2. Organization
1. Introduction.
2. Basic principle.
3. What is LASER Communication?
4. Free Space Laser Communication
5. Why Laser Communication?
6. A Simple Laser comm. System
6.1. Transmitter
6.2. Modulation
6.3. Receiver
7. Current Application.
8. FSO
7.1. What is FSO?
7.2. Advantages of FSO
9. SUMMARY
10. References
5. What Is Laser Communication?
“Laser communications systems are wireless
connections through the atmosphere. They
work similarly to fiber optic links, except the
fact that, in lasers, beam is transmitted
through free space. “
6. Free Space Laser Communication
Transmitting information via a laser beam
Video
Data
Sound
Terrestrial / Space based systems
010001100110
111011001111
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101110010001
111001011011
7. DIRECTIONAL TRANSMISSION:
Narrow divergence of the FSO transmit path
(shown in red) as compared to a typical Radio
Frequency (RF) path (shown in blue).
8. Free Space Optic Link Equation:
Preceived = received power
Ptransmit = transmit power
Areceiver = receiver area
Div = beam divergence (in radians)
Range = link length
10. What is the Transmitter?
►The transmitter involves:
Signal processing electronics (analog/digital)
Laser modulator
Laser (visible, near visible wavelengths)
11. What is the Receiver?
►The receiver involves:
Telescope (referred to as ‘antenna’)
Signal processor
Detector
-PIN diodes
-Avalanche Photo Diodes (APD)
-Single or multiple detectors
Often both ends will be equipped
with a receiver and transmitter
13. Laser Diode
“Laser Diodes include
Photodiodes for
feedback to insure
consistent output.”
14. Modulation
AM
Easy with gas lasers, hard with diodes
PWM (Pulse Width Modulation)
PFM (Pulsed FM)
Potentially the highest bandwidth (>100kHz)
15. AVALANCHE PHOTO DIODE
Avalanche photodiode-2
Stabilisation of working point of APD:
.
Gain =75
Temperature stabilisation.
Thermoelectrically cooler stabilisation
system is inside of APD module
17. Why not Fiber Optics?
Not always possible to lay fiber lines
Satellites
Combat zones
Physically / Economically not practical
Emergencies
LC being incorporated into fiber optic networks
when fiber is not practical.
18. Why not RF?
Bandwidth
for Laser Communication (LC) is 100 times greater than for
RF.
Power
in LC is directed at target, so much less transmission power
required. Also the power loss is less.
Size / Weight
LC antenna is much smaller than RF.
Security
Due to low divergence of laser beam, LC is more secure than
RF.
19. Current Applications
►Defense and sensitive areas.
►FSO Communication.
►At airports for communication across the
runways.
►Mass communication
►400 TV channels
►40,000 phone conversations
►NASA
Satellite - satellite
Earth - satellite
20. 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.
21. 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.
22. 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.
23. Opportunities For Student
Involvement
•LASER Research
•LASER Modulation Circuitry
•Encoding/Decoding Circuitry