A

Paper presentation

On

Free Space OpticsBy

J.SRI CHAKRADHAR B.VAMSI BHARGAVA

(Chakri_sree@yahoo.co.in) (vamsi.409@gmail.com)

III/IV E.C.E III/IV E.C.E

G.M.R.INSTITUTE OF TECHNOLOGY A.I.T.S.College of Engg;

RAJAM.

Abstract:

Imagine a communication technology that has unmatched reliability and high-

speed wireless connectivity, easy to install and does not require any spectrum licensing.

Such technology is Free Space Optics (FSO). Free Space Optics uses line-of-sight laser

technology to provide optical bandwidth connections. These FSO based wireless

communication systems have the capability to transmit voice, data, and video

transmissions of up to 2.5 Gbps over 4 km and in the future 10 Gbps using WDM, across

any protocol, which is not possible with any fixed wireless/RF technology. FSO systems

are completely secure and do not require any additional security software.

FSO systems have number of advantages over existing wireless technologies.

Here in this paper, an attempt has been made to explain FSO technology, its working,

challenges and safety measures required along with solutions.

1. Introduction:

A technology that offers full-duplex Gigabit Ethernet throughput. A technology

that can be installed license-free worldwide can be installed in less than a day. A

technology that offers a fast and can be used between cell-site towers. That technology is

free-space optics (FSO). Free space optical communications is a line-of-sight (LOS)

technology that transmits a modulated beam of visible or infrared light through the

atmosphere for broadband communications. In a manner similar to fiber optical

communications, free space optics uses a light emitting diode (LED) or laser (light

amplification by stimulated emission of radiation) point source for data transmission.

However, in free space optics, an energy beam is collimated and transmitted through

space rather than being guided through an optical cable. These beams of light, operating

in the Terahertz portion of the spectrum, are focused on a receiving lens connected to a

high sensitivity receiver through an optical fiber.

FSO is a line-of-sight technology that uses invisible beams of light to provide

optical bandwidth connections that can send and receive voice, video, and data

information. Today, FSO technology — the foundation of LightPointe's optical wireless

offerings — has enabled the development of a new category of outdoor wireless products

that can transmit voice, data, and video at bandwidths up to 1.25 Gbps. This optical

connectivity doesn't require expensive fiber-optic cable or securing spectrum licenses for

radio frequency (RF) solutions. FSO technology requires light. The

use of light is a simple concept similar to optical transmissions

using fiber-optic cables; the only difference is the medium. Light

Fig 1: prototype of FSO Tx/Rx

travels through air faster than it does through glass, so it is fair to classify FSO

technology as optical communications at the speed of light.

Historically, Free Space Optics (FSO) or optical wireless communications

was first demonstrated by Alexander Graham Bell in the late nineteenth century

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

experiment converted voice sounds into telephone signals and transmitted them

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

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

this optical technology – and not the telephone – his preeminent invention because it

did not require wires for transmission.

FSO has been used for more than three decades in various forms to provide

fast communication links in remote locations. LightPointe has extensive experience in

this area. While fiber-optic communications gained worldwide acceptance in the

telecommunications industry, FSO communications is still considered relatively new.

FSO technology enables bandwidth transmission capabilities that are similar to fiber

optics, using similar optical transmitters and receivers and even enabling WDM-like

technologies to operate through free space

2. FSO technology-Working Principle:

FSO technology is surprisingly simple. Free Space Optics (FSO) transmits invisible, eye-

safe light beams from one "telescope" to another using low power infrared laser 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. Like

fiber, Free Space Optics (FSO) uses lasers to transmit data, but instead of enclosing the

data stream in a glass fiber, it is transmitted through the air.

Fig 1: prototype of FSO Tx/Rx

Free Space Optics (FSO) works on the same basic principle as Infrared television

remote controls, wireless keyboards or wireless Palm® devices. It's based on

connectivity between FSO-based optical wireless units, each consisting of an optical

transceiver with a transmitter and a receiver to provide full-duplex (bi-directional)

capability. Each optical wireless unit uses an optical source, plus a lens or telescope that

transmits light through the atmosphere to another lens receiving the information. At this

point, the receiving lens or telescope connects to a high-sensitivity receiver via optical

fiber. Commercially available systems offer capacities in the range of 100 Mbps to

2.5 Gbps, and demonstration systems report data rates as high as 160 Gbps.

Free Space Optics (FSO) systems can function over distances of several

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

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

is possible.

2.1 FSO: OPTICAL OR WIRELESS? (Speed of fiber — flexibility of wireless).

Optical wireless, based on FSO-technology, is an outdoor wireless product

category that provides the speed of fiber, with the flexibility of wireless. It enables optical

transmission at speeds of up to 1.25 Gbps and, in the future, is capable of speeds of 10

Gbps using WDM.

This is not possible with any fixed wireless or RF technology. Optical wireless

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

Fig 2: FSO Tx/Rx mounted on a roof.

license approvals worldwide), which further distinguishes it from fixed wireless

technologies. Moreover, FSO technology’s narrow beam transmission is typically two

meters versus 20 meters and more for traditional, even

newer radio-based technologies such as millimeter-wave

radio. Optical wireless products' similarity with

conventional wired optical solutions enables the seamless

integration of access networks with optical core networks

and helps to realize the vision of an all-optical network.

2.2 HOW FREE SPACE OPTICS (FSO) CAN HELP:

Foss’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 customers 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.

2.3 BREAKING THE BANDWIDTH BOTTLENECK:

Why FSO?

The global telecommunications network has seen massive expansion over the last

few years. First came the tremendous growth of the optical fiber long haul, wide-

area network (WAN), followed by a more recent emphasis on metropolitan area

networks (MANs). Meanwhile, local area networks (LANs) and gigabit ethernet

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

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

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

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

network. Presently, however, most local loop network connections are limited to 1.5

Mbps. As a consequence, there is a strong need for a high-bandwidth bridge

between the LANs and the MANs or WANs.

Five years ago, downloading a simple Web site was slow. Now we want

streaming video. Tomorrow it will be HDTV. Consumers want bandwidth. Paradise for

service providers, bar one point—network traffic is outpacing network capabilities, and

upgrade costs are growing faster than revenues.

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

approaches for addressing the emerging broadband access market and its bottleneck.

Free Space Optics (FSO) systems offer many features, principal among them being

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

due to the optical nature of the technology.

2.4 BROADBAND BANDWIDTH ALTERNATIVES:

Access technologies in general use today include telco-provisioned copper wire,

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

connections (fiber to the building; fiber to the home). Telco/PTT telephone networks are

still trapped in the old Time Division Multiplex (TDM) based network infrastructure that

rations bandwidth to the customer in increments of 1.5 Mbps (T-1) or 2.024 Mbps (E-1).

DSL penetration rates have been throttled by slow deployment and the pricing strategies

of the PTTs. Cable modem access has had more success in residential markets, but

suffers from security and capacity problems, and is generally conditional on the user

subscribing to a package of cable TV channels. Wireless Internet access is still slow, and

the tiny screen renders it of little appeal for web browsing.

Broadband RF/microwave systems have severe limitations and are losing favor. The

radio spectrum is a scarce and expensive licensed commodity, simply unavailable due to

congestion. Furthermore, radio equipment is not inexpensive, the maximum data rates

achievable with RF systems are low compared to optical fiber, and communications

channels are insecure and subject to interference from and to other systems (a major

constraint on the use of radio systems).

Hard-wire alternatives such as cable modems and DSL (Digital Subscriber Line),

and broadband-wireless alternatives such as LMDS (Local Multipoint Distribution

Service) will not compete with the bandwidth offered by FSO, which is almost 20 times

that of LMDS services. Additionally, the FCC regulates radio frequencies (the kind used

in LMDS) to keep radio-frequency links from interfering with one another. Infrared

radiation does not penetrate its surroundings the way radio waves do, so regulation of

FSO is not an issue.

With a line of sight and the right optical equipment, FSO links can route signals to

destinations 1 to 5 kilometers away—no fiber, no construction permits, no installation

delays, and no FCC regulation. Free-space communication between neighboring

buildings requires only rooftop or indoor setups that play a hi-tech form of laser tag. FSO

links go up in days, they are not "sunk costs" (unbolting an FSO link is a lot easier than

unearthing half a mile of fiber).

2.5 FREE SPACE OPTICS (FSO) ADVANTAGES: Free space optical (FSO) systems offer a flexible networking solution that delivers on

the promise of broadband. Only free space optics or Free Space Optics (FSO) provides

the essential combination of qualities required to bring the traffic to the optical fiber

backbone – virtually unlimited bandwidth, low cost, ease and speed of deployment.

Freedom from licensing and regulation translates into ease, speed and low cost of

deployment. Since Free Space Optics (FSO) optical wireless transceivers can transmit

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

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

cabling, and permitting the equipment to operate in a very favorable environment. The

only essential for Free Space Optics (FSO) is line of sight between the two ends of the

link. Undersea communications—from offshore oil mining to counter-terrorism and

defense applications—need the speed FSO promises. Space-based FSO uses include

satellite links for defense use and devices that astronauts use to privately converse on

board shuttles.

In brief, the advantages can be given as

Requires no spectrum licensing

Is easy to upgrade and open interfaces support a variety of vendor’s equipment

Requires no security software

Unaffected by radio frequency interference or saturation

3. Free Space Optics (FSO) security:

The common perception of wireless is that it offers less security than

wireline connections. In fact, Free Space Optics (FSO) is far more secure than RF or

other wireless-based transmission technologies for several reasons:

Free Space Optics (FSO) laser beams cannot be detected with spectrum

analyzers or RF meters

Free Space Optics (FSO) laser transmissions are optical and travel along a

line of sight path that cannot be intercepted easily. It requires a matching

Free Space Optics (FSO) transceiver carefully aligned to complete the

transmission. Interception is very difficult and extremely unlikely

The laser beams generated by Free Space Optics (FSO) systems are narrow and invisible, making them harder to find and even harder to intercept and crack

Data can be transmitted over an encrypted connection adding to the degree of

security available in Free Space Optics (FSO) network transmissions.

4. Challenges in FSO technology:

While fiber-optic cable and FSO technology share many of the same attributes,

they face different challenges due to the way they transmit information. While fiber is

subject to 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. Getting a laser beam to hit the mark in a laboratory setting is

easy, but in the real world both ends of the FSO link are moving targets.

4.1 FOG:

The primary challenge to FSO-based communications is dense fog, 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. 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. The primary answer to counter fog is through a network design that

shortens FSO link distances and adds network redundancies.

4.2 ABSORPTION:

Absorption occurs when suspended water molecules in the terrestrial

atmosphere extinguish photons. This causes a decrease in the power density (attenuation)

of the FSO beam and directly affects the availability of a system. However, the use of

appropriate power, based on atmospheric conditions, and use of spatial diversity helps

maintain the required level of network availability.

4.3 SCATTERING:

Scattering refers to the "pinball machine" nature

of light trying to pass through the atmosphere. Light

scattering can drastically impact the performance of FSO

systems. Scattering is not related to a loss of energy due to

a light absorption process. Rather, it can be understood as a redirection or redistribution

of light that can lead to a significant reduction of received light intensity at the receiver

location. 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, this is known as Rayleigh scattering. When the scatterer is

of comparable size to the wavelength, this is known as Mie scattering. When the scatterer

is much larger than the wavelength, this is known as non-selective scattering. In

scattering — unlike absorption — there is no loss of energy, only a directional

redistribution of energy that may have significant reduction in beam intensity for longer

distances.

Even a clean, clear atmosphere is composed of oxygen and nitrogen molecules.

The weather can contribute large amounts of water vapor. Other constituents can exist, as

well, especially in polluted regions. These particles can scatter or absorb infrared photons

propagating in the atmosphere.

Although it is not possible to change the physics of the atmosphere, it is possible

to take advantage of optimal atmospheric windows by choosing the transmission

wavelengths accordingly. To ensure a minimum amount of signal attenuation from

scattering and absorption, FSO systems operate in atmospheric windows in the IR

spectral range. "Fundamentals of FSO Technology," today's commercially available FSO

systems operate in the near IR spectral windows located around 850 nm and 1550 nm.

Other windows exist in the wavelength ranges between 3–5 μmand 8–14 μm. However,

their commercial use is limited by the availability of devices and components and

difficulties related to the practical implementation such as low-temperature cooling.

4.4 SCINTILLATION:

When you have seen a mirage that appears as a lake in the middle of a hot

asphalt parking lot, you have experienced the effects of atmospheric scintillation. Of the

three turbulence effects, free-space optical systems might be most affected by

scintillation. Random interference with the wave front can cause peaks and dips, resulting

in receiver saturation or signal loss. "Hot spots" in the beam cross-section can occur of

the size, about 3 cm for an 850 nm beam 1 Km away. A great deal of work was done on

this topic for applications like telescope signals and earth-satellite links, where a majority

of the scintillation could be observed near the Earth's surface. FSO systems operate

horizontally in the atmosphere near the surface, experiencing the maximum scintillation

possible.

4.5 BEAM SPREADING:

Beam spreading — long-term and short-term — is the spread of an optical

beam as it propagates through the atmosphere. The wavelength dependency on beam

spreading is not strong. The spot size can often be observed to be twice that of the

diffraction-limited beam diameter. Many FSO systems incur approximately 1 m of beam

spread per kilometer of distance. In a perfect world with no environmental attenuators

present, beam spread would be the only distance-limiting variable.

5. Safety:

With FSO technology, safety can be a concern because the technology uses lasers

for transmission. The proper use and safety of lasers is required since FSO devices first

appeared in laboratories more than three decades ago. The two major concerns involve

eye exposure to light beams and high voltages within the light systems and their power

supplies. Strict international standards have been set for safety and performance

6. Solution #1:

Technologies to keep the beam on target include active beam steering (sometimes

involving Micro Electro-Mechanical Systems or MEMS), multiple laser beams,

automated power control, and calculated laser beam divergence. Pulsed laser beams start

out just centimeters wide and can diverge to diameters of 1 meter or more over typical

link distances—like using a really big bullet to hit a small target.

7. Solution #2—no lost data:

FSO link heads essentially "talk" to one another, and just as people repeat words in

conversation, data can be retransmitted in the event of a temporary beam blockage.

Several companies even have redundant radio-frequency links that simultaneously

transmit the FSO signal. When the radio or the FSO signal is blocked, the counterpart is

automatically used.

Conclusion:

Thus, unlike radio and microwave systems, free space optical

communications requires no spectrum licensing and interference to and from other

systems is not a concern. In addition, the point-to-point laser signal is extremely difficult

to intercept, making it ideal for covert communications. Free space optical

communications offer data rates comparable to fiber optical communications at a fraction

of the deployment cost while extremely narrow laser beam widths provide no limit to the

number of free space optical links that may be installed in a given location.

References:

1. www.google.com

2. www.howstuffworks.com

3. www.sonabean.com

4. Optical communication by Gerd Keiser.