INTRODUCTION

When we talk about optical communication, most people

think about optical-fiber. But optical communication is also

possible without optical-fiber. We know that light travels

through air for a lot less money. This makes possible the

optical communication without optical-fiber. Optical

communication without fiber is known as Free Space Optics.

It is used due to economic advantages. Since the introduction

of internet the backbone traffic is increasing at the rate greater

than 100%, hence the owner of the backbone infrastructure

(which is entirely based on fiber optics) are eagerly embracing

technologies that add of the capacity of the fiber optics

without adding mountains of optical cables.

FSO is not a new idea. 30-years back optical-fiber cables

are used for high-speed communication. In those days FSO

are used for high-speed connectivity over short distances.

Today’s FSO can carry full-duplex data at gigabit-per-second

rates over metropolitan distances.

What is Free Space Optics (FSO)?

Free Space Optics (FSO) is a line-of-sight technology that

uses lasers to provide optical bandwidth connections.

Currently, Free Space Optics are capable of up to 2.5 Gbps of

data, voice and video communications through the air,

allowing optical connectivity without requiring fiber-optic

cable or securing spectrum licenses. Free Space Optics require

light, which can be focused by using either light emitting

diodes (LEDs) 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 is the medium. Light travels through air faster

than it does through glass, so it is fair to classify Free Space

Optics as optical communications at the speed of light.

Free Space Optics (FSO) technology is relatively simple.

It's based on connectivity between FSO units, each consisting

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 (i.e. laser), plus a lens

that transmits light through the atmosphere to another lens

receiving the information. The receiving lens connects to a

high-sensitivity receiver via optical fiber. FSO technology

requires no spectrum licensing. FSO is easily upgradeable,

and its open interfaces support equipment from a variety of

vendors, which helps service providers protect their

investment in embedded telecommunications infrastructures.

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

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.

Light Beam Used for FSO System

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. 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.

Why FSO Now?

Substantial investments by carriers to augment the capacity

of their core fiber backbones have facilitated dramatic

improvements in both price and performance, and they have

also increased the capacity of these large backbone networks.

However, to generate the communications traffic and revenue

needed to fully utilize and pay for these backbone upgrades,

higher bandwidth connections must reach the end customers.

This requires substantial bandwidth upgrades at the network

edge. Essentially, to fully leverage their backbone

investments, service providers will also need to expand and

extend the reach of their metropolitan optical network to the

edge. FSO presents an opportunity that allows carriers to

achieve that goal for one-fifth the cost when compared to fiber

(if even available) and at a fraction of the time.

Increased competition: Regulation changes and

significant investments by various funds have

increased the competitive climate in these metro

networks. Each of the existing or new entrants is

racing to gain an advantage over their competition.

FSO is one of the evolutionary technologies that

allows a carrier to acquire and retain new customers

quickly and cost-effectively, thereby gaining an

entry point over competition. Metro optical

networks are expected to see $57.3 billion invested

by 2005.

International growth: Due to the growing number of

Internet-based applications, most countries are

experiencing tremendous growth in bandwidth

needs. In growing economies like Latin America

and China—where the ability to have high-

bandwidth connectivity outweighs standards for

reliability—the lack of infrastructure and rising

bandwidth demands offers a unique opportunity for

FSO.

Changing traffic patterns and protocol standards:

Multiple traffic types characterize metro networks.

Where voice was once the dominant traffic type,

data has emerged as the winner. Moreover, these

networks are also a mixture of multiple protocols

ranging from Ethernet, SONET, IP, ESCON,

FICON, etc. As a Layer One technology, FSO is

protocol agnostic.

Wireless world: With the rapid adoption and slow

deployment of wireless technologies such as LMDS

and MMDS in response to high bandwidth

communication needs in the metro area, many

service providers still find themselves short of

bandwidth to satisfy their needs. To better

understand this growing need for FSO, it is

important to understand the key drivers for FSO.

Applications of FSO

The applications of free-space-optics are many. Some of

them are as follows –

1:- Metro Network Extensions

Carriers can deploy FSO to extend existing

metropolitan-area fiber rings, to connect new networks,

and, in their core infrastructure, to complete Sonet

rings.

2:- Last-Mile Access

FSO can be used in high-speed links that connect end-

users with internet service providers or other networks.

It can also be used to bypass local-loop systems to

provide business with high-speed connections.

3:- Enterprise Connectivity

the ease with which FSO links can be installed makes

them a natural for interconnecting local-area network

segments that are housed in buildings separated by

public streets or other right-of-way property.

4:- Fiber Backup

FSO may 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

FSO can be also used to provide instant service to fiber-

optic customers while their fiber infrastructure is being

laid.

FSO: Optical or Wireless?

FSO is clearly an optical technology and not a wireless

technology for two primary reasons. One, FSO enables optical

transmission at speeds of up to 2.5 Gbps and in the future 10

Gbps using WDM. This is not possible using any fixed

wireless/RF technology existing today. Two, FSO obviates

the need to buy expensive spectrum (it requires no FCC or

municipal license approvals), which distinguishes it clearly

from fixed wireless technologies. Thus, FSO should not be

classified as a wireless technology. Its similarity to

conventional optical solutions will enable a seamless

integration of access networks with optical core networks and

help to realize the vision of an all-optical network.

Free-Space Optics (FSO) 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:

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

Challenges To Free-Space Optics

Fiber-optic cable and FSO share many similarities.

However, there is a difference in how each technology

transmits information. While fiber uses a relatively

predictable medium that is subject to outside disturbances

from wayward construction backhoes, gnawing rodents and

even sharks when deployed under sea, FSO uses an open

medium (the atmosphere) that is subject to its own potential

outside disturbances. Networks with FSO must be designed to

counter the atmosphere, which can affect an FSO system's

capacity. FSO is also a line-of-sight technology and

interconnecting points must be free from physical obstruction

and able to "see" each other.

1:- Scintillation

Scintillation is best defined as the temporal and spatial

variations in light intensity caused by atmospheric

turbulence. Such turbulence is caused by wind and

temperature gradients that create pockets of air with

rapidly varying densities and therefore fast changing

indices of optical refraction. These air pockets act like

prisms and lenses with time varying properties. Their

action is readily observed in the twinkling of stars in the

night sky and the shimmering of horizon on a hot day.

FSO communications systems deal with scintillation by

sending the same information from several separate laser

transmitters. These are mounted in the same housing, or

link head, separated from one another by distances of

about 200 mm. it is unlikely that in traveling to the

receiver , all the parallel beams will encounter the same

pocket of turbulence since the scintillation pockets are

usually quite small. Most probably, at least one of the

beams will arrive at the target node with adequate strength

to be properly received. This approach is called Spatial

Diversity.

2:- Mie-scattering

It is the scattering of beam due to fog. It is largely a matter

of boosting the transmitted power. Spatial diversity also

helps to deal with scattering. In areas with frequent heavy

fogs, it is often necessary to choose 1550-nm lasers

because of the higher power permitted at that wavelength.

Also, there seems to be some evidence that mie-scattering

is slightly lower at 1550-nm than at 850-nm. But some

studies shows that scattering is independent of the

wavelength under heavy fog conditions. Other atmospheric

disturbances, like snow and especially rain, are less of a

problem for free-space optics than fog.

3:- Swaying Buildings

One of the more common difficulties that arises when

deploying free-space optics links on tall buildings or

towers is sway due to wind or seismic activities. Both

storms and earthquakes can cause buildings to move

enough to affect beam aiming.

The problem of swaying buildings can be dealt with in two

ways.

Beam Divergence

With beam divergence, the transmitted beam is

purposely allowed to diverge, or spread, so that by the

time it arrives at the receiving link head, it forms a

fairly large optical cone. Depending on product

design, the typical free-space optics light beam

subtends an angle of 3-6 milliradians (10-20 minutes

of arc) and will have a diameter of 3-6 meters after

traveling 1 kilometer. If the receiver is initially

positioned at the center of the beam, divergence alone

can deal with many perturbations.

Active Tracking

This method is used when the link heads are mounted

on the top of extremely tall buildings or towers.

Active tracking is based on movable mirrors that

control the direction in which the beams are launched.

A feedback mechanism continuously adjust the

mirrors so that the beams stay on target. It is more

sophisticated and costly than beam divergence

method.

6:- Physical Obstructions

Flying birds can temporarily block a single beam, but this

tends to cause only short interruptions, and transmissions

are easily and automatically resumed. LightPointe uses

multi-beam systems (spatial diversity) to address this

issue, as well as other atmospheric conditions, to provide

for greater availability.

5:- Safety

To those unfamiliar with FSO, safety is often a concern

because the technology uses lasers for transmission. This

concern, however, is based on perception more than

reality. The proper use and safety of lasers have been

discussed since FSO devices first appeared in laboratories

more than two decades ago. The two major concerns

involve human exposure to laser beams (which present

much more danger to the eyes than any other part of the

human body) and high voltages within the laser systems

and their power supplies. Standards have been set for laser

safety and performance and FSO systems comply with

these standards.

Advantages Of Free-Space Optics

The FSO system requires less than one fifth of the

capital outlay of comparable ground-based fiber-optic

technologies. Optical-fibers are too costly.

Connecting the buildings with optical-fiber cost US

$100000 - $200000/km in metropolitan areas, 85

percent of the total figure tied to trenching and

installation. To install fiber you have to dig the road.

Street trenching and digging are not only expensive,

they cause traffic jams (which increase air pollution),

displace trees, and sometimes destroy historical areas.

Using FSO, a service provider can be generating

revenue while a fiber-based competitor is still seeking

municipal approval to dig up a street to lay its cable.

It is flexible, offers freedom, and is fast (speeds from

20 Mbps to 2.5 Gbps and beyond)

Demand for bandwidth is increasing and has been

increasing exponentially for the past few years.

Service providers have been struggling to keep up

with such demand. Service providers must extend the

reach of metro optical networks, and FSO offers

service providers the opportunity to accomplish this

objective.

The primary advantages of FSO are high throughput,

solid security, and low cost.

Conclusion

The entire face of the Free-Space Optics community is

about to change radically as driven by the need for high-speed

local loop connectivity and the costs and difficulties of

deploying fibers. FSO can be the ultimate solution for high-

speed access. Instead of hybrid fiber-coax system, hybrid

fiber-laser system may turn out to be the best way to deliver

the high capacity last-mile access. FSO provide higher

security, and throughput. FSO is capable to fulfill the

increasing demand of bandwidth.