Seminar on free space optics

7/29/2019 Seminar on free space optics

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THE TECHNOLOGY AT THE HEART OF OPTICAL WIRELESS

-REVANSIDDAPPA.S.R


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INTRODUCTION Simple point-to-point communication for longer

distance.

Environment friendly and least threat of tracingsignals.

Low investment , faster installation and easilyupgradable.

Large data transmission of about 2.5Gb/s for upto

Hundreds of kms in range without any licensing ofsignals.

It can be operated in highly sensitive technology likespace .


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TECHNOLOGY OF FSO Clear line-of-site technology.

Depended on clear atmosphere.

Use invisible modulated beams of light instead ofradio waves.

Transmission speed up to 1.25 Gbps.

They allows communication through windows without

the need for rooftop mounted antenna . 40 Gbps has been successfully tested in laboratories.


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How FSO works?

1 Network traffic

converted intopulses of

invisible light

representing 1s

and 0s

2 Transmitter projects thecarefully aimed light pulses

into the air

5 Reverse direction datatransported the same way.

Full duplex

3 A receiver at the other end of thelink collects the light using

lenses and/or mirrors

4 Received signal

converted back intofiber or copper and

connected to the

network

Anything that can be done in fiber can

be done with FSO


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How FSO works? Conti

Optical wireless unit uses an optical sources + transmitting lensor telescope that transits light through the atmosphere toreceiving lens.

It transmit invisible, eye-safe light beams form source to

destination using low power infrared laser in the THzspectrum.

Light focused on highly sensitive detector receives.

At this point, the receiving lens or telescope connect with

optical cable which gives our original information. Commercially, available systems offer capacities in the range

of 100 Mbps to 2.5 Gbps.

Most FSO system use simple ON-OFF keying modulationformat.

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Transmission IssuesAlignment:

FSO systems have to maintain transceivers alignment.

Transceivers transmit highly directional and narrow beams oflight that must impinge upon the receiver aperture.

FSO receivers have a limited cone of acceptance and is similar

to the cone of light projected by the transmitter.

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Transceiver Unit

The received laser beam (yellow) is much wider than the transmittedbeam (red). Thats why the receiver lens is so much larger than thetransmitter lens.

Both lenses, which share the same axis.

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Optical Source

The modulated light source, which is typically a LASER or

LED, provides the transmitted optical signal.

LASER

1. Modulates at 20 Mbps to 2.5 Gbps

2. Coherent

3. Operate in the 850 to 1550 wavelength band

LED

1. Very difficult to modulate high intensity

2. Non coherent

3. Less optical output

4. Less expensive & less complex circuitry

5. Longer lifetime than laser

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Optical Detectors

PIN photodiodes

Inexpensive

Sensitive in the red and infrared spectrum

Photomultiplier tubes (PMTs)

Expensive

Required a high voltage (about 1000 volt)

Many types have poor red wavelength sensitivity

Avalanche Photodiodes (APD)

Expensive

Excellent red sensitivity

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SOME IMPORTANT CHARACTERISTICS OF FSO

ATMOSPHERICATTENUATION: FSO system is typically dominated by the fog but can also be dependent

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

SCINTILLATION:It is defined as the changing of light intensities in time and space at the plane ofa 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 transmission path.

ALIGNMENT:

Due to the transmission of the highly directional narrow beams oflight that must reach receiver aperture on opposite transceiver,It transmits one or more beams which is 5-8 cm in diameter attransmitter and at receiver it spreads upto 1-5m in diameter for arange of 1km.


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TRANSMISSION :For transmission ,modulated light source is used which is typically a laser or LED ,Laser light source use wavelength band of 850-1550nm.

Some important characteristics of in transmission isAbility to operate at high power levels.

Small foot print and low power consumption.High speed modulation.Ability to operate over a wide temperature range without major performance degradation

VCSEL :(Vertical Cavity Surface Emitting Laser)It is low cost high reliable, hight speed laser technology.

Active cooling is not requiredIt emits light in the form of a circular bean instead of n elliptical beam.

APLIFICATION SOURCES:Sources such as EDFA and SOA are used to boost the power of lower-power laser sources.


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PEAK OUTPUT POWER:The maximum allowable output power of a transmission source.FSO peak rating should be 1. Approximately it should be twice the

average output power.

AVERAGE OUTPUT POWER:It is defined as the transmit power of FSO transceiver.

Average power is used for eye safety.Digital signal like 1s and 0s are transmitted using coding schemes with

50% duty cycle.

ENVIRONMENTAL FACTORS:The performance of a FSO link is primarily dependent upon the climatic

condition.It is capable of operating at arangeof two to three times that of the naked

eye in any particular environmental condition.


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FSO ARCHITECTURE

POINT-TO-POINT ARCHITECTURE:

Point-to-point architecture is a dedicatedconnection that offers higher bandwidth

claims it can deliver 10Gbits/ sec. "Terabeam"can provide up to 2Gbits/sec now

155Mbits/sec and I 0Gbits/sec at a distance of2 kilometers (km) to 4km

Access"

FSO can support speeds


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FSO ARCHITECHTURE

MESH ARCHITECTURE:

A meshed configured can support622Mbits/sec at a distance of 200

meters(m)To 450m. TeraBeam claims tohave successfully tested 160Gbits/secspeeds in its lab,But such speeds in the real world are surelya year or two off

Mesh architectures may offer redundancyand higher reliability with easy nodeaddition but restrict distances more thanthe other options.


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FSO RACHITECTURE

POINT-TO-MULTIPOINT ARCHITECTURE

Point-to-Multipoint architecture offers cheaperconnections and facilitates node addition butat the expense of lower bandwidth than the

point-to-point optionA single node serves as an originator andmultiple links emanate from it. The mosteffective method is to connect each FSO linkinto a layer 2 or 3 device located in a buildingcloset.

In a point-to-multipoint arrangement,FSO can support the same speeds as the point-to-point arrangement-155Mbits/sec to10Gbits/sec-at 1km to 2km.


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APPLICATION 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-loopsystems to provide

businesses with high-speed connections. ENTERPRISE CONNECTIVITY:

The ease with which FSO links can be installed makes them a naturalfor

interconnecting local-area network segments that are housed inbuildings separated by

public streets or other right-of-way property


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APPLICATION OF FSO BACKHAUL: FSO can be used to carry cellular telephone traffic from antenna towers back to facilities wired into the public switched telephone network.

SERVICE 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 newcustomers to high speed MANs resulting to MetroNetwork extension.


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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 rooftop rights and

spectral licenses.

7. Re-deployability


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LIMITATIONS OF FSO 1. 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 affect on FSO. Fog being microns in diameter, it hinder the passage of light by absorption, scattering and reflection .

2. 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.

3. SCINTILLATION 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.


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LIMITATIONS OF FSO 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. 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. * When the scatterer is of comparable size to the wavelength -Mie


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

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

delivers fiber quality connections, it provides the lowest cost

transmission capacity in the broadband industry. As a truly protocol-independent broadband conduit, FSO systems complement legacy network investments and work in harmony with

any protocol, saving substantial up-front capital investments. A FSO link can be procured and installed for as little as one-tenth of

the cost of laying fiber cable, and about half as much as comparable

microwave/RF wireless systems.

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Seminar on free space optics