Free space optics

CHAPTER 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 I have for u

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 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.

1

FSO TRANSMITTER:

Fig 1.1 FSO transmitter

FSO RECEIVER :

Fig 1.2 FSO reciever

2

CHAPTER 2

LITERATURE SURVEY

RELEVANCE OF FSO IN PRESENT DAY COMMUNICATION

Presently we are faced 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 pare 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.

ORIGIN OF FSO

Telegraphy is a word coming from ancient greek and means in Italian “scrivere a

distanza” while in English sounds more or less like “writing to a distant place”. The human being

has from the very beginning tried to increase his capabilities to communicate with his far away

fellow men and so to transmit. Under this point of view, the mythology is full of interesting

examples with the most famous and known that is Ermes, the Gods messenger, able to move

faster than the wind and responsible to carry informations to the Gods.

First experiences in the ancient past can be found in the IVth century b.C. (before Christ),

where Diodoro Crono reports on a human chain used by the Persian king Dario I (522-486 b.C)

to transmit informations from the Capital to the Empire’s districts. In the IVth Century b.C.,

Enea il Tattico, reports on an hydraulic telegraph probably invented by the Chartaginians. During

the Roman and Greek age, was used to place in geographical key points “fire towers” to be

switched on in case of security breaches and/or attacks on the borders. Eschilo (525-456 a.C.)

reports in the Orestea that the news about the falls of Troy arrived to Argo passing through the

Cicladi islands covering, more or less, 900 km (Eschilo, 458 b.C.). This sort of tradition

remained, for example, on the Italian territory assuming and adopting different schemes, fire or

3

mechanical systems, depending on the time period, the geography and the geopolicy (Pottino,

1976). In the Center-South of Italy, in particular, the use of fire based signals during night and of

smoke based signals during the daylight on the top of towers or hills, afterwards called

communications by the usage of fani, has been quite common in the XVI and XVII Centuries

a.C. (Agnello, 1963). During the day one smoke signal means the presence of one enemy vessel,

in the night was switched on a bundle of dry woods and moved up and down to inform about the

exact number of the enemy vessels. Several testimonies report on different communications links

and distances. The most interesting one has been established in 1657 between the city of Messina

and the Malta island with mid span vessels used to cover the Mediterranean sea (Castelli, 1700).

The use of mechanical systems to implement optical wireless systems is due to Claude Chappe in

1792 (Huurdeman, 2003). Chappe introduces the “optical telegraph” in France. The system was

based on a regulator, 4.5m long and 0.35m wide, to which two indicators were attached. This

systems was placed on the top of stations in LOS (Line Of Site) at 9 km each. Telescopes and

human repeaters were, of course, needed to move the regulator and the indicators via three

cranks and wire ropes. The time usage was short because the system was able to work only

during the daylight and with good weather conditions. On the other hand, it was long reach

considering an average coverage in France equal to around 4830km, with 29 cities connected

using around 540 towers. Security was to ensure by transmitting secret codes with short

preambles, this also to understand the accuracy of the transmission. Chappe introduced, infact, a

particular code in 1795, to increase the transmission speed. This system helped to reduced the

time to exchange informations from several days to minutes and has been adopted in 1794. The

subsequent studies on the electricity, the results from Volta (1745-1827) and from Ampere

(1775-1836) on the electrical pile and the introduction of the electrical telegraph in 1838

(Morse), will carry to the dismission of the Chappe system around the mid of 1800. The Chappe

system was introduced also in other European countries connecting the cities of Amsterdam,

Strasbourg, Turin, Milan and Brussels.

At the end of the 19th century, Alexander Graham Bell experienced with excellent results

the so called Photophone (Michaelis, 1965) (Bova and Rudnicki, 2001). This system worked

using the sound waves of the voice to move a mirror, responsible to send pulses of reflected

sunlight to the receiving instrument. In particular Bell modulated with his voice, by the use of an

4

acousto-optic transducer, a lens-collimated solar beam. Bell used to consider this invention to be

his best work, even better than “his demonstration of the telephone”. Although Bell’s

Photophone never became a commercial reality, it demonstrated the basic

principle of optical communications. Wireless Optical Communications, becomes from this point

and year by year more important boosting the research worldwide. We can in this case divide the

wireless optical experiments in three main areas depending on the time periods: in the 60s arrives

the laser concepts and rises up the idea of wireless communications, in the 90s becomes popular

the idea of ground to satellite and satellite to ground laser communications still using red and

green sources, after 2000 the explosion of the Free Space Optical technologies (FSO) faces civil

and military applications ranging from standard telecommunications up to inter satellites & inter

planets experiments and using different wavelengths from 1 up to 10 microns.

For these reasons, essentially all of the engineering of today’s FSO communications

systems, has been studied over the past 40 years, at the beginning for defense applications and

afterwards for civil ones. By addressing the principal engineering challenges FSO, this

aerospace/defense activity established a strong foundation upon which today’s commercial FSO

systems.

In particular, the realization of the first LASER, based on ruby, in 1960 by Maiman

opened wider possibilities for the communications involving beams propagating over long

distances in atmosphere. Low loss optical fibers (less than 20 dB/km), infact, will arrive only in

the 70s. In 1960s NASA started to perform preliminary experiments between the Goddard Space

Center and the Gemini 7 module. In 1968 the first experiment about FSO transmission of 12

phone channels along 4km had been demonstrated in Rome (Italy) by researchers from the

Istituto P.T, CNR and Fondazione Ugo Bordoni under the management of Prof. Sette, Phisic

Insitute University La Sapienza. A red laser source (0.8 microns) was used to connect two

buildings between the Colombo and Trastevere Streets in Rome (Unknown, 1968). In the same

year Dr. E. Kube in Germany published on the viability of free space optical communications

considering both green (0.6 microns) and red (0.8 microns) laser sources (Kube, 1968). The

introduction of semiconductor light sources working at room temperature, by Alferov in 1970,

were decisive for a further development of integrated and low cost FSO systems. On the point of

view of the research, the first experiment using a quantum cascade laser (Capasso 1994) can be

5

considered fundamental today speaking about new transmission wavelengths for FSO (up to 10

microns). Between 1994 and 1996 years the first demonstration of a bidirectional space to

ground laser link between the ETS-VI satellite and the Communications Research Laboratory

(CRL) in Koganey (Tokio) has been accomplished. 1Mbps using 0.5 microns and 0.8 microns

emitting lasers. With the ongoing intensive and worldwide studies on FSO communications,

especially re started after the September 11 tragedy where the communications were supported

by free space optics links, the related scenarios changed extremely fast covering today different

applications and environments like the followings: atmosphere, undersea, inter satellites, deep

space. We can infact report on the SILEX experiment (Semiconductor Intersatellite Link

Experiment) in 2001 demonstrating bidirectional GEO-LEO and GEO-ground communications.

ARTEMIS satellite (GEO) using a semiconductor laser at 0.8 microns directly driven at 2 Mbps

with an average output of 10mW towards a Si-APD on SPOT-4 satellite (LEO). In the same

year, the GeoLite (Geosyncronous Lightweight Technology Experiment) experiment

successfully demonstrated a bidirectional laser communication between GEO satellites, ground

and aircraft. We cannot forget afterwards the MLCD (Mars Laser Communication

Demonstration) program started in 2003 and ended in 2005 with the aim of covering the distance

between Earth and Mars planets using an optical parametric amplifier with an average output of

5W and photon counting detectors working at 1.06 microns (Majumdar and Ricklin, 2008).

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.

6

CHAPTER 3

WORKING OF FSO

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. 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.

DIFFERENCE BETWEEN FIBER OPTIC AND FREE SPACE CHANNEL:

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

7

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 .

Fig 2.1Mechanism of FSO device

8

Fig 2.2 Block Diagram of FSO operation

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. 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.

Characteristics of Optical Free-Space Communications

Very high data-rates (several Gbps)

Small beam divergence minimizes free-space losses

Small, light terminals (few centimetres) with low power-consumption

No regulatory issues since systems do not interfere due to the small divergence angle

Tap-proof due to minimal signal foot-print on the ground

Dependence on clear sky. Can be solved by a distributed network of optical ground

stations.

9

WAVELENGTH

Currently available FSO hardware are 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.

SUBSYSTEM

Fig 2.3: subsystems of FSO

In the transmitting section, the data is given to the modulator for modulating signal and the

driver is for activating the laser. In the receiver section the optical signal is detected and it is

converted to electrical signal, preamplifier is used to amplify the signal and then given to

demodulator for getting original signal. Tracking system which determines the path of the beam

and there is special detector (CCD, CMOS) for detecting the signal and given to pre amplifier.

The servo system is used for controlling system, the signal coming from the path to the processor

and compares with the Modulator Driver Laser Transmit optic Data in Demodulator preamplifier

detector Receive optic Data out preamplifier Special detector Tracking optic Processor Servo

systems Environmental condition.

10

Fig 2.4 Network of FSO

Generally equipment works at one of the two wavelengths: 850 nm or 1550 nm. Laser for 850

nm are much less expensive (around $30 versus more than $1000) and are favored for

applications over moderate distances. One question arises that why we use 1550 nm wavelength.

The main reason revolves around power, distance, and eye safety. Infrared radiation at 1550 nm

tends not to reach the retina of the eye, being mostly absorbed by the cornea. 1550 nm beams

operate at higher power than 850 nm, by about two orders of magnitude[6]. That power can

boost link lengths by a factor of at least five while maintaining adequate strength for proper link

operation. So for high data rates, long distances, poor propagation conditions (like fog), or

combinations of those conditions, 1550 nm can become quite attractive.

Laser eye safety

A laser beam operating with an irradiance (W/cm2) above a certain level can cause

damage to the human eye. The minimum permissible exposure level (MPE) is tabulated in the

11

ANSI standards.16 The standards, written for direct ocular view, are given as a function of

wavelength. This is important because for wavelengths under about 1.4 _m (1400 nm), the

optical radiation that enters the eye is focused onto the retina and increased in irradiance. For

wavelengths longer than about 1.4 _m, the light is absorbed by the cornea and vitreous humor

inside the eye. Taking these considerations into account, the ANSI standards indicate that the

MPE for a 10-s exposure is about 1 mW/cm2 for 0.8-_m wavelength and about 100 mW/cm2 for

1.55-_m wavelength. The eye safety level for a LED is higher since LEDs are non coherent

sources and will not be focused to a small, diffraction-limited spot on the retina. Most FSO

systems are designed to be eye safe, or to operate in areas in which a human will not intercept the

beam. They often use beams tens of centimeters in diameter, which helps reduce beam intensity.

FREE SPACE OPTICS SECURITY

The common perception of wireless is that it offers less security than wireline connections. In

fact, Free Space Optics (FSO) is far more secure than RF or other wireless-based transmission

technologies for several reasons:

1.Free Space Optics (FSO) laser beams cannot be detected with spectrum analyzers or RF meters

2.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.

3.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

4.Data can be transmitted over an encrypted connection adding to the degree of security

available in Free Space Optics (FSO) network transmissions

12

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 is many and varied but a few can be listed.

1. Metro Area Network (MAN):

FSO network can close the gap between the last mile customers, there by providing

access to new customers to high speed MAN’s resulting to Metro Network extension.

2. Last Mile Access:

End users can be connected to high speed links using FSO. It can also be used to bypass

local loop systems to provide business with high speed connections.

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 layed.

13

CHAPTER 4

CASE STUDY

Market:Telecommunication has seen massive expansion over the last few years. First came the

tremendous growth of the optical fiber. Long-haul Wide Area Network ( WAN ) followed by

more recent emphasis on Metropolitan Area Networks ( MAN ). Meanwhile LAN giga bit

Ethernet ports are being deployed with a comparable growth rate. Even then there is pressing

demand for speed and high bandwidth. The ‘connectivity bottleneck’ which refer the imbalance

between the increasing demand for high bandwidth by end users and inability to reach them is

still an unsolved puzzle. Of the several modes employed to combat this ‘last mile bottleneck’, the

huge investment is trenching, and the non- redeployability of the fiber has made it uneconomical

and non satisfying.

Other alternatives like LMDS, a RF technology has its own limitations like higher initial

investment, need for roof rights, frequencies, rainfall fading, complex set and high deployment

time. In the United States the telecommunication industries 5 percent of buildings are connected

to OFC. Yet 75 percent are with in one mile of fiber. Thus FSO offers to the service providers, a

compelling alternative for optical connectivity and a complement to fiber optics.

Free-space point-to-point optical links can be implemented using infrared laser light,

although low-data-rate communication over short distances is possible using LEDs.Infrared Data

Association (IrDA) technology is a very simple form of free-space optical communications. Free

Space Optics are additionally used for communications between spacecraft. Maximum range for

terrestrial links is of the order of 2 to 3 km (1.2 to 1.9 mi), but the stability and quality of the link

is highly dependent on atmospheric factors such as rain, fog, dust and heat. Amateur

radio operators have achieved significantly farther distances using incoherent sources of light

from high-intensity LEDs. One reported 173 miles (278 km) in 2007. However, physical

limitations of the equipment used limited bandwidths to about 4 kHz. The high sensitivities

required of the detector to cover such distances made the internal capacitance of the photodiode

used a dominant factor in the high-impedance amplifier which followed it, thus naturally forming

a low-pass filter with a cut-off frequency in the 4 kHz range.

14

In outer space, the communication range of free-space optical communication is currently of the

order of several thousand kilometers, but has the potential to bridge interplanetary distances of

millions of kilometers, using optical telescopes as beam expanders. In January 2013, NASA used

lasers to beam an image of the Mona Lisato the Lunar Reconnaissance Orbiter roughly 240,000

miles away. To compensate for atmospheric interference, error correction code algorithm similar

to that used in CDs was implemented. The distance records for optical communications involved

detection and emission of laser light by space probes. A two-way distance record for

communication was set by the Mercury laser altimeter instrument aboard

the MESSENGER spacecraft. This infrared diode neodymium laser, designed as a laser altimeter

for a Mercury orbit mission, was able to communicate across a distance of 15 million miles (24

million km), as the craft neared Earth on a fly-by in May, 2005. The previous record had been set

with a one-way detection of laser light from Earth, by the Galileo probe, as two ground-based

lasers were seen from 6 million km by the out-bound probe, in 1992.

Secure free-space optical communications have been proposed using a laser N-slit

interferometer where the laser signal takes the form of an interferometric pattern. Any attempt to

intercept the signal causes the collapse of the interferometric pattern. This technique has been

demonstrated to work over propagation distances of practical interest and, in principle, it could

be applied over large distances in space.

Visible light communication

Researchers used a white LED-based space lighting system for indoor local area

network (LAN) communications. These systems present advantages over traditional UHF RF-

based systems from improved isolation between systems, the size and cost of

receivers/transmitters, RF licensing laws and by combining space lighting and communication

into the same system. In 2003, a Visible Light Communication Consortium was formed

in Japan. A low-cost white LED (GaN-phosphor) which could be used for space lighting can

typically be modulated up to 20 MHz. Data rates of over 100 Mbit/s can be easily achieved using

efficient modulation schemes and Siemens claimed to have achieved over 500 Mbit/s in

2010. Research published in 2009 used a similar system for traffic control of automated vehicles

with LED traffic lights. In January 2009 a task force for visible light communication was formed

by the Institute of Electrical and Electronics Engineers working group for wireless personal area

15

network standards known as IEEE 802.15.7. A trial was announced in 2010 in St. Cloud,

Minnesota.

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-deployability

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.

16

Fig 3.1 FSO using High intensity LED

LIMITATIONS OF FSO:

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.

1. FOG AND FSO

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 affect 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. In areas of heavy fogs 1550nm lasers can be of more are. Fog can be

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

like san Francisco have successfully achieved carrier-class reliability.

17

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.

3. SCINTILLATION

Scintillation refers the variations in light intensity caused by atmospheric turbulence.

Such turbulence may be caused by wind and temperature gradients 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.

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.

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 redistribution of energy which may

cause reduction in beam intensity for longer distance.

18

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.

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

a. With beam divergence, the transmitted beam spread, forming optical cones which can take

many perturbations.

b. Active tracking is based on movable mirrors that controls the direction in which beams are

launched.

Fig 3.2 New variant of FSO

System and engineering trade-offs

19

Besides the scientific factors to be taken into consideration in FSO design, a considerable

number of system and engineering trade-offs must be evaluated. Some of the trade-off

parameters that need to be taken into account include: ease of modulation of the laser or LED

(direct modulation through the power supply or the need for an expensive external modulator)

detector bandwidth and cooling requirements in the case of IR detectors; increased laser beam

divergence and possible need to increase laser power versus increased cost of using adaptive

optics or active alignment of a narrow laser beam; cost of laser or LED system at different

wavelengths versus advantages of availability of cheaper detector components versus penetration

of beam through fog or rain; and eye safety versus laser beam size versus divergence of beam

and beam size at detector telescope. Although a significant number of engineering trade-offs

clearly have to be made, there are usually several different ways to build a successful solution for

a specified operating condition. There is usually no single wavelength or ultimate maximized

system, but rather several that will provide the communication link required.

Fig 3.3 : Difference between Telescopic antenna and RF antenna

Free Space Optics (FSO) Features: 

1. Easily upgraded

2. Roof-top or through window operation

3. No latency

4. Highly Secure (wide military applications.

20

5. Compatible with WDM technology

6. Low power consumption

7. Immunity from interference

8. Commercially available systems offer capacities in the range of 100 Mbps to 2.5 Gbps, and demonstration systems report data rates as high as 160 Gbps.

Free Space Optics (FSO) Applications:

1. LAN-to-LAN connections on campuses at Fast Ethernet or Gigabit Ethernet speeds.

2. To cross a public road or other barriers which the sender and receiver do not own.

3. Temporary network installation (for events or other purposes).

4. Reestablish high-speed connection quickly (disaster recovery).

5. For communications between spacecraft, including elements of a satellite constellation.

PERFORMANCE – TRANSMIT POWER & RECEIVER SENSITIVITY

 Free Space Optics (FSO) products performance can be characterized by four main parameters

(for a given data rate):

•Total transmitted power 

•Transmitting beamwidth

•Receiving optics collecting area

•Receiver sensitivity

High transmitted power may be achieved by using erbium doped fiber amplifiers, or by noncoher

ently combining multiple lower cost semiconductor lasers. Narrow transmitting beam width

(a.k.a. high antenna gain) can be achieved on a limited basis for fixed-pointed units, with the

minimum beam width large enough to

accommodate building sway and wind loading. Much narrower beams can beachieved with an

actively pointed system, which includes an angle tracker and fast steering mirror (or gimbal).

Ideally the angle tracker operates on the communication beam, so no separate tracking beacon is

required. Larger receiving optics captures a larger fraction of the total transmitted power, up to

21

terminal cost, volume and weight limitations. And high receiver sensitivity can be achieved by

using small, low-capacitance photo detectors, circuitry which compensates for detector

capacitance, or using detectors with internal gain mechanisms, such as APDs. APD receivers

can provide 5-10 dB improvement over PIN detectors, albeit with increased parts co stand

a more complex high voltage bias circuit. These four parameters allow links to travel over longer

distance, penetrate lower visibility fog, or

both.In addition, Free Space Optics (FSO) receivers must be designed to betolerant to

scintillation, i.e. have rapid response to changing signal levels and high dynamic range in the

front end, so that the fluctuations can be removed in the later stage limiting amplifier or AGC.

Poorly designed Free Space Optics (FSO) receivers may have a constant background error rate

due to scintillation, rather than perfect zero error performance.

FIXED-POINTING OR ACTIVE-POINTING?

Another element of Free Space Optics (FSO) system design that must be considered by a

prudent buyer is the challenge of maintaining sufficiently accurate pointing stability. A number

of Free Space Optics (FSO) systems employ an active pointing-stabilization approach, which

represents an effective approach for addressing this challenge. However, the cost, complexity,

and reliability issues associated with active-pointing approach can be avoided in some

applications (particularly for shorter ranges and lower data rates) by utilizing the fixed-pointed

approach schematically shown in the figure. According to this approach, the transmitted beam is

broadened significantly beyond its near-perfect minimum beam divergence angle, and the

receiver field of view is broadened to a comparable extent. The broadening of the transmitted

beam and receiver field of view leads to large pointing/alignment tolerances and a very low

probability of building motion being of sufficient magnitude to take the link down. Well

engineered hardware exploits this approach of designing for loose alignment tolerances.

Therefore, it is possible to perform initial alignment of the transceivers at opposite ends of the

link during installation and then leave them unattended for many years of reliable service.

Note that this approach is facilitated for systems operating at wavelengths >1400 nm,

because the higher allowable eye-safe powers at such wavelengths allow the transmitted beam to

be significantly broadened spatially while still maintaining an adequate intensity at the receiver.

Of primary importance to prospective buyers will be selecting the right system for the situation.

22

CHAPTER 5

CONCLUSION

23

We have discussed in detail how FSO technology can be rapidly deployed to provide

immediate service to the customers at a low initial investment, without any licensing hurdle

making high speed, high bandwidth communication possible.

Though not very popular in India at the moment, FSO has a tremendous scope for

deployment companies like CISCO, LIGHT POIN few other have made huge investment to

promote this technology in the market. It is only a matter of time before the customers realized,

the benefits of FSO and the technology deployed in

large scale.

CHAPTER 6

FUTURE SCOPE

24

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.

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.

REFERENCES

1. Kontogeorgakis, Christos; Millimeter Through Visible Frequency Waves Through Aerosols-Particle Modeling, Reflectivity and Attenuation

25

2. Analysis of Free Space Optics as a Transmission Technology, U.S. Army Information Systems Engineering Command, page 3.

3. A 173-mile 2-way all-electronic optical contact

4. http://www.esa.int/esaTE/SEMN6HQJNVE_index_0.html 12

5. http://silicium.dk/pdf/speciale.pdf Optical Communications in Deep Space, University of Copenhagen

6. F. J. Duarte, Secure interferometric communications in free space, Opt. Commun. 205, 313-319 (2002).

7. http://www.cs.utah.edu/cmpmsi/papers09/paper1.pdf CMP-MSI: 3rd Workshop on Chip Multiprocessor Memory Systems and Interconnects held in conjunction with the 36th International Symposium on Computer Architecture, June 2009.

26