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Laser Communication
PRAVIN KUMAR
07EC75
05-04-2011
Organization
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
Introduction
Basic principle
LASER action
Construction
What is LASER Communication?
Why Laser Communication?
A Simple Laser comm. System
7.1. Transmitter
7.2. Modulation
7.3. Receiver
Current Application
FSO
9.1. What is FSO?
9.2. Advantages of FSO
9.3. Last mile bottleneck
9.4. Signal Propagation Impedances
SUMMARY
References
INTRODUCTION

LASER ?
“LIGHT AMPLIFICATION BY SIMULATED
EMMISSION OF RADIATION”
BASIC PRINCIPLE
ABSORPTION
 SPONTANEOUS EMISSION
 STIMULATED EMISSION
 POPULATION INVERSION

LASER ACTION
LIGHT PROPOGATION
CONSTRUCTION
Laser Communication ?
•
Laser communications systems
- wireless connections.
- work similarly to fiber optic links.
- no fiber backbone required.
- lasers transmitted through free space.
Free Space Laser Communication

Transmitting information via a laser beam
◦ Video
◦ Data
◦ Sound

Terrestrial / Space based systems
010001100110
111011001111
001010000010
101110010001
111001011011
Why Laser Communication?

Current high speed communications technology:
◦ Radio
◦ Fiber Optics
Why not Fiber Optics?

Not always possible to lay fiber lines
◦
◦
◦
◦
•
Satellites
Combat zones
Physically / Economically not practical
Emergencies
LC incorporated into fiber optic
networks when fiber not practical.
Why not RF?

Bandwidth

Power

Size / Weight

Security
◦ LC >> RF
◦ transmit data @ 2.5-10 Gbps.
◦ LC directed at target.
◦ Much less transmission power required.
◦ Less free space power loss.
◦ LC antenna << RF antenna.
◦ LC more secure than RF
- low divergence laser beam.
Free Space Optic Link Equation:





Preceived = received power
Ptransmit = transmit power
Areceiver = receiver area
Div = beam divergence (in radians)
Range = link length
Directional transmission:
Narrow divergence of the FSO transmit path (shown in red) as
compared to a typical Radio Frequency (RF) path (shown in blue).
A Simple laser Comm. System
Signal
Transmitter
Laser
Signal
Receiver
laser
MCU
UART
Conditioning
Laser
Diode
MCU
PORT
Conditioning
A/D
High Level design
UART
Conditioning
Photo resistor
What is the Transmitter?
•
The transmitter involves:
o Signal processing electronics (analog/digital)
o Laser modulator
o Laser (visible, near visible wavelengths)
• Transmit antennae gain, transmit pointing losses.
• Laser characteristics
- peak and average optical power
- pulse rate
- pulse width
Laser Diode
Laser Diodes include
Photodiodes for
feedback to insure
consistent output.
Modulation

AM
◦ Easy with gas lasers, hard with diodes
PWM (Pulse Width Modulation)
 PFM (Pulsed FM)

◦ Potentially the highest bandwidth (>100kHz)
What is the Receiver?
•
o
o
o
The receiver involves:
Telescope (referred to as ‘antenna’)
Signal processor
Detector
-PIN diodes
-Avalanche Photo Diodes (APD)
-Single or multiple detectors
•Often both ends - equipped
with a receiver and transmitter
AVALANCHE PHOTO DIODE
Stabilisation of working point of APD:
.
Gain =75
Temperature stabilisation.
Thermoelectrically cooler stabilisation
system inside of APD module
Current Applications
• Defense and sensitive areas.
• FSO Communication.
• airports - communication across the
runways.
• Mass communication
400 TV channels
o 40,000 phone conversations
o
• NASA
Satellite – satellite
o Earth - satellite
o
Earth
FSO




line-of-sight technology.
uses LASERS and Photo detectors.
optical connections between two points—
without the fiber.
FSO units consist
-optical transceiver with a laser (transmitter)
-Photo detector (receiver)
-provide full duplex capability.
ADVANTAGES OF FSO SYSTEMS
No licensing required.
 Very low installation cost.
 No sunk costs.
 No capital overhangs.
 Highly secure transmission possible.
 High data rates @ 2.5 -10 Gbps.

LAST MILE BOTTLENECKS
Signal Propagation Impediments:

Absorption:
- suspended water molecules extinguish photons.

Scattering:
- wavelength collides with the scatterer.
- Rayleigh scattering (Lambda scatterer << wavelength)
- Mie scattering (Lambda scatterer ≈ wavelength)

Physical obstructions: Flying birds

Safety:
- technology uses lasers for transmission.

Scintillation:
- fluctuations in signal amplitude.
- heated air rising from the earth or man made
devices.
Summary
Basic principle of laser action discussed.
 Laser communication system used in
satellite communication.
 Provide higher data rates , high security &
lesser antenna size.
 FSO used for lesser link length ~ 4km.
 FSO links –designed carefully due to
safety issues.

References
www.mindstein.net Laser communication.pdf
 www.bestneo.com Lasercommunicationsystem.pdf
 server4.oersted.dtu.dk/courses/31825/Project11.pdf
 www.freepatentsonline.com/4717828.html
 www.mseconference.org/.../mse03_2P_Uherek_Microoptoele
ctronicscurricula.pdf
 opticalcomm.jpl.nasa.gov/PAPERS/ATP/gospi03b.pdf

5/6/2017
Technical Seminar-II
30