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FREE SPACE OPTIC COMMUNICATIONS

 Presented By: Ankur S.
Sharma

 Course: ECEE-641 Fiber
Optics and Optical
Communications I

 Instructor:
Dr. Timothy P. Kurzweg

PRESENTATION LAYOUT:

 Introduction to the concepts of Free Space Optics (FSO).
 Propagation concepts, Link Budget calculations.
 FSO: Last Mile Bottleneck Solution.
 Configurations of FSO systems.
 Chaining in FSO Systems
 DATA security/ Safety considerations for FSO systems.
 Signal Propagation impediments.
 Advantages of FSO as regards to other widely used
systems.
 Physical Applications of FSO systems
 Manufacturers/Players in field of FSO.

Requirements of a good
Transmission System:
 High Bandwidth
 High BER
 Low SNR
 Power efficient
 Provide Data Security.
 Low cost
 Easy to install and maintain.

Introduction to the concepts of Free
Space Optics (FSO)
 FSO is a line-of-sight technology
which uses LASERS and Photo
detectors to provide optical
connections between two points—
without the fiber.
 FSO can transmit data, voice or
video at speeds capable of
reaching 2.5 Gbps. Products
capable of speeds upto 10 Gbps
are expected to hit the markets
within one year.
 FSO units consist of an optical
transceiver with a laser
(transmitter) and a Photo detector
(receiver) to provide full duplex (bi-
directional) capability.
 FSO systems use invisible infrared
laser light wavelengths in the
750nm to 1550nm range.

Free Space Optic Link Equation:

 • Preceived = received power
 • Ptransmit = transmit power
 • Areceiver = receiver area
 • Div = beam divergence (in radians)
 • Range = link length

Denver, Colorado Fog/Snowstorm Conditions

LAST MILE BOTTLENECKS
 Less then 5% of all buildings in the
US have a direct connection to the
very high speed (2.5-10 Gbps) fiber
optic backbone, yet more than 75%
of businesses are within 1 mile of
the fiber backbone.
 Most of these businesses are
running some high speed data
network within their building, such
as fast Ethernet (100 Mbps), or
Gigabit Ethernet (1.0 Gbps).
 Yet, their Internet access is only
provided by much lower bandwidth
technologies available though the
existing copper wire infrastructure
(T-1 (1.5 Mbps), cable modem (5
Mbps shared) DSL (6 Mbps one
way) ), etc.
 The last mile problem is to connect
the high bandwidth from the fiber
optic backbone to all of the
businesses with high bandwidth
networks.

 DSL and cable modems cannot provide true
broadband services. Cable modems enjoy higher
capacity, yet the channel is shared and the
amount of bandwidth at any given time is not
guaranteed.
 Copper lines provide data rates to a fraction of 1
Mbps.
 T1 lines can reach upto a few Mbps but are still
far away from the Gbps speed which the fiber
backbone can support.
 The chart below shows how these technologies
address different market segments based on
technology, technical capabilities (reach,
bandwidth), and economic realities.

A high-bandwidth cost-effective solution to the last mile problem is to use free-
space laser communication (also known as or optical wireless) in a mesh
architecture to get the high bandwidth quickly to the customers.

DATA SECURITY

To overcome the security in a network two conditions are necessary:
 (1) Intercept enough of the signal to reconstruct data packets and


.
(2) Be able to decode that information

Preventing Interception of the Signal
Directional transmission:
Narrow divergence of the FSO transmit path (shown in red) as compared to a typical
Radio Frequency (RF) path (shown in blue). The tightly collimated FSO beam ensures
that the signal energy is focused on the receiving unit, making interception of the beam
extremely difficult.

Another view of the narrow beam divergence inherent in
FSO transmission. (For clarity only one transit beam is
shown.)

Signal Propagation Impediments:
 Fog: The major challenge to FSO communications is fog. The primary way to counter
fog when deploying FSO is through a network design that shortens FSO link
distances and adds network redundancies. FSO installations in foggy cities such as
San Francisco have successfully achieved carrier-class reliability.
 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.
 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, this is known as Rayleigh scattering. When the scatterer
is of comparable size to the wavelength, this is known as Mie scattering.
 Physical obstructions: Flying birds can temporarily block a single beam, but this
tends to cause only short interruptions, and transmissions are easily and
automatically resumed.
 Building sway/seismic activity: The movement of buildings can upset receiver and
transmitter alignment.
 Safety: To those unfamiliar with FSO, safety is often a concern because the
technology uses lasers for transmission.
 Scintillation: Heated air rising from the earth or man-made devices such as heating
ducts creates temperature variations among different air pockets. This can cause
fluctuations in signal amplitude which leads to image fluctuations at the FSO receiver
end.

 Rough Estimate of Power losses in the system Infrared ight (765 nm) :
 Clear, still air -1 dB/km -5 dB/km
 Scintillation 0 to -3 dB/km 0
 Birds or foliage Impenetrable 0 to -20 dB
 Window (double-glazed) -3 dB -1 dB
 Light mist (visibility 400m) -25 dB/km -1 dB/km
 Medium fog (visibility 100m) -120 dB/km -1 dB/km
 Thick fog (visibility 40m) -300 dB/km -1 dB/km
 Light rain (25mm/hour) -10 dB/km -10 dB/km
 Heavy rain (150mm/hour) -25 dB/km -40 dB/km

ADVANTAGES OF FSO SYSTEMS

 No licensing required.
 Installation cost is very low as compared to
laying Fiber.
 No sunk costs.
 No capital overhangs.
 Highly secure transmission possible.
 High data rates, upto 2.5 Gbps at present
and 10 Gbps in the near future.

 Applications Of FSO Systems
 Disaster management as was
exhibited during the Sept 11
attacks.
 Merill Lynch & Co. has set up FSO
system from its Vesey Street office
towers across the Hudson River to
an alternate site in New Jersey.
 TeraBeam, a major producer of
FSO equipment, successfully
deployed FSO at the Sydney
Summer Olympic Games.
 A network of FSO devices is fast
coming up in Seattle which is
touted as the Capital of Fog.
Manufacturers believe that if an
FSO system can successfully
work in Seattle then it can do so in
any part of the world.
 Affordably extend existing fiber
network.
 Disaster recovery and temporary
applications

Manufacturers/ Players in the Field of FSO:

 LightPointe: A San Diego based company which
received contributions from Cisco Systems and
Corning to the tune of $33 million. It has raised a
total of $51.5 million.
 AirFiber: Another San Diego based company
which has received contributions from Nortel
Networks to the tune of $50 million. It has raised
a total of $92.5 million.
 Terabeam: A Kirkland, WA based company has
received funding from Luscent technologies to
the tune of $450 million and has raised $585
million to date.

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