free space optics
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.pdf   FREE SPACE.pdf (Size: 84.14 KB / Downloads: 21)


Communication, as it has always been relied and simply depended
upon speed. The faster the means ! the more popular, the more effective the
communication is !
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
When we hear of optical communications we all think of optical
fibers, what I have for u today is AN OPTICAL COMMUNICATION
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.


Free space optics or FSO, free space photonics or optical wireless,
refers to the transmission of modulated visible or infrared beams through
the atmosphere to obtain optical communication. FSO systems can function
over distances of several kilometers.
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.



Presently we are faced 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


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


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.


Telecommunication 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 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 refer 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- redeployability of the fiber has made it uneconomical and nonsatisfying.
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.


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 .
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.


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 project and implimentationors, 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
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, if invisible, future !

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