Free Space Point to Point Laser and Optical Communications by hcj


									Free Space Point to Point Laser and
Optical Communications
M. Liu and Dr. S. L. Hill
  NTR (Centre for Networking and Telecommunications Research)
School of Acoustics and Electronic Engineering
University of Salford

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
                                                   ATMOSPHERIC EFFECTS [3]
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.[1] 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.



                                            Test environment

This test installation is conducted inside the       [1]     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             [2]     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.[5] The laser transmission
system is based on two Cablefree Lasers. They        [3]   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         [4]    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.   [5]    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.


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.[4]
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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