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.
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.