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MAR BASELIOS CHRISTIAN COLLEGE OF ENGINEERING & TECHNOLOGY PEERMADE DEPARTMENT OF ELECTRONICS & COMMUNICATION ENGINEERING SEMINAR REPORT ON FREE SPACE OPTIC TECHNOLOGY Submitted by Mr.Amal Dominic Reg No: 222006 under the guidance of Mr.Amol Joy (lecturer Dept: of ECE) Submitted in the partial fulfillment of the requirement for the award of Degree of Bachelor of Technology in

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Page 1: Free Space Optics

MAR BASELIOS CHRISTIAN COLLEGE OFENGINEERING & TECHNOLOGY

PEERMADE

DEPARTMENT OFELECTRONICS & COMMUNICATION ENGINEERING

SEMINAR REPORTON

FREE SPACE OPTIC TECHNOLOGY

Submitted by Mr.Amal Dominic

Reg No: 222006

under the guidance ofMr.Amol Joy

(lecturer Dept: of ECE)

Submitted in the partial fulfillment of the requirementfor the award of

Degree of Bachelor of Technology in

ELECTRONICS & COMMUNICATION ENGINEERING

Of the MAHATMAGANDHI UNIVERSITY

Page 2: Free Space Optics

MAR BASELIOS CHRISTIAN COLLEGE OF ENGINEERING & TECHNOLOGY, PEERMADE

Certificate This is to certify that this SEMINAR report on FREE SPACE OPTIC

TECHNOLOGY is a bonafide record of the miniproject presented by MrA.mal

Dominic(222006) and submitted in partial fulfillment of the requirement of the

degree of Bachelor of Technology in Electronics and Communication Engineering

of Mahatma Gandhi University.

Place: Peermade

Date :…………………

Project Coordinator Head of the Department

Internal Examiner External Examiner

Page 3: Free Space Optics

INTRODUCTION

Free Space Optics (FSO) communications, also called Free Space Photonics

(FSP) or Optical Wireless, refers to the transmission of modulated visible or infrared

(IR) beams through the atmosphere 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. Free Space Optics

(FSO) works on the same basic principle as Infrared television remote controls,

wireless keyboards or wireless Palm® devices.

HISTORY OF FREE SPACE OPTICS (FSO)

The engineering maturity of Free Space Optics (FSO) is often underestimated,

due to a misunderstanding of how long Free Space Optics (FSO) systems have been

under development. Historically, Free Space Optics (FSO) or optical wireless

communications was first demonstrated by Alexander Graham Bell in the late

nineteenth century (prior to his demonstration of the telephone!). Bell’s Free Space

Optics (FSO) experiment converted voice sounds into telephone signals and

transmitted them between receivers through free air space along a beam of light for a

distance of some 600 feet. Calling his experimental device the “photophone,” Bell

considered this optical technology – and not the telephone – his preeminent

invention because it did not require wires for transmission.

Although Bell’s photophone never became a commercial reality, it

demonstrated the basic principle of optical communications. Essentially all of the

engineering of today’s Free Space Optics (FSO) or free space optical

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

applications. By addressing the principal engineering challenges of Free Space

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Optics (FSO), this aerospace/defense activity established a strong foundation upon

which today’s commercial laser-based Free Space Optics (FSO) systems are based.

HOW FREE SPACE OPTICS (FSO) WORKS

Free Space Optics (FSO) transmits invisible, eye-safe light beams from one

"telescope" to another using low power infrared lasers in the teraHertz spectrum.

The beams of light in Free Space Optics (FSO) systems are transmitted by laser light

focused on highly sensitive photon detector receivers. These receivers are telescopic

lenses able to collect the photon stream and transmit digital data containing a mix of

Internet messages, video images, radio signals or computer files.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) systems can function over distances of several

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

destination, and enough transmitter power, Free Space Optics (FSO) communication

is possible.

FREE SPACE OPTICS (FSO) TECHNOLOGY

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Lasers are one of the most significant inventions of the 20th century - they

can be found in many modern products, from CD players to fiber-optic networks.

The word laser is actually an acronym for Light Amplification by Stimulated

Emiission of Radiation. Although stimulated emission was first predicted by Albert

Einstein near the beginning of the 20th century, the first working laser was not

demonstrated until 1960 when Theodore Maiman did so using a ruby. Maiman's

laser was predated by the maser - another acronym, this time for Microwave

Amplification by Stimulated Emission of Radiation. A maser is very similar to a

laser except the photons generated by a maser are of a longer wavelength outside the

visible and/or infrared spectrum.

A laser generates light, either visible or infrared, through a process known as

stimulated emission. To understand stimulated emission, understanding two basic

concepts is necessary. The first is absorption which occurs when an atom absorbs

energy or photons. The second is emission which occurs when an atom emits

photons. Emission occurs when an atom is in an excited or high energy state and

returns to a stable or ground state – when this occurs naturally it is called

spontaneous emission because no outside trigger is required. Stimulated emission

occurs when an already excited atom is bombarded by yet another photon causing it

to release that photon along with the photon which previously excited it. Photons are

particles, or more properly quanta, of light and a light beam is made up of what can

be thought of as a stream of photons.

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A basic laser uses a mirrored chamber or cavity to reflect light waves so they

reinforce each other. An excitable substance – gas, liquid, or solid like the original

ruby laser – is contained within the cavity and determines the wavelength of the

resulting laser beam. Through a process called pumping, energy is introduced to the

cavity exciting the atoms within and causing a population inversion. A population

inversion is when there are more excited atoms than grounded atoms which then

leads to stimulated emission. The released photons oscillate back and forth between

the mirrors of the cavity, building energy and causing other atoms to release more

photons. One of the mirrors allows some of the released photons to escape the cavity

resulting in a laser beam emitting from one end of the cavity.

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TERRESTRIAL LASER COMMUNICATIONS

CHALLENGES

Fog

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

near-infrared wavelengths that are employed in laser communications. Similar to the

case of rain attenuation with RF wireless, fog attenuation is not a “show-stopper” for

optical wireless, because the optical link can be engineered such that, for a large

fraction of the time, an acceptable power will be received even in the presence of

heavy fog. Laser communication systems can be enhanced to yield even greater

availabilities by combining them with RF systems.

Physical Obstructions

Laser communications systems that employ multiple, spatially diverse

transmitters and large receive optics will eliminate interference concerns from

objects such as birds.

Pointing Stability

Pointing stability in commercial laser communications systems is achieved by

one of two methods. The simpler, less costly method is to widen the beam

divergence so that if either end of the link moves the receiver will still be within the

beam. The second method is to employ a beam tracking system. While more costly,

such systems allow for a tighter beam to be transmitted allowing for higher security

and longer distance transmissions.

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Scintillation

Performance of many laser communications systems is adversely affected by

scintillation on bright sunny days. Through a large aperture receiver, widely spaced

transmitters, finely tuned receive filtering, and automatic gain control, downtime due

to scintillation can be avoided.

FSO: WIRELESS, AT THE SPEED OF LIGHT

Unlike radio and microwave systems, Free Space Optics (FSO) is an optical

technology and no spectrum licensing or frequency coordination with other users is

required, interference from or to other systems or equipment is not a concern, and

the point-to-point laser signal is extremely difficult to intercept, and therefore

secure. Data rates comparable to optical fiber transmission can be carried by Free

Space Optics (FSO) systems with very low error rates, while the extremely narrow

laser beam widths ensure that there is almost no practical limit to the number of

separate Free Space Optics (FSO) links that can be installed in a given location.

HOW FREE SPACE OPTICS (FSO) CAN HELP YOU

FSO’s freedom from licensing and regulation translates into ease, speed and

low cost of deployment. Since Free Space Optics (FSO) transceivers can transmit

and receive through windows, it is possible to mount Free Space Optics (FSO)

systems inside buildings, reducing the need to compete for roof space, simplifying

wiring and cabling, and permitting Free Space Optics (FSO) equipment to operate in

a very favorable environment. The only essential requirement for Free Space Optics

(FSO) or optical wireless transmission is line of sight between the two ends of the

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link.

For Metro Area Network (MAN) providers the last mile or even feet can be

the most daunting. Free Space Optics (FSO) networks can close this gap and allow

new customers access to high-speed MAN’s. Providers also can take advantage of

the reduced risk of installing an Free Space Optics (FSO) network which can later be

redeployed.

THE MARKET. WHY FSO? BREAKING THE

BANDWIDTH BOTTLENECK

Why FSO? The global telecommunications network 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 a more recent emphasis on

metropolitan area networks (MANs). Meanwhile, local area networks (LANs) and

gigabit ethernet ports are being deployed with a comparable growth rate. In order for

this tremendous network capacity to be exploited, and for the users to be able to

utilize the broad array of new services becoming available, network designers must

provide a flexible and cost-effective means for the users to access the

telecommunications network. Presently, however, most local loop network

connections are limited to 1.5 Mbps (a T1 line). As a consequence, there is a strong

need for a high-bandwidth bridge (the “last mile” or “first mile”) between the LANs

and the MANs or WANs.

A recent New York Times article reported that more than 100 million miles of

optical fiber was laid around the world in the last two years, as carriers reacted to the

Internet phenomenon and end users’ insatiable demand for bandwidth. The sheer

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scale of connecting whole communities, cities and regions to that fiber optic cable or

“backbone” is something not many players understood well. Despite the huge

investment in trenching and optical cable, most of the fiber remains unlit, 80 to 90%

of office, commercial and industrial buildings are not connected to fiber, and

transport prices are dropping dramatically.

Free Space Optics (FSO) systems represent one of the most promising

approaches for addressing the emerging broadband access market and its “last mile”

bottleneck. Free Space Optics (FSO) systems offer many features, principal among

them being low start-up and operational costs, rapid deployment, and high fiber-like

bandwidths due to the optical nature of the technology

BROADBAND BANDWIDTH ALTERNATIVES

Access technologies in general use today include telco-provisioned copper

wire, wireless Internet access, broadband RF/microwave, coaxial cable and direct

optical fiber connections (fiber to the building; fiber to the home). Telco/PTT

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telephone networks are still trapped in the old Time Division Multiplex (TDM)

based network infrastructure that rations bandwidth to the customer in increments of

1.5 Mbps (T-1) or 2.024 Mbps (E-1). DSL penetration rates have been throttled by

slow deployment and the pricing strategies of the PTTs. Cable modem access has

had more success in residential markets, but suffers from security and capacity

problems, and is generally conditional on the user subscribing to a package of cable

TV channels. Wireless Internet access is still slow, and the tiny screen renders it of

little appeal for web browsing.

Broadband RF/microwave systems have severe limitations and are losing

favor. The radio spectrum is a scarce and expensive licensed commodity, sold or

leased to the highest bidder, or on a first-come first-served basis, and all too often,

simply unavailable due to congestion. As building owners have realized the value of

their roof space, the price of roof rights has risen sharply. Furthermore, radio

equipment is not inexpensive, the maximum data rates achievable with RF systems

are low compared to optical fiber, and communications channels are insecure and

subject to interference from and to other systems (a major constraint on the use of

radio systems).

FREE SPACE OPTICS (FSO) ADVANTAGES

Free space optical (FSO) systems offers a flexible networking solution that

delivers on the promise of broadband. Only free space optics or Free Space Optics

(FSO) provides the essential combination of qualities required to bring the traffic to

the optical fiber backbone – virtually unlimited bandwidth, low cost, ease and speed

of deployment. Freedom from licensing and regulation translates into ease, speed

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and low cost of deployment. Since Free Space Optics (FSO) optical wireless

transceivers can transmit and receive through windows, it is possible to mount Free

Space Optics (FSO) systems inside buildings, reducing the need to compete for roof

space, simplifying wiring and cabling, and permitting the equipment to operate in a

very favorable environment. The only essential for Free Space Optics (FSO) is line

of sight between the two ends of the link.

Freedom from licensing and regulation leads to ease, speed and low cost of

deployment.

Since FSO units can receive and transmit through windows it reduces the need

to compete for roof space, simplifying wiring and cabling.

Only need is the line of sight between the two ends of the link.

Providers take advantage of the reduced risk in installing FSO equipment,

which can even be re-deployed.

Zero chances of network failure.

Virtually unlimited bandwidth.

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. Building a

network on the SONAbeam™ platform is one of the best ways to ensure that data

transmission between any two points is completely secure. Its focused transmission beam

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foils jammers and eavesdroppers and enhances security. Moreover, fSONA systems can

use any signal-scrambling technology that optical fiber can use.

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:

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

APPLICATIONS

Metro network extensions – FSO is used to extend existing metropolitan area

fiberings to connect new networks from outside.

Last mile access – FSO can be used in high-speed links to connect end users

with ISPs.

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Enterprise connectivity - The ease in which FSO can be installed makes them a

solution for interconnecting LAN segments, housed in buildings separated by

public streets.

Fiber backup - FSO may be deployed in redundant links to backup fiber in

place of a second fiber link.

Backhaul – Used to carry cellular telephone traffic from antenna towers back

to facilities into the public switched telephone networks.

FREE SPACE OPTICS (FSO) CHALLENGES

The advantages of free space optical wireless or Free Space Optics (FSO) do

not come without some cost. When light is transmitted through optical fiber,

transmission integrity is quite predictable – barring unforseen events such as

backhoes or animal interference. When light is transmitted through the air, as with

Free Space Optics (FSO) optical wireless systems, it must contend with a a complex

and not always quantifiable subject - the atmosphere.

FOG AND FREE SPACE OPTICS (FSO)

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

on the near-infrared wavelengths that are employed in Free Space Optics (FSO)

systems. Note that the effect of fog on Free Space Optics (FSO) optical wireless

radiation is entirely analogous to the attenuation – and fades – suffered by RF

wireless systems due to rainfall. Similar to the case of rain attenuation with RF

wireless, fog attenuation is not a “show-stopper” for Free Space Optics (FSO)

optical wireless, because the optical link can be engineered such that, for a large

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fraction of the time, an acceptable power will be received even in the presence of

heavy fog. Free Space Optics (FSO) optical wireless-based communication

systems can be enhanced to yield even greater availabilities.

PHYSICAL OBSTRUCTIONS AND FREE SPACE OPTICS (FSO)

Free Space Optics (FSO) products which have widely spaced redundant

transmitters and large receive optics will all but eliminate interference concerns

from objects such as birds. On a typical day, an object covering 98% of the

receive aperture and all but 1 transmitter; will not cause an Free Space Optics

(FSO) link to drop out. Thus birds are unlikely to have any impact on Free Space

Optics (FSO) transmission.

FREE SPACE OPTICS (FSO) POINTING STABILITY –

BUILDING SWAY, TOWER MOVEMENT

Fied pointed Free Space Optics (FSO) systems are designed to be

capable of handling the vast majority of movement found in deployments on

buildings. The combination of effective beam divergence and a well matched

receive Field-of-View (FOV) provide for an extremely robust fixed pointed Free

Space Optics (FSO) system suitable for most deployments. Fixed-pointed Free

Space Optics (FSO) systems are generally preferred over actively-tracked Free

Space Optics (FSO) systems due to their lower cost.

SCINTILLATION AND FREE SPACE OPTICS (FSO)

Performance of many Free Space Optics (FSO) optical wireless systems

is adversely affected by scintillation on bright sunny days; the effects of which

are typically reflected in BER statistics. Some optical wireless products have a

unique combination of large aperture receiver, widely spaced transmitters, finely

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tuned receive filtering, and automatic gain control characteristics. In addition,

certain optical wireless systems also apply a clock recovery phase-lock-loop time

constant that all but eliminate the affects of atmospheric scintillation and jitter

transference.

SOLAR INTERFERENCE AND FREE SPACE OPTICS (FSO)

Solar interference in Free Space Optics (FSO) free space optical

systems operating at 1550 nm can be combatted in two ways. The first is a long-

pass optical filter window used to block all optical wavelengths below 850 nm

from entering the system; the second is an optical narrowband filter proceeding

the receive detector used to filter all but the wavelength actually used for

intersystem communications. To handle off-axis solar energy, two spatial filters

have been implemented in SONAbeam systems, allowing them to operate

unaffected by solar interference that is more than 1.5 degrees off-axis.

FREE SPACE OPTICS (FSO) COMPARISONS

Free space optical communications is now established as a viable approach for

addressing the emerging broadband access market and its “last mile” bottleneck..

These robust systems, which establish communication links by transmitting laser

beams directly through the atmosphere, have matured to the point that mass-

produced models are now available. Optical wireless systems offer many features,

principal among them being low start-up and operational costs, rapid deployment,

and high fiber-like bandwidths. These systems are compatible with a wide range of

applications and markets, and they are sufficiently flexible as to be easily

implemented using a variety of different architectures. Because of these features,

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market projections indicate healthy growth for optical wireless sales. Although

simple to deploy, optical wireless transceivers are sophisticated devices.

The many sub-systems require a multi-faceted approach to system engineering

that balances the variables to produce the optimum mix. A working knowledge of

the issues faced by an optical wireless system engineer provides a foundation for

understanding the differences between the various systems available. This paper

aims to examine the many elements considered by the system engineer when

designing a product so that the buyer can ask those same questions about the systems

they are evaluating for purchase.

WHICH WAVELENGTH?

Currently available Free Space Optics (FSO) hardware can be classified into

two categories depending on the operating wavelength – systems that operate near

800 nm and those that operate near 1550 nm. There are compelling reasons for

selecting 1550 nm Free Space Optics (FSO) systems due to laser eye safety, reduced

solar background radiation, and compatibility with existing technology

infrastructure.

EYE-SAFETY

Laser beams with wavelengths in the range of 400 to 1400 nm emit light that

passes through the cornea and lens and is focused onto a tiny spot on the retina while

wavelengths above 1400 nm are absorbed by the cornea and lens, and do not focus

onto the retina, as illustrated in Figure 1. It is possible to design eye-safe laser

transmitters at both the 800 nm and 1550 nm wavelengths but the allowable safe

laser power is about fifty times higher at 1550 nm. This factor of fifty is important

Page 18: Free Space Optics

as it provides up to 17 dB additional margin, allowing the system to propagate over

longer distances, through heavier attenuation, and to support higher data rates.

ATMOSPHERIC ATTENUATION

Carrier-class Free Space Optics (FSO) systems must be designed to

accommodate heavy atmospheric attenuation, particularly by fog. Although longer

wavelengths are favored in haze and light fog, under conditions of very low

visibility this long-wavelength advantage does not apply. However, the fact that

1550 nm-based systems are allowed to transmit up to 50 times more eye-safe power

will translate into superior penetration of fog or any other atmospheric attenuator.

RECEIVER

There are a number of factors to consider when examining the effectiveness of

the receiver in an FSO system; these include the type of detector used, the sensitivity

rating and size of the detector, the size and design of the receiver optics, and the

operating wavelength itself. In order to correctly assess the efficiency of the overall

system, one must also take into account the number and power of the lasers being

used to generate the signal.

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Types of optical detectors used in FSO equipment come in two flavors: PIN

and APD. The PIN detector is a lower cost detector that has no internal gain, while

the APD is a more expensive but also more sensitive detector with internal gain. The

Benefits of using APD over PIN technology will vary, but real-world results indicate

the benefits to be an improvement in sensitivity of approximately 4x that of a PIN

detector. Although at first glance it would seem that systems using APD detectors

should have a performance advantage; however, the performance of a system must

also take into consideration the transmit characteristics. As an example, the

SONAbeam155-M uses the lower-cost PIN detectors but because it produces 20-40

times the laser power of competing systems the SONAbeam155-M is still 5-10 times

more effective than those systems utilizing APD based receivers. Thus, the

SONAbeam is a much more powerful system, which allows it to outperform other

products at the same distance, under the same weather conditions.

The size of the receiver optics is also important; a larger area receive optic

contributes to reducing errors due to scintallation. Scintillation is atmospheric

turbulence due to solar loading and natural convection, causing temporally and

spatially varying refractive index changes in the air. As a laser beam propagates

through the atmosphere, there is a time-varying intensity at the receiver due to this

phenomenon; this is referred to as 'scintillation'. This is quite similar to the apparent

twinkling of the stars or distant city lights, which is due to the same effect. The

result is that an FSO communications receiver can experience error bursts due to

surges and fades in the receive signal strength. One way to combat this scintillation

effect, and thus improve the error-rate performance, is to use a large aperture

receiver. A collecting aperture that is much larger than the spatial scale of the

scintillation provides an averaging effect of the localized surges and fades, thus

improving the error rate. This large-aperture approach is more effective for

scintillation reduction than multiple smaller apertures, which perform less averaging

at each lens. Another way to mitigate the effects of scintillation is to use multiple

transmitters, each of which takes a slightly different path through the atmosphere,

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which also contributes an averaging effect. The net result is that a properly designed

system can defeat scintillation impairments.

The operating wavelength of an FSO system also contributes to the

performance of the receiver. It is generally true that high-quality photodiodes at both

800nm and 1550nm achieve comparable quantum efficiencies. However, longer

wavelengths enjoy an advantage in the receiver due to their lower photon energies.

Specifically, a 1550nm photon has half the energy of a 800nm photon.

Consequently, for the same total energy (i.e. Watts of power), a beam of 1550nm

light has twice the number of photons as a beam of 800nm light. This results in

twice the photoelectrons (photocurrent) from the receiver photodiode. Since a certain

minimum number of photoelectrons is required to detect an optical pulse, a pulse at

1550nm can be detected with ~ 3 dB less optical power. Hence, 1550nm has a

fundamental 3 dB advantage over 800nm in receiver sensitivity.

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 non-coherently combining multiple lower cost semiconductor

lasers. Narrow transmitting beamwidth (a.k.a. high antenna gain) can be achieved on

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a limited basis for fixed-pointed units, with the minimum beamwidth large enough to

accommodate building sway and wind loading. Much narrower beams can be

achieved 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 terminal cost, volume and weight

limitations. And high receiver sensitivity can be achieved by using small, low-

capacitance photodetectors, 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 cost

and 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 be

tolerant 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

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(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.

RELIABILITY

Systems are designed, engineered and tested to ensure exceptional reliability.

Building on their extensive experience in laser communications systems for military and

space applications, our design engineers have ensured that critical sub-systems are

manufactured using high-reliability components. Component reliability is further ensured

by rigorous vendor qualification and incoming inspection procedures.

Our equipment reliability analysis is performed using the stringent

Bellcore/Telcordia guidelines applicable to carrier equipment. This is further backed up

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by exhaustive qualification testing in our in-house test facilities, where subsystems are

severely stressed and operational performance is validated at extremes ranging from -

50°C to 75°C. The combination of active laser cooling, high-reliability components,

sealed housings and rugged mechanical design enables us to offer carriers superior

products with outstanding communications performance and a rated service life of 15

years.

Built for Dependability and Longevity

Depending on their bandwidth and operating range, NAbeam™ systems are

designed with two-, four- or eight-fold redundancy of lasers, laser drivers, laser coolers

and cooler controllers. SONAbeam's™ environmentally sealed cast-aluminum

exterior housings, unique in the market, are impervious to water, sun and other

environmental hazards. fSONA's rugged transceiver mounting structures maintain

pointing accuracy through Class 1 hurricanes of 120 km/hr, and survive Class 2

hurricanes of 160 km/hr.

COST OF DEPLOYEMENT

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Higher performances with little extra cost penalty, provides the best value. The

key factor that affects the cost are system design, minimization of manual labour and

bulk manufacturing. An 850 nm laser can cost up to $5000 while a 1550 nm laser can go

up to $50,000.

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.

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REFERENCES

Websites:

1. http://www.lightpointe.com

2. http://www.spie.org

3. http://www.osa.org

Journals

1. IEEE Spectrum August 2001

2. IEEE Intelligent System May-June 2001

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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 tranceivers without risk of mutual interference in a given location. FSO has roles

to play as primary access madium 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...

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CONTENTS

1. INTRODUCTION 1

2. HISTORY OF FREE SPACE OPTICS (FSO) 1

3. HOW FREE SPACE OPTICS (FSO) WORKS 2

4. FREE SPACE OPTICS (FSO) TECHNOLOGY 3

5. TERRESTRIAL LASER

COMMUNICATIONS CHALLENGES 5

6. FSO: WIRELESS, AT THE SPEED OF LIGHT 6

7. THE MARKET. WHY FSO?

BREAKING THE BANDWIDTH BOTTLENECK 7

8. FREE SPACE OPTICS (FSO) ADVANTAGES 10

9. FREE SPACE OPTICS (FSO) SECURITY 11

10. APPLICATIONS 12

11. FREE SPACE OPTICS (FSO) CHALLENGES 12

12. COST OF DEPLOYEMENT 22

13. CONCLUSION 23

14. REFERENCES 24

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