Laser Communication

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Laser Communication

• Presented by,
• P.Anitha(08MCS204)

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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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How does it Work
Signal
Transmitter
Laser
Receiver
Signal
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Laser Transmitter and Receiver
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A one-way Laser communication system.
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Laser Transmitter

• The transmitter involves
• Signal processing electronics(analog/digital)
• Laser modulator
• Laser( Visible, near visible wavelengths)

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Modulation

• AM
• Easy with gas lasers,hard with diodes
• PWM
• PFM
• Potentially the highest bandwidth(gt100kHz)

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Receiver

• The receiver involves
• Telescope(antenna)
• Signal processor
• Detector
• PIN diodes
• Avalanche Photo Diodes(APD)
• Single or multiple detectors

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Gain Systems

• Transmitter
• Maximum output power
• Minimum divergence
• Receiver
• Maximum lens area
• Clarity
• Tight focus on detector

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Laser Diode

• Laser Diodes include Photodiodes for feedback to
  insure consistent output

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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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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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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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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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Laser Communication System
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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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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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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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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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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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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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Channel Parameters

• Consists of
• Range, associated loss
• background spectral radiance
• spectral irradiance

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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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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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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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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 -gt EarthGEO

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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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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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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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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_Microopt
  oelectronicscurricula.pdf
• opticalcomm.jpl.nasa.gov/PAPERS/ATP/gospi03b.pdf