11-09-2014, 02:26 PM
Space Laser Communications:
Systems, Technologies, and Applications
[attachment=67836]
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.
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.
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
Systems, Technologies, and Applications
[attachment=67836]
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.
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.
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