Laser Communications


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

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Laser Communications

  1. 1. Introduction • Laser communications systems are wireless connections through the atmosphere. • Use Laser Beams to transmit information between two locations • No fibers 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.
  2. 2. How does it Work ? Signal Transmitter Laser Receiver Signal Visit to download
  3. 3. Laser Transmitter and Receiver Laser Transmitter Receiver Optical fiber link Direct Link
  4. 4. One-way Laser communication system
  5. 5. Laser Transmitter • 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.
  6. 6. Receiver The receiver involves: • Telescope(‘antenna’) • Signal processor • Detector  PIN diodes  Avalanche Photo Diodes(APD)  Single or multiple detectors
  7. 7. Modulation • AM  Easy with gas lasers, hard with diodes • PWM • PFM  Potentially the highest bandwidth(>100kHz)
  8. 8. Gain Systems Transmitter  Maximum output power  Minimum divergence Receiver  Maximum lens area  Clarity  Tight focus on detector
  9. 9. 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
  10. 10. 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.
  11. 11. • Thus system is set up to send voice data. • A person's voice gets put into a conditioning circuit so that the full eight bit range of the analogue to digital converter is utilized. • Once the digital signal is obtained by the ADC, the MCU passes the signal to the uart.
  12. 12. • The UART sets a transmit pin high or low according to the serial protocol. • Some conditioning is applied to this signal as well in order to ensure constant current to the laser. • On the receiver side, the signal is read by a photo transistor and basically the signal goes through an opposite sequence to output a sound instead of receiving one and using the DAC will change the digital signal to an analog one.
  13. 13. • 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. Why not Fiber Optics?
  14. 14. • 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. Why not RF?
  15. 15. Applications ISP (Internet Service Provider) Industrial Use
  16. 16. Applications • Defense and sensitive areas. • At airports for communication across the runways. • Mass communication • Free-space optical communication • Space probe are being designed to use optical rather than radio communication. • Laser communication has also been demonstrated on aircraft and high altitude platforms.
  17. 17. Lunar Atmosphere and Dust Environment Explorer (LADEE) • Lunar Laser Communication Demonstration (LLCD) equipment on LADEE set a space communication bandwidth record in October 2013. • Early tests using a pulsed laser beam to transmit data over the 385,000 kilometres (239,000 mi) between the Moon and Earth. • Passed data at a "record-breaking download rate of 622 megabits per second (Mbps)“ • Demonstrated an error-free data upload rate of 20 Mbps from an Earth ground station to LADEE in Lunar orbit.
  18. 18. 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 interruption of the data transmission. • The existence of laser beams cannot be detected with spectrum analyzers.
  19. 19. 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(s)[citation needed] • No Fresnel zone necessary
  20. 20. 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
  21. 21. Conclusion • 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.
  22. 22. Thank YouVisit to download