Free space optics

VISVESVARAYA TECHNOLOGICAL UNIVERSITY
BELGAUM, KARNATAKA.
A Technical Seminar on
FREE SPACE OPTICS
By:-
Kartik K Benageri
ARKA Educational and Cultural Trust(R)
Jain Institute of Technology, Davangere
Department Of Electronics and Communication Engineering

Supervised by:
Prof. Magdy Ibrahim
Prepared by:
Ahmed Ashraf Abdel-Haseb
Ahmed-Houssam Mahmoud
Ahmed Magdy El-Sayed
Amr Atef Hussein
Mohamed Khaled Abo-Seif

• Introduction
• Key Features
• Working of FSO
• Advantages of FSO
• Limitations of FSO
• Conclusion
• References
• Acknowledgment

 FSO is a line-of-sight technology which uses LASERS and
Photo detectors to provide optical connections between
two points-without the fibre.
 FSO can transmit data, voice or video at speeds capable of
reaching 2.5 Gbps.
 An FSO unit consists of an optical transceiver with a
laser transmitter and a receiver to provide full duplex (bi-
directional) capability.
 FSO systems use invisible infrared laser light wavelengths in
the 750nm to 1550nm range
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 This mode of communication was first used in the 8th century by
the Greeks.They used fire as the light source ,the atmosphere as
the transmission medium and human eye as receiver.
 Optical wireless communication was used by Alexander Graham
Bell in the late 19th century even before his telephone.
 Bell 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

 AT TRANSMITTER
Electrical signal is converted to optical energy using
LED or Laser diodes and is transmitted through air.
 AT RECIEVER
a. Optical concentrator
b. Optical Filter
c. Photo detector
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Transmitter Principle
FSOTRANSMITTER FSO RECIEVER

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1 Network traffic
converted into
pulses of
invisible light
representing 1’s
and 0’s
2 Transmitter projects the
carefully aimed light pulses
into the air
5 Reverse direction data
transported the same way.
• Full duplex
3 A receiver at the other end of
the link collects the light using
lenses and/or mirrors
4 Received signal
converted back into
fiber or copper and
connected to the
network
Anything that can be done in fiber
can be done with FSO

DRIVER
CIRCUI
T
SIGNAL
PROCESSING
PHOTO
DETECTOR
Link Range L
FSO LINK EQUATION
 Cloud
 Rain
 Smoke
 Gases
 Temperature variations
 Fog and aerosol
Transmission of optical radiation through the atmosphere obeys the Beer-
Lamberts’s law:
α : Attenuation coefficient dB/km – Not controllable and is roughly independent of
wavelength in heavy attenuation conditions.
d1 and d2:Transmit and receive aperture diameters (m)
D: Beam divergence (mrad)(1/e for Gaussian beams; FWHA for flat top beams),
This equation fundamentally ties FSO to the atmospheric weather conditions
 Range
RangeDiv
A
PP
receiver
tr .exp.
)( 2



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 Serial to parallel converter(Independent data streams ).
 Parallel encoder.
 Parity generator(Parity check bits).
 Modulation(OOK or PPM).
At the modulator these code sequences modulate each diode with
a different wavelength and are multiplexed. In the multiplexer each
optical signal from channels is focused on an optical fiber.
 TransmitOptics
Size
Power
Beam quality

 Receive Optics
Aperture size
f-number
 Demodulator
The multiplexed signals are de-multiplexed with their carrier
wavelength. The optical ﬁlter is used as the de-multiplexer.
 Photo diode array.
 Demodulator (pulse demodulator).
 Parity checker.
 Parallel decoder
Parallel data blocks are parallel to serial converted to retrieve
the original data.

 Free space optics offers a flexible networking solution that
delivers on the promise of broadband
 No licensing required like RF.
 Deployment of FSO systems quickly and easily.
 Security
 Immunity from electromagnetic interference
 Lower costs as compared to fiber networks.
 High Speed data upto 2.5Gbps

As tahe medium is air and the light pass through it,
some environmental challenges are inevitable.
1. FOG : Fog substantially attenuates visible radiation,
and it has a similar affect on the near-infrared
wavelengths that are employed in FSO systems.
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.

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.
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.

4. SCATTERING: In scattering there is no loss of energy, only
a directional redistribution of energy which may cause reduction
in beam intensity for longer distance.
5. SOLAR INTERFERENCE
6. ABSORPTION
7. BUILDING SWAY / SEISMIC ACTIVITY

Secure and undetectable FSO system can
connect large areas safely with minimal planning
and deployment time.

2. Wireless Service Provider
Unlike microwave or fiber, deployment of FSO
does not require spectrum licensing, physical
disruption to a location, or government zoning
approvals. Carriers are free to grow their
business.

3. Enterprise connectivity
Companies, airports, hospitals and schools
can use safe, secure Free space optical
wireless links to connect buildings within their
campus environments.

Criteria FSO Optical Fiber Microwave
Data rate Up to 10 Gbps 100 Mbps to 100 Gbps 275 Mbps
Installation Easy Difficult Moderate
Cost Moderate High Moderate
Maintenance low High low
Most common
uses
Between buildings
Short distance
Point-to point
Long distance
Point-to-point
Short distance
Advantages Price
performance
No license
Security
capacity and speed
Immunity to EMI
speed
Disadvantages Can be intercepted Difficult to splice
determinate
Can be intercepted
Requires radio license
Security Moderate Excellent Poor 22

 Clear, still air -1 dB/km -5 dB/km
 Scintillation 0 to -3 dB/km
 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
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REFRENCES
 [1] S.V. Kartalopoulos , “ Disaster Avoidance in
the Manhattan Fiber Distributed
Data Interface Network , ” Globecom ’ 93,
Houston, TX, December 2, 1993 .
 [2] Scott Bloom, “The Physics of Free Space
Optics”, AirFiber, Inc.
 [3] “Free-Space Optical Communications on
HAPs”, www.hapcos.org , accessed on : 13-
May-2012

We thank our Seminar Guide Prof. Halesh.M.R. for
his valuable guidance and directions in making the
seminar resourceful.
I extend my sense of gratitude to Dr.Nagaraja B.G.
HOD, Department of E&CE, JIT, Davangere for
extending support and cooperation which helped
me in completion of the seminar.
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Free space optics

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