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Laser Communication PRAVIN KUMAR 07EC75 05-04-2011 Organization 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 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 INTRODUCTION LASER ? “LIGHT AMPLIFICATION BY SIMULATED EMMISSION OF RADIATION” BASIC PRINCIPLE ABSORPTION SPONTANEOUS EMISSION STIMULATED EMISSION POPULATION INVERSION LASER ACTION LIGHT PROPOGATION CONSTRUCTION Laser Communication ? • Laser communications systems - wireless connections. - work similarly to fiber optic links. - no fiber backbone required. - lasers transmitted through free space. Free Space Laser Communication Transmitting information via a laser beam ◦ Video ◦ Data ◦ Sound Terrestrial / Space based systems 010001100110 111011001111 001010000010 101110010001 111001011011 Why Laser Communication? Current high speed communications technology: ◦ Radio ◦ Fiber Optics 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. Why not RF? Bandwidth Power Size / Weight Security ◦ LC >> RF ◦ transmit data @ 2.5-10 Gbps. ◦ LC directed at target. ◦ Much less transmission power required. ◦ Less free space power loss. ◦ LC antenna << RF antenna. ◦ LC more secure than RF - low divergence laser beam. Free Space Optic Link Equation: Preceived = received power Ptransmit = transmit power Areceiver = receiver area Div = beam divergence (in radians) Range = link length Directional transmission: Narrow divergence of the FSO transmit path (shown in red) as compared to a typical Radio Frequency (RF) path (shown in blue). A Simple laser Comm. System Signal Transmitter Laser Signal Receiver laser MCU UART Conditioning Laser Diode MCU PORT Conditioning A/D High Level design UART Conditioning Photo resistor What is the Transmitter? • The transmitter involves: o Signal processing electronics (analog/digital) o Laser modulator o Laser (visible, near visible wavelengths) • Transmit antennae gain, transmit pointing losses. • Laser characteristics - peak and average optical power - pulse rate - pulse width Laser Diode Laser Diodes include Photodiodes for feedback to insure consistent output. Modulation AM ◦ Easy with gas lasers, hard with diodes PWM (Pulse Width Modulation) PFM (Pulsed FM) ◦ Potentially the highest bandwidth (>100kHz) What is the Receiver? • o o o 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 AVALANCHE PHOTO DIODE Stabilisation of working point of APD: . Gain =75 Temperature stabilisation. Thermoelectrically cooler stabilisation system inside of APD module Current Applications • Defense and sensitive areas. • FSO Communication. • airports - communication across the runways. • Mass communication 400 TV channels o 40,000 phone conversations o • NASA Satellite – satellite o Earth - satellite o Earth 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. 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. LAST MILE BOTTLENECKS 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) 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. 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. 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_Microoptoele ctronicscurricula.pdf opticalcomm.jpl.nasa.gov/PAPERS/ATP/gospi03b.pdf 5/6/2017 Technical Seminar-II 30