Space Laser Communications: Systems, Technologies, and Applications
#1

Abstract:
Laser communication links in space are attractive alternatives to present-day microwave links. This tutorial
describes the basic concept and the functions of an optical terminal on board a spacecraft. It points out the differences
between free-space optical links on one hand and glass fiber systems and microwave directional links on the other
hand. The requirements on data transmitters and receivers as well as on optical antennas and pointing, acquisition and
tracking mechanisms are discussed. Typical application scenarios are outlined, experimental systems and their
technologies are cited.
Key Words: laser communications, free space, intersatellite links, space communication, space networks
1. 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 terahertz since we
employ lasers in fiber systems. The main driving force
was that the usable bandwidth - and hence
transmission capacity - increases proportional to the
carrier frequency. Another asset comes into play in
free-space point-to-point links. The minimum
divergence obtainable with a freely propagating beam
of electromagnetic waves scales proportional to the
wavelength. The jump from microwaves to light
waves therefore means a reduction in beamwidth by
orders of magnitude, even if we use transmit antennas
of much smaller diameter. The reduced beamwidth
does not only imply increased intensity at the receiver
site but also reduced cross talk between closely
operating links and less chance for eavesdropping.
Space communication, as employed in satellite-tosatellite
links, is traditionally performed using
microwaves. For more than twenty five years,
however, laser systems are being investigated as
alternatives. 1-3) One hopes that mass, power
consumption, and size of an optical transceiver
module will be smaller than that of a microwave
transceiver. Also, fuel consumption for satellite
attitude control when quickly re-directing antennas
should be less for optical antennas. On the other hand,
a new set of problems had to be addressed in
connection with the extreme requirements for
pointing, acquiring, and tracking the narrow-width
laser beams.
In this tutorial we will first discuss the basics of an
optical free space page link (Sect. 2) and then point out the
differences to terrestrial fiber systems and to
microwave links in Sect. 3. Section 4 presents the
requirements for and the available technologies to
implement transmitters, receivers, optical antennas, as
well as the PAT system (PAT...pointing, acquisition,
and tracking). Next we sketch application scenarios,
and we conclude with both a glimpse onto past and
future system technologies.
2. System Layout
A scenario typical for the transmission system in
question asks for point-to-point data transfer between
two spacecraft (see Fig. 1). The distances to be
bridged may extend anywhere from a few hundred
kilometers to 70 000 km (e.g. in near-earth
applications) up to millions of kilometers in case of
signals transmitted by a space probe.4) Today the data
rates in mind range from several hundred kbit/s to
some 10 Gbit/s.
Terminals for optical communication in space are
mostly designed for bi-directional links, at least
concerning the optical tracking function. They
comprise both a transmitter and a receiver that
generally share the optical antenna. Another
peculiarity is the necessity of beam steering (or
pointing) capability with sub-microradian angular
resolution and possibly with an angular coverage
exceeding a hemisphere.
These requirements lead to a transceiver block
diagram as shown in Fig. 2. The light source S is a
laser, preferably operating in a single transverse mode
in order to achieve the highest possible antenna gain.
If the laser operates continuously or in a pulsed mode
producing a periodic pulse train, an external modulator
(M) is utilized to impress the data signal onto the
beam. Alternatively, internal modulation may be
employed with some lasers. The modulated beam
passes an optical duplexer (DUP) and a fine pointing
assembly (FPA) before it enters a telescope acting as
Fig. 1 A scenario of laser communication links in
space.
Fig. 2 Block diagram of optical transceiver for
space-to-space links (S..laser source,
M..modulator, DUP..optical duplexer,
FPA..fine pointing assembly,
ANT..antenna, CPA..coarse pointing
assembly, PAA..point ahead assembly,
BS..beam splitter, DD..data detector,
DE..data electronics, ATD..acquisition and
tracking detector, ATE..acquisition and
tracking electronics).
transmit antenna (ANT). The telescope increases the
beam diameter and thus reduces the beam divergence.
A coarse pointing assembly (CPA) provides for
steering the antenna.
The received radiation also passes the antenna and
the fine pointing assembly, and is then directed to the
receive part of the terminal with the aid of the
duplexer. A beam splitter (BS) directs one part of the
received beam to the data detector (DD) for
demodulation and further signal processing in the data
electronics unit (DE). Another part of the received
power is used for controlling the fine and coarse
pointing mechanisms in such a way that the
acquisition and tracking detector (ATD) is always hit
centrally. A point-ahead assembly (PAA) has to be
inserted in either the transmit path or the receive path
to allow electronic control of the internal angular
alignment between transmission and reception (see
Sect. 3.2).
It should be stressed that the block diagram of Fig.
2 shows only a basic outline and that it may be
modified in several respects. Among such
modifications are:
– the provision of separate laser sources to generate
extra beams for acquisition and for tracking
(beacon lasers),
– separate antennas for the outgoing and the
incoming beam,
– means to deliberately increase the divergence of
the beam used as beacon in order to illuminate
the opposite terminal during the acquisition
process,
– the provision of separate photodetectors for
acquisition and for tracking, or the use of a single
photodetector for data detection, acquisition, and
tracking,
– the installation of an optical booster amplifier to
increase the output power.
In any case, the task of engineering a laser terminal
may be divided into three major complexes, namely
– one covering the data transmission aspects,
– one providing for pointing, acquiring and
tracking (PAT) the very narrow laser beams,
– and one of designing space-qualifiable optomechanical
structures and proper interfacing with
the spacecraft platform.
While each of them requires a sophisticated concept, it
should be stressed here that the problems associated
with PAT are generally underestimated.

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http://publik.tuwien.ac.at/files/pub-et_4235.pdf
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#2
[attachment=14699]
Abstract:
Laser communication links in space are attractive alternatives to present-day microwave links. This tutorial
describes the basic concept and the functions of an optical terminal on board a spacecraft. It points out the differences
between free-space optical links on one hand and glass fiber systems and microwave directional links on the other
hand. The requirements on data transmitters and receivers as well as on optical antennas and pointing, acquisition and
tracking mechanisms are discussed. Typical application scenarios are outlined, experimental systems and their
technologies are cited.
Key Words: laser communications, free space, intersatellite links, space communication, space networks
1. 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 terahertz since we
employ lasers in fiber systems. The main driving force
was that the usable bandwidth - and hence
transmission capacity - increases proportional to the
carrier frequency. Another asset comes into play in
free-space point-to-point links. The minimum
divergence obtainable with a freely propagating beam
of electromagnetic waves scales proportional to the
wavelength. The jump from microwaves to light
waves therefore means a reduction in beamwidth by
orders of magnitude, even if we use transmit antennas
of much smaller diameter. The reduced beamwidth
does not only imply increased intensity at the receiver
site but also reduced cross talk between closely
operating links and less chance for eavesdropping.
Space communication, as employed in satellite-tosatellite
links, is traditionally performed using
microwaves. For more than twenty five years,
however, laser systems are being investigated as
alternatives. 1-3) One hopes that mass, power
consumption, and size of an optical transceiver
module will be smaller than that of a microwave
transceiver. Also, fuel consumption for satellite
attitude control when quickly re-directing antennas
should be less for optical antennas. On the other hand,
a new set of problems had to be addressed in
connection with the extreme requirements for
pointing, acquiring, and tracking the narrow-width
laser beams.
In this tutorial we will first discuss the basics of an
optical free space page link (Sect. 2) and then point out the
differences to terrestrial fiber systems and to
microwave links in Sect. 3. Section 4 presents the
requirements for and the available technologies to
implement transmitters, receivers, optical antennas, as
well as the PAT system (PAT...pointing, acquisition,
and tracking). Next we sketch application scenarios,
and we conclude with both a glimpse onto past and
future system technologies.
2. System Layout
A scenario typical for the transmission system in
question asks for point-to-point data transfer between
two spacecraft (see Fig. 1). The distances to be
bridged may extend anywhere from a few hundred
kilometers to 70 000 km (e.g. in near-earth
applications) up to millions of kilometers in case of
signals transmitted by a space probe.4) Today the data
rates in mind range from several hundred kbit/s to
some 10 Gbit/s.
Terminals for optical communication in space are
mostly designed for bi-directional links, at least
concerning the optical tracking function. They
comprise both a transmitter and a receiver that
generally share the optical antenna. Another
peculiarity is the necessity of beam steering (or
pointing) capability with sub-microradian angular
resolution and possibly with an angular coverage
exceeding a hemisphere.
These requirements lead to a transceiver block
diagram as shown in Fig. 2. The light source S is a
laser, preferably operating in a single transverse mode
in order to achieve the highest possible antenna gain.
If the laser operates continuously or in a pulsed mode
producing a periodic pulse train, an external modulator
(M) is utilized to impress the data signal onto the
beam. Alternatively, internal modulation may be
employed with some lasers. The modulated beam
passes an optical duplexer (DUP) and a fine pointing
assembly (FPA) before it enters a telescope acting
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#3
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#4

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