Laser communications systems use lasers to transmit data through free space instead of fiber optic cables. They require line-of-sight between a transmitter and receiver. The transmitter includes a laser diode and signal processing, while the receiver has a telescope, detector, and signal processor. Modulation techniques include AM, PWM, and PFM. Advantages over radio frequency include higher bandwidth, directability, and security from a narrow beam. Applications include satellite links, underwater networks, and space communications where fiber is impractical.

1. Laser Communications

2. Contents
• Introduction
• Block Diagram
• Laser Transmitter And Receiver
• Laser Transmitter
• Receiver
• Modulation
• Gain Systems
• Filters
• Mounting System
• Pointing
• Why not Fiber Optics??
• Why not R.F??
• Advantages
• Disadvantages
• Applications
• Conclusion
• Reference

3. Introduction
• Laser communications systems are wireless connections through
the atmosphere.
• They work similarly to fiber optic links, except the beam is
transmitted through free space.
• While the transmitter and receiver must require line-of-sight
conditions, they have the benefit of eliminating the need for
broadcast rights and buried cables.

4. BLOCK DIAGRAM

5. LaserTransmitter and Receiver
LaserTransmitter Receiver
Optical fiber link
Direct Link

6. LaserTransmitter
• The Transmitter involves a signal
processing circuit, and a laser.
• A laser diode is used to create the laser
signal.
• Laser Diodes include Photodiodes for
feedback to insure consistent output.

7. Receiver
The receiver involves:
• Telescope(‘antenna’)
• Signal processor
• Detector
 PIN diodes
 Avalanche Photo Diodes(APD)
 Single or Multiple detectors

8. Modulation :
• AM :
Easy with gas lasers, hard with diodes
• PWM (Pulse Width Modulation) :
Used by Ramsey in their kit
• PFM (Pulsed FM) :
Potentially the highest bandwidth (>100kHz)

9. Gain Systems :
Transmitter:
• Maximum output power
• Minimum divergence
Receiver:
• Maximum lens area
• Clarity
• Tight focus on detector

10. Filters :
• Sun shade over detector
• Shade in front of lens
• Detector spectral response
• Colored filters
 Absorb ~50% of available light
 Difficult to find exact frequency

11. Mounting System :
• Mounts and stands need only be as accurate as beam
divergence.
• Good laser diodes will be 1-2mR (milli-radian)
• A 32 pitch screw at the end of a 2' mount will yield 1mR per
revolution.
• Higher thread pitches allow shorter mounts which may be more
stable (against wind, vibration, wires)
• 1mR is 1.5' of divergence every 1000', 3' at 2000 ', etc.

12. Pointing
• GPS and Compass
• Scopes and Binoculars
• Strobe lights, large handheld floods, headlights
• HTs to yell when laser light is seen at remote location

13. Why not Fiber Optics??
• Not always possible to lay fiber lines :
 Satellites
 Combat zones
 Physically/Economically not practical
 Emergencies
 Laser Communication being incorporated into fiber optic
networks when fiber is not practical.

14. Why not R.F??
• Bandwidth :
 For laser communication is 100times larger than R.F.
• Power :
 In LC is directed at target, so much less transmission power
 Also the power loss is less.
• Size/Weight :
 LC antenna is much smaller than R.F.
• Security :
 Due to low divergence of laser beam ,LC is more secure than R.F.

15. Advantages :
• Ease of deployment
• Can be used to power devices
• License-free long-range operation (in contrast with radio
communication)
• High bit rates
• Low bit error rates
• Immunity to electromagnetic interference
• Full duplex operation
• Protocol transparency
• Increased security when working with narrow beam.

16. Disadvantages :
For terrestrial applications, the principal limiting factors are:
• Beam dispersion
• Atmospheric absorption
• Rain
• Fog (10..~100 dB/km attenuation)
• Snow
• Scintillation
• Interference from background light sources (including the Sun)
• Shadowing
• Pointing stability in wind
• Pollution / smog

17. Applications :
• Laser light is used in optical fiber communications to send information
over large distances with low loss.
• Laser light is used in underwater communication networks.
• Lasers are used in space communication, radars and satellites.

18. Conclusion
Laser communications offers a viable alternative to RF
communications for inter satellite links and other applications
where high-performance links are a necessity.

19. References
• www.studymafia.org
• www.google.com
• www.wikipedia.com