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Free Space Point to Point Laser and Optical Communications M. Liu and Dr. S. L. Hill M.Liu@pgt.salford.ac.uk C NTR (Centre for Networking and Telecommunications Research) School of Acoustics and Electronic Engineering University of Salford Salford UK M5 4WT ABSTACT ADVANTAGES In this paper a three-year research programme investigating the climatic effects on long-term The main benefit of point-to-point laser data transmission using high bandwidth lasers connection is that bandwidth is dedicated is described. This work involves the theoretical between the points, so this can make high- analysis and the implementation of experiments speed communication possible between to investigate the effects of weather conditions different places on earth. Small beam on the data transmission. Design solutions divergence, small size, and large information resulting from this will be tested to compensate bandwidth due to operation at a higher for distortions encountered during the frequency are all advantages of a laser transmission according to the results from the system. Additionally, the advantages of light experiments. This paper describes possible weight, small volume, and lower power effects and possible solutions; however, the consumption provides laser communication a results from the experiments are not yet ready potential edge over RF communication. The to be analysed. laser communication also has no FC requirement, is easy to install and can be safer. INTRODUCTION ATMOSPHERIC EFFECTS  Rapidly growing use of the Internet and multimedia services has created congestion in Absorption and Scattering: Absorption is the telecommunications networks and placed caused primarily by the water vapour (H2O) many new requirements on carriers. Customers and carbon dioxide (CO2) in the air along the require rapid service so they can obtain high- transmission path. Gases in the atmosphere capacity links as needed. Laser transmitters have many resonant bands, called offer an immediate, low-risk way to introduce transmission windows, which allow specific desired network functionalities with high frequencies of light to pass through. These bandwidth. The atmospheric distortion windows occur at various wavelengths. encountered when a laser beam transmits Absorption is not generally a big concern in through the air produces errors and affects the an infrared laser transmission system. quality of communication. However, space Scattering has a greater effect than absorption. and ground based optical communications offer The atmospheric scattering of light is a potential advantages in bandwidth over function of its wavelength and the number traditional RF communications and and size of scattering elements in the air. The conventional microwave technology. most common scattering elements in the air that affect laser beam transmission are fog and smog, rain, and snow. times less than that of fog. The effects of Weather: The biggest issue for free space laser snow on a laser transmission fall somewhere communication is fog. While the laser beam in between those of fog and rain, depending transmits through fog, the moisture particles are on the degree of water particles in the snow. so small and dense and like many tiny prisms to distort and dissipate the signal. This problem Shimmer: This is the direct result of a makes the light with information distort and combination of factors, including atmospheric produces an error bit. Although the liquid turbulence, air density, light refraction, cloud content of a heavy shower is 10 times that of a cover, and wind. The combination of factors dense fog, the radius of a raindrop is about will cause a similar disturbance when a laser 1000 times that of a fog droplet. This is the beam is transmitted through the atmosphere. primary reason that attenuation via rain is 100 W0 is initial wavelength of the laser beam W is the wavelength of the laser beam transmitting through the moisture particle N is the refraction ratio of the moisture particle Moving building: This is because of the natural movements of buildings. Although we are not aware of the movement, buildings often sway from side to side even settle into the ground. This problem can offset the laser beam and make the receiver laser receive less power. LABORATORY TEST APPARATUS, PROCEDURE INITIAL TEST SETUP Test environment REFERENCES This test installation is conducted inside the  Brian R. Strickland, M. J. L., Eric campus to collect the data information using Woodbridge, and Victor Chan (1999). Effects HP ProBER 2. HP ProBER 2 test set provides a of fog on the bit-error rate of a free-space powerful hand-held solution for testing 2 Mb/s laser communication system. Applied optics. and 64 kb/s digital circuits. It offers extensive 38: 3: 424-431. BER (bit error rate) test functions plus a unique range of signal quality measurements. HP  Roberta A. Ewart, M. E. (2000). Free ProBER 2 can rapidly identify signal quality space laser communictions. IEEE problems such as frequency, level, pulse shape Communictions Magazine: 124-125. or jitter problem. The laser transmission system is based on two Cablefree Lasers. They  Bates, R. J. Wireless Network are manufactured by Cablefree Solution Communications: concepts, technology, and Limited. The model is Cablefree 500 that can implementation. ISBN: 0-07-004674-3 be conducted greater than 500m ranges at 155Mbps, and data rates supported are 64kbps  Tyson, R. K. (1996). Adaptive optics to over 622Mbps. The transmitter and the and ground-to-space communications. receiver laser are exactly same. The receiver is Applied optics. 35: 3640-3646. connected by wire to make a loopback so that all data can be collected at the transmitter site.  V. A. Skormin, M. A. T., T. E. Busch HP ProBER 2 is performing data logging and (1997). Demonstration of a jitter rejection then the data is stored in the computer to technique for free-space laser communication. waiting for analysis. The weather monitor IEEE TRANSACTIONS ON AEROSPACE provides sophisticated monitoring and logging AND ELECTRONIC SYSTEMS. 33: 568- of essential weather conditions such as inside 575 and outside temperature, barometric pressure, wind direction, wind speed and wind chill corresponding to the transmission error data acquired at the time. PROPOSED SOLUTIONS Although there are no immediate solutions to the absorption problems other than the use of light intensification the use of adaptive optics systems similar to those employed in astronomy may provide sufficient wavefront correction to maintain high bandwidths in situation of bulk atmospheric movements. These ideas will be investigated more fully when full details of the atmospheric effects on the error rates has been analysed. .
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