Free space optics- A technical seminar report

Applications of FSO

1 Sri venkateshwara college of engineering

KARTHIK DC

(DEPT of ECE)

Applications of FSO


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 bit rate signals through the air using laser transmission.

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 forgetting 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 environmental condition, if there is any

change in the signal then the servo system is used to correct the signal.

Applications of FSO


CONTENTS:

Ä INTRODUCTION

Ä WHAT IS FSO?

Ä WHY FSO?

Ä METHOD OF OPERATION

Ä SYSTEM DESIGN

Ä FSO ISSUES

Ä COMPARISION

Ä FSO ADVANTAGES

Ä FSO CHALLENGES

Ä FSO APPLICATIONS

Ä RECENT DEVELOPMENTS IN FSO

Ä CONCLUSION

Ä REFERENCES

Applications of FSO


INTRODUCTION

Mention optical communication and most people think of fiber optics. But

light travels through air for a lot less money. So it is hardly a surprise that clever

entrepreneurs and technologists are borrowing many of the devices and

techniques developed for fiberoptic systems and applying them to what some call

fiber-free optical communication. Although it only recently, and rather suddenly,

sprang into public awareness, free-space optics is not a new idea. It has roots that

go back over 30 years--to the era before fiber optic cable became the preferred

transport medium for high-speed communication. In those days, the notion that

FSO systems could provide high-speed connectivity over short distances seemed

futuristic, to say the least. But research done at that time has made possible

today's free-space optical systems, which can carry full-duplex (simultaneous

bidirectional) data at gigabit-per-second rates over metropolitan distances of a

few city blocks to a few kilometers.

FSO first appeared in the 60's, for military applications. At the end of

80's, it appeared as a commercial option but technological restrictions prevented

it from success. Low reach transmission, low capacity, severe alignment

problems as well as vulnerability to weather interferences were the major

drawbacks at that time. The optical communication without wire, however,

evolved! Today, FSO systems guarantee 2.5 Gb/s taxes with carrier class

availability. Metropolitan, access and LAN networks are reaping the benefits.

FSO success can be measured by its market numbers: forecasts predict it will

reach a USS 2.5 billion market by 2006.

The use of free space optics is particularly interesting when we perceive

that the majority of customers does not possess access to fibers as well as fiber

installation is expensive and demands long time. Moreover, right-of-way costs,

difficulties in obtaining government licenses for new fiber installation etc. are

further problems that have turned FSO into the option of choice for short reach

applications.

Applications of FSO


FSO uses lasers, or light pulses, to send packetized data in the terahertz

(THz) spectrum range. Air, or fiber is the transport medium. This means that

urban businesses needing fast data and Internet access have a significantly lower-

cost option. An FSO system for local loop access comprises several laser

terminals, each one residing at a network node to create a single, point-to-point

link; an optical mesh architecture; or a star topology, which is usually point-to-

multipoint. These laser terminals, or nodes, are installed on top of customers'

rooftops or inside a window to complete the last-mile connection. Signals are

beamed to and from hubs or central nodes throughout a city or urban area. Each

node requires a Line-Of-Sight (LOS) view of the hub.

WHAT IS FSO?

FSO technology is implemented using laser device .These laser devices or

terminals can be mounted on rooftops, Corners of buildings or even inside offices

behind windows. FSO devices look like security video cameras. Low-power

infrared beams, which do not harm the eyes, are the means by which

Free-space optics technology transmits data through the air between transceivers,

or link heads, mounted on rooftops or behind windows. It works over distances of

several hundred meters to a few kilometers, depending upon atmospheric

conditions.

Commercially available free-space optics equipment provides data rates

much higher than digital subscriber lines or coaxial cables can ever hope to offer.

And systems even faster than the present range of 10 Mb/s to 1.25 Gb/s have

been announced, though not yet delivered.

Generally the equipment works at one of two wavelengths: 850 nm or

1550 nm. Lasers for 850 nm are much less expensive (around $30 versus more

than $1000) and are therefore favored for applications over moderate distances.

But a 1550 nm lasers are also used. The main reasons revolve around power,

distance, and eye safety. Infrared radiation at 1550 nm tends not to reach the

Applications of FSO


retina of the eye, being mostly absorbed by the cornea. Regulations accordingly

allow these longer-wavelength beams to operate at higher power than the 850-nm

beams, by about two orders of magnitude. That power increase can boost link

lengths by a factor of at least five while maintaining adequate signal strength for

proper link operation. Alternatively, it can boost data rate considerably over the

same length of link. So for high data rates, long distances, poor propagation

conditions (like fog), or combinations of those conditions, 1550 nm can become

quite attractive.

As the differences in laser prices suggest, such systems are quite a bit

more expensive than 850-nm links. An 850-nm transceiver can cost as little as

$5000 (for a 10-100-Mb/s unit spanning a few hundred meters), while a 1550-nm

unit can go for $50 000(for gigabit-per-second setups encompassing a kilometer

or two).

Air fibre, a major FSO vendor, says it can get a link up and running

within two to three days at one-third to one-tenth the cost of fiber (about $20,000

per building). FSO is not only cost-effective and easy to deploy but also fast. The

technology is not for everyone. A major reason companies might not adopt FSO

is its confinement to urban areas. FSO deployments must be located relatively

close to big hubs, which means only customers in major cities will be eligible-at

least initially. Businesses in more remote locations are out of luck, unless a

provider sets up hubs in their area, which seems like a distant reality right now.

When fiber was compared with free-space optics, deployment costs for service to

the three buildings worked out to $396 500 versus $59 000, respectively. The

fiber cost was calculated on a need for 1220 meters: 530 meters of trunk fiber

from the CLEC's central office to its hub in the office park plus an average of 230

meters of feeder fiber for each of the runs from the hub to a target building, all at

$325 per meter. Free-space optics is calculated as $18 000 for free-space optics

equipment per building and $5000 for installation. Supposing a 15 percent annual

revenue increase for future sales and customer acquisition, the internal rate of

return for fiber over five years is 22 percent versus 196 percent for free-space

optics.

Applications of FSO


WHY FSO?

The increasing demand for high bandwidth in metro networks is

relentless, and service providers' pursuit of a range of applications, including

metro network extension, enterprise LAN-to-LAN connectivity, wireless

backhaul and LMDS supplement has created an imbalance. This imbalance is

often referred to as the "last mile bottleneck." Service providers are faced with

the need to turn up services quickly and cost-effectively at a time when capital

expenditures are constrained. But the last mile bottleneck is only part of a larger

problem. Similar issues exist in other parts of the metro networks. "Connectivity

bottleneck" better addresses the core dilemma. As any network planner will tell

you, the connectivity bottleneck is everywhere in metro networks.

From a technology standpoint, there are several options to address this

"connectivity bottleneck," but most don't make economic sense.

The first, most obvious choice is fiber-optic cable. Without a doubt, fiber

is the most reliable means of providing optical communications. But the digging,

delays and associated costs to lay fiber often make it economically prohibitive.

Moreover, once fiber is deployed, it becomes a "sunk" cost and cannot be re-

deployed if a customer relocates or switches to a competing service provider,

making it extremely difficult to recover the investment in a reasonable timeframe.

Another option is radio frequency (RF) technology. RF is a mature

technology that offers longer ranges distances than FSO, but RF-based networks

require immense capital investments to acquire spectrum license. Yet, RF

technologies cannot scale to optical capacities of 2.5 gigabits. The current RF

bandwidth ceiling is 622 megabits. When compared to FSO, RF does not make

economic sense for service providers looking to extend optical networks.

Applications of FSO


The third alternative is wire- and copper-based technologies, (i.e. cable

modem, T1s or DSL). Although copper infrastructure is available almost

everywhere and the percentage of buildings connected to copper is much higher

than fiber, it is still not a viable alternative for solving the connectivity

bottleneck. The biggest hurdle is bandwidth scalability. Copper technologies may

ease some short-term pain, but the bandwidth limitations of 2 megabits to 3

megabits make them a marginal solution, even on a good day.

The fourth-and often most viable-alternative is FSO. The technology is an

optimal solution, given its optical base, bandwidth scalability, speed of

deployment (hours versus weeks or months), re-deployment and portability, and

cost-effectiveness (on average, one-fifth the cost of installing fiber-optic cable).

Only 5 percent of the buildings in the United States are connected to

fiber-optic infrastructure (backbone), yet 75 percent are within one mile of fiber.

As bandwidth demands increase and businesses turn to high-speed LANs, it

becomes more frustrating to be connected to the outside world through lower-

speed connections such as DSL, cable modems or T1s. Most of the recent

trenching to lay fiber has been to improve the metro core (backbone), while the

metro access and edge have completely been ignored. Studies show that

disconnects occurs in the metro network core, primarily due to cost constraints

and the deployment of such non-scalable, non-optical technologies such as

LMDS. Metro optical networks have not yet delivered on their promise. High

capacity at affordable prices still eludes the ultimate end-user

METHOD OF OPERATION

FSO systems operate very much like a fiber optic connection using a cable. The

main difference being the attenuation in a cable is known and controllable, whereas

in a FSO link that uses the atmosphere as the media, the exact attenuation of the

link can vary by the second and is unknowable. To make this type of system work a

Applications of FSO


device known as a laser diode is used to produce a signal in the first part of the near

infrared range, which is just above visible light at 700 nanometers or nm. The most

common wavelengths used are 780 nm to 900 nm and 1500 to 1600 nm. The laser

diode differs from the traditional laser in that it is simpler, smaller, and lower

powered. The device on the other end that receives the signal is a photodiode. A

transceiver has both devices so that the units can send and receive, either a LED –

Light Emitting Diode or semiconductor laser can be used to generate the signal

LEDs are used for low data rate applications as the beam is not as precise as the

laser beam. These operate in the 700 to 900 nm range whereas, Semiconductor

laser based systems can operate at short or long wavelengths. A laser can produce a

more coherent beam which allows longer distances to be connected. A laser can

also cycle on and off faster than an LED, which produces higher data rates.

Regardless, the beam between the two units is transmitted as a narrow infrared

light beam. A telescope consisting of either lenses or a mirror is used to narrow and

direct the beam produced by the LED or laser. This beam is conical in shape and

diverges from one side of the link to the other. The amount of beam spreading is

determined by the size of the transmitting lens. A typical FSO transmitter will

generate a beam from 5 to 8 cm in diameter. At the other end of the link typical

beam diameters at one kilometer range from 1 meter to 6 meters, depending on the

type of beam alignment system used.

Applications of FSO


BEAM DIVERGENCE

The ability of the unit to receive this beam is determined by the optical receiver

sensitivity and the size of the receiving lens which focuses the beam on a photo

detector. Beam divergence angles or spreading of the signal is typically expressed

as a milliradiant value

The conversion is

a. 1 radiant or rad = 57.3 degrees

b. 1 milliradiant or mrad = 0.0573 degrees

A FSO system operates at layer 1 of the OSI model as a wireless repeater.

Therefore, it can carry any of the higher layer protocols. In these systems, when

they are sending data, the logical one is represented by a narrow pulse of optical

energy and the logical zero by no energy

SYSTEM DESIGN

Let’s switch now to a more detailed discussion of the design of these systems and

the parts used to create a laser beam that will carry information through the open

air. The important design considerations include

a. Transmitter type

b. Transmitting power

c. Beam divergence

d. Receiver diode type and characteristics

TRANSMITTER TYPE

The first consideration in system design is the optical light source, which

modulates the light signal so as to generate a logical one or zero, either a LED or a

semiconductor laser is used for this LEDs are used for the shorter wavelengths,

over shorter distances, and to keep costs down. When a laser is used, it can be of

three types, either a FP - FabryPerot or DFB - Distributed-Feedback laser is used

Applications of FSO


for longer wavelengths. For wavelengths around 850 nm a VCSEL – Vertical

Cavity Surface Emitting Laser is used. VCSELS are lower in cost, but with

excellent performance as compared to FP and DFB lasers.

TRANSMITTER POWER

The power of the transmitting device is the next system design consideration. In

general the higher the power the longer and more stable the link, but at a higher

cost. But, due to safety considerations, the amount of power that can be used is

restricted, as discussed below FSO beams can damage the eye. There are constant

arguments concerning the best wavelength to use as longer wavelengths do not

damage the eye therefore, they can use higher transmitter power. In practice, the

exact wavelength used does not matter

BEAM DIVERGENCE

Beam divergence is a function of the transmitting lens. A more focused beam is

desired, but this means a higher cost for the transmitting unit. The diameter of the

receiver must be matched to the beam being transmitted. The larger the diameter

the better the receiving unit is in maintaining a link as the building moves or the

atmosphere alters the path of the transmitted beam

RECIEVER DIODE TYPE AND CHARACTERISTICS

The type of diode used makes a difference in both performance and cost. A

photodiode is used to convert the incoming light signal into an electrical

signal, two types of detectors are used, Si and InGaAs. Si or sodium based

detectors are used at the shorter wavelengths. For the longer wavelengths

InGaAs – indium/gallium/arsenide detectors are used in all cases. These are

not used below 850 nm. Both of these types of detectors are then one of two

designs, either PIN or APD. A PIN diode is a p-n junction diode with a doping

Applications of FSO


profile tailored so that an intrinsic layer or i region is sandwiched between a p

layer and an n layer. These type of detectors are not as sensitive as the APD

APD - Avalanche Photo Diodes provide 100 times the level of sensitivity and

greater speed than PIN diodes, regardless of the type of detector device used,

the more sensitive the detector is the better. Greater sensitivity will allow a

marginal signal to be read. Higher receiver sensitivity is desirable to read in

the transmitted signal, but always at a higher cost.

FREE SPACE OPTICS (FSO) ISSUES

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. The different elements considered by the

system engineer when designing the product are discussed below.

Applications of FSO


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. However the near

800nm technology is very cheap and hence it is used for small distance

communication.

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. 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

Applications of FSO


more eye-safe power will translate into superior penetration of fog or any

other atmospheric attenuator.

NETWORK PROTOCOL – TRANSPARENT OR

MANAGED?

For carriers today the issue of interoperability of systems within their

multi-faceted networks made up of both legacy and next generation networks

is crucial. Most Free Space Optics (FSO) systems currently available are

physical layer devices that act the same way as fiber optic cables and

receivers and are therefore able to work with all protocols while not being

limited to any of them. There are systems on the market that incorporate

ATM into the device but most designers of Free Space Optics (FSO) systems

have opted for a protocol ‘transparent’ approach for both deployment

flexibility and cost-reduction. Should a carrier wish to add such switching

functionality to networks incorporating physical layer products there are

many switches available on the market, all of which will interoperate with a

physical layer device.

PERFORMANCE - TRANSMIT POWER &

RECEIVER SENSITIVITY

Free Space Optics (FSO) products performance can be characterized

by four main parameters (for a given data rate):

Ä Total transmitted power

Ä Transmitting beamwidth

Ä Receiving optics collecting area

Ä Receiver sensitivity

Applications of FSO


A figure of merit (FOM) can be used to compare competing systems,

based on the basic physics of this equation:

Figure of Merit =

(Power*Diameter2)/ (Divergence2*Sensitivity);

Where

Power = Laser power in milliwatts

Diameter = effective diameter in cm (excluding any obscuration losses)

Divergence = beam divergence in milliard

Sensitivity = receiver sensitivity in nanowatts

High transmitted power may be achieved by using erbium doped fiber

amplifiers, or by non-coherently 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 be achieved with an actively pointed

system, which includes an angle tracker and fast steering mirror (or

gimbals). 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 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 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.

Applications of FSO


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.

Comparison between RF, FSO and optical fiber:

Communication

technology

Free space optics RF wireless Optical fiber

Bandwidth 9.6 Gbps 622 Mbps 40 Gbps

Deployment 5-50K 5-50K Very high

concerns

Fog absorption Rain and fading Very reliable

Distance in miles 1.24 3-12.42 7-12.42

FREE SPACE OPTICS (FSO) ADVANTAGES

ULTRA HIGH BANDWIDTH:

The laser systems operate in the terahertz frequency spectrum and usually

operate in the 194 THz or 375 THz range. Their performance is comparable to

the best fiber optic system available; giving speeds between 622 Mbps and 1.25

Gbps. This technology uses devices and techniques developed for fiber optic

systems.

Applications of FSO


RAPID DEPLOYMENT TIME:

Installing a FSO system can be done in a matter of days even faster if the

gear can be placed in offices behind windows instead of on rooftops. A fibre

based competitor has to seek municipal approval to dig up a street to lay its cable.

Unlike most of the lower frequency portion of the electromagnetic spectrum, the

part above 300 GHz is unlicensed worldwide. So no extra time is needed to

obtain right-of-way permits or trench up the streets or to obtain FCC frequency

licenses.

SECURITY:

FSO is far more secure than RF or other wireless-based transmission

technologies for several reasons:

Ä Laser beams cannot be detected with spectrum analyzers or RF meters

Ä Laser transmissions travel along a line of sight path that cannot be

intercepted easily. It requires a matching transceiver carefully aligned

to complete the transmission. Interception is very difficult and

extremely unlikely

Ä The laser beams are narrow and invisible, making them harder to find

and even harder to intercept and crack

Ä Data can be transmitted over an encrypted connection adding to the

degree of security available in FSO network transmissions

RELIABILITY:

Employing an adaptive laser power (Adaptive Power Control or APC)

scheme to dynamically adjust the laser power in response to weather

conditions will improve the reliability of Free Space Optics (FSO) optical

Applications of FSO


wireless systems. In clear weather the transmit power is greatly reduced,

enhancing the laser lifetime by operating the laser at very low-stress

conditions. In severe weather, the laser power is increased as needed to

maintain the optical link - then decreased again as the weather clears. A TEC

controller that maintains the temperature of the laser transmitter diodes in the

optimum region will maximize reliability and lifetime, consistent with power

output allowing the FSO optical wireless system to operate more efficiently

and reliably at higher power levels.

EQUIPMENT:

Ä Only a few pieces of equipment are required to create a FSO link

Ä All the parts are mounted together within a few feet of each other

Ä The outdoor equipment consists of

o Main FSO unit to generate the beam

o Outdoor box to convert the signal from the LAN to the FSO unit,

control the unit, and manage the connection

o Cables, which can be fiber optic or UTP, used to connect the FSO

equipment to the local area network

o Electrical power to the FSO outdoor unit using an exterior electrical

outlet or power over Ethernet

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 unforeseen

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 complex and not always quantifiable subject - the

atmosphere.

Applications of FSO


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 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.

Applications of FSO


FREE SPACE OPTICS (FSO) POINTING

STABILITY – BUILDING SWAY, TOWER

MOVEMENT

Fixed 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 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 combated in two ways. The first is a long-pass

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

Applications of FSO


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.

FSO APPLICATIONS

Typical applications for FSO include

a. Connecting sites in a campus setting

b. Extending a fiber optic cable network to nearby buildings

c. Local loop bypass

d. Backhaul

e. Disaster recovery

f. Last Mile

CAMPUS APPLICATIONS

The most common use of FSO is in connecting buildings within a campus

area network. These buildings are typically well within the range where laser

based signals are highly reliable.

The reasons for using FSO in this type of application include

a. Create a temporary connection, such as during construction or

for an exhibition

b. Avoid the time, expense, and right-of-way issues of digging in

order to use fiber

c. Avoid interference from nearby radio frequency signals or high

levels of EMI or RFI in a factory

d. Avoid generating radio signals, such as near an airport

Applications of FSO

Sri venkateshwara college of engineering

EXTENDING A FIBER NETWORK APPLICATION

Extending the reach of a fiber network is a new use for FSO

existing metropolitan area fiber optic cable installation is extended to

buildings that are not directly served by the fiber in the ground

more buildings on the loop, connections are made to buildings not on the loop

This is done not by burying more fiber, but through the air


EXTENDING A FIBER NETWORK APPLICATION

Extending the reach of a fiber network is a new use for FSO. In this case an


are not directly served by the fiber in the ground.


This is done not by burying more fiber, but through the air.

22

In this case an


. From one or


Applications of FSO


LOCAL LOOP BYPASS APPLICATION

Bypassing the local loop is also a new use for FSO links

rather than connecting via copper or fiber to the incumbent carrier, any nearby

point of presence for a competing provider can be reached

requirement to rent access from

than one carrier is within range, this allows competition in the last mile


LOCAL LOOP BYPASS APPLICATION

local loop is also a new use for FSO links. In this application,


point of presence for a competing provider can be reached. This is without the

requirement to rent access from the incumbent carrier. In areas where more


23

In this application,


This is without the

In areas where more


Applications of FSO


BACKHAUL APPLICATION

When used as a backhaul connection the FSO link connects a remote antenna

site, such as a cellular

where the connection to the wired telecommunications network is made

is done when a wired data communications circuit either cannot be made to

the remote antenna tower or it is too costly to do so

to avoid radio frequency interference at either site


BACKHAUL APPLICATION


site, such as a cellular telephone network tower, back to a central location

where the connection to the wired telecommunications network is made


the remote antenna tower or it is too costly to do so. Another reason might be

to avoid radio frequency interference at either site.

24


telephone network tower, back to a central location

where the connection to the wired telecommunications network is made. This


Another reason might be

Applications of FSO


DISASTER RECOVERY APPLICATION

Unlike the previous examples, in this application the FSO equipment is not

deployed until needed and hopefully it will never be needed as this use is for

when the main telecommunication system is down, for example, after the 2001

terrorist attack in New York city destroyed the main telecommunications hub

in the area, several business were able to get back online by connecting via

FSO links to areas of the city that were unaffected. When used in this manner

the usual equipment as discussed below can be deployed on a temporary basis

or a kit kept in a case as shown next can be stored. Of course this assumes the

undamaged area is within the distance limits for a FSO link

LAST MILE APPLICATION

A new application that is being promoted by companies such as Omnilux is to

deliver last mile service using a mesh style of deployment. In their model they

abandon the long range, high bandwidth approach to attack the larger market

Applications of FSO


of delivering service to a group of end users. This is in contrast to the more

typical point-to-point single customer links. A lower cost product can then be

produced. This may widen the market for FSO systems. Cost has been the

major factor in holding back deployment of these systems

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

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 customer’s access to high-speed MAN’s. Providers also

can take advantage of the reduced risk of installing a Free Space Optics

(FSO) network which can later be redeployed.

Recent Developments in the field of FSO

1) Short distance communication Using FSO:

The speed and complexity of integrated circuits are increasing rapidly. For instance,

today’s mainstream processors have already surpassed gigahertz global clock

frequencies on-chip. As a consequence, many algorithms proposed for applications

Applications of FSO


in embedded signal-processing (ESP) systems, e.g. radar and sonar systems, can be

implemented with a reasonable number(less than 1000) of processors, at least in

terms of computational power. An extreme inter-processor networks is required,

however, to implement those algorithms. The demands are such that completely new

interconnection architectures must be considered. In the search for new architectures,

developers of parallel computer systems can actually take advantage of optical

interconnects. The main reason for introducing optics from a system point of view is

the strength in using benefits that enable new architecture concepts ,e.g. free-space

propagation and easy fan-out, together with benefits that can actually be exploited by

simply replacing the electrical links with optical ones without changing the

architecture,

Algorithms recently proposed for applications in embedded signal-processing (ESP)

systems, e.g. in radar and sonar systems, demand sustained performance in the range

of 1 GFLOPS to 50TFLOPS [1]. As a consequence, several processors must work

together, thus increasing inter-processor communication. Moreover, the data transfer

time increases quickly if an incorrect network topology is chosen, especially in

systems with frequent use of all-to-all communication structures. The choice of a

scalable high-speed network is therefore essential. Other requirements that must be

fulfilled in ESP systems are real-time processing, low power consumption, small

physical size, and multimode operation. New parallel computer architectures are

necessary to be able to cope with all these constraints at the same time.

In this sort of environment, free space optics can be used to communicate with each

other. The high data rate of free space optics provides a better alternative compared

to wired networks. As the size of the chips is getting smaller and smaller the

interconnection of the chips becomes the main concern. And unlike electrical signals

the optical signals don’t suffer from electrical interferences and signal dependent

attenuations. The free space optics also has the advantage of high fan-in and fan-out

property which is very useful in transmission between thousands of channels

between chips of a microprocessor.

Applications of FSO


By placing electronic chips (including optoelectronic devices) and optical elements

on a substrate where light beams can travel, we get a planar free space system

Electronic chips are placed in a two-dimensional plane, while light beams travel in a

three-dimensional space. In this way, optical systems can be integrated

monolithically, which brings compact, stable and potentially inexpensive systems.

The interconnection pattern in a planar free space system can, for example, be chosen

with respect to a pipelined dataflow between chips. Another possibility is to have a

more general topology, such as the two-dimensional mesh, or to have special optical

or optoelectronic devices dedicated to switch functions. Current optical interconnect

research efforts focus on using planar optical waveguides, which will be integrated

onto the same chip as the electronics. This in-plane waveguide approach, however,

presents some significant challenges. Pure optical switching and storage devices in

silicon technologies remain far from practical. Without these capabilities, routing and

control in a packet-switched network, as typically envisioned for an on-chip optical

interconnect system, require repeated optoelectronic (O/E) and electro optic (E/O)

conversion, which can significantly increase signal delay, circuit complexity, and

energy consumption. Simultaneously, efficient silicon electro-optic modulators

Applications of FSO


remain challenging due to the inherently poor nonlinear optical properties of silicon,

and silicon electro-optic modulators have to rely on other weaker physical

mechanisms such as the plasma dispersion effect (refractive index change induced by

free carriers) [7]. Hence the modulator design requires a long optical length, which

results in large device size

2) FSO in space applications:

This involves the application of free space optics technology for space applications.

The optical communication has various advantages over the RF in terms of wider

bandwidth, a larger capacity, lower power consumption, more compact equipment,

greater security against eavesdropping, and immunity from interference. The main

criterion for deep space communication is signal to noise ratio. Another important

aspect is that the use of optical radiation as a carrier between the satellites permits

very narrow beam divergence. Due to the narrow divergence and the large distance

between the satellites, pointing from one satellite to another is difficult. The pointing

task is further complicated by vibration of the pointing system caused by tracking

noise and mechanical impacts.

The use of optical intersatellite links has some advantages over microwave

intersatellite links: Smaller size and weight of the terminal, Smaller transmitter

power, Greater immunity to interference, Larger data rate and Smaller transmitter

beam divergence. The main disadvantage of optical intersatellite links is the complex

pointing system required. The complexity of the pointing system derives from the

necessity to point from one satellite to another satellite over a distance of tens of

thousands of kilometers with a beam divergence angle of micro radians as the

satellites move and vibrate. The pointing system compensates the movement of the

satellites using the known ephemerides data. Coupling of satellite mechanical

vibration and tracking noise to the pointing system causes vibration of the satellite

transmitted beam in the receiver plane. Such vibrations decrease the received signal.

The decrease of the signal increases the bit error probability (BEP) in optical satellite

Applications of FSO


networks the problem is more complicated because all the satellites continually

vibrate randomly. Possible solutions to satellite vibrations include: Increasing the

transmitter power, use of a more complicated pointing system, and an adaptive

model that adapts the system parameters such as power, bandwidth, and telescope

gain to the vibration amplitude. These solutions have some or all of the

disadvantages of Larger size, More weight, Higher energy consumption, heat-

transfer problems, Greater expense and More complexity. All the above

disadvantages raise the price of the mission and decrease the reliability of the system.

Network of free space optical channels

Even though the free space technology has many advantages, if we consider long

distance communication such as 107 km the RF fares better when compared to the

free space optical communication because of lack of technology to support it. For

near-Earth communication links such as the Moon-to-Earth communication link at

around 4 × 105 km, an optical system with optical preamplification is best suited. It is

Applications of FSO


also best suited for high-data-rate, short-link communications. The choice of a

suitable space communication system must also take into account the terminals'

mass, power, volume, regulatory restrictions, etc. RF communication systems will be

preferable in deep space communication links for the time being, maybe until

quantum communications technology makes a big leap forward.

3) Deep sea communication using FSO:

The growing need for underwater observation and subsea monitoring systems has

stimulated considerable interest in advancing the enabling technologies of

underwater wireless communication and underwater sensor networks. This

communication technology is expected to play an important role in investigating

climate change, in monitoring biological, biogeochemical, evolutionary, and

ecological changes in the sea, ocean, and lake environments, and in helping to

control and maintain oil production facilities and harbors using unmanned

underwater vehicles , submarines, ships, buoys, and divers. However, the present

technology of underwater acoustic communication cannot provide the high data rate

required to investigate and monitor these environments and facilities. Optical

wireless communication has been proposed as the best alternative to meet this

challenge.

The present technology of acoustic underwater communication is a legacy

technology that provides low-data-rate transmissions for medium-range

communication. Data rates of acoustic communication are restricted to around tens

of thousands of kilobits per second for ranges of a kilometer, and less than a

thousand kilobits per second for ranges up to 100 km, due to severe, frequency-

dependent attenuation and surface-induced pulse spread. In addition, the speed of

acoustic waves in the ocean is approximately 1500 m/ s, so that long-range

communication involves high latency, which poses a problem for real-time response,

synchronization, and multiple-access protocols. As a result, the network topology is

simple and throughput is low. In addition, acoustic waves could distress marine

Applications of FSO


mammals such as dolphins and whales. As a result, acoustic technology cannot

satisfy emerging applications that require around the clock, high-data-rate

communication networks in real time. Examples of such applications are networks of

sensors for the investigation of climate change; monitoring biological,

biogeochemical, evolutionary, and ecological processes in sea, ocean, and lake

environments; and unmanned underwater vehicles used to control and maintain oil

production facilities and harbors. An alternative means of underwater

communication is based on optics, wherein high data rates are possible. However, the

distance between the transmitter and the receiver must be short, due to the extremely

challenging underwater environment, which is characterized by high multiscattering

and absorption. Multiscattering causes the optical pulse to widen in the spatial,

temporal, angular, and polarization domains. Although high data rates are threatened

by extremely high absorption and scattering, there is evidence that broadband Links

can be achieved over moderate ranges. It has been shown that at close distances a

data rate of 1Gb/s can be achieved using the wireless optical communication.

However, subsea FSO is challenged by high extinction and the immense variability

of background illumination in shallow waters. This has stimulated us to investigate

the potential of underwater FSO in the UV solar-blind spectral range, where

background illumination is nearly nonexistent and considerable scattering occurs.

The achievable performance is compared to transmission at 520 nm, where, in Clear

Ocean, data rates of 100 Mbps can be transmitted over distances of 170 m, falling to

under 15 m in harbor waters. It is anticipated that ranges of 12 m can also be

obtained with UV solar-blind wavelengths,

The solar blind UV rays are rays which are completely reflected by the earth’s

atmosphere (240–285 nm). These have special properties such as low interference

when used for communication compared to other regions. This makes it possible to

operate very sensitive wide field-of-view quantum noise limited photon counting

receivers, and provide communication systems that perform very differently than free

space optical systems that operate in other spectral regions. Because of these unique

Applications of FSO


properties this is used in the deep sea communication where interference and losses

are more.

Although the use of free space optics seems to give many advantages, it has to be

proved practically. However getting accurate data on the transmission and reception

in under water is very difficult. Even though the FSO provides high data rate in

normal conditions its data rate comes down drastically in the case of harbor water.

CONCLUSION

Free space optics (FSO) provides a low cost, rapidly deployable method

of gaining access to the fiber optic backbone. FSO technology not only delivers

fiber-quality connections, it provides the lowest cost transmission capacity in

the broadband industry.

As a truly protocol-independent broadband conduit, FSO systems

complement legacy network investments and work in harmony with any

protocol, saving substantial up-front capital investments.

A FSO link can be procured and installed for as little as one-tenth of the

cost of laying fiber cable, and about half as much as comparable microwave/RF

wireless systems. By transmitting data through the atmosphere, FSO systems

dispense with the substantial costs of digging up sidewalks to install a fiber

link. Unlike RF wireless technologies, FSO eliminates the need to obtain costly

spectrum licenses or meet further regulatory requirements.

Applications of FSO


REFERENCES

1. International Journal on Science and Technology (IJSAT) Volume 1, Issue I, (Oct.-

Nov.) 2010

2. JOURNAL OF COMMUNICATIONS, VOL. 4, NO. 8, SEPTEMBER 2009

3. An Intra-Chip Free-Space Optical Interconnect

Jing Xue, Alok Garg, Berkehan Ciftcioglu, ShangWang, Jianyun Hu, Ioannis Savidis, yManish Jain,

Michael Huang, Hui Wu, Eby G. Friedman, yGary W. Wicks, yDuncan Moore

4. Subsea ultraviolet solar-blind broadband free-space optics communication

Optical Engineering 48_4_, 046001 _April 2009

5. Performance limitations of free-space optical communication satellite networks

due to vibrations: direct detection digital mode 3148 Opt. Eng. 36(11) 3148–3157

(November 1997)

6. Wavelength Diversity in Free-Space Optics to Alleviate Fog Effects.

John Heidemann, Wei Ye, Jack Wills, Affan Syed, Yuan Li Information Sciences Institute,

University of Southern California

Applications of FSO


Free space optics- A technical seminar report

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