ABSTRACT

Free space optics ( FSO ) is a line-of-sight technology that currently

enables optical transmission up to 2.5 Gbps of data, voice, and video

communications through the air , allowing optical connectivity without

deploying fiber optic cables or securing spectrum licenses. FSO system can

carry full duplex data at giga bits per second rates over Metropolitan

distances of a few city blocks of few kms. FSO, also known as optical wireless,

overcomes this last-mile access bottleneck by sending high –bitrate signals

through the air using laser transmission .

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 4

2. HISTORY OF FREE SPACE OPTICS (FSO) 4

3. HOW FREE SPACE OPTICS (FSO) WORKS 5

4. FREE SPACE OPTICS (FSO) TECHNOLOGY 6

5. TERRESTRIAL LASER

COMMUNICATIONS CHALLENGES 8

6. FSO: WIRELESS, AT THE SPEED OF LIGHT 9

7. THE MARKET. WHY FSO?

BREAKING THE BANDWIDTH BOTTLENECK 10

8. FREE SPACE OPTICS (FSO) ADVANTAGES 13

9. FREE SPACE OPTICS (FSO) SECURITY 14

10. APPLICATIONS 15

11. FREE SPACE OPTICS (FSO) CHALLENGES 15

12. COST OF DEPLOYEMENT 25

13. CONCLUSION 26

14. REFERENCES 27

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ACKNOWLEDGEMENT

We are deeply indebted to Mr. S. MAHESWAR REDDY, Head of the Department of Electronic And Communication Engineering, Hi-tech College of Engineering and Technology, Hyderabad for his valuable guidance, support in this project. We thank him for his valuable help and co-operation.

We wish to express our sincere thanks to our guide Mr. SURENDRA DUSTAKAR, Department of Electronics and Communication Engineering, Hi-Tech College of Engineering and Technology, Hyderabad for his valuable support, guidance and co-operation throughout the course of this project. He helped us immensely throughout the course of our project and was continuous inspiration to us.

We would also like to express our gratitude to one and all, who directly or indirectly helped us in bringing this effort to present form. 

RAMAKANTH (06J11A0406)

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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

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defense applications. By addressing the principal engineering challenges of

Free Space 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.

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FREE SPACE OPTICS (FSO) TECHNOLOGY

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

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optical wireless transmission is line of sight between the two ends of the

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

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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 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

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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 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

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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 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

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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 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

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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.

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)

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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 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

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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 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

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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, 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.

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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 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

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account the number and power of the lasers being used to generate the

signal.

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

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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, 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

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~ 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 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

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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 (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

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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 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

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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

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

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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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