laser communication

1. Laser Communication
Presented by,
P.Anitha(08MCS204)

2. Introduction
 Laser communications systems are wireless connections
through the atmosphere.
 Use Laser Beams to transmit information between two
locations
 No fibres need, a wireless technology
 Communication over long distances, e.g. between planets
 Laser Communication Terminals (LCTs) transmit a laser beam
and are capable of receiving laser beams
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3. How does it Work
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Signal Transmitter Laser
Receiver Signal

4. Laser Transmitter and Receiver
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5. A one-way Laser communication system.
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6. Laser Transmitter
 The transmitter involves:
 Signal processing electronics(analog/digital)
 Laser modulator
 Laser( Visible, near visible wavelengths)
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7. Modulation
 AM
 Easy with gas lasers,hard with diodes
 PWM
 PFM
 Potentially the highest bandwidth(>100kHz)
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8. Receiver
 The receiver involves:
 Telescope(‘antenna’)
 Signal processor
 Detector
 PIN diodes
 Avalanche Photo Diodes(APD)
 Single or multiple detectors
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9. Gain Systems
 Transmitter
 Maximum output power
 Minimum divergence
 Receiver
 Maximum lens area
 Clarity
 Tight focus on
detector
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10. Laser Diode
 Laser Diodes
include
Photodiodes for
feedback to insure
consistent output
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11. 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
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12. Mounting Systems
 Mounts and stands need only be as accurate as beam
divergence
 Good laser diodes will be 1-2mR (milliRadian)
 A 32 pitch screw at the end of a 2' mount will yield 1mR per
revolution. Since quarter turns (even eighth turns) are
possible, this is more than accurate enough
 Higher thread pitches allow shorter mounts which may be
more stable (against wind, vibration, wires)
 1mR is 1.5 of divergence every 1000, 2000 etc.
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13. Security Aspects
 Free space laser communications systems have narrow optical
beam paths, which are not accessible unless viewing directly
into the transmitter path.
 Any potential eavesdropping will result in an inter-ruption of
the data transmission.
 The existence of laser beams cannot be detected with
spectrum ana-lyzers.
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14. Safety Aspects
 The free space laser communications systems do not
require certification for handling or operation.
 Although the emitted laser beam is invisible to the
unaided eye, it can cause eye damage if viewed directly
at close range for extended periods of time
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15. Laser Communication System
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16. Laser Communication System
 Input: Digital data
 Direct or indirect modulation
 Source output passes through the optical system into the
channel
 Optical system: transfer, beam shaping, telescope optics
 Receiver beam comes through optical system and
passed to detectors and signal processing electronics
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17. System Characteristics
 Link parameters:
 Type of laser, wavelength, type of link
 Semiconductor laser diodes, solid state lasers,
fiber amplifier lasers.
 Lasers operate in single or multiple longitudinal
modes.
 Single longitudinal mode
 laser emits radiation at a single frequency
 Multiple longitudinal mode
 multiple frequencies are emitted
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18. Link parameters
 Semiconductor laser diodes
 Reliable operations as direct sources
 Operating in 800-900 nm range
 High efficiency of about 50%
 Small size
 Output power
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19. Link parameters
 Solid state lasers
 Higher power levels, high peak power mode
 Operating at 1064 nm
 Increase in complexity and reliability
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20. Link parameters
 Types of link:
 Acquiston
 Acquiston time,false alarm rate,probability of detection
 Tracking
 Amount of error induced in the signal circuitry
 Communications
 Bit error rates
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21. Transmitter Parameters
 Laser characteristics, losses incurred in the
transmit optical path, transmit antennae gain,
transmit pointing losses.
 Laser characteristics
 peak and average optical power
 pulse rate
 pulse width
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22. Channel Parameters
 Consists of
 Range, associated loss
 background spectral radiance
 spectral irradiance
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23. Receiver Parameters
 The receiver parameters are the
 Receiver antenna gain
 proportional to the square of effective receiver diameter in meters
and inversely proportional to the square of the wavelength.
 Receive optical path loss
 optical transmission loss for systems employing the direct detection
techniques.
 Optical filter bandwidth
 the spectral width of the narrow band pass filter employed in optical
inter satellite links
 Receiver field of view
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24. Advantages of Laser Communication
Technology
 Higher data rates
 Compared to RF technology LC provides much higher data rates
 Higher data rates are essential as more and more data is moved between
diff. locations
 Key Driver for investments in Laser Communication Technology
 High security regarding interception
 A focused laser beam is hard to intercept without notice
 Path to Quantum Cryptography
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25. Advantages of Laser Communication
Technology
 Less frequency restrictions
 RF spectrum is crowed and heavily used
 Smaller aperture dimensions and thus reduced size and mass
 Less weight and power per bit
 Autonomous alignment agility resulting in less platform
manoeuvres
 Less fuel or more flexibility
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26. Applications for Laser
Communication
 Data Relay Services for UAVs
 UAV transmits its data to a GEO Stationary Satellite
 Data Relay Services for Satellites
 LEO Satellite transmit their data to a GEO Satellite
 Inter-Satellite Links
 Data Exchange between GEO/LEO Satellites
 Deep Space Data Transmissions
 Scientific data is transmitted down to Earth, e.g. Mars -> EarthGEO
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27. Situation
 Situation
 Demand for more information requires more and higher resolution
sensors/cameras on UAVs
 Data transmission becomes the limiting factor to acquire and distribute
information from UAV to Operation Center at diff. location
 RF solution reach data rate limits
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28. Problem, Need
 Problem
 Limited information is available to Operation Centers
 Information not available when needed as transmission time
is a bottle neck
 Real-time decision making not possible or only limited
possible
 Need
 Solution for higher data rate transmissions from UAV to
Operation Center of long distances (span continents)
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29. Conclusions
 With the dramatic increase in the data handling
requirements for satellite communication services, laser
inter satellite links offer an attractive alternative to RF
with virtually unlimited potential and an unregulated
spectrum.
 The system and component technology necessary for
successful inter satellite link exists today.
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30. 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.qsl.net/k7kw/DEMOS/LaserCommunications.ppt
 www.mseconference.org/.../mse03_2P_Uherek_Microop
toelectronicscurricula.pdf
 opticalcomm.jpl.nasa.gov/PAPERS/ATP/gospi03b.pdf
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