16-01-2013, 10:40 AM
RESEARCH REPORTS Free Space Optical Laser Communication Link
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ABSTRACT
A Free Space Optical (FSO) LASER Communication Link is presented. This project deals with the development of a full-duplex FSO
analogue / digital transceiver. In this information age, the demand for high speed, high bandwidth communications channel, is ever
increasing. FSO is presented as a solution to these demands in that it is free to implement, easy to install and of very high bandwidth.
The reader is introduced to the FSO system of communication and the development of a small scale communicator using laser as the
carrier signal for information transfer. Experimental results explain the performance of the completed system and offer methods of
maximizing efficiency of such FSO-based communication systems.
INTRODUCTION
An initial hurdle faced by early means of laser communication
was the enormous heat generated by pumped laser action.
However, in the late 1960’s, semiconductor laser was developed
and ever since, the possibilities for laser communication have
grown. Though developed for carriers, new laser technologies are
finding a place in private networks [1][2]. Recent breakthroughs
in wireless technology and the need for a wireless extension of
the Internet have increased the demand for faster, higher
bandwidth wireless access networks [3][4]. The two wireless
options nowadays are either radio or optical networks. Radio
frequency has been the primary medium of communication for a
long period of time. However, in this day and age, the RF
spectrum has become congested and may no longer be sufficient
for broadband high-speed applications [5][6]. In addition to this
radio communication requires the leasing of frequencies in order
to be legally permitted to use them. On the other hand, optical
communication is the key to supply the ever-increasing demand
for higher bandwidth, without the associated hassles or
interference experienced with radio communication
A FREE SPACE OPTICAL LASER COMMUNICATION
LINK
The free space optical laser communication link developed is
comprised of four circuits: the analogue transmitter and receiver,
the digital transceiver and the Voice Unit (VU) meter. This
combination of circuits will enable line-of-sight (LOS), fullduplex
voice and RS232-compatible data communication, over a
laser beam in free space.
System Overview
The completed system consists of two transceivers, each capable
of simultaneously transmitting and receiving either analogue or
digital information. The user has the option to manually select
between either analogue or digital information via means of two
switches, which control the laser transmitter and phototransistor
receiver. This translates to a system where, if necessary,
analogue information can travel in one direction while digital
information travels in the opposite direction. In normal operation,
the system is a full-duplex analogue/digital communicator. The
system overview is depicted in Figure 2.
The Transmitter Side
The transmitter side is made up of two dedicated circuits that
perform amplitude modulation of the laser diode when supplied
with either a digital or analogue input signal. The circuits are
designed with built-in protection for the laser diode in case the
input signal amplitude is too high. As a result, this portion of the
circuit is more complex than the receiver side.
The Receiver Side
The receiver side is again made up of two dedicated circuits that
perform the demodulation of the analogue signal from the
phototransistor. The digital section of the circuit incorporates the
MAX232 IC to generate the RS232-compatible signals necessary
for interfacing PC and the circuit. In addition to this, there is an
onboard VU meter designed to make the alignment process
easier and protect the listener from unexpected high pitch sounds
during the alignment process.
CONCLUSION
A full-duplex, combined analogue and digital FSO Laser
Communication Link was successfully designed and
implemented. It was also possible to obtain experimental data
from the designed system, which proved to be very useful and
informative. The results of testing and experiments created new
areas within this study’s scope that can still be explored.
Recommendations for future developments of the FSO laser link
are based mainly on alignment and focusing systems to improve
performance, in addition to the use of laser diodes specifically
designed for modulation.