Free Space Optics

ABSTRACT

This paper deals with communication through optics using one of the latest technologies

called the free space optics (FSO).

FSO may sound new and experimental but in fact it predates optical fiber and has its roots in

wartime efforts to develop secure communication systems that did not require cable and

could withstand radio jamming.

As a commercial communications technology, FSO has been around for more than a decade,

but it is only recently that interest in this technology has started to grow. It was only recently

developed for use in metropolitan area networks.

The technology has its roots in military applications that reach back as far as the 1940s.

It was not until the 1960s, however, that the first significant FSO technology advancements

began to occur in the United States, Europe and Middle East, where military researchers,

engineers and technicians applied the use of infrared lasers in communications devices with

the aim of providing secure data and voice transmission that would not be susceptible to

“jamming” of radio frequency-based communications systems.

These early FSO systems were capable of transmitting merely a handful of kilobits over the

air, but the advent of the Internet and its impact on telecommunications was decades away.

In fact, European researchers of FSO systems in the 1960s experimented with ways to send

FSO signals through both underground and underwater pipes, seeking to bend the invisible

light beams with mirrors where a straight line-of-site could not be established.

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Free Space Optics

1.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 fiberoptic

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.

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 obataining

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Free Space Opticsgovernment licenses for new fiber installation etc. are further problems that has turned FSO

into the option of choice for short reach application.

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

2. FUNDAMENTALS OF FREE SPACE OPTICS

FSO is an optical wireless, point-to-point, line-of-sight broadband solution.

a) Lasers Through Free Space

FSO is an optical technology and simple concept involving the transmission of voice, video

and data through the air using lasers. It is not a disruptive technology; it is more of an

enabling technology that promises to deliver that ever-eluding high-speed optical bandwidth

to the ultimate end users. FSO offers many advantages when compared to fiber. It is a zero

sunk-costs solution. The principle advantages of free space optics (FSO) are:

1. Significantly lower cost on average than the build out of a new fiber optical solution, or

leasedlines.

2. FSO can be deployed in days to weeks vs. months to years

3. Bandwidth can easily be scaled (10 Mbs to 1.25 Gbps) per link

As opposed to fiber, FSO can be redeployed if the customer moves or cancels service. It is

also a fraction of the cost and time, allowing carriers to generate revenue, while also taking

advantage of the high capacity of optical transmissions. FSO allows service providers to

accelerate their deployment of metro optical networks as well as extend the reach of such

optical capacity to anyone who needs it.

b) FSO: Optical or Wireless?

FSO systems share several characteristics with fiber optics. FSO can use the same optical

transmission wavelengths as fiber optics, namely 850nm and 1550nm and they use the same

components such as lasers, receivers and amplifiers. Some systems already include fiber

3

Free Space Opticsconnections inside the transmission link heads, to separate electronics and optics. Similar to

fiber optics, FSO systems also target the high-bandwidth market. However, while fiber optics

can be used over longer distances, FSO targets shorter distances due to the variability of the

terrestrial atmosphere as a transmission medium.

One common feature of FSO equipment commercially available today is that most of these

systems perform optical to electrical back to optical (O-E-O) conversion steps in the process

of sending and receiving information through the air and connecting back to the attached

networking interface fiber. This feature does not automatically constitute a performance

limitation, but O-E-O conversion can impact the ability to scale an FSO system easily to

ultra-high bandwidth capabilities. The fiber optic communications industry realized from the

start the importance of an all-optical system approach, as higher backbone capacity — along

with wavelength division multiplex technology. An important breakthrough to reach this goal

occurred when fiber systems with erbium doped fiber amplifier (EDFA) became

commercially available. It was then, that the concept carrying multiple wavelengths over a

single piece of optical fiber achieved commercial attention. The invention of EDFA amplifier

technology paved the way for optical transmission at multiple wavelengths over longer

distances without the need to perform expensive O-E-O conversion and separate electrical

amplification of each specific wavelength at every repeater station.

c) Bandwidth Drivers/Trends

The push to build more high-speed networks was spurred by unprecedented growth in

bandwidth usage. Telecommunications carriers will implement multiple technologies in their

networks and will use the best access technology for the particular situation. The chart below

shows how these technologies address different market segments based on technology,

technical capabilities (reach, bandwidth), and economic realities.

d) Bit Error rates, Data Rates, and Range

4

Free Space OpticsAs shown in the building below(Fig.b), where fog is effecting the visibility range are

illustrated. A building is about 300m from the viwer.first pic shows clear air ,a t

6.5dB/km(2000m visibility range),similarly at 150dB/km and 225 dB/km is shown having

visibility range of 113m and 75m respectively, we can see the difference in the visibility

range, same reason effect the Transmitter and Receiver in such conditions, Hence

Bit error , or the Data rate will be different for these types of the condition

The data rate will be slower in this situation, where as it will fast for clear sky.

Fig.a

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Free Space Optics

Fig.b

3. 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

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Free Space Opticscompeting 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.

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

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.

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Free Space Optics

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

affordable prices still eludes the ultimate end-user.

Optical communications are in the process of evolving Giga bits/sec to terabits/sec and

eventually to pentabits/sec. The explosion of internet and internet based applications has

fuelled thenbadwidth requirements. Business applications have grown out of the physical

boundaries of the enterprise and gone wide area linking remote vendors, suppliers, and

customersin a new web opf business applications.

4.THE TECHNOLOGY OF FSO

The concept behind FSO is simple. FSO uses a directed beam of

light radiation between two end points to transfer information (data, voice

or even video). This is similar to OFC (optical fiber cable) networks, except

that light pulses are sent through free air instead of OFC cores.

An FSO unit consists of an optical transceiver with a laser

transmitter and a receiver to provide full duplex (bi-directional) capability.

Each FSO unit uses a high power optical source ( laser ) plus a lens that

transmits light through the atmosphere to another lens receiving

information. The receiving lens connects to a high sensitivity receiver via

optical fiber. Two FSO units can take the optical connectivity to a

maximum of 4kms.

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Free Space Optics

SUBSYSTEMS

In the transmitting sectin, the data is givento themodulator for modulating signal and the

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

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

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

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

preamplifier. The servo sustem is used for controlling system,the signal coming from the

path to theprocessor and compares with the environmental condition, if there is an change in

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

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Free Space Optics

5. FSO ARCHITECTURES

POINT-TO-POINT ARCHITECTURE:

Point-to-point architecture is a dedicated connection that offers higher bandwidth but is less

scalable In a point-to-point configuration, FSO can support speeds between 155Mbits/sec and

I 0Gbits/sec at a distance of 2 kilometers (km) to 4km. "Access" claims it can deliver

10Gbits/ sec. "Terabeam" can provide up to 2Gbits/sec now, while "AirFiber" and

"Lightpointe" have promised Gigabit Ethernet capabilities sometime in

2001..

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Free Space Optics

MESH ARCHITECTURE:

Mesh architectures may offer redundancy and higher reliability with easy node

addition but restrict distances more than the other options.

A meshed configured can support 622Mbits/sec at a distance of 200 meters(m)To 450m.

TerBeam claims to have successfully tested 160Gbits/sec speeds in its lab,

But such speeds in the real world are surely a year or two off

POINT-TO-MULTIPOINT ARCHITECTURE:

Point-to-Multipoint architecture offers cheaper connectins and facilitates node addition

but at the expense of lower bandwidth than thepoint-to-point option.

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Free Space Optics

In a point-to-multipoint arrangement, FSO can support the same speeds as the point-

to-point arrangement-155Mbits/sec to 10Gbits/sec-at 1km to 2km.

A single node serves as an originator and multiple links emanate from.it. The most

effective method is to connect each FSO link into a layer 2 or 3 device located in a building

closet. Then thelinks are fiber coupled to the switch or router and placed at arbitrary

locations either on the building rooftop or in an interior room or office there in.

Attemots have been made to sectorize theoptical beam to serve more than one customer

at atime from asongle node, as done in LMDS systems, but this rchitectureis restricted by

power limitations imposed by regulatory authorities like the CDRH( Center for Devices and

Radiological Health) and International Electrotechnical Commission .Point-to-Multipoint

Architecture offers cheaper connections and facilitates node addition but at the expense of

lower bandwidth than the point to point option.

MULTIPLE PTP ARCHITECTURE:

Multiple PTP architecture is suitable in cases where its desirable to create an extensive link

path that exceeds he product range limit or the recommended weather constrained distance

for an optical link.

It is a dedicated connection that offershigher bandwith.

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Free Space Optics

PETERNET ARCHITECTURE:

As mentioned, bandwidth and end-user servicesare two important features of NexGen,

packet-centric

cellular networks, which call for a major paradigmshift in network design and deployment.

While services are more related to the business perspectives,bandwidth involves core

technological issues. Under this changing scenario, the fundamental requirementsof any

NexGen backhaul topology can be

identified as:

_ It must provide very high bandwidth (155Mbps and above) to support new services.

_ It should be able to provide carrier class reliabilityto ensure the stringent quality of

service(QoS) requirements of future technologies.

_ It should render itself scalable and reconfigurable under dynamically changing

requirementswithout compromising on performance issues.

. _It must not inject unacceptable communication latency between the different units

_ It should incur low capital expenditures (CAPEX) and operational

expenditures

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Free Space Optics

(OPEX) i.e low cost of building construction,

tower installation and maintainence.

Keeping these key issues as the driving force, we propose a novel backhaul architecture,

called the PeterNet, based on the well known Petersen graph that has been extensively

studied . This graph with 10 vertices and 15 edges, has many distinctive properties: all

vertices have degree 3; every vertex can be reached from every other vertex by at most 2

hops (i.e, diameter 2) and the length of any cycle is 5; and node connectivity is 3. In order to

deploy the Petersen graph as a backhaul

network, we consider a typical hexagonal cell framework where each cell site has omni-

directional

6.WORKING OF FSO

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

14

Free Space Optics"telescope" to other 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 datarates 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 enoughtransmitter power, Free Space Optics (FSO) communication is possible.

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Free Space Optics

Free space optics 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 communication is possible,

The FSO remains simple: a narrow beam of light is launched at a transmission

station, transmitted through the atmosphere, and subsequently received at the receive

station. In wireless optical system it uses the infrared of visual range frequencies to

transmit data

Free Space Optics (FSO) is a telecommunication technology that uses light

propagating in free space to transmit data between two points. The

technology is useful where the physical connection of the transmit and

receive locations is difficult, for example in cities where the laying of fibre

optic cables is expensive. Free Space Optics is also used to communicate

between space-craft, since outside of the atmosphere there is little to distort

the signal.

Fso , have come about in response to a need fo greater and bandwidth and improved

communicatins systems. In as much as FSO and fiber-optic transmission systems use similar

infrared wavelengths of light and hae similar transmission bandwidth capabilities, FSO is

often referred to as “fiber optics” or “optical wireless” transmission. Furthermore , given the

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Free Space Opticsfact that theoptical spectrum is unlicensed with frequencies of the order of hundreds of

terahertz, most Fso system use simple ON-OFF keying (OOK) as a modulation format

The same standard modulation technique that is used in digital fiber optics systems, where in

data are typically transmitted in a digital format with light “ON” representing a “1” and light

“OFF” representing a “0” . This simple modulatin scheme allows FSO systems tobe designed

as bandwidth-and protocol-transparent physical layer connection.

Some of the Factors that has to go through with this Technlogy is:

2. Environmental Factors

The performance of a FSO link is primarily dependent upon the climatology and the physical

characteristics of its installation location. In general, weather and installation characteristics

that impair or reduce visibility also effect FSO link performance. A typical FSO

system is capable of operating at a range of two to three times that of the naked eye in

any particular environmental condition. The primary factors affecting performance include

atmospheric attenuation, scintillation, window attenuation, alignment or building motion,

solar interference, and line-of-sight obstructions.

3. Atmospheric Attenuation

Atmospheric attenuation of FSO systems is typically dominated by fog but can also be

dependent

upon low clouds, rain, snow, dust, and various combinations of each. The effects

of fog on visibility and range can be seen in Fig. 1, which presents a series of photographs

taken during a fog event in Denver, Colorado. The tall building in the foreground (on the

right-hand side) is located approximately 300 m from the camera. The first panel shows

clear atmospheric conditions with a visibility range of >2000 m as measured with a

nephelometer

mounted at the camera site. This corresponds to an attenuation of approximately

6.5 dB/km at near-IR wavelengths and according to the 5% contrast standard for visibility

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Free Space Opticsand as defined by the World Meteorological Organization (WMO). The distant mountain

range is clearly visible, even though it is many kilometers away. The second panel depicts

the onset of a fog event, at which time visibility is measured at approximately 113 m (115

dB/km). The near building is still visible at 300 m; all buildings and landmarks beyond this

range are obscured. In the third panel, with a visibility range of approximately 75 m (173

dB/km), the building in the foreground is completely obscured.

4. Scintillation

Atmospheric scintillation can be defined as the changing of light intensities in time and

space at the plane of a receiver that is detecting a signal from a transmitter located at a

distance. The received signal at the detector fluctuates as a result of the thermally induced

changes in the index of refraction of the air along the transmit path. These index changes

cause the atmosphere to act like a series of small lenses that deflect portions of the light

beam into and out of the transmit path. The time scale of these fluctuations is of the order

of milliseconds, approximately equal to the time that it takes a volume of air the size of the

beam to move across the path, and therefore is related to the wind speed.

Scintillation can change by more than an order of magnitude during the course of a day,

being the worst, or most scintillated, during midday when the temperature is the highest.

Some experiments have shown that, depending upon the atmospheric conditions along the

beam path, the magnitude of scintillation-induced fades reaches a maximum that does not

continue to increase with distance.

Overall, scintillation causes rapid fluctuations of received power and, in a worst case,

results in high-error-rate FSO performance. However, at ranges less than 1 km, most FSO

systems have enough dynamic range or margin to compensate for scintillation effects. In

addition, FSO installations capable of 99.9% or better availability typically have enough

margin to compensate for large amounts of atmospheric attenuation and thus have more

than enough margin to compensate for scintillation. For longer, lower-availability links,

transceiver design features such as the use of multiple laser transmitters can substantially

reduce the effects of scintillation.

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Free Space Optics5. Window Attenuation

One of the advantages of FSO systems is that they allow communication through windows

without the need for rooftop-mounted antennas. This is especially advantageous for

connecting

individual customers who may or may not have access to a building’s roof and also

may have to pay for access to the riser wiring of a building.

Even though windows allow optical signals to pass through them, they all add some

amount of attenuation to the signal. Uncoated glass windows usually attenuate 4% per

surface, because of reflection. This means that a perfectly clear double-pane window

attenuates

all optical signals at least 15% (four surfaces, each with 4% reflection). Windows

that are tinted or coated can have much greater attenuation, and the actual magnitude is

typically quite wavelength dependent.

For a high-availability FSO deployment behind windows, it is recommended that installers

measure the actual attenuation of the window so that the expected link performance

can be accurately calculated. In addition, when planning an installation on tall buildings,

an installer may want to weigh the possibility of low clouds interrupting a rooftop-mounted

system against the reduction in link performance that results from the lower-altitude window

attenuation. In many cases, the window attenuation may have a lesser effect on overall

link availability.

6. Alignment

One of the key challenges with FSO systems is maintaining transceiver alignment. FSO

transceivers transmit highly directional and narrow beams of light that must impinge upon the

receive aperture of the transceiver at the opposite end of the link. A typical FSO

transceiver transmits one or more beams of light, each of which is 5–8 cm in diameter

at the transmitter and typically spreads to roughly 1–5 m in diameter at a range of 1 km.

Adding to the challenge is the fact that FSO receivers have a limited FOV, which can be

thought of as the receiver’s “cone of acceptance” and is similar to the cone of light projected

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Free Space Opticsby the transmitter. For a FSO link to function, it is very important that both the transmitted

beam of light and the receive FOV cone encompass the transceiver at the opposite end of

the link.

7. Low-Frequency Base Motion

Thermal gradients induce bending and twisting in buildings, the magnitude of which varies

greatly with the building size, shape, and structural type. Generally, this motion is so

insignificant and slow that it goes unnoticed by building occupants. A correlation has been

shown to exist between low-frequency base motion and daily temperature changes. As

would be expected, the motion tends to increase with height in a building and can be

significant for rooftop installations—even for installations on shorter buildings. Also, it is

more pronounced in elevation angles than in azimuth angles.

8. Moderate-Frequency Base Motion

Moderate-frequency base motion is caused by wind and can be quite significant in tall

buildings. Fortunately, minimizing building motion in strong winds is usually a key goal in

the structural design of skyscrapers. Thus, only the most severe winds are likely to result

in large building motions. FSO outages that result from building motion will be short in

duration inasmuch as once the wind gust tapers off, the building will return to its original

position and alignment. Wider-beam transceivers and transceivers with sufficiently capable

automatic pointing and tracking systems will be able to “reject” even these rare large

motions without outage.

9. High-Frequency Base Motion

High-frequency base motion is caused by vibration. Base motion faster than a few hertz is

highly dependent on how and where a FSO terminal is mounted. Floor, wall, and rooftop

(i.e., surface of roof or parapet wall) can all yield quite different levels of base motion.

Figure 2 presents power spectral density plots of vibration for several buildings, including

two rooftop mounts (surface of roof), two tall office buildings (floor mount), and a small

wood-frame building (floor mount). The curves show the large variability in vibration from

building to building. In addition, the magnitude of vibration due to occupant activity (e.g.,

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Free Space Opticswalking, shutting doors) will vary greatly over time within the same building. It is interesting

to note that almost all the integrated motion is due to frequency content below 10 Hz.

Measurements show that peak angular base motion due to vibration above 1 Hz should

rarely exceed 1 mrad and in many environments will rarely approach half this value.

However, mounting hardware must be carefully designed (and installed) so that the mount

does not amplify the base motion that the FSO terminal experiences.

10. Sample Pointing and Tracking

Total tracking and pointing errors must be determined by means of combining base

motion (as described above) with other criteria, such as initial field alignment errors (for

nontracking systems), coalignment errors, and terminal thermal drift. Table 1 presents

examples of pointing and tracking error budgets for nontracking and automatic pointing and

tracking FSO terminals. The tracking terminal uses residual base motion, which accounts

for the expected compensation of the tracking system as a function of base motion frequency.

The low-, moderate-, and high-frequency base motions are not independent. Therefore

this budget, which directly adds the base motion components, has been tailored for

conditions that emphasize low-frequency building motion with non-extreme moderate- and

high-frequency motion.

The above types of base motion can be summarized in a few rules of thumb for evaluating

whether a particular FSO system will experience motion-induced outages.

• Short concrete structures (fewer than three stories) typically move less than taller

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Free Space Opticsbuildings or wooden structures.

• Motion of the transceiver mount may dominate motion of the building.

• Less than 15% of buildings move more than 4-mrad full side to side over a 1-yr

period.

• Less than 5% of buildings move more than 6-mrad full side to side over a 1-yr period.

• Less than 1% of buildings move more than 10-mrad full side to side over a 1-yr

period.

14. Transmission

The modulated light source, which is typically a laser or light-emitting diode (LED), provides

the transmitted optical signal and determines all the transmitter capabilities of the

system. Only the detector sensitivity plays an equally important role in total system

performance.

For telecommunication purposes, only lasers that are capable of being modulated at

20 Mbit/s to 2.5 Gbit/s can meet current marketplace demands. In addition, how the device

is modulated and how much modulated power is produced are both important to the selection

of a device. Lasers in the 780–925-nm and 1525–1580-nm spectral bands meet frequency

requirements and are available as off-the-shelf products. Although other operating

wavelengths are used in commercial FSO systems, this discussion will focus on lasers that

operate in the 850- and 1550-nm wavelength bands.Within these two wavelength windows,

FSO systems should have the following characteristics:

• Ability to operate at higher power levels (important for longer-distance FSO systems).

• High-speed modulation (important for high-speed FSO systems).

• Small footprint and low power consumption (important for overall system design and

maintenance).

• Ability to operate over a wide temperature range without major performance degradation

(important for outdoor systems).

• Mean time between failure (MTBF) that exceeds 10 yr.

To meet the above requirements, FSO manufacturers generally use VCSELs for operation

in the shorter-IR wavelength range and Fabry–Perot (FP) or distributed-feedback (DFB)

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Free Space Opticslasers for operation in the longer-IR wavelength range. Several other laser types are not

suitable for high-performance FSO systems.

15. VCSEL Lasers

The VCSEL (850-nm wavelength) is an outgrowth of fiber communications development

and has many attractive features. VCSELs revolutionized the transmission component market

because of their exceptional cost and performance advantages over previously available

technology. Most notably, VCSELs have a reasonable, nominal average power level of

several milliwatts of output at high-speed operation and high reliability numbers for MTBF.

The average power, not the peak power, determines the link margin. Inasmuch as the 850-

nm VCSEL is cheaper than many of its alternatives, the 850-nm products dominate the

low-price FSO systems because operation speeds are generally below 1 Gbit/s for off-theshelf

systems. Because of their high efficiency, power dissipation is typically not an issue

for VCSELs, and active cooling is not required. In addition, VCSELs emit light in the

form of a circular beam instead of an elliptical beam. The round shape of the beam pattern

perfectly matches the round core of an optical fiber, facilitating the coupling process and

improving coupling efficiency. The success of VCSEL technology has been so tremendous

that many VCSEL manufacturers can produce shorter-wavelength 850-nm laser structures

with direct modulation speeds beyond 3 Gbit/s. The direct electrical modulation of VCSELs

beyond 10 Gbit/s has been demonstrated and commercialized for OC-48 (STM-16)

and 10-GigE operations.

17. Amplification Sources

Amplification sources, such as EDFAs and semiconductor optical amplifiers (SOAs), are

used to boost the power of lower-power laser sources. EDFA and SOA technologies also

can amplify both single and multiple closely spaced wavelengths simultaneously, which is

known as WDM.With high optical gains that exceed 30 dB, EDFAs can drive the 1550-nm

optical output power of a FSO system up to between 1 and 2 W. At this time, EDFAs are

quite expensive, and their use tends to be limited to very high-end performance systems

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Free Space Opticsthat operate at or above 1 Gbit/s. As 1550-nm technology becomes more widely used in

telecommunications, it is anticipated that the costs will come down.

18. Peak Output Power

The peak output power refers to the maximum allowable output power of a transmission

source. The peak output power value is often important in pulsed laser operations, in which

a high-power laser pulse is required for a short period of time. In general, the peak power

and pulse repetition frequency are closely tied, and high peak power is usually coupled

with a low pulse repetition frequency to prevent damage to the device. However, for most

communication systems, the peak power capability of a laser source is irrelevant because

most applications do not use high-power pulse/low duty cycle modulation schemes but

instead typically rely on 50% duty cycle modulation schemes. For most FSO systems, the

peak power rating would refer to the transmit power of a “1” and would be approximately

twice the average output power of the signal.

19. Average Output Power

The average output power of a transmission system is a key factor in determining the system

link margin because, as with the majority of digital fiber communication systems, most

FSO systems are digital in terms of their transmission of bits across the link. These

communication

systems typically use a coding scheme (e.g., 8B/10B coding) to ensure that an

approximately equal number of digital “1s” and “0s” are transmitted, thus maintaining a

50% duty cycle. In this case, where peak power is transmitted for a “1” and zero power is

transmitted for a “0,” the average power is approximately half the peak power. This average

power is what is used for eye-safety classification and is typically used to define the

transmit power of a FSO transceiver.

20. Beam Divergence

One of the primary advantages of FSO transmission is the narrowness of the transmitted

laser beam that can be achieved with well-designed optics. This narrow beam allows for

24

Free Space Opticssecure and efficient transmission with a major fraction of the transmitted power being

collected

by the receiver. Typically, the optical beam width from a FSO transceiver will be relatively

wide (2–10-

mrad divergence, which is equivalent to a beam spread of 2–10 m at 1 km), as is generally

the case in nontracking applications. For such applications, the system must compensate

for any platform motion by having a beam width and total FOV (TFOV) that is larger

than either transceiver’s anticipated platform motion. If the system provides for automatic

pointing and tracking, then the beam width can be narrowed significantly (typically, 0.05–

1.0 mrad of divergence, which is equivalent to a beam spread of 5 cm to 1 m at 1 km),

further improving link margin and providing the system with greater link margin to combat

adverse weather conditions. However, the cost for the additional tracking feature can be

significant.

28. Atmospheric Effects and Availability

The link equation for a FSO system is actually quite simple at a high level (if we exclude

optical efficiencies, detector noises, and so on). The equation is

where

P = power,

d1 = transmit aperture diameter (m),

d2 = receive aperture diameter (m),

D = beam divergence (mrad)(1/e for Gaussian beams; FWHA for flat top beams),

R = range (km),

a = atmospheric attenuation factor (dB/km).

In Eq. (1), the amount of received power is proportional to the amount of power transmitted

and the area of the collection aperture but inversely proportional to the square of the beam

divergence and the square of the link range. It is also inversely proportional to the

exponential

25

Free Space Opticsof the product of the atmospheric attenuation coefficient (in units of 1/distance) times

the link range.

30. Metrics

One of the biggest challenges faced by potential users of FSO systems is accurately

comparing

the relative performance of products from different manufacturers. The reason for

this is that each manufacturer tends to quote slightly different versions of similar performance

specifications. For example, transmit power can be defined as either peak or average,

and transmit divergence can be defined to either the 1/e, 1/e2, or FWHA points.

As discussed in the abstract, one of our goals in this paper is to suggest a common set

of metrics for FSO systems that will allow for the accurate comparison of performance. To

that end, we would like to suggest that the following six metrics be used by manufacturers

of FSO equipment when specifying performance.

• Transmit power

• Transmit beam divergence

• Receive sensitivity

• Receive field of view

• Pointing and tracking field of regard and closed-loop bandwidth

• Link margin and attenuation versus range curves

By using these six metrics, a potential end user should be able to fully evaluate the expected

performance of a given FSO transceiver in a particular installation and be able to make a

quantitative comparison of different systems.

Transmit Power

We would like to suggest that the industry adopt the standard metric “Total Maximum

Average Power at the Output Aperture.” Using this guideline, each FSO transceiver

specification

sheet would list the integrated sum of the emitted power over the entire output

aperture measured, with an averaging time of at least 25 times the operational bit period.

Alternatively, a FSO manufacturer could list the “Maximum Average Output Power of the

26

Free Space OpticsTransmit Laser” in conjunction with “Transmit System Optical Losses.”

Transmit Beam Divergence

We would like to suggest that the industry adopt the standard metric “Transmit Beam

Divergence

to the 1/e, 1/e2, and FWHA points.” This combined information will allow a

potential user to make an approximate determination of the transmit beam profile and be

able to predict power distribution at various ranges.

Receive Sensitivity

We would like to suggest that the industry adopt the standard metric “Average Required

Power at the Receive Aperture for a 10-9 Bit Error Rate.” Extrapolating to other error rates

is pretty straightforward, with 10-12 approximately corresponding to 1 dB less sensitivity

and 10-6 approximately corresponding to 1 dB more sensitivity. Alternatively, a FSO

manufacturer could list the “Average Required Power at the Detector” in conjunction with

“Receive System Optical Losses.”

Receive Field of View

We would like to suggest that the industry adopt the standard metric “Receive Field of View

to the FWHA Points.”

Pointing and Tracking

For pointing and tracking systems, we would like to suggest that the industry adopt the

standard metrics “Tracking System Field of Regard” and “Tracking System Closed-Loop

Bandwidth.” We fully acknowledge that these two metrics would not provide sufficient

information to fully quantify the capabilities of a pointing and tracking system, but they

are typically fairly representative of overall performance and should allow for the accurate

comparison of different transceivers.

27

Free Space Optics

7.APPLICATIONS OF FSO

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

businesses with high-speed connections.

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

FIBER BACKUP:

FSO may also be deployed in redundant links to back up fiber in place of a second

fiber link.

BACKHAUL:

FSO can be used to carry cellular telephone traffic from antenna towers back to

facilities wired into the public switched telephone network.

28

Free Space OpticsSERVICE ACCELERATION:

FSO can be also used to provide instant service to fiber-optic customers while

their fiber infrastructure is being laid.

Metro Area Network ( MAN ):

FSO network can close the gap

between the last mile customers, there by providing access to new

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

8.MERITS OF FSO

1. Free space optics offers a flexible networking solution that delivers on

the promise of broadband.

2. Straight forward deployment-as it requires no licenses.

3. Rapid time of deployment.

4. Low initial investment.

5. Ease of installation even indoors in less than 30 minutes.

6. Security and freedom from irksome regulations like roof top rights and

spectral licenses.

7. Re-deployability

Unlike radio and microwave systems FSO is an optical technology

and no spectrum licensing or frequency co-ordination with other users is

required. Interference from or to other system or equipment is not a

concern and the point to point laser signal is extremely difficult to

intercept

and therefore secure. Data rate comparable to OFC can be obtained with

very low error rate and the extremely narrow laser beam which enables

unlimited number of separate FSO links to be installed in a given location.

FSO works in a completely unregulated frequency spectrum (THz), unlike

LMDS or MMDS. Because there's little or no traffic currently in this range, the FCC

hasn't required licenses above 600GHz. This means FSO isn't likely to interfere with

29

Free Space Opticsother transmissions. Regulation could come about, however, when and if FSO carriers

start to fill up the spectrum. License free frequency band is an advantage of FSO.

Cost is one of the major advantage of this technology. Airfiber has prepared a

cost model based on deploying an FSO mesh in Boston. According to its analysis,

deployment would cost about $20,000 per building, with an average link length of 55

meters and a maximum length of 200 meters. The mesh would also provide full

redundancy. A comparable fiber network would run between $50,000 to $200,000 per

building.

LIMITATIONS OF FSO

The advantages of free space optics come without some cost. As

the medium is air and the light pass through it, some environmental

challenges are inevitable.

1. FOG AND FSOFog substantially attenuates visible radiation, and it has a similar

affect on the near-infrared wavelengths that are employed in FSO

systems.

Rain and snow have little affect on FSO. Fog being microns in diameter, it

hinder the passage of light by absorption, scattering and reflection .

Dealing with fog – which is known as Mie scattering, is largely a matter of

boosting the transmitted power. In areas of heavy fogs 1550nm lasers can

be of more are. Fog can be countered by a network design with short FSO

link distances. FSO installation in foggy cities like san Francisco have

successfully achieved carrier-class reliability.

2. PHYSICAL OBSTRUCTIONSFlying birds can temporarily block a single beam, but this tends to

cause only short interruptions and transmissions are easily and

automatically re-assumed. Multi-beam systems are used for better

30

Free Space Opticsperformance.

3. SCINTILLATIONScintillation refers the variations in light intensity caused by

atmospheric turbulence. Such turbulence may be caused by wind and

temperature gradients which results in air pockets of varying diversity act

as

prisms or lenses with time varying properties.

This scintillation affects on FSO can be tackled by multi beam

approach exploiting multiple regions of space- this approach is called

spatial diversity.

4. SOLAR INTERFERENCE

This can be combated in two ways.

The first is a long pass optical filter window used to block all

wavelengths below 850nm from entering the system.

The second is an optical narrow band filter proceeding the receive

detector used to filter all but the wavelength actually used for

intersystem communications.

5. SCATTERING

Scattering is caused when the wavelength collides with the

scatterer. The physical size of the scatterer determines the type of

scattering.

* When the scatterer is smaller than the wavelength-Rayleigh scattering.

31

Free Space Optics* When the scatterer is of comparable size to the wavelength -Mie

scattering.

*When the scatterer is much larger than the wavelength -Non-selective

Scattering In scattering there is no loss of energy, only a directional

redistribution

of energy which may cause reduction in beam intensity for

longer distance.

6. ABSORPTION

Absorption occurs when suspended water molecules in the terrestrial

atmosphere extinguish photons. This causes a decrease in the power

density

of the FSO beam and directly affects the availability of a system.

Absorption occurs more readily at some wavelengths than others.

However, the use of appropriate power, based on atmospheric conditions.

10.FUTURE DEVELOPMENTS

The free space optical wireless or Free Space Optics (FSO) do not come without

some cost. So FSO will reduce the costs.

Fog: The primary challenge to FSO-based communications is dense fog. Rain and snow

have little effect on FSO technology, but fog is different. Fog is vapor composed of water

droplets, which are only a few hundred microns in diameter but can modify light

characteristics or completely hinder the passage of light through a combination of

absorption, scattering, and reflection. The primary answer to counter fog when deploying

FSO-based optical wireless products is through a network design that shortens FSO link

distances and adds network redundancies. FSO installations in extremely foggy cities

such as San Francisco have successfully achieved carrier-class reliability.

Absorption: Absorption occurs when suspended water molecules in the terrestrial

32

Free Space Opticsatmosphere extinguish photons. This causes a decrease in the power density (attenuation)

of the FSO beam and directly affects the availability of a system. Absorption occurs more

readily at some wavelengths than others. However, the use of appropriate power, based

on atmospheric conditions, and use of spatial diversity (multiple beams within an FSObased

unit) helps maintain the required level of network availability.

Currently researches are being conducted to overcome the disadvantages of FSO

such as fog and absorption.

Infrared technology is as secure or cable applications and can be

more reliable than wired technology as it obviates wear and tear on the

connector hardware. In the future it is forecast that this technology will be

implemented in copiers, fax machines, overhead projectors, bank ATMs,

credit cards, game consoles and head sets. All these have local

applications

and it is really here where this technology is best suited, owing to the

inherent difficulties in its technological process for interconnecting over

distances.

10.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 fiberquality

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

33

Free Space Opticssystems. 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.

We have discussed in detail how FSO technology can be rapidly

deployed to provide immediate service to the customers at a low initial

investment, without any licensing hurdle making high speed, high

bandwidth communication possible. Though not very popular in India at

the

moment, FSO has a tremendous scope for deployment companies like

CISCO, LIGHT POIN few other have made huge investment to promote

this technology in the market.

It is only a matter of time before the

customers realized, the benefits of FSO and the technology deployed in

large scale.

34

Free Space Optics

BIBLIOGRAPHY

www.freespaceoptics .org

www.networkmagazin.corn

www.freespaceoptics.com

www.wikepidea.corn

www.howstuff.com

General Literature Covering Various Aspects of Free-Space Optics

FSO systems have been studied in detail for several decades. One can find numerous

technical

papers in the SPIE proceedings on Free-Space Laser Communication Technologies

and in the SPIE proceedings on Optical Wireless Communications.

Specific Literature Covering Various Aspects of Laser Safety

American National Standards Institute (ANSI), Standard for the Safe Use of Lasers,

ANSI Z136.1, http://www.ansi.org.

European Committee for Electrotechnical Standardization (CENELEC), Safety of

Laser Products—Part 1: Equipment Classification, Requirements, and User’s Guide,

EN 60825-1:1994 (CENELEC, 1994), http://www.cenelec.org.

European Committee for Electrotechnical Standardization (CENELEC), Safety of

Laser Products—Part 2: Equipment Classification, Requirements, and User’s Guide,

EN 60825-1:1994 (CENELEC, 1994), http://www.cenelec.org.

International Electrotechnical Commission (IEC), Safety of Laser Products— Part 1:

Equipment Classification, Requirements, and User’s Guide, IEC 825-1 (IEC, 1993),

35