i) Optical Communication through the Atmosphere
Compared
to
radiowave
or
microwave
systems
optical
communication
systems
through
the
at
-
mosphere
are
characterized
by
a
higher
transmission
bandwidth
and
a
much
higher
direction
-
ality
of
the
radiation.
But
gener
ally
they
also
suffer
from
stronger
damping
losses
and
turbulence
effects in the atmosphere.
Therefore,
optical
systems
are
favorably
applied
for
high
data
transmission
over
shorter
point-to-
point
connections,
e.g.,
between
urbanic
computer
centers
or
factory
buildings,
and
on
the
other
side they have proven for high-performance data transfer between satellite links in space.
The fundame
ntals of Optical Communications are composed in these lecture notes:
Set-Up of an Optical Communication System
The
basic
components
of
an
optical
communication
system
can
be
represented
by
this
block-
diagram.
The
transmitter
is
a
cw
or
pulsed
laser,
dependent
on
the
applied
modulation
type,
and
is
con
-
troll
ed
by
an
optical
modulator
or
an
analog-digital
converter
to
transfer
an
input
information
on
the
laser
carrier
frequency.
For
the
further
transmission
and
detection
similar
components
are
used as for a range finder.
Within
their
master
theses
students
were
developing
and
testing
different
optical
communica
-
tion
systems,
mainly
using
pulsed
semiconductor
lasers
in
combination
with
pulse-gated-binary-
modulation.
These
systems
were
successfully
designed
and
applied
for
medium
bandwidths
with
parallel transmission channels and tested over transmission distances up to 5 km.
Propagation of Visible and Infrared Radiation through the Atmosphere
For
longer
distances
not
only
transmission
losses
due
to
absorption
but
also
Rayleigh-
and
Mie-
scattering
as
well
as
turbulence
effects
have
to
be
considered,
which
are
strongly
dependent
on
the laser wavelength as well as on the environmental and weather conditions.
In
addition,
scattering
and
turbulence
effects
have
a
direct
influence
on
the
long-term
signal
qua
-
lity and
error rate.
To
quantify
the
size
of
all
these
effects,
extensive
calculations
for
different
configurations
and
modulation techniques were performed.
Radiation
losses
on
the
laser
wavelength
due
to
molecular
absorption,
scattering
and
diffraction
are
simulated
with
our
program
platform
MolExplorer
(see
next
section),
which
uses
the
HITRAN-
or
GEISA-database
with
almost
6
Mio
spectral
lines
of
all
relevant
components
and
trace
gases
in
the
atmosphere,
and
which
directly
calculates
the
transmitted
power
from
the
transmitter
with
cross-sectional
area
A
S
to
the
receiver
with
area
A
E
.
Dependent
on
the
propagation
direction
and
path
length
through
the
atmosphere
the
program
accounts
for
the
changing
molecular
density
and tempe
rature with altitude above ground.
The
upper
Figure
shows
an
example
for
an
Earth-Satellite
data
transmission
to
the
geo-stationary
orbit
in
36,000
km
height.
A
secure
data
exchange
is
only
possible
within
the
optical
windows
a
-
round
0.85
µm,
1.1
µm,
1.24
µm
and
1.6
µm.
Outside
these
windows
the
molecular
absorption
is
strongly increasing, primarily caused by water vapor.
Also
the
attenuation
in
the
different
windows
is
increasing
with
shorter
wavelength
due
to
scat
-
tering
and
turbulence,
but
partially
compensated
by
lower
diffraction
losses.
For
this
simulation
a
transmitting
and
receiving
area
of
A
S
=
A
E
=
0.03
m
2
(cross-section
=
20
cm)
is
assumed,
which
al
-
most causes the same diffraction as the beam expansion by turbulences.
Under
these
conditions
free-space
damping
and
turbulence
with
-62
dB
at
a
wavelength
of
1.06
µm
gives
the
main
contribution
to
the
transmission
losses,
while
scattering
at
medium
visibility
(V = 10 km) causes additional -16 dB.
With
a
Nd
+
:YAG
laser
at
wavelength
of
1.064
µm,
an
average
output
power
of
4
W,
a
pulse
rate
of
1
GBit/s
and
using
pulse-code-modulation,
under
clear
sky
conditions
a
signal/noise
ratio
of
20
dB
is
achievable
and
still
allows
communication
even
at
cirrostratus
overcast
in
15
km
altitude,
but
at
stronger
cloudiness,
e.g.
altostratus
with
an
attenuation
of
-20dB/km
completely
breaks
down.
References
H. Harde
Optical Communications
Script of Advanced Course, Helmut-Schmidt-University Hamburg, 2008
H. Harde
Laser Communication through the Atmosphere and Optical Fibers
Lecture, Carl-Cranz-Gesellschaft, Hamburg, 1988
H. Harde
Transmitters for Optical Communications
Lecture, Carl-Cranz-Gesellschaft, Hamburg, 8. März 1990
H. Harde
Laser Transmission from GEO-Sat to Earth
Research Study and Lecture, Helmut-Schmidt-University, funded by EADS Astrium, 01.Feb. 2011
H. Harde, J. Pfuhl
MolExplorer: A Tool for Computation and Display of Molecular Spectra from the HITRAN and GEISA Database
Helmut-Schmidt-University, 2018