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INTRODUCTION
Communication technology has experienced a continual development to higher
and higher carrier frequencies, starting from a few hundred kilohertz at
Marconi's time to several hundred tera-hertz since we employ lasers in fiber
systems
...
Another
asset comes into play in free-space point-to-point links
...
The jump from microwaves to light
waves therefore means a reduction in beam width by orders of magnitude, even
if we use transmit antennas of much smaller diameter
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Lasers have been considered for space communications since their realization in
1960
...
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 small antenna size requires only a small
increase in the weight and volume of host vehicle
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The smaller antennas, with diameters typically less than 30cm, create
less momentum disturbance to any sensitive satellite sensors
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The
narrow beam divergence of affords interference-free and secure operation
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The carrier used for
the transmission signal is typically generated by a laser diode
...
Laser
communications systems are wireless connection through the atmosphere
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In Laser Communication the transmitter
and receiver must require a line-of-sight conditions and Laser communications
systems have the benefit of eliminating the need for broadcast rights and buried
cables
...

The carrier used for the transmission signal is typically generated by a laser
diode
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METHODOLOGY
Features of the Laser Communication System
A laser communications system basically consist of six key system
characteristics or parameters which when quantified, together gives a detailed
description of a laser communications system
...
The critical parameters are grouped into
five major categories:

i
...


Transmitter parameters

iii
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Receiver parameters

v
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Link Parameters
The link parameters include the type of laser, wavelength, type of link, and
the required signal criterion
...
Laser sources are described as operating in either in
single or multiple longitudinal modes
...
Semiconductor
lasers have been in development for three decades and have only recently
(within the past 7 years) demonstrated the levels of performance needed for
the reliable operation as direct sources typically operating in the 800-900 nm
range(gallium arsenide/gallium aluminum arsenide)their inherently high
efficiency(50%)and small size made this technology attractive
...

Solid state lasers have offered higher power levels and the ability to operate
in high peak power modes for the acquisition
...
A variety of materials have been proposed for laser
transmitters: neodyminium doped yttrium aluminum garnet (Nd: YAG) is
the most widely used
...
With
the rapid development of terrestrial fibre communications, a wide array of
components is available for the potential applications in space
...

Operating at 1550nm erbium doped fibre amplifiers have been developed for
commercial optical fibre communications that offer levels of performance
consistent with many free space communications applications
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The major
differences between the link types are reflected in the required signal
criterion for each
...
For the tracking link the key
considerations are the amount of error induced in the signal circuitry
...
For the
communications link, the required data and the bit error rates are of prime
importance
...
The key laser characteristics include peak and average
optical power, pulse rate and pulse width
...
Transmit optical path loss is
made up of optical transmission losses and the loss due to the wave front
quality of the transmitting optics
...
The optic
transmit antenna gain is analogous to the antenna gain in the RF systems and
describes the on axis gain relative to an isotropic radiator with the
distribution of the transmitted laser radiation defining the transmit antenna
gain
...
The reduction in
the far field signal strength due to the transmitter pointing is the transmitter
pointing losses
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Channel Parameters
The channel parameters for an optical inter-satellite link(ISL) consist of
range and associated loss ,background spectral radiance and spectral
irradiance
...


Receiver Parameters
The receiver parameters are the receiver antenna gain, the receive optical
path loss, the optical filter bandwidth and the receiver field of view
...
The receiver optical path loss is simply the optical transmission
loss for systems employing the direct detection techniques
...
The preservation of the wave front quality is
essential for the optimal mixing of the received signal and the local oscillator
fields on the detector surface
...
Optical filters reduce the amount of unwanted background
entering the system
...
The minimum width will be
determined by the acceptable transmission level of the filter
...
To
maximize the rejection, the FOV should be as small as possible
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The
minimum FOV is limited by optical design constraints and the receiver
pointing capability
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For optical ISL
systems based on semiconductor laser diodes or Nd:YAG lasers, the detector
of choice is a p type intrinsic n type (PIN) or an avalanche photodiode(APD)
APIN photo diode can be operated in the photovoltaic or photoconductive
mode and has no internal gain mechanism
...
The quantum efficiency of the detector is the
efficiency with which the detector converts the incident photons to electrons
...
By definition the quantum efficiency is always less than
unity
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Typically in a detector there is a DC current even in the absence of signal or
background
...
In an APD there are two contributors to this
DC dark current; a multiplied and an un multiplied current
...
Being the first element past the detector, the noise due to the
preamplifier can have a significant effect on the systems sensitivity
...


Principle of Operation of the Laser Communications System
Laser Communications System mainly comprises of 2 sections such as
Transmitter section and Receiver section
...

The receiver section is used to receive the laser beam, using photo transmitter,
which incorporated with the data or sound signals from the transmitter
comprised of Conditioning, MCR and Digital to Analog converter to extract the
data signals from the received laser beam and given as a input to the speaker
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For this we used an electric microphone
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The signal coming off the mic was far too low to be read (with any
degree of precision) by the analog to digital converter
...
Before the signal is put
through the amplifier, first put the signal through a capacitor to remove DC, and
then through a voltage divider to appropriately bias the signal
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UART (Universal Asynchronous Receiver/Transmitter): It is a piece of
computer hardware that translates data between parallel and serial forms
...
The
universal designation indicates that the data format and transmission speeds are
configurable and that the actual electric signaling levels and methods typically
are handled by a special driver circuit external to the UART
...
As soon as data is deposited in the
shift register after completion of the previous character, the UART hardware
generates a start bit, shifts the required number of data bits out to the line,
generates and appends the parity bit(if used), and appends the stop bits
...

Laser Driver: After the A/D converter translates the mic signal into 8 bits, the
MCU generates the appropriate bits to send (including start and stop bits) and
applies them to the laser driver circuit a 5V and 0V signals
...

Receiver: A photo diode detects the laser pulses in a different (distant) location
...


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Laser Driver: Once the signal is put through the DAC, it is boosted and low
pass filtered (to improve sound quality)
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The transmitting satellite should transmit the
narrowest possible beam for maximum power concentration
...
The
pointing error ultimately decides the minimal beam size
...
This depends on the accuracy to which one satellite knows
the location of the other, the accuracy with which it knows its own orientation
in space and the accuracy to which it can aim its beam, knowing the required
direction
...
Determination of the satellite
location can be aided by using an optical beacon transmitted from the receiving
antennae back to the transmitting satellite
...
The uncertainty in absolute satellite location is transferred
to smaller uncertainty in reading beacon arrival direction
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When the beams are
extremely narrow there is a possibility that the receiving satellite may have
moved out of transmitters beam width during the round trip transmission time
...

α=Vt /150 μ radians
Where α is the point ahead required and
Vt is the tangential velocity of the satellite in m\s
...
This
means that the transmitting laser cannot transmit back through the same optics
from which the beacon is received
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The use of a beacon modifies the optical hardware on each satellite,
since the transmitting and receiving satellite must contain both a transmitting
laser and a optical receiver
...
The modulated laser beam can
serve as a beacon for the return direction
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Separate
wavelengths are used for optical beams in each direction
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If point ahead is needed then command control
(either stored or received from the earth station) must adjust transmitting
direction relative to receiving direction
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Optical Noise
Noise characteristics play an important role in laser communication systems
...
In the RF domain, quantum noise is quite low, while thermal
noise predominates and does not vary with frequency in the microwave region
...
Because there is so little energy in a photon at radio
frequencies, it takes many problems to equal the thermal noise
...
However, in optical receivers employing direct
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detection and avalanche photodiodes, the detection process does not approach
the quantum limit performance
...
Free space optical communication links, atmospheric turbulence
causes fluctuations in both the intensity and the phase of the received light
signal, impairing link performance
...
In homogeneities in the temperature and pressure of the
atmosphere lead to variations of the refractive index along the transmission
path
...
These fluctuations can lead to an increase in the link error
probability, limiting the performance of communication systems
...
The primary
background noise is the sun
...

Atmospheric scattered sunlight, sunlit clouds, the planets, the moon, and the
Earth background have similar radiances; the sun’s radiance is much higher and
a star field’s much lower
...
If one were able to look only at an individual star, one would
find brightness similar to that of the sun; but a star field as a whole is composed
of small point sources of light, the stars in the field, against a dark area having
no background level
...
For direct detection systems,

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narrow field of view spectral filters on the order of 20A*(2 nm) are typical
...
However, some systems can be signal-quantum-noise
limited, rather than background-limited, without having to resort to heterodyne
detection
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As the equipments operate within the near
infrared spectrum, they are not subject to government licensing and no spectrum
fees have to be paid (it requires only the use of the frequency spectrum below
3’000 GHz a licence)
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The narrow laser beam width
precludes interference with other communication systems of this type
...
Laser communication systems offer many
advantages over radio frequency (RF) systems
...
RF wavelengths are thousands of times longer than those at
optical frequencies are
...
First, the beam-width attainable with the laser
communication system is narrower than that of the RF system by the same ratio
at the same antenna diameters (the telescope of the laser communication system
is frequently referred as an antenna)
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Taking advantage of this brighter
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beam or higher gain, permits the laser communication designer to come up with
a system that has a much smaller antenna than the RF system and further, need
transmit much less power than the RF system for the same receiver power
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Advantages of laser communications system
i
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ii
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So it overtakes the LAN or
wireless LAN comprehensively
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Laser communications systems can be easily deployed since they are
inexpensive, small, low power and do not require any radio interference
studies
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Two parallel beams are needed, one for transmission and
one for reception
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The transmitting and receiving station are smaller and lighter for given
range
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Higher data rate may be achieved for given distance and power output
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A tiny light detector may allow for superfast broadband communication
over interplanetary distances
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Disadvantages of laser communications system
i
...

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ii
...


iii
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Laser communications systems provide only interconnection between
points that have direct line-of-sight
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Occasionally, short interruptions or unavailability events lasting from
some hours up to a few days can occur
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The systems are protocol
transparent allowing transmission of digital computer data (LAN interconnect),
video, and voice over IP, multiplexed data, or ATM
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Some of the
applications of this system include;

i
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ii
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iii
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Direct
communication between high-rise buildings in a crowded city
would become easy
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85
μm), up to those employing diode-pumpedNd:YAG lasers (λ = 1
...
5 μm
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The strengths and weaknesses of the various types of lasers
presently available for laser communications should be carefully considered
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The system and component technology necessary for successful
inter satellite link exists today
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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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One should also give serious thoughts to
use the large, mature, and reliable technology base commercially available in
the 1
...
Only then one can hope to achieve economy in medium-scale
applications like inter-satellite networks

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