Transcript EE 566 Optical Communications

EE 566
Optical Communications
Free Space Optics
An overview
Snehil Tiwari
[email protected]
Free Space Optics (FSO)??
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FSO is a line of sight technology that uses
devices such as lasers to establish connectivity
for video and voice communication
 Currently, It can allow upto 2.5 Gbps of data rate
but can be increased to 10 Gbps using WDM
 FSO is based on connectivity between two
stations consisting of optical transceiver to
achieve full duplex communication
Technology: How it works?
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FSO is a pretty old and relatively simple
technology which has been used for more than
30 years. FSO enables similar bandwidth
transmission abilities as fiber optics, using
similar optical transmitters and receivers
 It involves two FSO units (similar to BTS of
wireless technology) each consisting of high
power laser transmitters and receivers, a
telescope is used in conjunction to guide the
light and capture it at the receiving end. It is then
typically interface with network switches, hub,
bridge or router via multimode fiber
Transceiver Design
Transmitter source:
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Semiconductor lasers and LEDs(short range).
Solid-state lasers e.g. Yd:Yag (bulky, power)
Gas lasers e.g. CO2 (bulky, lifetime)
Quantum Cascade (QC) lasers (cooling for higher power operation)
Operation within optical windows
Basic Requirements:
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High power operation (average power,not peak power!)
High modulation speed
Operation over wide temperature range
Contd.. (Receivers)
Detector:
Basic Requirement:
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Compensation for lower power lasers (High gain)
Capability to Reject Background noise(S/N ratio)
High Acceptance Angle (NA)
A typical FSO unit
http://www.freespaceoptics.org/images/flightstrata_illustration.gif
Typical City-Wide Broadcast FSO Deployment
Technical Specification for laser
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Wavelength used are ~780nm to ~850 nm for
short ranges and less intensity typically 20mw
1550nm (class 1M)for long range data links
typically 640 mw .
1550 nm wavelength is safe for eye .
1550 nm wavelength is also being used for
optical fiber communication so compatible also.
Attenuation is less at longer wavelengths.
FSO As A Last-Mile Solution
Low investment in infrastructure
typical fiber optical cable costs $ 200,000 per mile approximately for
installation and support. FSO based networks can be implemented for a fraction of
the cost
Low delay in deployment of FSO based networks
Typical deployment of a fiber based network takes well over an year, while FSO
based networks may be deployed with no substantial over-heads
License Free Bandwidth
Unlike microwave links, optical bandwidth isn’t licensed – i.e , bandwidth is free.
This amounts to lower operational costs
Supports High Data Rates
Technological Challenges
FSO is a LOS technology where it is essential that
interconnecting points must see each other without any
physical obstruction. Some of the potential disturbances
are:-
Fog:
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Wavelength dependence of fog attenuation change with
density Between 0.5-4um attenuation by dense fog
increases with wavelength.
Atmospheric Absorption:
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Molecular absorption (gases)
Aerosol absorption (dust, smoke, water drops)
Scattering:
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Rayleigh Scattering (gases)
Mie scattering (aerosol, fog)
Geometrical optics (snow, rain)
Scintillation:
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The variation of refractive index
along the propagation path
caused by slight temperature
variations among different air
pockets.
 Acts like series of small lenses
that deflect the beam into and
out of the transmission path
causes amplitude fluctuations
at the receiver.
 Can impact BER performance
,increases with distance
.therefore for long distance
communication Multiple beam
technology is used.
Limitations
•Unreliable Bandwidth availability : Variations in
weather conditions affect available bandwidth
•Requires Line of Sight link
•Limited Range
•Need sophisticated mechanism for alignment of the
transmitter and receiver
•Even slight mechanical disturbances may lead to
loss of alignment, and may result in complete link
break-down
Industrial Applications
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Short Range Applications
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Long Range Applications
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Weather monitors; fog, snow, rain using light back-scatter
Traffic counting and monitoring
Optical Interconnects.
Deep space probe communications; distances measured in lightyears
Building to building computer data links; very high data rates.
Ship to ship communications; high data rates with complete
security.
Telemetry transmitters from remote monitors; weather,
geophysical.
Electronic distance measurements; hand held units out to 1000
ft.
Optical radar; shape, speed, direction and range.
Remote telephone links; cheaper than microwave
Wide Area Applications
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Campus wide computer networks
City-wide information broadcasting
Inter-office data links
Markets For FSO
The industry is expected to grow from approximately $120
million in 2000 to more than $2 billion annually by 2006,
according to a study conducted by the Strategis Group, a
Washington, D.C.- based telecommunications research firm.
Companies working in FSO
• Terabeam
Corp
• Lightpointe
• Nortel
• fSONA Communications
• Crinis Networks
References
 http://www.mrv.com/technology
 http://www.freespaceoptics.org/index.cfm
 http://www.imagineeringezine.com/
 http://www.sciam.com/
 http://www.wcai.com