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FREE SPACE OPTICAL COMMUNICATION

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A Report by,Harish.RFinal year (UG) B.E, ECE- Dept, JJCET, Trichy-09Contact Mail: harishshaheb@gmail.com

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INTRODUCTIONLasers have been considered for space communications since their realization in 1960.It was soon recognized that, although the laser had potential for the transfer of data at extremely high rates, specific advancements were needed in component performance and systems engineering, particularly for space-qualified hardware. Advances in system architecture, data formatting, and component technology over the past three decades have made laser communications in space not only a viable but also a attractive approach to inter satellite link applications. The high data rate and large information throughput available with laser communications are many times greater than in radio frequency (RF) systems.

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The extremely high antenna gain made possible by the narrow beams enables small

telescope apertures to be used.

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OPERATIONFree space laser communications systems are wireless connections through the atmosphere. They work similar to fiber optic cable systems except the beam is transmitted through open spaceThe carrier used for the transmission of this signal is generated by either a high power LED or a laser diodeThe laser systems operate in the near infrared region of the spectrum. The laser light across the link is at a wavelength of between 780 – 920 nm.Two parallel beams are used, one for transmission and one for reception.

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FREE SPACE LASER COMMUNICATION

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Simulation Layout

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Modulation TechniquesModulation is simply a process of super imposing the modulating or low frequency

signal into the high frequency carrier signal.The optical laser light here act as the carrier wave .It does not contain any data with it.Here normally we employ Pulse Position Modulation or On-Off keying techniques for modulating the carrier wave.

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Radiation Pattern ( A comparison)

The antenna radiation pattern is the display of the radiation properties of the antenna

as a function of spherical co ordinates.

A typically radiation pattern is characterized by a main beam with 3 db beam width and

side lobes at different levels.

For an effective communication the minor lobes that is side lobe levels must be

minimized.

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R.P of RF antenna vs R.P of optical antenna

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Optical antenna produce non ionizing radiation

Ionizing radiations : These are the radiations which has enough ability to ionize an atom or a molecule when passing through matter where as non ionizing radiation do not.

Even though the optical antennas operating frequency range is in the range of few Hz ,the wavelength is reduced.

Since here laser is used for communication which falls under non ionizing radiation which is reflected from the body surface.

A question might arise that higher frequency component cause more harmless. But this is not actually the case

Each particle has its own absorbing and radiating properties. Human and animals body skin have tendency to reflect the incident light or optical energy where as plants absorbs most of the light energy.

Thus we clear that laser energy is no more harmful as RF.

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OPTICAL ANTENNAAn optical antenna is a device that efficiently couples the energy of free-space radiation to a confined region of sub wavelength size. While antennas are widespread in the radio wave and microwave regimes they are basically unexplored at optical frequencies.Because nano scale devices need to interface with optical radiation it is likely that optical antennas will have a broad impact on future technology. The light beam from an optical antenna is highly directed in nature since it uses coherent light form(Laser) for its transmission. Whereas the Electro-Magnetic waves from a RF antenna takes multiple path to reach its destination antenna.

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OPTICAL ANTENNA

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APPLICATIONS Temporary network installation for events or other purpose as disaster recovery For communications between spacecraft, including elements of satellite constellation Security applications Military application: (its potential for low electromagnetic emanation when transferring sensitive data for air forces) Enterprise connectivity: FSO links can be installed makes them for interconnecting local area network segments that are housed in buildings separated by public streets. Telecommunication and computer networking Point-to-point LOS links

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ADVANTAGES AND DISADVANTAGES High Directivity & Gain achieved.Larger Bandwidth of operation.High Information throughput is available.Increased Data rate.No need of trenching & digging lands.Minor lobe level reduced Data is highly secured.No E-M interference.

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CONCLUSION The implementation of any of these systems in an inter-satellite link will require a substantial development effort.Most of the recent large effort of digging up the ground and laying down new fiber has been directed towards extending the fiber optic backbone to new central offices, and not laying fiber directly to the customerThe growing requirements for the efficient and secure communications has led to an increased interest in the operational deployment of laser cross-links for commercial and military satellite systems in both low earth and geo-synchronous orbits. 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.

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REFERENCES IEEE communications Magazine. August 2000, free space laser communications: Laser cross-link systems and technology by: David L. Begley, Ball Aerospace & technologies corporationFree Space Optics or Laser Communication through the Air BY: Dennis Killinger Optics & Photonics NewsHigh data-rate laser transmitters for free-space laser Communications. BY:A. Biswas, H. Hemmati and J. R. Lesh Optical Communications Group Jet Propulsion Laboratory.

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