free space optics

Seminar Report 2012-2013 Free Space Optics

Chapter 1

INTRODUCTION

Free Space Optics (FSO) communications is also called Free Space

Photonics (FSP) or Optical Wireless, refers to the transmission of modulated

visible or infrared (IR) beams through the atmosphere to obtain optical

communications. Free Space Optics (FSO) uses lasers to transmit data, but

instead of enclosing the data stream in a glass fiber, it is transmitted through the

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

television remote controls, wireless keyboards or wireless Palm devices.

Presently in the twenty-first centaury wireless networking is gaining

because of speed and ease of deployment and relatively high network robustness.

Modern era of optical communication originated with the invention of LASER in

1958 and fabrication of low loss optical fiber in 1970.

Free space optics or FSO although it only recently and rather suddenly

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

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

transport medium for high speed communication.

FSO technology has been revived to offer high band width last mile

connectivity for today’s converged network requirements.

Dept. of E&C 1G.P.T.C. Tirurangadi


Figure 1.1: Free Space Optics structure

FSO is a line-of-sight technology, which enables optical transmission up to

2.5 Gbps of data, voice and video communications, allowing optical connectivity

without deploying fiber optic cable or securing spectrum licenses. Free space

optics require light, which can be focused by using either light emitting diodes

(LED) or LASERS(light amplification by stimulated emission of radiation). The

use of lasers is a simple concept similar to optical transmissions using fiber optic

cables, the only difference being the medium.

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

destination and enough transmitter power, communication is possible virtually at

the speed of light. Because light travels through air faster than it does through

glass, so it is fair to classify FSO as optical communications at the speed of light.

FSO works on the same basic principle as infrared television remote controls,

wireless keyboards or wireless palm devices.



Chapter 2

HISTORY AND RELEVANCE OF FSO

2.1 History

It is said that this mode of communication was first used in the 8 th centaury

by the Greeks. They used fire as the light source, the atmosphere as the

transmission medium and human eye as receiver.

FSO or optical wireless communication by Alexander Graham Bell in the

late 19th centaury even before his telephone. Bells FSO experiment converted

voice sounds to telephone signals and transmitted them between receivers

through free air space along a beam of light for a distance of some 600 feet, this

was later called PHOTOPHONE. Although Bells photo phone never became a

commercial reality, it demonstrated the basic principle of optical

communications.

The engineering maturity of Free Space Optics (FSO) is often under

estimated, due to a misunderstanding of how long Free Space Optics (FSO)

systems have been under development. Historically, Free Space Optics (FSO) or

optical wireless communications was first demonstrated by Alexander Graham

Bell in the late nineteenth century (prior to his demonstration of the telephone).

Bell’s Free Space Optics (FSO) experiment converted voice sounds into

telephone signals and transmitted them between receivers through free air space

along a beam of light for a distance of some 600 feet.



Calling his experimental device the photophone. Bell considered this

optical technology and not the telephone, his preeminent invention because it

did not require wires for transmission.

Although Bell’s photophone never became a commercial reality, it

demonstrated the basic principle of optical communications. Essentially all of the

engineering of today’s Free Space Optics (FSO) or free space optical

communications systems was done over the past 40 years or so, mostly for

defense applications. By addressing the principal engineering challenges of Free

Space Optics (FSO), this aerospace/defense activity established a strong

foundation upon which today’s commercial laser based Free Space Optics (FSO)

systems are based.

Essentially all of the engineering of today’s FSO or free space optical

communication systems was done over the past 40 years or so mostly for defense

applications.

2.2 Relevance of FSO in communication

Presently we are facing with a burgeoning demand for high bandwidth and

differentiated data services. Network traffic doubles every 9-12 months forcing

the bandwidth or data storing capacity to grow and keep pare with this increase.

The right solution for the pressing demand is the untapped bandwidth potential of

optical communications.

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 the bandwidth requirements.



Business applications have grown out of the physical boundaries of the

enterprise and gone wide area linking remote vendors, suppliers, and customers

in a new web of business applications. Hence companies are looking for high

bandwidth last mile options. The high initial cost and vast time required for

installation in case of OFC speaks for a wireless technology for high bandwidth

last mile connectivity there FSO finds its place.

2.3 FSO as a future technology

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.

Outdoors two its use is bound to grow as communications companies,

broadcasters and end users discovers how crowded the radio spectrum has

become. Once infrared’s image issue has been overcome and its profile raised,

the medium will truly have a bright future.



Chapter 3

WORKING OF FSO SYSTEMS

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.

Optical systems work in the infrared or near infrared region of light and the

easiest way to visualize how the work is imagine, two points interconnected with

fiber optic cable and then remove the cable. The infrared carrier used for

transmitting the signal is generated either by a high power LED or a laser diode.

Two parallel beams are used, one for transmission and one for reception, taking

a standard data, voice or video signal, converting it to a digital format and

transmitting it through free space.

Today’s modern laser system provide network connectivity at speed of 622

Mega bits/sec and beyond with total reliability. The beams are kept very narrow

to ensure that it does not interfere with other FSO beams. The receive detectors

are either PIN diodes or avalanche photodiodes.



The FSO transmits invisible eye safe light beams from transmitter to the

receiver using low power infrared lasers in the tera hertz spectrum. FSO can

function over kilometers.

3.1 FSO subsystem

Figure 3.1 FSO Subsystem

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 Dept. of E&C 7

G.P.T.C. Tirurangadi


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 pre

amplifier. The servo system is used for controlling system, the signal coming

from the path to the processor and compares with the environmental condition, if

there is any change in the signal then the servo system is used to correct the

signal.

Figure 3.2 FSO Transmitter



Fig 3.3 FSO Receiver

3.2 Performance-Transmit power and Receiver sensitivity

Free Space Optics (FSO) products performance can be characterized by

four main parameters (for a given data rate).

Total transmitted power

Transmitting beam width

Receiving optics collecting area

Receiver sensitivity



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 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.

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.



Chapter 4

FSO ARCHITECTURES

4.1 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 10Gbits/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.



Figure 4.1 Point-to-point architecture

4.2 Mesh architecture

Mesh architectures may offer redundancy and higher reliability with easy

node addition but restrict distances more than the other options.

Figure 4.2 Mesh architecture

4.3 Point-to-multipoint architecture



Point-to-multipoint architecture offers cheaper connections and facilitates

node addition but at the expense of lower bandwidth than the point-to-point

option.

Figure 4.3 Point-to-multipoint architecture



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.

Chapter 5

NEED OF FSO

The increasing demand for high bandwidth in metro networks is relentless,

and service provider’s pursuit of a range of applications, including metro network

extension, enterprise LAN-to-LAN connectivity, wireless backhaul and Local

Multipoint Distribution System (LMDS) supplement has created an imbalance.

This imbalance is often referred to as the "last mile bottle neck”. 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 bottle neck 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



redeployed 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.

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), redeployment 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. 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.

5.1 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. Redeploy ability.

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.

5.2 Market

Telecommunication has seen massive expansion over the last few years.

First was the tremendous growth of the optical fiber. Long haul Wide Area

Network (WAN) followed by more recent emphasis on Metropolitan Area

Networks (MAN). Meanwhile LAN giga bit Ethernet ports are being deployed

with a comparable growth rate. Even then there is pressing demand for speed and

high bandwidth.

The ‘connectivity bottleneck’ which refers the imbalance between the

increasing demand for high bandwidth by end users and inability to reach them is

still an unsolved puzzle. Of the several modes employed to combat this ‘last mile

bottleneck’, the huge investment is trenching, and the non redeploy ability of the

fiber has made it uneconomical and non satisfying.

Other alternatives like LMDS, a RF technology has its own limitations like

higher initial investment, need for roof rights, frequencies, rainfall fading,

complex set and high deployment time.



In the United States the telecommunication industries 5 percent of

buildings are connected to OFC. Yet 75 percent are with in one mile of fiber.

Thus FSO offers to the service providers, a compelling alternative for optical

connectivity and a complement to fiber optics.

5.3 Applications of FSO

Optical communication systems becoming more and more popular as

the interest and requirement in high capacity and long distance space

communications grow. FSO overcomes the last mile access bottleneck by

sending high bit rate signals through the air using laser transmission.

Applications of FSO system are many and varied but a few can be listed.

1. 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.

2. Last Mile Access: End users can be connected to high speed links using

FSO. It can also be used to bypass local loop systems to provide business

with high speed connections.

3. Enterprise connectivity: As FSO links can be installed with ease, they

provide a natural method of interconnecting LAN segments that are housed

in buildings separated by public streets or other right of way property.

4. Fiber backup: FSO can also be deployed in redundant links to backup

fiber in place of a second fiber link.



5. Backhaul: FSO can be used to carry cellular telephone traffic from

antenna towers back to facilities wired into the public switched telephone

network.

6. Service acceleration: Instant services to the customers before fiber being

laid.

5.4 FSO at the speed of light

Unlike radio and microwave systems, Free Space Optics (FSO) is an

optical technology and no spectrum licensing or frequency coordination with

other users is required, interference from or to other systems or equipment is

not a concern, and the point-to-point laser signal is extremely difficult to

intercept, and therefore secure. Data rates comparable to optical fiber

transmission can be carried by Free Space Optics (FSO) systems with very

low error rates, while the extremely narrow laser beam widths ensure that

there is almost no practical limit to the number of separate Free Space Optics

(FSO) links that can be installed in a given location.

5.5 Construction of FSO

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.

Chapter 6

FSO CHALLENGES

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.

6.1 Operating 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.



6.2 Eye safety

Laser beams with wavelengths in the range of 400 to 1400 nm emit

light that passes through the cornea and lens and is focused onto a tiny spot

on the retina while wavelengths above 1400 nm are absorbed by the cornea

and lens, and do not focus onto the retina, as illustrated in Figure 1. It is

possible to design eye-safe laser transmitters at both the 800 nm and 1550 nm

wavelengths but the allowable safe laser power is about fifty times higher at

1550 nm.

This factor of fifty is important as it provides up to 17 dB additional

margin, allowing the system to propagate over longer distances, through

heavier attenuation, and to support higher data rates.

6.3 Fog

Fog 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 effect 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.

Fog can be countered by a network design with short FSO link distances. FSO

installation in foggy cities like San Francisco has successfully achieved carrier-

class reliability.

6.4 Physical obstructions

Flying 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 performance.

6.4.1 ScintillationDept. of E&C 22



Scintillation 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.

6.4.2 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.

6.4.3 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.

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 re-distribution of

energy which may cause reduction in beam intensity for longer distance.

6.4.4 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, and use

of spatial diversity helps to maintain the required level of network availability.

6.5 Building sway / Seismic activity

One of the most common difficulties that arises when deploying FSO

links on tall buildings or towers is sway due to wind or seismic activity Both

storms and earthquakes can cause buildings to move enough to affect beam

aiming. The problem can be dealt with in two complementary ways through

beam divergence and active tracking.

With beam divergence, the transmitted beam spread, forming optical

cones which can take many perturbations.

Active tracking is based on movable mirrors that control the direction in

which beams are launched.

6.6 FSO Security



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

technologies for several reasons:

Laser beams cannot be detected with spectrum analyzers or RF meters

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.

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

and even harder to intercept and crack

6.7 FSO 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

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.



Chapter 7

CONCLUSION

Free space optics (FSO) provides a low cost gaining access to the fiber

optic backbone. FSO technology can be rapidly deployed to provide immediate

service to the customers at low initial investment without any licensing hurdle

making high speed high band width communication possible. Though not very

popular in India at the moment, FSO has 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, benefits of FSO and the technology deployed in large scale.



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.

REFERENCES

1.Heinz A. Willebrand, Baksheesh S. Ghuman "Fiber Optics Without Fiber."

2.http://www.freespaceoptics.org

3.http://www.networkmagazin.com

4.http://www.furtera.com/fsofaq.html

5.http://www.sans.org/rr/wireless/optics.php

6.http://www.freespaceoptic.com/



QUESTIONS AND ANSWERS

1.What is the difference between Rayleigh scattering &Mie scattering?

When the scatterer is smaller than the wavelength Rayleigh

scattering.When the scatterer is of comparable size to the wavelength Mie

scattering.

2.What is scintillation?

Scintillation refers the variations in light intensity caused b atmospheric

turbulence.

Dept. of E&C 28



3.What is the wavelength rage of FSO?

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.


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free space optics