All basic things about laser communication.

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EETT 22

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

3. INTRODUCTION
 LASER ?
“LIGHT AMPLIFICATION BY SIMULATED
EMMISSION OF RADIATION”

4. BASIC PRINCIPLE
 ABSORPTION
 SPONTANEOUS EMISSION
 STIMULATED EMISSION
 POPULATION INVERSION

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
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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

9. How Does it Work?
laser

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

12. RUBY LASER

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

16. Why Laser Communication?
 Current high speed communications technology:
 Radio
 Fiber Optics

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