Tech Guide

Space Laser Communications:

Systems, Technologies, and Applications
Walter R. LEEB
Institut für Nachrichtentechnik und Hochfrequenztechnik, Technische Universität Wien,
Gusshausstrasse 25/389, A-1040 Wien, Austria

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-to satellite 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 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 2 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.
3. Peculiarities
Some of the readers may be more familiar with
fiber-based optical transmission systems, others with
conventional satellite links employing microwaves.
The following two sections serve to point out basic
differences of laser space communications with these
systems.
3.1 Differences to fiber systems
While in fiber systems dispersion and non-linearity
is a major concern, no such effects exist for the free
space channel. Coupling the transmit signal into the
channel - which is free space - requires an antenna,
usually in the form of a telescope. Further, background
radiation - e.g. caused by the Sun - may pose a
problem, and, of course, no in-line amplifiers or
regenerators can be implemented.
If a space link is to sustain data transmission in
both directions simultaneously and if the terminal has
only a single antenna, a duplexing element must
separate the transmit and the receive beam in the
transceiver (see Fig.2). The degree of isolation it has
to provide for the transmit beam not to reach the own
terminal's data receiver is quite large: For a transmit
power of 500mW and a receive power of 5 nW, the
degree of isolation should clearly exceed 90 dB to
make cross talk negligible. Extremely low stray light
DUP
S M
FPA ANT
CPA
PAA
DD ATE
BS ATD
DE
RECEIVED
DATA
OPTICAL BEAM
ELECTRICAL
CONNECTION
ACQUISITION
SEQUENCE
INITIALISTATION
POINT
AHEAD
INPUT
TRANSMIT
DATA
IN
OUT
3
levels of the duplexer are an essential prerequisite.
Duplexers can be based on spectral discrimination (i.e.
filtering), on polarization diversity, or on both. Hence
a common suggestion is to use left hand and right
hand circularly polarized light for the two directions,
respectively. This also makes the transmission
insensitive against rotation of the terminals along their
antenna axes. Because polarization duplexing will
provide only some 15 dB of isolation, wavelength
duplexing must be designed into the system in any
case.
For the general case that both terminals experience
a relative velocity along the line-of-sight, vD, the
Doppler effect will yield a frequency shift Δf in the
received signal. As long as vD << c (c ... velocity of
light), one has Δf = vD/λ where λ is the carrier
wavelength. In a LEO-GEO* link, vD may amount up
to some ± 8⋅103 m/s. Because of the small wavelength,
the resulting Doppler shift is large and amounts up to
± 7.5 GHz at λ = 1.06 μm for the example cited. Such
a large frequency shift might be negligible in a direct
detection receiver (as long as no extremely narrow
optical filtering is applied). In a heterodyne receiver,**
however, the frequency shift has to be compensated by
either tuning the local laser oscillator, by tuning the
electrical oscillator in a second intermediate frequency
stage, or by both.
In space applications - even more than in undersea
fiber systems - reliability and lifetime is of special
importance. As examples, the laser source itself or a
(cooled) detector may represent a weak point
concerning reliability and thus require redundancy.
Other subunits, like the telescope or the coarse
pointing assembly may be too bulky and present such
a high fraction of the mass budget that a single failure
point is accepted in their case.
3.2 Differences to microwave systems
At a first glance, the equation governing the
amount of power received in an optical directional
link, PR, is the same as one knows from microwave
links, namely
R T T R LT LR LP
R
P P G G
2
4




Ï€
= λ . (1)
Here PT is the optical output power generated at the
transmitter, GT and GR are the gain values of the
transmit and receive antenna, λ is the carrier
wavelength, R the distance between the terminals and
the factors LT and LR cover the loss within the transmit
and receive terminal. However, the last factor, LP,
which accounts for loss caused by non-ideal pointing,
may correspond to several dB in a free-space laser
link: Because of the extremely small beamwidths
involved in optical links, transmit and receive antenna
will, in general, not yield their maximum gain. Despite
the implementation of an active tracking control loop
* LEO..low earth orbiting (satellite), GEO..geostationary (satellite),
see also Sect. 5
** see Sect. 4.2
to align the antenna axes, some mispointing will
persist and the receive intensity will vary statistically.
To a first approximation, the antenna gains GT, GR
are related to the diameters of the (circular) transmit
and receive antenna, DT, DR as
2
 

 

λ
Ï€
= T,R
T,R
D
G . (2)
Substituting (2) into (1) reveals the 1/λ2-dependence
of receive power PR which makes the optical regime
so attractive compared to microwaves. Equation (2) is
applicable in case of diffraction limited antenna
operation. The full beam divergence then obtained is
on the order of
DT
θ ≈ λ . (3)
The very small beamwidths θ at optical frequencies
(some 5 μrad for typical values of λ and DT) are, of
course, the reason for the high antenna gain achievable
(some 115 dB). However, this advantage is not gained
for free: Establishing and maintaining contact with
extremely narrow beams is a tough task, especially if
transmitter and receiver change their relative position
(see Sect. 4.4).
One critical aspect of intersatellite laser
communications with narrow beams results from the
need to introduce a point ahead angle. Because of the
finite velocity of light (c) and the relative angular
velocity of two communication terminals moving in
space, the transmit beam must be directed towards the
receiver's position it will have at some later time. This
point ahead angle is given by 5)
c
2 vR
β = , (4)
where vR is the relative velocity component of
transmitter and receiver, orthogonal to the line-ofsight,
as illustrated in Fig. 3. Point ahead is generally
required in both dimensions. It amounts up to 40 μrad
for a GEO-GEO link and up to 70 μrad for a LEOGEO
link and may thus be appreciably larger than the
beamwidth. The point ahead angle can be introduced
in either the receive or the transmit path of each
transceiver and must be adjustable if vR varies with
time. It is difficult to design a control loop for
automatic adjustment of point ahead. Therefore today's
concepts rely on the calculation of point ahead angles
from known ephemeris data and on open loop
implementation.
4. Requirements and technology
4.1 Data transmitter
The main parameters characterizing the optical
source are wavelength, output power, transverse mode,
4
Fig. 3 Point ahead angle β for space craft S1 and
S2 that have a relative velocity component
vR orthogonal to the line of sight. Shown
with dotted lines: position of S2 at time
instants indicated (L..distance, c..velocity of
light).
polarization, linewidth, and modulation capability. A
smaller wavelength requires increased surface quality
of optical elements which in turn asks for bulkier
devices if diffraction limited operation is essential.
Thus the mass of the antenna (and hence the load for
the coarse pointing assembly) is strongly influenced
by the choice of λ. Also, the wavelength dependence
of the sensitivity of available optical receivers must be
considered. The output power will have to be in the
range of 100 mW and 1 W, depending on the link
distance and data rate. It should be available in a
single transverse mode to achieve maximum on-axis
antenna gain, and in a single longitudinal mode to
obtain optimum spectral efficiency. For coherent
reception, phase noise is detrimental and thus a narrow
linewidth of both the transmitter laser and the local
laser oscillator in the receiver is required. The usually
linear state of polarization emitted by the laser source
is to be converted into circular polarization before the
beam leaves the terminal (see Sect. 3.1). Modulation
may be achieved directly (e.g. in case of diode lasers
and moderate data rates) or with an external
modulator. Especially in connection with a subsequent
optical booster amplifier, the insertion loss introduced
by an electro-optic or acousto-optic modulator may be
tolerable. As with fiber systems, binary modulation
formats are envisaged for space links. In connection
with a coherent receiver, phase shift keying (and
possibly frequency shift keying) is an attractive
alternative to on-off keying, as it makes better use of
the carrier power.
4.2 Data receiver
For space applications, good receiver sensitivity is
an extremely valuable asset, not at least because no inline
amplification is possible. It is often characterized
by the minimum number of input photons per bit to
achieve a bit error probability of 10-6. If other sources
of noise than that due to the quantum nature of
radiation are negligible, a direct detection receiver
needs n = 6.6 photons/bit. As an example for a
coherent receiver, a homodyne receiver with PSK
modulation would require n = 5.6 photons/bit.*** To
what extent this quantum limit is reached in practice
depends on the engineer’s ability to make negligible
the effect of other noise contributions, as there is
– excess noise in avalanche photodiodes (APDs),
– optical preamplifier noise (amplified spontaneous
emission),
– transistor noise and circuit noise in the receiver
electronics,
– laser phase noise,
– transmit-receive cross coupling,
– background radiation.
Today direct receivers employing APDs can be
used up to 2.5 Gbit/s. Their sensitivity is determined
by electronic and by multiplication noise and may be
less than 100 photons/bit at low data rates.6) With
optical preamplification by an Erbium-doped fiber
amplifier, direct receivers have shown sensitivities of
40 photons/bit at 10 Gbit/s.7)
With coherent reception, the received optical field
is transposed into the electrical regime (intermediate
frequency, IF) by mixing it with the field of a local
laser oscillator.8) A photodetector serves as mixer
element. Information is preserved not only about
amplitude but also about frequency and phase of the
received field, hence frequency and phase modulated
optical signals can be detected, too.✝ As optical mixers
have sensitive areas with dimensions large compared
to the wavelength, in the optical regime the spatial
modes of received and local field have to be matched
to obtain maximum IF signal. Matching requires
identical polarization and asks for equal amplitude and
phase distribution, the latter two optimized with
respect to the mixer element.
Coherent receivers perfectly reject radiation from
other than the nominal input direction. Equally well
they discriminate against unwanted spectral
components by their IF filter. Therefore they are a
priori less sensitive against background radiation and
cross talk.✝✝ An experimental heterodyne receiver with
phase shift keying at 565 Mbit/s has demonstrated a
sensitivity of 22 photons/bit.9)
4.3 Antennas
The transmit antenna is essentially a telescope
which magnifies the diameter of the beam emitted by
the laser (or by a booster amplifier). This beam is
generally well modeled by a Gaussian intensity
distribution. The antenna will not only introduce
truncation via its finite diameter DT but may also cause
some central obscuration, depending on the telescope's
construction. These two effects reduce the ideal onaxis
antenna gain given by equ. (2) by typically 1.5
*** In both cases binary signaling and equally probable marks and
zeros are assumed.
✝ For high-data rate frequency-shifted and differential-phase-shifted
signals, optical demodulation in combination with a direct
receiver is feasible, too.
✝✝ The same degree of spatial and spectral filtering is obtained with a
direct receiver equipped with a singlemode spatial filter (e.g. a
fiber preamplifier) and an optical filter matched to the data
spectrum.
t
s1 t
0
0
t -
t +
0
0
L
L
c
c
s2
β
L
vR
5
dB.10) The antenna pattern resembles that of an Airy
pattern. Alignment tolerances of the optical elements
constituting the telescope are usually very tight, as the
output beam has to be perfectly collimated for
maximum gain.
The main specifications of the optical antenna are:
diameter of primary mirror (or lens), magnification,
aberrations, wavelength dependence of throughput,
sensitivity to temperature changes and gradients, and
stray light level. Usually, refractive telescopes are
envisaged in case of small diameters while reflective
systems are preferred for diameters exceeding several
centimeters. With increasing antenna aperture it
becomes more and more difficult (and expensive) to
meet specifications. Large antennas will also increase
the mass and size of an optical transceiver
considerably, as the telescope and the coarse pointing
assembly do contribute appreciably to these
characteristics. Presently it is felt that the diameter of
diffraction limited antennas should not exceed some
25 cm for free-space laser links. Coarse pointing may
be accomplished via gimbal mounting the antenna or
by a separate unit consisting of two orthogonally
mounted steering mirrors or one gimbaled reflector.
4.4 Pointing, acquisition, and tracking.
To establish an optical link in space, a
sophisticated spatial pointing and acquisition
procedure must be initiated. Information on the
position of the two space terminals has to be
available. Still, because of position uncertainty and
incomplete knowledge of the spacecraft's orientation
(attitude uncertainty), one terminal's beam width has
to be widened deliberately as to illuminate the second
terminal despite the uncertainty in position. A spatial
search operation by the (narrow beam) receive path of
the second, and subsequently, of the first terminal
have to follow before acquisition is completed and
switching to the tracking mode can occur. Wide-fieldof-
view acquisition detectors in the form CCDs are
most helpful.
During data transmission, the angle between the
line-of-sight and the transmit beam axis must be kept
to within a fraction of the transmit beamwidth θ which
may be as small as a few μrad. To maintain sufficient
alignment of the transmit and receive antennas despite
platform vibrations, both terminals have to be
equipped with a tracking servo loop. Optical beacons
have to be provided in both directions to render input
information for the control loops. The data carrying
beams themselves may serve as beacon, or separate
optical beams may be implemented, e.g. in a one-way
link. Tracking should ensure a mispointing of typically
less than 1 μrad. Whenever the tracking loop signals
optimum receive position, the transmitted beam (or
beacon) will be correctly directed to the opposite
terminal. This would require a perfect coaxial
alignment for the optical transmit and receive path
within each transceiver. However, some bias, or point
ahead angle, has in general to be introduced into the
alignment, as was discussed in Sect. 3.2. To ensure
short acquisition time and adequate tracking accuracy,
sufficient optical power for the acquisition and the
tracking process must be received.
5. Application scenarios
One of the first scenarios considered was a bidirectional,
symmetric link between two geostationary
satellites (GEOs). The orbital distance between the
GEO satellites may lie anywhere between a few
degrees and some 120°, corresponding to distances
between a few thousand kilometers and 75 000 km
(see Fig. 4a). Such a link has the attractive features of
a single (or very seldom) acquisition process, of a
nominally zero Doppler shift, and of low angular
tracking velocities. Connections to ground stations
could be performed with microwaves.
Fig. 4 Two geostationary satellites (GEO1,
GEO2) are connected by a laser duplex link (a).
A low-earth orbiting satellite (LEO) transmits
data via a laser link to a GEO acting as data relay
(b). In both cases the downlink is via microwaves
(μW).
Large data streams generated on a low-earthorbiting
satellite (a LEO, with a distance to ground of
less than 1000 km) may advantageously be transmitted
to a GEO acting as a relay before being directed to the
earth via microwaves (see Fig. 4b). Distances for this
asymmetric link may be as large as 45 000 km. The
concept allows continual data transfer to a single earth
station for at least half a LEO orbit.
Another use of a laser data link was already
included in the upper part of Fig. 1. Characterized by
very large distances (e.g. millions of kilometers) and
by relatively low data rates (e.g. some 100 kbit/s),
such a link would serve to transfer data from
interplanetary and deep space probes to relay satellites
orbiting the earth. This relay could be equipped with a
large receive telescope. Further transport to ground
stations would use microwaves. As an alternative, an
optical ground station would receive the probe's data
after passage through the atmosphere.
For satellite networks now being planned or
established to serve mobile data transfer,
interconnectivity at very high data rates could be
achieved by optical links (see Fig. 1). Frequency
GEO1
GEO2
LEO
GEO
LASER
LASER
μW
μW
μW
a)
b)
6
allocation problems - as they persist increasingly for
radio links - are practically non-existent, with the
merit of negligible mutual interference. Another
advantage is the expected smaller mass and volume of
optical terminals.
6. System technologies
The almost three decades of efforts towards
intersatellite laser links have seen various
technologies,1-3) starting from those based on lamppumped,
mode-locked Nd:YAG lasers,11) on CO2
lasers 5) operating at λ = 10 μm, on GaAlAs diodes
(0.85 μm), up to those employing diode-pumped
Nd:YAG lasers (λ = 1.06 μm) 12) and InGaAsP
semiconductors operating at λ = 1.5 μm.
Only a few experimental systems have been
launched so far. The European space agency, ESA, has
put a terminal on SPOT IV, a LEO earth observation
satellite.13) It employs a diode laser at λ = 0.85 μm and
shall transmit data at 50 Mbit/s. The counter terminal
still awaits its launch on board of the GEO satellite
ARTEMIS. The development of this system, dubbed
SILEX (semiconductor laser intersatellite link
experiment), started as early as 1985. Japan will
participate in this experiment by launching, in 2001, a
dedicated satellite named OICETS. This LEO satellite
is equipped with an optical terminal to communicate
with ARTEMIS. - Between 1994 and 1996 a laser link
was tested between a terminal placed on the Japanese
test satellite ETS-VI and ground stations in Tokyo and
in California, although the satellite did not reach the
intended GEO orbit but a highly elliptical one.14) The
down link operated with a diode laser, the up link with
an Argon laser.
For future applications, systems based on
Nd:YAG lasers 12) and on diode lasers at 1.5 μm in
connection with Erbium-doped fiber amplifiers 7) are
investigated presently. With the specific properties
inherent to these laser sources, they lend themselves
especially to coherent detection and to optically preamplified
direct detection, respectively.
In the future one should take into consideration not
only recent technological developments like optical
demodulation of phase modulated signals, the use of
low-duty-cycle return-to-zero coding, or a
combination of both. One should also give serious
thoughts to use the large, mature, and reliable
technology base commercially available in the 1.5 μm
band. Only then one can hope to achieve economy in
medium-scale applications like intersatellite networks.
Acknowledgment
The author appreciates stimulating discussions with K.
Kudielka and P. Winzer.
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