensure that it does not interfere with other FSO beams. The receive detectors are

either PIN diodes or avalanche photodiodes.

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The FSO transmits invisible eye safe light beams from transmitter to the

receiver using low power infrared lasers in the tera hertz spectrum. FSO can function

over kilometers.

WAVELENGTH:

Currently available FSO hardware is of two types based on the operating

wavelength – 800 nm and 1550 nm. 1550 FSO systems are selected because of

more eye safety, reduced solar background radiation and compatibility with

existing technology infrastructure.

7. FSO BLOCK DIAGRAM

The block diagram of a typical terrestrial FSO link is shown below. Like any

other communication technology, an FSO system essentially comprises the

following three parts: transmitter, channel and receiver. Further discussion of each

of these blocks is given below.

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7.1 The transmitter

This functional block has the primary duty of modulating the source data onto

the optical carrier, which is then propagated through the atmosphere to the receiver.

The most widely used modulation type is the intensity modulation (IM), in which the

source data is modulated onto the irradiance of the optical radiation. This is achieved

by varying the driving current of the optical source directly in sympathy with the

data to be transmitted or via an external modulator, such as the symmetric Mach-

Zehnder interferometer. The use of an external modulator guarantees a higher data

rate than direct modulation, but an external modulator has a nonlinear response.

Other properties of the radiated optical field such as its phase, frequency and state of

polarisation can also be modulated with data/information through the use of an

external modulator. The transmitter telescope collects, collimates and directs the

optical radiation towards the receiver telescope at the other end of the channel.

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7.2 The atmospheric channel

An optical communications channel differs from the conventional Gaussian-

noise channel in that the signal x(t) represents power rather than amplitude. This

leads to two constraints on the transmitted signal: i) x(t) must be non-negative; and

ii) the average value of x(t) must not exceed a specified maximum power Pmax, that

is . In contrast to the conventional channels, where the signal-to-noise ratio (SNR) is

proportional to the power, in optical systems the received electrical power and the

variance of the shot noise are proportional to Ad2 and Ad, respectively, where Ad is

the receiver detector area. Thus, for a shot noise limited optical system, the SNR is

proportional to Ad. This implies that for a given transmitted power, a higher SNR

can be attained by using a large area detector. However, as Ad increases so does its

capacitance, which has a limiting effect on the receiver bandwidth.

The atmospheric channel consists of gases, and aerosols – tiny particles

suspended in the atmosphere. Also present in the atmosphere are rain, haze, fog and

other forms of precipitation. The amount of precipitation present in the atmosphere

depends on the location (longitude and latitude) and the season. The highest

concentration of particles is obviously near the Earth surface within the troposphere

and this decreases with increasing altitude up through to the ionosphere. With the

size distribution of the atmospheric constituents ranging from sub-micrometres to

centimetres, an optical field that traverses the atmosphere is scattered and/or

absorbed resulting in power loss.

Another feature of interest is the atmospheric turbulence. When radiation

strikes the Earth from the Sun, some of the radiation is absorbed by the Earth’s

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surface thereby heating up its (Earth’s) surface air mass. The resulting mass of warm

and lighter air then rises up to mix turbulently with the surrounding cooler air mass.

This culminates in small (in the range of 0.01 to 0.1 degrees) but spatially and

temporally fluctuating atmospheric temperature. The temperature inhomogeneity of

the atmosphere causes corresponding changes in the index of refraction of the

atmosphere, resulting in eddies, cells or air packets having varying sizes from ~0.1

cm to ~10 m. These air packets act like refractive prisms of varying indices of

refraction. The propagating optical radiation is therefore fully or partially deviated

depending on the relative size of the beam and the degree of temperature

inhomogeneity along its path. Consequently the optical radiation traversing the

turbulent atmosphere experiences random variation/fading in its irradiance

(scintillation) and phase. Familiar effects of turbulence are the twinkling of stars

caused by random fluctuations of stars. irradiance, and the shimmer of the horizon

on a hot day caused by random changes in the optical phase of the light beam

resulting in reduced image resolution .Atmospheric turbulence depends on i)

atmospheric pressure/altitude, ii) wind speed, and iii) variation of index of refraction

due to temperature inhomogeneity.

Known effects of atmospheric turbulence include:-

a) Beam steering – Angular deviation of the beam from its original LOS causing

the beam to miss the receiver.

b) Image dancing – The received beam focus moves in the image plane due to

variations in the beam.s angle of arrival.

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c) Beam spreading – Increased beam divergence due to scattering. This leads to a

reduction in received power density.

d) Beam scintillation – Variations in the spatial power density at the receiver plane

caused by small scale destructive interference within the optical beam.

e) Spatial coherence degradation – Turbulence also induces losses in phase

coherence across the beam phase fronts. This is particularly deleterious for

photomixing (e.g. in coherent receiver) .

f) Polarisation fluctuation – This results from changes in the state of polarisation

of the received optical field after passing through a turbulent medium. However

for a horizontally travelling optical radiation, the amount of polarisation

fluctuation is negligible.

The modelling of the fluctuation of an optical radiation traversing a turbulent

atmosphere will be examined in Chapter Four, with the view to understanding the

statistical behaviour of the signal received at the receiver plane.

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7.3 The receiver

This essentially helps recover the transmitted data from the incident optical

field. The receiver is composed of the following:-

a) Receiver telescope – collects and focuses the incoming optical radiation onto the

photodetector. It should be noted that a large receiver telescope aperture is desirable

as it collects multiple uncorrelated radiations and focuses their average on the

photodetector. This is referred to as aperture averaging but a wide aperture also

means more background radiation/noise.

b) Optical bandpass filter – to reduce the amount of background radiation.

c) Photodetector – p-i-n diode (PIN) or avalanche photodiode (APD) that converts

the incident optical field into an electrical signal. Germanium only detectors are

generally not used in FSO because of their high dark current.

d) Post-detection processor/decision circuit – this is where the required

amplification, filtering and signal processing necessary to guarantee a high fidelity

data recovery are carried out.

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FSO TRANSMITTER

FSO RECEIVER

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8. FSO ARCHITECTURES

8.1 POINT-TO-POINT ARCHITECTURE

Point-to-point architecture is a dedicated connection that offers higher

bandwidth but is less scalable .In a point-to-point configuration, FSO can support

speeds between 155Mbits/sec and 10Gbits/sec at a distance of 2 kilometers (km) to

4km. “Access” claims it can deliver 10Gbits/ sec. “Terabeam” can provide up to

2Gbits/sec now, while “AirFiber” and “Lightpointe” have promised Gigabit Ethernet

capabilities sometime in 2001..

8.2 MESH ARCHITECTURE

Mesh architectures may offer redundancy and higher reliability with easy

node addition but restrict distances more than the other options.

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A meshed configuration can support 622Mbits/sec at a distance of 200 meters

(m) to 450m. TeraBeam claims to have successfully tested 160Gbit/sec speeds in its

lab, but such speeds in the real world are surely a year or two off.

8.3 POINT- . TO-MULTIPOINT ARCHITECTURE

Point-to-multipoint architecture offers cheaper connections and facilitates

node addition but at the expense of lower bandwidth than the point-to-point option.

In a point-to-multipoint arrangement, FSO can support the same speeds as

the point-to-point arrangement -155Mbits/sec to 10Gbits/sec-at 1km to 2km.

9. FREE SPACE OPTICS (FSO) SECURITY

Security is an important element of data transmission, irrespective of the

network topology. It is especially important for military and corporate applications.

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Building a network on the SONA beam platform is one of the best ways to ensure

that data transmission between any two points is completely secure. Its focused

transmission beam foils jammers and eavesdroppers and enhances security.

Moreover, SONAR systems can use any signal-scrambling technology that optical

fiber can use.

The common perception of wireless is that it offers less security than wire

line connections. In fact, Free Space Optics (FSO) is far more secure than RF or

other wireless-based transmission technologies for several reasons:

Free Space Optics (FSO) laser beams cannot be detected with spectrum

analyzers or RF meters.

Free Space Optics (FSO) laser transmissions are optical and travel

along a line of sight path that cannot be intercepted easily. It requires a

matching Free Space Optics (FSO) transceiver carefully aligned to

complete the transmission. Interception is very difficult and extremely

unlikely.

The laser beams generated by Free Space Optics (FSO) systems are

narrow and invisible, making them harder to find and even harder to

intercept and crack.

Data can be transmitted over an encrypted connection adding to the

degree of security available in Free Space Optics (FSO) network

transmissions.

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10. MERITS OF FSO

1. Free space optics offers a flexible networking solution that delivers on the

promise of broadband.

2. Straight forward deployment-as it requires no licenses.

3. Rapid time of deployment.

4. Low initial investment.

5. Ease of installation even indoors in less than 30 minutes.

6. Security and freedom from irksome regulations like roof top rights and

spectral licenses.

7. Re-deploy ability.

Unlike radio and microwave systems FSO is an optical technology and

no spectrum licensing or frequency co-ordination with other users is required.

Interference from or to other system or equipment is not a concern and the point to

point laser signal is extremely difficult to intercept and therefore secure. Data rate

comparable to OFC can be obtained with very low error rate and the extremely

narrow laser beam which enables unlimited number of separate FSO links to be

installed in a given location.

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11. APPLICATIONS OF FSO

Optical communication systems are becoming more and more popular as

the interest and requirement in high capacity and long distance space

communications grow. FSO overcomes the last mile access bottleneck by sending

high bit rate signals through the air using laser transmission.

Applications of FSO system are many and varied but a few can be listed.

1. Military and government: Secure and undetectable FSO systems can

connect large areas safely with minimal planning and deployment time

2. Wireless service provider: unlike microwaves or fiber ,FSO does not require

spectrum licensing, physical disruption to a location or government zoning

approval. Carriers are free to grow their business

3. Enterprise connectivity: As FSO links can be installed with ease, they

provide a natural method of interconnecting LAN segments that are housed in

buildings separated by public streets or other right-of-way property.

4. Fiber backup: FSO can also be deployed in redundant links to backup fiber

in place of a second fiber link

5. Backhaul: FSO can be used to carry cellular telephone traffic from antenna

towers back to facilities wired into the public switched telephone network.

6. Service acceleration: Instant services to the customers before fiber being

laid.

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12. FSO CHALLENGES

The advantages of free space optics come without some cost. As the medium

is air and the light pass through it, some environmental challenges are inevitable.

12.1 FOG

Fog substantially attenuates visible radiation, and it has a similar affect

on the near-infrared wavelengths that are employed in FSO systems. Rain and snow

have little effect on FSO. Fog being microns in diameter, it hinder the passage of

light by absorption, scattering and reflection. Dealing with fog – which is known as

Mie scattering, is largely a matter of boosting the transmitted power. Fog can be

countered by a network design with short FSO link distances. FSO installation in

foggy cities like San Francisco has successfully achieved carrier-class reliability.

12.2 PHYSICAL OBSTRUCTIONS

Flying birds can temporarily block a single beam, but this tends to cause

only short interruptions and transmissions are easily and automatically re-assumed.

Multi-beam systems are used for better performance.

12.3 SCINTILLATION

Scintillation refers the variations in light intensity caused by atmospheric

turbulence. Such turbulence may be caused by wind and temperature gradients

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which results in air pockets of varying diversity act as prisms or lenses with time

varying properties. This scintillation affects on FSO can be tackled by multi beam

approach exploiting multiple regions of space- this approach is called spatial

diversity.

12.4 SOLAR INTERFERENCE

This can be combated in two ways:

The first is a long pass optical filter window used to block all wavelengths

below 850nm from entering the system.

The second is an optical narrow band filter proceeding the receive detector

used to filter all but the wavelength actually used for intersystem

communications.

12.5 SCATTERING

Scattering is caused when the wavelength collides with the scatterer.

The physical size of the scatterer determines the type of scattering.

When the scatterer is smaller than the wavelength-Rayleigh scattering.

When the scatterer is of comparable size to the wavelength -Mie scattering.

When the scatterer is much larger than the wavelength-Non-selective

scattering

In scattering there is no loss of energy, only a directional re-distribution of

energy which may cause reduction in beam intensity for longer distance.

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12.6 ABSORPTION

Absorption occurs when suspended water molecules in the terrestrial

atmosphere extinguish photons. This causes a decrease in the power density of the

FSO beam and directly affects the availability of a system. Absorption occurs more

readily at some wavelengths than others.

However, the use of appropriate power, based on atmospheric conditions, and use of

spatial diversity helps to maintain the required level of network availability.

12.7 BUILDING SWAY / SEISMIC ACTIVITY

One of the most common difficulties that arises when deploying FSO

links on tall buildings or towers is sway due to wind or seismic activity Both

storms and earthquakes can cause buildings to move enough to affect beam

aiming. The problem can be dealt with in two complementary ways: through

beam divergence and active tracking

With beam divergence, the transmitted beam spread, forming optical cones

which can take many perturbations.

Active tracking is based on movable mirrors that control the direction in which

beams are launched.

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13. FSO! AS A FUTURE TECHNOLOGY

Infrared technology is as secure or cable applications and can be more

reliable than wired technology as it obviates wear and tear on the connector

hardware. In the future it is forecast that this technology will be implemented in

copiers, fax machines, overhead projectors, bank ATMs, credit cards, game consoles

and head sets. All these have local applications and it is really here where this

technology is best suited, owing to the inherent difficulties in its technological

process for interconnecting over distances.

Outdoors two its use is bound to grow as communications companies,

broadcasters and end users discovers how crowded the radio spectrum has become.

Once infrared’s image issue has been overcome and its profile raised, the medium

will truly have a bright, if invisible, future!

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14. CONCLUSION

FSO enables optical transmission of voice video and data through air at very

high rates. It has key roles to play as primary access medium and backup

technology. Driven by the need for high speed local loop connectivity and the cost

and the difficulties of deploying fiber, the interest in FSO has certainly picked up

dramatically among service providers world wide. Instead of fiber coaxial systems,

fiber laser systems may turn out to be the best way to deliver high data rates to your

home. FSO continues to accelerate the vision of all optical networks cost effectively,

reliably and quickly with freedom and flexibility of deployment.

15.REFERENCES

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[1] Wasiu Oyewole Papoola, “Subcarrier intensity modulated free-space optical communication systems”, School of Computing, Engineering and Information Sciences, University of Northumbria at Newcastle for the degree of Doctor of Philosophy,September 2009

[2] Manzur, T. “Free Space Optical Communications (FSO)”Avionics, Fiber-Optics

and Photonics Technology Conference, 2007 IEEE

[3] Juarez, J.C. ; Dwivedi, A. ; Hammons, A.R. ; Jones,S.D. ;Weerackody, V. ; Nichols,R.A. “FreeSpace Optical Communications for Nextgeneration Military Networks”Communications Magazine, IEEE Volume: 44 , Issue: 11

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