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Laser Communication

Laser Communication. PRAVIN KUMAR 07EC75 05-04-2011. Organization. Introduction Basic principle LASER action Construction What is LASER Communication? Why Laser Communication? A Simple Laser comm. System 7.1. Transmitter

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Laser Communication

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  1. Laser Communication PRAVIN KUMAR 07EC75 05-04-2011

  2. Organization • Introduction • Basic principle • LASER action • Construction • What is LASER Communication? • Why Laser Communication? • A Simple Laser comm. System 7.1. Transmitter 7.2. Modulation 7.3. Receiver • Current Application • FSO 9.1. What is FSO? 9.2. Advantages of FSO 9.3. Last mile bottleneck 9.4. Signal Propagation Impedances • SUMMARY • References

  3. INTRODUCTION • LASER ? “LIGHT AMPLIFICATION BY SIMULATED EMMISSION OF RADIATION”

  4. BASIC PRINCIPLE • ABSORPTION • SPONTANEOUS EMISSION • STIMULATED EMISSION • POPULATION INVERSION

  5. LASER ACTION

  6. LIGHT PROPOGATION

  7. CONSTRUCTION

  8. Laser Communication ? • Laser communications systems - wireless connections. - work similarly to fiber optic links. - no fiber backbone required. - lasers transmitted through free space.

  9. Free Space Laser Communication • Transmitting information via a laser beam • Video • Data • Sound • Terrestrial / Space based systems 010001100110111011001111001010000010101110010001111001011011

  10. Why Laser Communication? • Current high speed communications technology: • Radio • Fiber Optics

  11. Why not Fiber Optics? • Not always possible to lay fiber lines • Satellites • Combat zones • Physically / Economically not practical • Emergencies • LC incorporated into fiber optic networks when fiber not practical.

  12. Why not RF? • Bandwidth • LC >> RF • transmit data @ 2.5-10 Gbps. • Power • LC directed at target. • Much less transmission power required. • Less free space power loss. • Size / Weight • LC antenna << RF antenna. • Security • LC more secure than RF - low divergence laser beam.

  13. Free Space Optic Link Equation: • Preceived = received power • Ptransmit = transmit power • Areceiver = receiver area • Div = beam divergence (in radians) • Range = link length

  14. Directional transmission:Narrow divergence of the FSO transmit path (shown in red) as compared to a typical Radio Frequency (RF) path (shown in blue).

  15. A Simple laser Comm. System Transmitter Laser Signal Signal Receiver laser

  16. High Level design Photo resistor MCU MCU Conditioning PORT A/D UART UART Conditioning Conditioning Laser Diode

  17. What is the Transmitter? • The transmitter involves: • Signal processing electronics (analog/digital) • Laser modulator • Laser (visible, near visible wavelengths) • Transmit antennae gain, transmit pointing losses. • Laser characteristics • - peak and average optical power • - pulse rate • - pulse width

  18. Laser Diode Laser Diodes include Photodiodes for feedback to insure consistent output.

  19. Modulation • AM • Easy with gas lasers, hard with diodes • PWM (Pulse Width Modulation) • PFM (Pulsed FM) • Potentially the highest bandwidth (>100kHz)

  20. What is the Receiver? • The receiver involves: • Telescope (referred to as ‘antenna’) • Signal processor • Detector -PIN diodes -Avalanche Photo Diodes (APD) -Single or multiple detectors • Often both ends - equipped with a receiver and transmitter

  21. AVALANCHE PHOTO DIODE Stabilisation of working point of APD: . Gain =75 Temperature stabilisation. Thermoelectrically cooler stabilisation system inside of APD module

  22. Current Applications • Defense and sensitive areas. • FSO Communication. • airports - communication across the runways. • Mass communication • 400 TV channels • 40,000 phone conversations • NASA • Satellite – satellite • Earth - satellite Earth

  23. FSO • line-of-sight technology. • uses LASERS and Photo detectors. • optical connections between two points—without the fiber. • FSO units consist -optical transceiver with a laser (transmitter) -Photo detector (receiver) -provide full duplex capability.

  24. ADVANTAGES OF FSO SYSTEMS No licensing required. Very low installation cost. No sunk costs. No capital overhangs. Highly secure transmission possible. High data rates @ 2.5 -10 Gbps.

  25. LAST MILE BOTTLENECKS

  26. Signal Propagation Impediments: • Absorption: - suspended water molecules extinguish photons. • Scattering: - wavelength collides with the scatterer. - Rayleigh scattering (Lambda scatterer << wavelength) - Mie scattering (Lambda scatterer ≈ wavelength)

  27. Physical obstructions: Flying birds • Safety: - technology uses lasers for transmission. • Scintillation: - fluctuations in signal amplitude. - heated air rising from the earth or man made devices.

  28. Summary • Basic principle of laser action discussed. • Laser communication system used in satellite communication. • Provide higher data rates , high security & lesser antenna size. • FSO used for lesser link length ~ 4km. • FSO links –designed carefully due to safety issues.

  29. 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.mseconference.org/.../mse03_2P_Uherek_Microoptoelectronicscurricula.pdf • opticalcomm.jpl.nasa.gov/PAPERS/ATP/gospi03b.pdf Technical Seminar-II

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